Porous Electroactive Material
The present invention relates to an electroactive material comprising silicon; the use of such
a material in the preparation of an electrode; an electrode including the electroactive silicon
material of the invention; the use of an electrode in the preparation of an electrochemical cell
and to an electrochemical cell or battery including such an electrode.
1.Background
Lithium ion rechargeable batteries are well known. The basic construction of a lithium ion
rechargeable battery is shown in Figure 1. The battery cell includes a single cell, but may
include multiple cells.
The battery cell generally comprises a copper current collector 10 for the anode and an
aluminium current collector 2 for the cathode, which are externally connectable to a load or
to a recharging source as appropriate. It should be noted that the terms "anode" and
"cathode" are used in the present specification as those terms are understood in the context
of batteries placed across a load, i.e. the term "anode" denotes the negative pole and the
term "cathode" the positive pole of the battery. A graphite-based composite anode layer 4
overlays the current collector 10 and a lithium containing metal oxide-based composite
cathode layer 16 overlays the current collector 12. A porous plastic spacer or separator 20
is provided between the graphite-based composite anode layer 14 and a lithium containing
metal oxide-based composite cathode layer 16: a liquid electrolyte material is dispersed
within the porous plastic spacer or separator 20, the composite anode layer 14 and the
composite cathode layer 16. In some cases, the porous plastic spacer or separator 20 may
be replaced by a polymer electrolyte material and in such cases the polymer electrolyte
material is present within both the composite anode layer 14 and the composite cathode
layer 16.
When the battery cell is fully charged, lithium has been transported from the lithium
containing metal oxide in the cathode via the electrolyte into the graphite-based anode
where it is intercalated by reacting with the graphite to create a lithium carbon compound,
typically LiC . The graphite, being the electrochemically active material in the composite
anode layer, has a theoretical maximum capacity of 372 mAh/g.
The use of silicon as an active anode material in secondary batteries such as lithium ion
batteries is well known (see, for example, Insertion Electrode Materials for Rechargeable
Lithium Batteries, M. Winter, J.O. Besenhard, M.E. Spahr, and P. Novak in Adv. Mater.
1998, 10, No. 10 and also Wang, Kasavajjula et al, J. Power Source.s 163 (2007) 1003-
1039). It is generally believed that silicon, when used as an active anode material in a
lithium-ion rechargeable cell, can provide a significantly higher capacity than the currently
used graphite anode materials. Silicon, when converted to the compound Li2iSi 5 by.reaction
with lithium in an electrochemical cell, has a theoretical maximum capacity of 4,200 mAh/g,
considerably higher than the maximum capacity for graphite.
Early approaches of using silicon or silicon based active anode materials in a lithium ion
electrochemical cell included the use of bulk silicon anodes, silicon powder anodes
comprising nanometer and micron sized silicon powders, thin film silicon anodes and silicon
anodes comprising silicon structures other than or in addition to powders. Composite anodes
comprising a dispersion of silicon in an inactive or active matrix material have also been
investigated. However, many of the approaches have failed to show sustained or adequate
capacity over the required number of charge/discharge cycles.
Electrodes comprising bulk silicon failed to exhibit good capacity retention and cycle-ability
over a number of charging and discharging cycles. This poor performance was attributed to
the mechanical stresses that arise within the electrode structure during the charging cycle.
Intercalation or insertion of lithium ions into the bulk silicon structure during the charging
cycle causes a massive expansion of the silicon containing material, which leads to a build¬
up of mechanical stress within the electrode structure and eventually causes cracking,
delamination and loss of contact within and between the components of the electrode
structure and the current collector respectively.
It should be understood that the term "intercalation" when used in relation to electroactive
materials, particularly the silicon-containing materials, referred to herein includes a process
where lithium is inserted into and disrupts the structure of the crystalline or amorphous
silicon-containing material as well as a process in which lithium is dispersed between crystal
planes defining the silicon-containing structure. The former process is more properly referred
to as lithium insertion and is observed for materials comprising pure or substantially pure
crystalline, amorphous and/or polycrystalline silicon. Some compounds or alloys of silicon
will, however, also exhibit this form of behaviour. The dispersion of lithium between crystal
planes within a crystalline or polycrystalline silicon-containing material is more often referred
to as "intercalation" and is usually observed for materials comprising compounds or alloys of
silicon.
In an attempt to overcome the stresses associated with bulk silicon anodes, anodes
including silicon structures that are more easily able to accommodate the volume changes
that occur on charging have been fabricated.
One of the earlier approaches employed anodes comprising pure silicon powder. Although it
was expected that anodes fabricated from silicon powder would be better able to
accommodate the volume expansion associated with lithium intercalation or insertion
compared to bulk silicon electrodes, it was found that, in practice, these electrodes fared
little better than bulk silicon electrodes and breakdown of the electronically conductive
network due to the expansion of silicon powder particles was also observed.
In an attempt to improve the electronic contact between anode components during the
charging and discharging of the cell, composite anodes comprising a mixture of powdered
silicon and additional components such as a conductive material, a binder and optionally a
further electroactive material were prepared. It was anticipated that these further
components would be able to suppress and/or accommodate the large volume changes
associated with the silicon species during the charging and discharging cycles of the cell.
However, these electrodes were found to exhibit a reduced capacity compared with
electrodes comprising silicon only and were unable to maintain this capacity over a required
number of charging and discharging cycles.
In one prior art approach described by Ohara et al. (Journal of Power Sources 136 (2004)
303-306) which addresses the problems associated with the expansion and contraction of
silicon during the charging and discharging cycles of the battery, silicon is evaporated onto a
nickel foil current collector as a thin film and this structure is then used to form the anode of
a lithium ion cell. However, although this approach gives good capacity retention, this is the
case for only very thin films and thus the structures do not give usable amounts of capacity
per unit area and increasing the film thickness to give usable amounts of capacity per unit
area causes the good capacity retention to be eliminated due to mechanical breakdown as a
result of the large volume expansion within the film.
Another approach used to address the problems associated with expansion of the silicon film
is described in US 6,887,51 1: Silicon is evaporated onto a roughened copper substrate to
create medium thickness films of up to 10 m. During the initial lithium ion insertion process
the silicon film breaks up to form columns of silicon. These columns can then reversibly react
with lithium ions and good capacity retention is achieved. However, the process does not
function well with thicker films and the creation of the medium thickness film is an expensive
process. Furthermore the columnar structure caused by the break-up of the film has no
inherent porosity, which means that over time the pillars will, themselves, begin to crack and
the electrode structure will likely not exhibit long term capacity retention.
In an attempt to overcome the problems associated with the bulk silicon, silicon powder and
thin film silicon anodes described above, many workers have investigated alternative silicon
and anode structures for the fabrication of anodes for lithium ion batteries. Examples of
silicon structures investigated include arrays of silicon pillars formed on wafers and particles;
silicon fibres, rods, tubes or wires; and complete porous particles comprising silicon. Anode
structures having pores or channels formed therein have also been investigated.
US 6,334,939 and US 6,514,395 each disclose silicon based nano-structures for use as
anode materials in lithium ion secondary batteries. Such nano-structures include cage-like
spherical particles and rods or wires having diameters in the range 1 to 50nm and lengths in
range 500nm to 10 m. Similar nanostructures are disclosed in KR 1020027017125 and ZL
01814166.8. JP 04035760 discloses silicon based anode materials comprising carboncoated
silicon fibres having diameters in the range 10nm to 50 m for use in lithium ion
secondary batteries. Batteries prepared using these nano-structures exhibited a total first
cycle charging capacity of 1300 mAh/g and a reversible capacity of 800mAh/g.
US 2007/0281216 discloses an anode active material for a lithium secondary battery
comprising a mixture of silicon nano-particles, graphite, carbon black and a binder. The
silicon nano-particles comprise either thread-like aggregates (a chain of connected
spheroidal particles) having a primary particle size in the range 20 to 200nm and a specific
surface area of 11m /g or spherical particles having a primary particle size in the range 5 to
50nm and a specific surface area of 170m /g. The silicon particles and threads are prepared
using techniques such as chemical vapour deposition. Anodes exhibiting a capacity of up to
1000mAh/g over 50 cycles are illustrated. The life of the battery is significantly increased if
the battery is operated at a limited voltage level.
Polycrystalline silicon nano-wires and wires having cross-sectional diameters in the range 20
to 500nm and aspect ratios of greater than 10, 50 or 100 and which have been prepared
using epitaxial and non-epitaxial growth techniques are disclosed in US 7,273,732.
Single crystalline silicon fibres, pillars or rods having diameters in the range 0.1 to 1 m and
lengths in the range 1 to 0 m can also be prepared using lithographic and etching
techniques as disclosed in US 7,402,829. Alternative etching techniques such as those
disclosed in WO 2007/083155, WO 2009/010758 and WO 2010/040985 can also be used.
The fibres, wires and rods described above are typically formed into a composite material
containing, in addition to the silicon rods, wires and fibres, additional ingredients such as a
binder, a conductive material and optionally a further electroactive material other than
silicon. The composite material is also known as an anode mix and is typically used in the
fabrication of anodes for lithium ion batteries. In accordance with the disclosure of the
present inventors in WO 2009/010758 and WO 2009/010757 anode materials comprising
silicon fibres or rods are preferably in the form of an entangled "felt" or "mat" in which silicon
fibres are randomly connected with each other either directly or indirectly through the other
components of the mix, and are also connected with the copper foil which acts as the current
collector of the electrode.
By the term "felt or mat" it should be understood to mean a structure in which any one of the
components of the structure is connected in a random or ordered manner with one or more
other components of the structure so that there are multiple interconnections between the
components. The mat may be provided in the form of a coating layer which is directly or
indirectly applied, bonded or connected to a current collector or it may be in the form of a
self-supporting structure, although this is less preferred. Preferably a felt or mat comprises
one or more species of fibre as these help to strengthen the overall structure.
It has been observed by the present inventors that these felt structures produced using the
silicon rod, wire and fibre products described above have an inherent porosity, (that is they
contain voids or spaces between the fibres) as a result of the maximum attainable packing
density of a random arrangement of fibres within a defined volume. These inherently porous
electrodes were found to exhibit better capacity retention and cycling lifetimes compared to
electrodes produced from bulk silicon, silicon powders and silicon films, for example. Without
wishing to be constrained by theory, it is believed that the inherent porosity of these
electrode structures provides at least some of the silicon components of the anode with
space to expand into the voids or pores that are part of the electrode structure rather than
push against each other during lithium intercalation or insertion (charging). The pores of the
electrode are therefore able to accommodate the expansion of these silicon components
during lithium intercalation or insertion within the volume initially occupied by the uncharged
anode material, thereby reducing the volume increase within the electrode structure, the
build up of stress and the application of pressure on the other cell components during the
charging and discharging cycle As a result there will be less cracking of the silicon structures
within the anode and a reduction in the extent of delamination of the electrode coating from
the current collector, leading to better capacity retention and cycle-ability. The pores or voids
also facilitate penetration of and therefore contact of the electrolyte with as much of the
surface of the silicon material as possible during charging and discharging of the anode. This
porosity is therefore believed to be important as it provides a path by which the lithium can
be intercalated (or inserted) into the whole of the silicon material so that the lithiation of the
silicon is as uniform as possible throughout the anode mass.
In addition to using silicon rods and fibres for the fabrication of porous electrode structures, it
is also known to use silicon components which are themselves porous in the fabrication of
porous electrodes or to form holes or channels into silicon based electrode structures having
minimal porosity.
US 2009/0253033 discloses anode active materials having an inherent porosity for use in
lithium ion secondary batteries. The anode material comprises silicon or silicon alloy
particles with dimensions of between 500nm and 20pm and a binder or binder precursor.
These particles are manufactured using techniques such as vapour deposition, liquid phase
deposition or spraying techniques. During anode fabrication, the silicon/binder composite is
heat treated to carbonise or partially carbonise the binder component thereby providing the
anode with an inherent porosity. In a preferred embodiment the anodes of US 2009/0253033
include pores having dimensions in the range 30nm to 5000nm in order to accommodate the
expansion of the silicon material during the charging and discharging phases of the battery.
Anodes prepared using such silicon materials exhibit a capacity retention of from 70 to 89%
and an expansion coefficient of 1 to 1.3.
Porous silicon anodes created by electrochemically etching channels into a silicon wafer
have also been prepared. See, for example, HC Shin et al, J. Power Sources 139 (2005)
314-320. Electrolyte penetration was observed for channels having a pore diameter of 1 to
1.5 m. It was observed that the peak current and the charge transferred during cyclic
voltammetry increased with channel depth up to a limit. The amount of charge transferred for
channels having an aspect ratio (channel depth to pore diameter) of the order of 1 was found
to be only marginally less than those having an aspect ratio of 5. It was suggested that the
channel walls were able to participate in the lithiation/delithiation and that the presence of
channels effectively increased the reactive area of the electrode. The porous structure
remained essentially the same after a number of charge/discharge cycles despite the
volume changes occurring as a result of the intercalation or insertion and release of lithium
during these cycles. The channels created by electrochemical etching of a silicon wafer differ
from the pores or voids created upon formation of a meshed electrode material using silicon
fibres, wires and rods as described above in WO 2009/101758 and WO 2009/040985. The
electrochemically etched electrode material is rigid and the entire volume of the electrode
material will expand upon lithium intercalation or insertion. In contrast the voids within the
meshed electrode material are able to contract and expand in response to the increase and
decrease in the volume of the mesh comprising silicon components during lithium
intercalation or insertion and release respectively. This means that silicon mesh type
electrodes are more able to accommodate volume changes within the electrode structure
upon lithium intercalation or insertion.
Rigid electrode structures such a s those prepared by Shin et al tend to be associated with a
build up of stress within the electrode structure on lithium intercalation or insertion as a result
of the isotropic volume expansion of the entire electrode material. Providing the voids within
the electrode structure are sufficiently open, the silicon mesh provides access for the
electrolyte into the bulk of the electroactive anode. In contrast the more flexible meshed
electrode structures including voids as described above are more able to accommodate
expansion of the silicon material on lithium intercalation or insertion due the contraction and
expansion of voids as described above. The overall expansion of a meshed electrode
structure is therefore significantly less than that of the rigid channelled electrode structure
described by Shin et al. This means that there will less build up of stress within meshed
electrode structures compared to rigid electrode structures.
Porous silicon particles are also known and have been investigated for use in lithium ion
batteries. The cost of manufacturing these particles is believed to be less than the cost of
manufacturing alternative silicon structures such as silicon fibres, ribbons or pillared
particles, for example. However, the life cycle performance of many of the composite
electrodes prepared to date, which comprise porous silicon particles needs to be significantly
improved before such electrodes could be considered to be commercially viable.
Porous silicon particles having dimensions in the range 4 to m, an average pore sizes of
6 to 8A and a BET surface area of from 4 1 to 1 3m /g have been prepared for use in fields
such as drug delivery and explosive design (Subramanian et al, Nanoporous Silicon Based
Energetic Materials, Vesta Sciences NJ 08852 Kapoor and Redner, US Army RDE-COMARDEC
Picatinny Arsenal NJ 07806, Proceedings of the Army Science Conference (26 th)
Orlando, Florida, 1-4 December 2008). There is no indication in Subramanian et al that their
silicon containing porous particles would be suitable for use in the fabrication of lithium ion
batteries.
Silicon nanosponge particles having a network of pores extending through the particle
structure have also been prepared, US 7,569,202. Nanosponge particles having a diameter
of 1 to 4 m and pore diameters of 2 to 8nm are prepared by stain etching metallurgical
grade silicon powders to remove both silicon material and impurities. It is believed that the
impurities in the metallurgical grade silicon are preferentially etched away to give particles
having a network of pores distributed throughout. The nanosponge particles can be surface
treated to introduce functional groups onto the silicon surface. US 7,569,202 teaches that
the surface functional groups enable the nanosponge particles to be used for a broad range
of applications from drug delivery to explosives. US 7,569,202 does not teach the application
of nanosponge particles in lithium ion batteries.
US 7,244,513 discloses a partially porous silicon powder comprising silicon particles having
a solid silicon core and an outermost layer of porous silicon. These partially porous silicon
particles are prepared by stain etching particles having a dimension in the range m to
1mm to give partially porous particles having a porous outer shell in which the pore
dimensions in the range 1nm to 100nm. The partially porous particles are then subjected to
ultrasonic agitation to give silicon nanoparticles having a dimension in the range 10nm to
50nm. US 7,244,513 teaches that the nanoparticles could be used in applications such as
sensors, floating gate memory devices, display devices and biophysics. There is no
suggestion that these nanoparticles could be used in the fabrication of a lithium ion battery.
US 2004/0214085 discloses an anode material comprising an aggregate of porous particles
that is capable of withstanding pulverization during the charging and discharging cycles of
the battery. According to US 2004/0214085, the reason why the particles are able to
withstand pulverisation is because the external volume of the porous particle is maintained
during the charging and discharging cycle of the battery due to compression of the particle
voids when the particle expands during the process of intercalating lithium ions into silicon.
The porous particles in the aggregate have an average particle size in the range 1 m to 100
pm and pore sizes in the range 1n to 10 . For particles having diameters of less than
1pm the relative volume of the pores within the particle is excessive and the hardness of the
particle is compromised. Particles having a diameter of more than 100 m are unable to
accommodate the volume changes associated with the intercalation or insertion and
deintercalation or release of lithium and cannot prevent pulverisation of the particle. The
particles are prepared by quenching an alloy of silicon with another element, M to form a
quenched alloy particle comprising an amorphous silicon phase and an element, M, which
can be eluted from the particle to provide a porous particle. 50:50 and 80:20 silicon-nickel
alloys and 70:30 AI:Si alloys were used to prepare alloy containing particles using a gas
atomisation technique in which a helium gas pressure of 80kg/cm 2 and a quenching rate of
1x105 K/s was used. The quenched particles were washed in acid (H2S0 4 or HCI) to remove
either the Ni or the A to give porous particles, which contained a mixture of both amorphous
and crystalline silicon. Batteries prepared using the Si porous materials of US 2004/0214085
have a capacity retention of between 83 and 95% over 30 cycles.
The porous particle of US 2004/0214085 is characterised by the ratio of the pore diameter,
n, to the particle diameter, N and the volume ratio of the voids to the porous particle. n/N is
preferably in the range 0.001 to 0.2 so that the diameter of the pores within the particles is
very small in order that the hardness of the particle can be maintained. The volume ratio of
the voids to the porous particle is preferably in the ratio 0.1% to 80% so that the expansion
and contraction of the silicon volume during intercalation or insertion and deintercalation or
releaseof lithium is fully compensated by the voids, the entire volume of the porous particle
is maintained and the particles are not degenerated.
US 7,581 ,086 discloses an electrode material comprising porous silicon particles, which
particles are prepared by quenching a eutectic alloy of silicon and another metal (typically
aluminium) using a roll solidification method at a cooling rate of greater than 0OK/s to give a
thin film alloy sheet. The thin film is pulverised to give alloy particles having a typical
diameter of m i, which are typically etched in HCI to give porous Si particles. Electrode
materials prepared from these powder particles exhibited a capacity retention of
approximately 68% at 0 cycles.
US 2009/01 86267 discloses an anode material for a lithium ion battery, the anode material
comprising porous silicon particles dispersed in a conductive matrix. The porous silicon
particles have a diameter in the range 1 to 0pm, pore diameters in the range 1 to 00nm
(preferably 5nm), a BET surface area value in the range 140 to 250m 2/g and crystallite sizes
in the range 1 to 20n . The porous silicon particles are mixed with a conductive material
such as carbon black and a binder such as PVDF to form an electrode material, which can
be applied to a current collector (such as a copper foil) to give an electrode. Although US
2009/0186267 suggests that these materials could be used for the manufacture of a battery,
there is no data in this document to suggest that a battery has actually been manufactured.
Kim et al teaches the preparation of three-dimensional porous silicon particles for use in high
performance lithium secondary batteries in Angewandte Chemie Int. Ed. 2008, 47, 10 15 1-
10 154. Porous silicon particles are prepared by thermally annealing composites of butyl
capped silicon gels and silica (Si02) nano-particles at 900°C under an argon atmosphere
and etching the silica particles out of the annealed product to give a carbon coated porous
amorphous silicon particle having a pore wall thickness of 40nm, pore diameters of the order
of 200nm and an overall particle size of greater than 20 m. Silicon crystallites having a
diameter of less than 5nm were observed within the structure. Half cells prepared using
these amorphous porous particles exhibited improved first cycling efficiency, which was
thought to be due to the carbon coating. It was also suggested that an amorphous silicon
structure could act as a buffer against the expansion of crystalline silicon upon intercalation
or insertion.
Although anode structures comprising silicon fibres, rods and wires have been found to
exhibit both a better capacity retention and an improved cycle life compared to bulk silicon
and silicon powder anodes, an improvement in their absolute capacity and cycle life is still
desired. Depending on the shape and dimension of the silicon elements, there can be a limit
to the achievable packing density in the composite mix which can restrict the maximum
achievable electrode capacity. Furthermore, the methods and costs associated with the
manufacture of these silicon structures needs to be further refined and reduced respectively.
Even with inherent porosity, electrode structures comprising silicon fibres, rods and wires
have been observed to exhibit an effect known as "heave" in which the bulk of the silicon
electrode material expands away from the surface current collector during intercalation,
which may result in delamination. This bulk does appear to survive the heave process and is
able to substantially resume its original configuration on release of the lithium from the
silicon fibres, but it exerts pressure on other cell components during cycling.
Further it has been found to be difficult to prepare anode structures comprising porous
silicon particles that are able to provide adequate performance in terms of absolute capacity,
capacity retention and cycle-ability. Anode structures comprising porous particles of
diameter ess than 500nm, for example, do not exhibit good capacity characteristics because
the particle pores are generally too small to facilitate electrolyte penetration and efficient
intercalation or insertion and release of lithium into the silicon structure. Further, the small
particles tend to agglomerate within the electrode structure, which leads to delamination over
a number of charging and discharging cycles. In addition because the pore wall thickness
(average thickness of material separating any one void or pore within a particle structure
from its adjacent pore or void) of these particles tends to be very low (less than 50nm), their
associated surface area tends to be high. A high surface area is associated with significant
first cycle losses of lithium in the electrode structure due to the formation of an excessive
Solid Electrolyte Interphase layer (SEI) as a result of the consumption of lithium in the
formation of these layers. Particles containing pores of a sufficiently large size to
accommodate electrolyte penetration and which have thicker pore walls of 0.1 to 2 m tend
themselves to have diameters that are too large to be successfully accommodated into an
electrode structure having an overall uniform thickness of around 50 m.
The fibres or wires used in the formation of silicon mesh electrode structures are also
believed to have a high surface area to volume ratio. These mesh like electrode structures
are also believed to be associated with high first cycle losses for the reasons given above.
It will be appreciated from the foregoing that the majority of approaches used to date for
creating porous particles result in the production of approximately spheroidal-shaped
particles with relatively smooth curved surfaces. Such shapes are not ideal for creating
networks of electronically connected particles in an electrode. This is because the surface
area of contact between one spheroidal particle and another or between one spheroidal
particle and a conductive additive particle is small; this means that the electronic conductivity
throughout the connected mass of active particles is relatively low, reducing performance.
Many of the electrodes produced using the electro-active silicon materials discussed herein
above are not able to exhibit the characteristics of uniform thickness, homogeneity and
porosity. Such electrodes do not comprise a strongly connected network of active particles
that are able to accommodate the expansion and contraction of the silicon material into its
own volume without cracking or de-lamination during the charging cycles of the battery.
There is a need, therefore, for an electroactive material and an electrode structure that
addresses the problems associated with the silicon based electrodes outlined above.
2. Compositions Comprising Porous Particle Fragments
A first aspect of the invention provides a composition comprising a plurality of electroactive
porous particle fragments comprising an electroactive material selected from the group
comprising silicon, tin, germanium, gallium, aluminium and lead. Preferably the porous
particle fragments comprise silicon (which will hereafter also be referred to as silicon
containing porous particle fragments). By the term "porous particle" it should be understood
to include a particle comprising a plurality of pores, voids or channels within a particle
structure, wherein each of the pores, voids or channels within the particle structure is
defined, bound, partially bound or separated by the electroactive material from which the
particle is formed. The term "porous particle" should also be understood to include a
particulate material comprising a random or ordered network of linear, branched or layered
elongate elements, wherein one or more discrete or interconnected void spaces or channels
are defined between the elongate elements of the network; the elongate elements suitably
include linear, branched or layered fibres, tubes, wires, pillars, rods, ribbons or flakes.
Layered elongate elements include structures in which the elongate elements are fused
together. The individual branched elongate elements typically have a smallest dimension in
the range 50 to 100nm with branches every 100 to 400nm. The porous particles from which
the porous particle fragments are derived can further be defined in terms of a smallest
dimension (or pore wall thickness), this being the average thickness of material separating
any one pore or void within a pore containing porous particle structure from an adjacent void,
or where the particle comprises a network of elongate elements, the average thickness (this
being the average smallest dimension) of an elongate element within the network. By the
term porous particle fragment it should be understood to include all fragments derived from a
porous particle, preferably a porous particle formed from an electroactive material such as
silicon, tin, germanium, gallium, aluminium and lead. Silicon containing porous particles are
especially preferred. Such fragments include structures having a substantially irregular
shape and surface morphology, these structures being derived from the electroactive
material originally defining, bounding, partially bounding or separating the pores or network
of pores within the porous particle from which the fragment structures are derived, without
themselves comprising pores, channels or a network of pores or channels. Preferably these
fragments are derived from the electroactive material, preferably the silicon material either
(a) defining the network of elongate elements or (b) originally defining bounding, partially
bounding or separating the pores or network of pores within the porous particle from which
the fragment structures are derived, without the fragments themselves comprising pores,
channels or a network of pores or channels. These fragments will hereafter be referred to as
fractals. The appearance of the fractals may or may not resemble the porous particles from
which they are derived. Typically the term "fractal" as described herein describes a structure
obtained through the random fragmentation of a larger porous particle. The surface
morphology of these fractal structures (which are devoid of pores or channels or a network
of pores or channels) may include an ordered or disordered array of indentations or
irregularities arising from the pores or channels or network of pores or channels originally
bound or partially bound by the electroactive material structure, preferably the silicon
structure of the parent porous particle. These fractal fragments will typically be characterised
by the presence of peaks and troughs extending over the surface thereof and will include
particles having a spiky appearance as well as those including a plurality of ridges or bumps
extending from the surface of the particle. The peaks are characterised by a peak height and
a peak width. The peak height is defined as the distance between the base of the peak (the
place where the peak merges with the body of the fractal) and the apex of the peak. The
peak width is defined as the minimum distance between one side of the peak and the other
at half height. The fractal can also be defined by the average thickness of the fractal body;
this value is typically identical to the average thickness (smallest dimension) of an elongate
element derived from a porous particle comprising a network of elongate elements or the
average thickness (preferably the pore wall thickness) of the electroactive material originally
separating any two adjacent pores within the pore containing porous particle from which the
fractal is derived.
The term porous particle fragment also includes porous particle fragments comprising a
network of pores and/or channels defined and separated by the electroactive material
defining the walls of the particle. Pore-containing porous particle fragments can also be
defined in terms of the average thickness of the electroactive material separating two
adjacent pore structures within the parent particle (also referred to herein as the pore wall
thickness). Preferably the electroactive material is a silicon containing electroactive material
and the term "silicon-containing electroactive material" should be interpreted to include
electroactive materials comprising essentially substantially pure or metallurgical grade
silicon, alloys of silicon with both electroactive and non-electroactive elements as well as
materials comprising electroactive compounds of silicon. Suitable silicon alloys include alloys
of silicon with one or more metallic elements selected from aluminium, copper, titanium,
strontium, nickel, iron, antimony, chromium, cobalt, tin, gold, silver, beryllium, molybdenum,
zirconium and vanadium. These fragments will herein after be referred to as pore containing
fragments. By the term "pore" or "channel" as defined in relation to the particles from which
the fragments are derived as well as the porous particle fragments themselves, it should be
understood to mean a void or channel enclosed or partially enclosed within the total volume
of the particle as well as a channel extending into the interior of the particle from its surface.
These pore and/or channel comprising porous particle fragments are also generally but not
exclusively characterised by an irregular shape and surface morphology. In contrast, the
particles from which the fragments are derived are generally but not exclusively
characterised by a disc-like or substantially spherical shape and a relatively smooth outer
surface morphology (inbetween the surface voids). Where the fractals and pore containing
porous particle fragments are described together hereinafter they will collectively be referred
to as either porous particle fragments or silicon containing porous particle fragments as
appropriate. The network of pores and/or channels suitably comprises a three dimensional
arrangement of pores and/or channels extending through the volume of the particle in which
the pore and/or channel openings are provided on two or more planes over the surface of
the pore containing porous particle fragment.
As indicated above, the porous particle fragments comprising the composition of the first
aspect of the invention suitably comprise an electroactive material selected from the group
silicon, tin, germanium, gallium, aluminium and lead and mixtures thereof as well as alloys of
these elements with each other and/or with other electroactive or non-electroactive
elements, providing the composition still exhibits electroactive properties. Silicon-containing
electroactive materials are preferred. The silicon-containing porous particle fragments may
be in the form of an alloy with or include additives such as Al, Sb, Cu, Mg, Zn, , Cr, Co,
Ti, V, Mo, Ni, Be, Zr, Fe, Na, Sr and P. Preferably the electroactive material is silicon, tin,
aluminium or gallium. More preferably the electroactive material is an alloy of silicon and
aluminium. Porous particle fragments comprising silicon or a silicon aluminium alloy are
especially preferred. Porous silicon particle fragments prepared from porous particles, which
porous particles were formed by etching particles of an aluminium silicon alloy comprising
from 11 to 30wt% silicon, for example 12wt%, 26wt%, 27wt% and 30wt% silicon and
fabricated using either a gas atomisation or melt spinning technique are especially preferred.
The nature of the porous particles will in turn depend upon the technique used to fabricate
the alloy particles and the processing conditions employed, the composition of the alloy and
the particle size of the alloy droplets. Fragments prepared from porous particles formed by
etching alloy particles of diameter less than 90 m made using a gas atomisation technique
and comprising up to 12wt% silicon are characterised by a network of fine silicon structures
having a fractal thickness of from 50 to 00nm. Fragments prepared by etching alloy
particles of diameter 90 to 1500 m formed using either the gas atomisation technique or the
melt spinning technique and comprising up to 12wt% silicon are characterised by a coarser
network of silicon structures having a fractal thickness of from 00 to 200nm. Fragments
prepared by etching alloy particles of a similar diameter but containing a hypereutectic
concentration of silicon, e.g. 2 to 30wt% silicon, have similar dimensional characteristics
but with the addition of low aspect ratio crystalline silicon particles with typical dimensions of
1-3mh . Compositions comprising silicon structures having a fractal thickness of from 100 to
200nm are particularly preferred. The invention will hereafter be described with reference to
electroactive materials comprising silicon and alloys thereof and will typically be referred to
as silicon-containing porous particles fragments. It should, however, be appreciated that
although silicon-containing porous particle fragments as described herein above are
especially preferred, the present invention extends to porous particle fragments comprising
alternative electroactive materials such as tin, germanium, gallium, aluminium and lead and
the term "silicon-containing" should be interpreted to extend in the context of the present
invention to electroactive materials comprising tin, germanium, gallium, aluminium and lead.
The composition of the first aspect of the invention is an electroactive material that is able to
form an alloy with lithium (either by insertion and/or by intercalation) and which can also be
used in the fabrication of anodes for use in lithium ion secondary batteries or batteries based
around alternative ions as the charge carrier, for example alkali metal ions such as sodium
or potassium ions or magnesium ion batteries. By the term "electroactive material" it should
be understood to mean that the material is able to accommodate and release lithium or other
alkali metal ions, or magnesium ions into or from its structure during the charging and
discharging cycles of a battery.
Suitably the composition according to the first aspect of the invention is provided in the form
of an electrode material, preferably a composite electrode material, which is connected or
applied to a current collector and used in the manufacture of an electrode. By the term
"electrode material" it should be understood to mean a material comprising an electroactive
material, which can be applied, bonded, adhered or connected to a current collector. By the
term "composite electrode material" it should be understood to mean a material comprising a
mixture, preferably a substantially homogeneous mixture, of an electroactive material, a
binder and optionally one or more further ingredients selected from the group comprising a
conductive material, a viscosity adjuster, a cross-linking accelerator, a coupling agent and an
adhesive accelerator. The components of the composite material are suitably mixed
together to form a homogeneous composite electrode material that can be applied as a
coating to a substrate or current collector to form a composite electrode layer. Preferably the
components of the composite electrode material are mixed with a solvent to form an
electrode mix, which electrode mix can then be applied to a substrate or current collector
and dried to form the composite electrode material.
By the term "electrode mix" it should be understood to mean compositions including a slurry
or dispersion of an electroactive material in a solution of a binder as a carrier or solvent. It
should also be understood to mean a slurry or dispersion of an electroactive material and a
binder in a solvent or liquid carrier.
Further the term "composite electrode" should, in the context of the present invention, be
understood to mean an electrode structure comprising a current collector having an
electroactive material or a composite electrode material applied, bonded, adhered or
connected thereto. The current collector may be provided in the form of a sheet or a mesh.
The electroactive material may be in the form of a coating applied thereto. The coating may
be provided in the form of a felt or a mat, the felt or mat being applied, bonded, adhered or
connected to the current collector.
In a first preferred embodiment of the first aspect of the invention, the silicon containing
porous particle fragments are fractals. Without wishing to be constrained by theory, it is
believed that composite electrodes comprising fractals are associated with a greater
capacity, improved cycle-ability (cycling characteristics) and an optimum porosity compared
to bulk silicon and silicon powder composite electrodes, for example. The fractals are
believed to pack together more closely within the electrode structure compared to nonfragmented
spherical particles. This close packing is believed to contribute to an improved
capacity. The irregular surface morphology of the fractals is believed to contribute to both
improved connectivity and improved porosity within the electrode structure. The presence of
peaks on the surface of the fractals provides contact points on each fractal surface for
connection to other electroactive and conductive components within an electrode mix. In
addition, the irregular surface morphology means that a number of voids will be created
within the electrode structure as a result of the incomplete overlap of the fractals either with
each other or with the other components of the electrode structure with which they are
packed.
It has been found that the structure of the fractal material depends on the structure of the
porous particle from which it is derived (parent porous particle). The structure of the parent
porous particle depends upon the composition of the material from which it is formed and the
method of formation. If the parent porous particles are prepared from alloy particles, such as
a silicon aluminium alloy, the particle structure will depend on both the alloy composition and
the method used to form the alloy particle. Cooling techniques (and variations of the cooling
rates employed in the techniques) such as gas atomisation techniques and melt spinning
techniques can be used to form alloy particles of different dimensions and morphologies.
The range of achievable cooling rates will vary with each technique. In general, faster
cooling rates will tend to produce finer morphologies in the final fractal material though the
overall size of the alloy particles can also have an effect. The use of the gas atomisation
technique typically produces particles having finer morphologies that those produced using
melt spinning. Finer morphologies are also observed in alloy particles prepared from eutectic
alloy compositions compared to hypereutectic compositions.
Compositions having fine silicon structures (fractal thickness of 50 to 100nm) were prepared
by cooling silicon aluminium alloys comprising 2wt% silicon using a gas atomisation
technique and selecting the alloy particles having a diameter of 0mpi to 90 m for etching to
form the porous particles. Coarser silicon structures (fractal thickness of 100 to 200nm) were
prepared by either cooling silicon aluminium alloys comprising 12wt% silicon using a gas
atomisation technique and selecting the alloy particles having a diameter in the range 90 to
1500 m or through the use of a melt spinning technique.
Coarser silicon structures having a fractal thickness of from 100nm to 200nm were also
prepared by cooling silicon aluminium alloys comprising 27 to 30wt% silicon using either a
gas atomisation or melt spinning technique.
The dimensions of the silicon containing porous particle fragments suitability facilitate their
accommodation into an anode having an active layer thickness (excluding any current
collector or supporting substrate) of the order of 40 m, without compromising the anode
structure and capacity. Because the silicon containing porous particle fragments are
relatively small, they are inherently suitable for use in the preparation of a homogenous
electrode or anode material that can be used to provide a smooth and continuous electrode
coating or mat.
2.2 Characteristics of Porous Particle Fragments
As indicated above, the porous particle fragments according to the first aspect of the
invention are suitably characterised by an average pore wall or fractal thickness of between
50nm and 2 m, preferably 100nm to 1 m, especially 100nm to 200nm. Suitably porous
particle fragments having a pore wall or fractal thickness in the range 50nm to 2pm,
preferably 100nm to 1 m, especially 100nm to 200nm comprise at least 10% of the volume
of the porous particle fragments used in the composition of the first aspect of the invention.
Preferably porous particle fragments having a pore wall or fractal thickness in the range
50nm to 2pm, preferably 100nm to 1pm, especially 100nm to 200nm comprise at least 30%
of the volume of the porous particle fragments used in the composition of the first aspect of
the invention. More preferably porous particle fragments having a pore wall or fractal
thickness in the range 50nm to 2 m, preferably 100nm to 1 m, especially 100nm to 200nm
comprise at least 50% of the volume of the porous particle fragments used in the
composition of the first aspect of the invention. Most preferably porous particle fragments
having a pore wall or fractal thickness in the range 50nm to 2 m, preferably 100nm to 1pm,
especially 0nm to 200nm comprise at least 70% of the volume of the porous particle
fragments used in the composition of the first aspect of the invention. It is especially
preferred that porous particle fragments having a pore wall or fractal thickness in the range
50nm to 2pm, preferably lOOnm to 1pm, especially 100nm to 200nm comprise at least 90%
of the volume of the porous particle fragments used in the composition of the first aspect of
the invention. In a most preferred embodiment of the first aspect of the invention porous
particle fragments having a pore wall or fractal thickness in the range 50nm to 2pm,
preferably 100nm to 1pm, especially 100nmto 200nm comprise 10 to 100% of the volume of
the porous particle fragments used in the composition of the first aspect of the invention,
preferably 30 to 100%, more preferably 50 to 90% and especially 70 to 90%.
The size of the particle fragments is suitably determined using laser diffraction techniques
that can be carried out using instruments such as the Malvern Master Sizer™. Such
techniques are well known to a skilled person. Laser diffraction calculates the equivalent
diameter of a sphere having the same volume as a non-spherical particle and provides a
volume distribution of the particles within a sample. Alternative techniques that can be used
to measure size distributions include digital imaging and processing such as Malvern
Morphologi™ where the diameter of a sphere or ellipse of a particle of the same projected
cross-sectional area of the particle being measured is calculated and a number or volume
distribution of a sample can be provided. The pore sizes of the pore containing porous
particle fragments suitably accommodates both the expansion of the pore wall on
intercalation or insertion of silicon and the penetration of electrolyte during the charging and
discharging cycles of the battery. The thickness of the fractal material and that of the pore
walls for both the fractals and pore containing porous particle fragments respectively is
believed to be an important parameter in the context of the present invention and needs to
be sufficient to impart to the anode structure, of which it forms a part, enough capacity to
reversibly intercalate and release lithium. The pore wall thickness and the thickness of the
fractal material must not be too thin as this would lead to excessive lithium loss due to the
formation of an SEI layer and high first cycle losses. However, the fractal material and that of
the pore walls must not be too thick as this would lead to a build-up of stress within the
structure, which can cause the particle to crumble, and an increased resistance to the
passage of ions into the bulk of the silicon. In this respect, the facile fabrication of a good
quality homogeneous coating or mat requires the use of silicon containing porous fragments
having a maximum overall dimension in the range 1 to 40 m, preferably 1 to 20 m and
especially 3 to 10m h . Porous particle fragments having diameters of less than 50nm are less
preferred as particles of this size tend to agglomerate, which results in the formation of an
inhomogeneous mat or coating. Suitably porous particle fragments having a maximum
overall dimension in the range 1 to 40 m, preferably 1 to 20mhi and especially 3 to Om h
comprise at least 10% of the volume of the porous particle fragments used in the
composition of the first aspect of the invention. Preferably porous particle fragments having a
maximum overall dimension in the range 1 to 40pm, preferably 1 to 20pm and especially 3 to
10pm comprise at least 30% of the volume of the porous particle fragments used in the
composition of the first aspect of the invention. More preferably porous particle fragments
having a maximum overall dimension in the range 1 to 40pm, preferably 1 to 20mhti and
especially 3 to 10pm comprise at least 50% of the volume of the porous particle fragments
used in the composition of the first aspect of the invention. It is especially preferred that
porous particle fragments having a maximum overall dimension in the range 1 to 40pm,
preferably 1 to 20 m and especially 3 to 10mhi comprise at least 70% of the volume of the
porous particle fragments used in the composition of the first aspect of the invention. Where
the silicon containing porous particle fragment is a pore containing fragment, each pore
containing fragment comprises a three dimensional arrangement of pores having pore
diameters in the range 60nm to 10pm, preferably 100nm to 5pm and especially 150nm to
2pm. The fractal material and the walls separating the pores within the porous particle
fragments suitably have a thickness in the range 0.05 to 2pm, preferably 0.1 to 1pm,
especially 100nm to 200nm.
Typically the ratio of pore diameter to wall thickness for pore containing fragments is suitably
greater than 2.5:1 and is preferably in the range 3:1 to 25:1 . The ratio of the voluhne of the
pores to the volume of the fragment (otherwise known as particle porosity) is suitably in the
range 0.2 to 0.8, preferably 0.25 to 0.75 and especially 0.3 to 0.7.
The porous particle fragments suitably have a BET surface area of greater than 0.5 m /g,
preferably at least 5m2/g. A higher surface area improves the electrical connectivity and
ionic reactivity of the fragments as the active material in an electrode and generally
increases the rate at which lithium can be inserted into the silicon. However a higher surface
area leads to a larger amount of SEI layer being formed during charge and discharge of the
electrode and consequently a higher lithium loss and reduced cycle life of the
electrochemical cell. Therefore a suitable balance needs to be made between these
competing effects. Accordingly, preferably the porous particle fragments have a surface area
of less than 50m2/g , more preferably less than 30m2/g. Preferred porous particle fragments
having a BET surface area in the range 4 to 50m /g, more preferably 4 to 40m2/g, especially
5 to 30m /g. Silicon structures prepared from melt spun 12wt% Si-AI alloy were found to
have a BET surface area in the range 10.51 to 15.97m /g. Silicon structures prepared from
around 90-1500 mh alloy particles prepared by gas atomisation of a silicon aluminium alloy
comprising 12wt% silicon and having a fractal thickness of 100 to 200nm are typically
characterised by a BET surface area of 7 to 22m /g. However finer silicon structures
prepared from 10-90pm alloy particles prepared by gas atomisation of a silicon aluminium
alloy comprising 12wt% silicon typically have a higher BET for example in the region of 40 to
70m /g, depending on the mix of the alloy particle dimensions in the measured sample. BET
values around the upper end of this range are less preferred. Silicon structures prepared
from alloy particles prepared by gas atomisation of a silicon aluminium alloy comprising
30wt% silicon and having a fractal thickness of 100 to 200nm are typically characterised by a
BET surface area of 10 to 15m /g.Pore diameters and BET surface area values for pore
containing porous particle fragments and fractals can be measured using both mercury and
gas adsorption porosimetry techniques that are well known to a person skilled in the art.
Mercury porosimetry can be used to determine the pore size distribution, the pore volume,
the pore area and porosity of both a meso-porous (pore size of between 2 to 50nm) and a
macro-porous (pore size of greater than 50nm) sample. Gas adsorption porosimetry (using
gases such as helium, argon or nitrogen, preferably nitrogen) can be used to determine the
specific surface area and porosity of micro-porous sample (down to pore size of 2nm or
less). The specific surface area and porosity of compositions comprising porous particle
fragments according to the first aspect of the invention can suitably be measured using
mercury porosimetry techniques.
The porosity of the particle should be distinguished from the porosity of the composition of
the first aspect of the invention. Particle porosity is (as indicated above) defined by the ratio
of the volume of the pores to the total volume of the particle. Where the porous particle
fragments are prepared by alloy etching, the particle porosity can most easily be determined
by knowing the density of the alloy particle from the compositional ratios and comparing the
mass of a sample before and after the etching and partial crushing steps, if all the metal
matrix (e.g. aluminium) material is removed during etching. Tap density measurements
before and after etching can also provide comparative values of particle porosity. The
composition porosity can be defined a s the ratio of the volume of voids i the composition to
the total volume of the composition and is the sum of both the particle porosity and the
packing density of the porous particle fragments and other elements within the composition.
Where the composition comprises fractals only, the composition porosity defines the bulk
porosity of the fractals per se; the fractals do not have significant inherent porosity.
The silicon containing porous particle fragments can further be characterised by one or more
parameters including bulk resistivity, powder resistivity, Seebeck Coefficient, and the 111
plane lattice spacing and crystallite size of the silicon crystallites within the silicon material a s
measured by X-ray diffraction spectrometry.
The Seebeck Coefficient of the porous particle fragments or fractals can be measured by
placing a sample of the particle fragments in a circular pressure cell of approximate
dimensions 5 mm diameter by 5mm thick and applying a pressure of 40MPa. A small
temperature gradient is then formed across the thickness direction of the cell using a heater
in the cell base. Measurement of the resulting thermal voltage generated across the cell
thickness provides the Seebeck Coefficient, S, in V/K at room temperature (e.g. 2 1 deg C).
For a material such a s silicon, the Seebeck Coefficient is dependent on the carrier density,
i.e. the number of free electrons or holes with the silicon. The sign of S depends on the type
of the majority carrier - it is positive for p-type (holes) and negative for n-type (electrons). A
smaller magnitude of S relates to a higher carrier density indicating a higher level of doping
and a higher conductivity. For the active material of a electrochemical cell electrode, a higher
conductivity and therefore, a lower value of S is preferred. Preferably the absolute
magnitude of S (at room temperature, |S|) is less than 300pV/K, more preferably less than
250 v k and especially less than 00mn7K. The method of making porous particle fragments
described herein where a molten alloy comprising an active material and a metal matrix
material is rapidly quenched to form alloy particles from which the metal matrix material is
removed by etching, is particularly advantageous in producing an active material with a low
Seebeck Coefficient (plus a high level of doping and low resistivity) when the metal matrix
material in the alloy comprises a doping element of the active material. Making porous
particle fragments from Al-Si alloy particles is preferred because aluminium is a p-type
dopant for silicon and this produces a very highly doped silicon material which is believed to
be beneficial for electrode performance without the need for any additional doping process
step.
Where the silicon containing porous particle fragments are characterised in terms of their
resistivity, this is value may be determined for a bulk sample (suitably, but not exclusively, a
sintered bulk sample) including these materials. Suitably a bulk sample of the porous particle
fragments has a resistivity of less than IOW/cm, preferably less than 1 W/cm, more preferably
less than O. W/cm and especially less than O.O W/ h.
The bulk resistivity of the silicon porous particle fragments can also be estimated from the
measurement of the Seebeck Coefficient, S. A suitable method is as follows: the carrier
density, p, is calculated using the equation S = (k/q)*(2.5 - ln(p/Nv)) where Nv=1 .8x10 9/cm 3,
k is Boltzmann's constant (1.38065 x 10"23 J/K) and q is the elementary charge of an electron
( 1.6021 76 x 10'19 C). Using the calculated value of the carrier density, the resistivity can be
estimated using one of the methods described in ASTM Standard F723-99 using the
assumptions that the dopant density is equal to the carrier density and the conversion
factors given in ASTM F723-99 for boron doped silicon can be used for aluminium doped
silicon. For the calculations given herein, the graphical method for boron doped silicon using
the resistivity-dopant density conversion from Thurber et al., NBS special Publication 400-64
(April 1981), as described in ASTM F723-99, is used.
The tap density refers to the bulk density of a powder comprising porous particle fragments
after a specified compaction process, usually involving vibration of container holding a
sample of the powder. For a bulk sample comprising porous particle fragments typical tap
densities after 6000 taps are in the range of 0.1 to 0.6 g/cm3.
X-ray diffraction spectrometry analysis provides an indication regarding the crystallinity and
average crystallite size of a sample. X-ray diffraction spectrometry analysis (using an X-ray
wavelength of 1.5456nm) of the porous particfe fragments of the present invention indicates
that the fragments comprise a polycrystalline material having a crystallite size of between 45
and 55nm with a crystal plane 1 lattice spacing of between 3.14 and 3.16A. The crystallite
size is calculated using the Scherrer equation where the shape constant K is taken to be
0.94. Silicon material prepared from a gas atomised 2wt% silicon aluminium alloy particles
of size 10-90pm (that typically produce finer particle fragments with higher BET values) is
typically characterised by a crystallite size of 51nm and a crystal plane 111 lattice spacing of
3.156A. Silicon material prepared from gas atomised or melt spun 12wt% silicon aluminium
alloy particles of size 90 to 1500pm (that typically produce coarser particle fragments with
lower BET values) is typically characterised by a crystallite size of 45.5nm and a crystal
plane 111 lattice spacing of 3.145A. Silicon material prepared from a melt spun 30wt%
silicon aluminium alloy that also typically produces coarser particle fragments with lower BET
values is typically characterised by a crystallite size of 49.2nm and a crystal plane 111 lattice
spacing of 3. 2A. The silicon material with a 111 lattice spacing of less than 1.5 Ais
preferred. It would appear that the porous particle fragments prepared from rapidly cooling
and then etching silicon aluminium alloys comprising from 12 to 30wt% possess significant
crystallinity. Porous particle fragments with a crystallite size of at least 20nm are preferred,
more preferably at least 30nm.
2.2.1 Characterisation of Parent Porous Particles
As indicated, the silicon containing porous particle fragments of the composition of the first
aspect of the invention are suitably derived from larger porous particles (including porous
particles comprising a network of elongate elements) having a diameter in excess of 40 m,
preferably at least 60 m and more preferably at least 100 m. Porous particles having
diameters up to 1000 m or 500 m can also be used to prepare the porous particle
fragments according to the first aspect of the invention. Preferably the porous particle
fragments are derived from porous particles (including porous particles comprising a network
of elongate elements) having a diameter in the range 40 to 200pm, preferably 60 to 80 m,
more preferably 70 to 150 m and especially 100 to 150pm. Preferably the porous particle
fragments of the first aspect of the invention are prepared from porous particles (including
porous particles comprising a network of elongate elements) having a diameter in the range
60 to 1500pm, more preferably 150 to 1000pm. Preferably the fragments are derived from
spheroidal and non-spheroidal-based larger porous particles (including porous particles
comprising a network of elongate elements) and have, themselves, an essentially nonspheroidal
shape with one or more surfaces of low curvature. Preferably the fragments have
an average smallest dimension (pore wall thickness or elongate element thickness) which is
less than half the value of the largest dimension (usually length). It has been found that
anodes prepared using particle fragments of the type and dimensions specified above, which
themselves have been derived from larger porous particles having a diameter specified
above exhibit improved capacity and cycling characteristics compared to bulk and powdered
silicon anodes and anodes comprising whole porous particles as an active material. Without
wishing to be constrained by theory it is believed that the superior characteristics of an
anode fabricated from porous particle fragments according to the first aspect of the invention
are due in part to factors such as overall particle size, surface morphology, pore wall
thickness or elongate element thickness (as defined herein above), shape and packing
density that are believed to be associated with fragments derived from larger porous
particles. In particular, it is believed that due to their non-spheroidal shape and irregular
surface morphology, the porous particle fragments of the first aspect of the invention are
characterised by a greater packing density within the electrode structure compared to the
porous particles from which they are derived due to a greater overlap of fragments
compared to the particles from which they are derived. The irregular surface morphology is
also thought to improve the connectivity between the electroactive and conductive elements
within the electrode structure compared to the porous particles from which they are derived
due to the additional connections created by the peaks and troughs on the surface of the
fragment structure; it is also thought to be responsible for the porosity of the electrode. It has
been found, for example, that it is very difficult to prepare porous particles fragments suitable
for use as anode materials using whole porous particles having an average diameter of less
than 4 m ϊ . This is because, for porous particle fragments derived from whole particles
having a diameter of less than 40 m and a porosity in the range 0.2 to 0.8, either the pore
size is insufficient to accommodate electrolyte penetration or the pore wall thicknesses is
insufficient to minimise losses arising from SEI (surface electrolyte interphase) formation or
to impart to the particle the capacity and resilience required to withstand the stresses
associated with intercalation or insertion and release of lithium or other ions during cycling of
the battery. Although larger whole particles having a diameter of greater than 40 m,
preferably greater than 60 miti, more preferably greater than 00mhh and especially greater
than 120 p (up to and including particle having a diameter of 000 m or 500 m) and a
porosity in the range 0.2 to 0.8 are unsuitable for use as a anode material due to their size
(their diameters are comparable to or greater than the thickness of the electrode), the pore
size and pore wall thicknesses associated with fragments derived from such particles
facilitates both electrolyte penetration and good lithium storage capability. The diameter of
the pores within the larger porous particles tend be greater than those in whole particles
having a diameter of less than 40 m. In addition, the pore wall thickness also tends to be
greater, which means that the fragments derived from such larger particles have a greater
packing density within the electrode structure, a greater capacity for lithium intercalation or
insertion compared to smaller particles and (due to the thickness of the fractal material or
that of the pore walls) are more resilient. Capacity loss is also minimised due to the reduced
surface area per unit volume that is available for formation of an SEI layer for thicker
fragments. Furthermore, without wishing to be constrained by theory, it is believed that the
porous particle fragments derived from particles having a diameter of greater than 40prn,
preferably greater than 60 pm , more preferably greater than 100 m, especially greater than
120 pm and up to and including particles having a diameter of 000 or 1500 m provide a
more open structure to the access of electrolyte into their pores compared to the whole
particles and their shape and form promotes better electronic connectivity across the
network of active particles in the anode composite. Additionally, it is believed that the shape
and form of the fragments leads to a more uniform thickness of the electrode layer and
results in an electrode having smoother top surface compared to electrodes fabricated from
a layer made with the larger whole porous particles. It is easier to calendar the anode
surface (further improving uniformity of thickness and providing some control over packing
density) without breaking particles. Composite electrode materials (suitably anode materials)
prepared from fragments derived from such particles are therefore associated with improved
capacity and cycling characteristics compared to bulk and powdered silicon anodes and
greater resilience. In addition because the active silicon mass is able to substantially expand
into its own volume (into the voids created by the pores in the pore containing fragments and
the troughs on the surface of the fractal), individual particles tend to impinge less on
neighbouring particles which reduces the stresses within the electrode structure. Composite
electrode materials (suitably anode materials) prepared from silicon containing porous
particle fragments of the type described herein above exhibit less build-up of heave over
lifetime of the battery compared to silicon fibre containing electrode or anode materials and
consequently, a longer battery life has been observed.
2.3 Coatings
The silicon containing porous particle fragments may include a coating. The coating suitably
improves one or more of the conductivity, resilience, capacity and lifetime of a battery
including a composition according to the first aspect of the invention. The coating may also
affect the ease with which the silicon-containing porous particle fragments are dispersed
within the electrode mix and their adherence to the other components within the electrode
mix (such as the binders). Carbon coatings and coatings formed from lithium salts are
preferred. Where the coating is a carbon coating, it is preferably a coating made with carbon,
such as graphite, electroactive hard carbon, conductive carbon or carbon black. Coatings
comprising a lithium salt include but are not limited to lithium salts selected from the group
comprising lithium fluoride, lithium carbonate and the lithium salts obtained through the
reaction of lithium ions with cyclic carbonates selected from the group comprising ethylene
carbonate, propylene carbonate, diethylene carbonate and vinyl carbonate. Coats are
typically applied to the silicon structures to a thickness of between 5 and 40% by weight of
the coated silicon structure. Methods of coating silicon particles and elongate elements are
known to a person skilled in the art and include chemical vapour deposition, pyrolysis and
mechanofusion techniques. Carbon coating of silicon structures through the use of Chemical
Vapour Deposition techniques is disclosed in US 2009/02391 5 1 and US 2007/0212538.
Pyrolysis methods are disclosed in WO 2005/011030, JP 2008/186732, CN 101442124 and
JP 04035760. Without wishing to be constrained by theory, it is believed that carbon
coatings are able to assist in controlling the formation and stability of SEI layers on the
surface of the anode. Lithium based coatings can be obtained by reacting silicon with a
solution of LiF or exposing silicon to a solution comprising a mixture of lithium ions and a
cyclic or acyclic carbonate.
2.4 Composite Electrode Material
The silicon containing porous particle fragments of the first aspect of the invention are
preferably formed into a composite electrode material, which is suitably provided on the
current collector in the form of a cohesive mass in which the short term order of the
components of the material is substantially retained over at least 100 charging and
discharging cycles of a battery including the composite material. The cohesive mass of the
composite electrode material may be provided in the form of a coating or layer, in which the
porous particle fragments are arranged in a random or ordered fashion. The coating is
typically applied or bonded to a current collector. Alternatively, the composite electrode
material may be provided in the form of a felt or mat comprising silicon containing porous
particle fragments and fibres of an electroactive or a conductive species which are randomly
arranged in a composite material. The felt or mat is typically applied, adhered, bonded or
connected to the current collector.
2.4.1 Additional Components of the Composite Electrode Material
The composite electrode material comprising porous particle fragments of the composition of
the first aspect of the invention may optionally include, i addition to the silicon, tin, gallium,
germanium, aluminium or lead containing porous particle fragments (first electroactive
material) referred to herein above, additional components such as a binder, a conductive
material and optionally second electroactive material. Preferably the composition and/or
structure of the second electroactive material is different to that of the first electroactive
material. Examples of the second electroactive material include but are not limited to
graphite, hard carbon, silicon, tin, gallium, germanium, aluminium and lead containing
material. In a first preferred embodiment of the first aspect of the invention, the composition
comprises a plurality of silicon containing porous particle fragments, a binder, a conductive
material and optionally a non-silicon containing electroactive material. Alternatively, in a
second preferred embodiment of the first aspect of the invention the composition may also
contain, in addition to the components of the composition of the first embodiment, one or
more silicon containing components having a minimal or negligible porosity, said
components being selected from the group comprising native silicon containing particles;
silicon containing tubes, wires, fibres, rods, sheets and ribbons and silicon containing
pillared particles. By the term "minimal or negligible porosity" it should be understood to
mean silicon structures having a porosity of less than 0.2. The terms "minimally porous
silicon containing particles, wires, nano-wires, fibres, rods, sheets and ribbons" may include
solid elongate elements such as wires, fibres, rods, sheets, ribbons and particles having a
silicon-based core respectively as well as wires, fibres, rods, sheets, ribbons and particles
having a silicon coating provided on a core other than silicon. Where the silicon containing
elongate elements and particles comprise silicon coated elongate elements, tubes and
particles, the cores of these coated elements can be selected from electronically and
ionically conductive materials such as carbon, preferably hard carbon or graphite or a
suitable metal. The silicon containing elongate elements, tubes and particles can be formed
from a silicon, a silicon-alloy or a silicon oxide material. When the elongate elements, tubes
and particles are formed from a silicon material they are suitably formed from a silicon
material comprising less than 99.99%, preferably less than 99.95% silicon because higher
purity silicon is more expensive to process, but have a silicon content of greater than 90% to
avoid significant reduction in performance from having high levels of impurities in the cell.
When they are silicon-alloy material, the alloy preferably contains at least 50wt% of silicon,
preferably at least 75wt% silicon, more preferably at least 80wt% of silicon and especially at
least 95wt%. Suitable alloy materials are disclosed herein above. Alloys of silicon with
metals such as aluminium, copper, titanium, nickel, iron, tin, gold and silver are preferred.
Preferred alloys have a resistivity of less than 10Qcm, preferably less than 1Qcm and
especially less than 0.1 Qcm. Where the composition contains one or more silicon containing
components in addition to the silicon containing porous particle fragments, it is preferred that
one or more these components are themselves electroactive.
2.4.2 Additional Components
As indicated above, additional components such as a binder, a conductive material, a nonsilicon
containing electroactive material, a viscosity adjuster, a cross-linking accelerator, a
coupling agent and an adhesive accelerator may also be present in the mix. These nonsilicon
containing components generally comprise carbon as a major constituent, but may
comprise silicon as a minor constituent. As indicated above, the compositions according to
the first and second embodiments are suitably used in the preparation of composite
electrodes, preferably composite anodes and for this reason each composition may also be
referred to as an electrode material or an anode material respectively. The electrode or
anode material is suitably provided as a cohesive mass that can be formed into a free¬
standing mat that can be connected to a current collector or may be formed as a mat that
can be applied, bonded or adhered to a current collector. In order to fabricate an electrode,
the electrode or anode material is typically combined with a solvent to form an electrode or
anode mix and then cast either directly onto a substrate (for subsequent removal) or directly
onto a current collector and subsequently dried to remove the solvent. The electrode or
anode mix is preferably prepared by combining whole porous particles as described above
with the other components of the electrode or anode mix and a solvent to form a slurry and
treating the slurry to partially crush the whole porous particles to give an electrode or anode
mix as described herein. The slurry is suitably treated using a high shear mixer, a ball mill
mixer or an ultra-sonic probe. When the electrode or anode material is formed into a free¬
standing mat or applied to a current collector as described above the silicon containing
porous particle fragments are randomly connected to each other, either directly or indirectly
through any other components present in the mix. By the term connected it should be
understood to mean, in relation to composition or composite electrode material of the
present invention, that substantially all silicon containing porous particle fragment is in
electrical contact, either via physical connections or interfaces, with the electrolyte and
optionally with one or more other electroactive elements and/or with one or more conductive
elements that may be present in the mix as well as the current collector.
It will be appreciated that where the composition comprises minimally porous silicon
structures as described above, these may make contact with each other and also with the
silicon containing porous particle fragments during the charging cycle of the battery, due to
the expansion in silicon volume arising from lithium intercalation or insertion during the
charging cycle. This contact between the components of the electrode or anode material
may result in a network having enhanced ionic and electrical conductivity.
The composition may also include metal bridging elements, which promote contact between
the electroactive silicon components and which also enhance the connectivity within the
electrode structure. One or more metal bridging elements selected from but not limited to
the group comprising copper, aluminium, silver and gold may be used. The provision of
metal bridging elements is well known to a person skilled in the art and is described in WO
2009/010757.
2.5 Composition Porosity
It will further be appreciated fom the foregoing that the anode structures formed from
compositions according to the first aspect of the invention possess an inherent porosity
arising from both the maximum packing density associated with the components of the
electrode material and the inherent porosity of the silicon containing porous particle
fragments. By controlling the porosity of the parent porous particles, the degree to which
they are fragmented and the relative amounts of porous and minimally porous components
(electro-active and non-electroactive) in the electrode material, it is possible to control the
bulk porosity of the silicon containing electrode or anode. This control of porosity is important
as the overall performance of the electrode relies on providing sufficient voids within the
electrode structure to accommodate both the expansion of the silicon material and
penetration of electrolyte into the voids during lithiation.
The total volume of the anode material, V can be expressed in terms of the volume taken up
by the solid elements such as silicon, graphite, conductive material and binder that may be
present in the material as well as the volume defined by the empty spaces generated within
the material as a result of the random packing of the solid elements. The total volume can
therefore be expressed as follows:
VT = VSi + VB + V + v + V + Vp
Where VT is the total volume of the anode material; V is the total volume of the minimally
porous electro-active silicon elements in the anode material; sip is the volume of the silicon
porous particle fragments including the pores contained within them; VB is the total volume of
the binder; Vc is the total volume of conductive material (where present), VG is the total
volume of additional electroactive material (such as graphite, where present) and is the
total volume occupied by the pores or voids generated by the packing arrangement of the
components of the anode material (excluding the pore volume of the porous particle
fragments). The porosity of the anode material is calculated in terms of the total volume of
pores or voids present in the anode material as a percentage of the total volume of the
anode material. This pore volume is made up of the volume of pores or voids created as a
result of the random packing of the components of the anode material into the electrode
structure (VP) as well as the volume of pores or voids present within the silicon containing
porous particle fragments (Vp
s'p).
VP + VSIP
Anode Porosity = x 100
T
It will be further appreciated that because silicon expands by a factor of up to approximately
400% when the material is charged, the porosity of the electrode decreases as a result. The
expansion of the other components of the electrode material, such as the binder, conductive
material and optional non-silicon containing electroactive materials is negligible in
comparison. The silicon of the pore containing porous particle fragments is believed to
expand substantially into its pore structure; for fractals, the silicon substantially expands into
the surface troughs or indentations. Without being constrained by theory, it is believed that
the total porosity of the electrode in the charged state should be in the range 15% to 50%,
more preferably 25% to 50% to ensure that electrolyte penetration within the electrode
structure is not inhibited.
The porosity of the uncharged electrode material will depend, in part, on the nature of
components used in the formation of the anode material and the relative proportions in which
they are present. It is important, however, that the nature of the components and the relative
proportions in which they are present is sufficient to achieve a porosity of between 15 and
50% when the electrode material is in the charged state. In order to achieve this, the
electrode material will typically have a porosity of at least 10%. Suitably the electrode
material will have a porosity of less than 75%. The electrode material will typically have a
porosity of between 0 to 75%, preferably 20 and 75%, more preferably 20 to 60% and
especially 30 to 60% in the uncharged state.
The anode porosity, n , of an uncharged anode material comprising an electroactive
material consisting of a volume of minimally porous silicon, silicon containing porous particle
fragments and a further electroactive material can be reduced relative to the anode porosity,
usi, of an uncharged anode material of equivalent volume comprising an electroactive
material comprising only minimally porous silicon and silicon containing porous particle
fragments.
This reduction in porosity in the uncharged state can be expressed as follows:
where is the volume occupied by pores in an uncharged material comprising an
electroactive material comprising minimally porous silicon, silicon containing porous particle
fragments and a further electroactive material, V Si is the volume occupied by pores in an
uncharged material comprising an electroactive material comprising minimally porous silicon
and silicon containing porous particle fragments only, V G is the volume of the additional
electroactive material, and a is volume expansion factor of the silicon-containing
electroactive material (in other words, the volume V of the silicon containing electroactive
material increases to aV at the end of the charge cycle with the insertion of lithium ions).
This calculation assumes that the silicon containing electroactive material has the same
volume expansion factor in each case, that the volume expansion of the further electroactive
material is minimal and can be neglected and that the porosity of each anode material in the
charged state is the same.
Without wishing to be constrained by theory, it is believed that the overall structure of the
electrode or anode material of the first aspect of the invention and hence its electrical and
mechanical properties will depend upon the relative dimensions, shapes and morphologies
of all the components (silicon and non-silicon containing components) from which the
material is formed as well as the proportions in which they are present and their individual
porosities. In other words, it is believed that the structure of the electrode material will be
governed by the packing density, surface morphology and porosities of the components of
the material. An electrode material comprising only particulate components will tend to
exhibit a higher packing density compared to an electrode material containing a mixture of
fibre and particulate components. It will, therefore be appreciated that since the electrode
material must accommodate the expansion of silicon during lithium intercalation or insertion
in order to minimise the build up of stress within the electrode structure, the porosity of the
pore containing porous particle fragments or the morphology of a fractal in a particle only
material must be greater or rougher, respectively, than that for pore containing porous
particle fragments or fractals in a fibre and powder material, since there is more inherent
porosity within a fibre and powder material into which the silicon components can expand.
2.6 Electrode Materials
Without further wishing to be constrained by theory, it is believed that because the pore
containing porous particle fragments expand into their own pores and the fractals expand
into their surface troughs during lithium intercalation, the volume fraction occupied by the
silicon containing porous particle fragment in the electrode structure does not significantly
change between the charging and discharging cycles of the battery. By the term
"significantly change" it should be understood that t e overall volume of the silicon
containing porous particle fragment, VSip, does not increase by more than 50% during the
charging cycle. This means that the total volume of an electrode material comprising silicon
containing porous particle fragments as the only silicon containing component does not differ
significantly between the charged and the uncharged state. Where the electrode material
comprises a mixture of silicon containing porous particle fragments and silicon components
having minimal porosity, it is believed that the packing density of the electrode material in the
uncharged state must be inversely proportional to the volume of minimally porous silicon
material present within the anode mix in order to provide sufficient pore volume within the
electrode structure to accommodate the expansion of the minimally porous silicon structure
upon lithium intercalation or insertion.
An electrode or anode material comprising a composition according to any of the preferred
embodiment of the first aspect of the invention will suitably comprise 50 to 90% of an
electroactive material by weight of the electrode or anode material, preferably 60 to 80% and
especially 70 to 80%. The electroactive material suitably comprises from 10 to 100% silicon
containing porous particle fragments by weight of the electroactive material, preferably from
20 to 100wt%, more preferably 40 to 100wt% silicon, most preferably 50 to 90wt% and
especially 60 to 80wt%. The electroactive material may include additional components
selected from the group comprising non-silicon containing electroactive materials; silicon
powders; elongate silicon containing elements such as silicon rods, fibres, wires, ribbons
and sheets; and silicon containing pillared particles. Examples of further electroactive
materials that may be present include graphite and transition metal oxides or chalcogenides
such as Mo0 2, 0 , MnV20 6 and TiS2; aluminium and its compounds, tin and its
compounds; germanium compounds, including germanium nano-wires; and ceramics such
as, for example, titanate ceramics and bismuth selenide. These additional components
suitably comprises up to 50wt%, for example from 5 to 40% by weight of the electrode or
anode material or mix.
In a preferred embodiment of the first aspect of the invention, the composition comprises, in
addition to the silicon containing porous particle fragments, an electroactive carbon material.
These electroactive carbons may be present in an amount comprising 8 to 90% of the total
weight of the electroacive material, preferably 8 to 80% and especially 8 to 50%. Examples
of suitable electroactive carbons include graphite, hard carbon, carbon microbeads and
carbon flakes, nanotubes and nanographitic platelets. Suitable graphite materials include
natural and synthetic graphite materials having a particle size in the range 5 to 30 m.
Electroactive hard carbon suitably comprises spheroidal particles having a diameter in the
range 2 to 50 m, preferably 20 to 30 m and an aspect ratio of 1:1 to 2:1. Carbon
microbeads having a diameter in the range 2 to 30 m can be used. Suitable carbon flakes
include flakes derived from either graphite or graphene.
In a first more preferred embodiment of the first aspect of the invention, the composition
comprises 5 to 40wt%, preferably 10 to 30wt% and especially 15 to 25wt% of siliconcontaining
electroactive material including silicon-containing porous particle fragments and
60 to 95wt%, preferably 70 to 90wt% and especially 75 to 85wt% of an electroactive carbon
material. Preferably an electroactive composition comprising 20wt% of a silicon-containing
electroactive material including silicon-containing porous particle fragments and 80wt% of
graphite can be used to manufacture an electrode material. The silicon-containing
electroactive material may include electroactive silicon-containing structures such as native
particles, fibres, threads, tubes, wires, nano-wires, pillared particles and the like as well as
porous particle fragments. Preferably the silicon-containing electroactive material comprises
porous particle fragments.
A second more preferred embodiment of the first aspect of the invention, the composition
comprises 60 to 80wt%, preferably 70 to 80wt% and especially 80wt% of silicon-containing
porous particle fragments and 20 to 40wt%, preferably 20 to 30wt% and especially 20wt% of
an electroactive carbon material. Preferably an electroactive composition comprising 80wt%
of silicon-containing porous particle fragments and 20wt% of graphite can be used to
manufacture an electrode material. The silicon-containing electroactive material may include
electroactive silicon-containing structures such as native particles, fibres, threads, tubes,
wires, nano-wires, pillared particles and the like as well as porous particle fragments.
Preferably the silicon-containing electroactive material comprises porous particle fragments.
The binder is a component used to bind the components of the anode mix together either
upon formation of the felt like mat or on application of the components to the current
collector. The binder helps to maintain the integrity of the anode mix according to the second
aspect of the invention when used in battery cells. It also functions to help adhere the anode
mix to the current collector. The binder can be added in an amount of 0 to 30% by weight
based on the weight of the anode material. Examples of binders include, but are not limited
to, polyvinylidene fluoride, polyacrylic acid, modified polyacrylic acid,
carboxymethylcellulose, modified carboxymethylcellulose, polyvinyl alcohol,
fluorocopolymers such as copolymers of hexafluoroethylene, polyimide, styrehe butadiene
rubber and thermo or photopolymerizable materials including, but not limited to, monomers,
oligomers and low molecular weight polymers and mixtures thereof which are polymerizable
by light irradiation and/or heat treatment. Examples of polymerizable monomers include
epoxy, urethane, acrylate, silicon and hydroxy! based monomers and acrylic derivatives
which may be used alone or in combination. Polymerisation of these materials is initiated
with light irradiation or heat treatment. The polymerizable oligomer is a polymerisation
product of from 2 to 25 monomers and may be formed into polymers having a higher degree
of polymerisation by light irradiation or heat treatment. The term polymerisable low molecular
weight polymer includes linear polymers and cross-linked polymers having a low degree of
polymerisation or a low viscosity. Examples of such polymers include polyester acrylate,
epoxy acrylate, urethane acrylate and polyurethane.
Preferably the binder is selected from one or more of a polyacrylic acid, a modified
polyacrylic acid or alkali metal salts thereof. Lithium and sodium salts are preferred.
Polyacrylic acid binders and sodium polyacrylic acid binders are able to bind to silicon
materials containing impurities. The silicon-containing porous particle fragments according to
the first aspect of the invention suitably have a silicon purity of between 75% and 100%.
Preferably the silicon-containing porous particle fragments have a silicon purity of at least
80%, more preferably at least 95%. It is especially preferred that the silicon-containing
porous particle fragments of the composition of the first aspect of the invention have a silicon
purity of less than 99.99%, preferably less than 99.95% because these materials can be
cheaper and the impurities can improve conductivity within the electrode structure. However
if the level of impurities is too high the performance of the active material in the cell can be
reduced and it has been found that a purity in the range 90% to 99.99% is preferred, more
preferably 90% to 99.95%, especially 95% to 99.9%. It will be appreciated therefore, that the
silicon containing porous particle fragments and other silicon containing components used in
the preparation of compositions according to the first aspect of the invention may be derived
from metallurgical grade silicon. Batteries including electrodes containing compositions of
the first aspect of the invention, which include a binder comprising polyacrylic acid, a
modified polyacrylic acid or an alkali salt thereof exhibit a significant reduction in first cycle
loss.
A particularly preferred embodiment of the first aspect of the invention provides a
composition comprising 10 to 95% by weight of silicon containing components, including
silicon containing porous particle fragments, 5 to 85% by weight of non-silicon containing
components and 0.5 to 5% by weight of a binder comprising polyacrylic acid and/or an
alkali metal salt thereof. Preferred alkali metal salts include those derived from lithium,
sodium or potassium. Preferably the silicon containing components have a purity in the
range 90 to 99.95% or in the range 95 to 99.9%, and optionally in the range 95 to 99.99%.
An especially preferred embodiment according to the first aspect of the invention provides a
composition comprising 70wt% of silicon-containing porous particle fragments, 12wt% of a
binder, 12wt% graphite and 6wt% of a conductive carbon. The composition is provided in the
form of an electrode material. Half cells prepared using this electrode material as an anode
material and charged to either 1200mAh/g or 1400mAh/g exhibit a capacity retention of
almost 100% over at least 80 cycles.
Half cells including electrode compositions comprising 70wt% silicon-containing porous
particle fragments, 18wt% of a binder, 4% graphite and 6wt% of a conductive carbon
exhibited a capacity retention of almost 100% when charged to 1400mAh/g.
A viscosity adjuster is a component used to adjust the viscosity of the anode mix so that the
mixing process and the application of the material to a current collector can be easily carried
out. The viscosity adjuster can be added in an amount of 0 to 30% by weight based on the
total weight of the anode mix. Examples of viscosity adjusters include, but are not limited to,
carboxymethylcellulose, polyvinylidene fluoride and polyvinyl alcohol. Where appropriate, in
order to adjust the viscosity of the anode mix, a solvent such as N-methyl pyrrolidone (NMP)
may be used in an amount of 0 to 30% based on the total weight of the anode mix. In this
case the solvent is removed before or after any polymerization or curing process.
The compositions of the first aspect of the invention preferably include a conductive material.
The conductive material is a component used to further improve the conductivity of the
anode material and may be added in an amount of 1 to 20% by weight based on the total
weight of the anode mix. There is no particular limit to the conductive material so long as it
has suitable conductivity without causing chemical changes in a battery in which it is
included. Suitable examples of conductive materials include hard carbon; graphite, such as
natural or artificial graphite; carbon blacks such as carbon black, acetylene black, ketjen
black, channel black; conductive fibres such as carbon fibres (including carbon nanotubes)
and metallic fibre; metallic powders such as carbon fluoride powder, aluminium powder and
nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive
metal oxides such as titanium oxide and polyphenylene derivatives. Suitably the total
amount of conductive carbon and electroactive carbon (such as graphite) comprises 0 to
60% of the total electroactive material by weight.
The compositions according to the first aspect of the invention may also include a coupling
agent and an adhesive accelerator. The coupling agent is a material used to increase
adhesive strength between the active material and the binder and is characterised by having
two or more functional groups. The coupling agent may be added in an amount of up to 0 to
30% by weight based on the weight of the binder. There is no particular limit to the coupling
agent so long as it is a material in which one functional group forms a chemical bond via
reaction with a hydrpxyl or carboxyl group present on the surface of the silicon, tin or
graphite-based active material, and the other functional group forms a chemical bond via
reaction with the nanocomposite according to the present invention. Examples of coupling
agents that can be used in the present invention include silane based coupling agents such
as triethoxysilylpropyl tetrasulphide, mercaptopropyl triethoxysilane, aminopropyl
triethoxysilane, chloropropyl triethoxysilane, vinyl triethoxysilane, methacryloxypropyl
triethoxysilane, glycidoxypropyl triethoxysilane, isocyanopropyl triethoxysilane and
cyanopropyl triethoxysilane.
The adhesive accelerator may be added in an amount of less than 0% by weight based on
the weight of the binder. There is no particular limit to the nature of the adhesive accelerator
so long as it is a material that improves the adhesive strength of the anode mix to the current
collector. Examples of adhesive accelerators include oxalic acid, adipic acid, formic acid,
acrylic acid and derivatives, itaconic acid and derivatives and the like.
2.6.2 Minimally Porous Silicon Containing Additives
As indicated above, the compositions of the first aspect of the invention may also include as
an electroactive material one or more minimally porous silicon containing components
selected from the group comprising elongate elements; native particles, pillared particles,
substrate particles, scaffolds and particles comprising a columnar bundle of nano-rods
having a diameter of 50 to 100 nm and a length of 2 to 5 m, wherein each nano-rod has a
diameter of at least 10nm.
Where the electrode material contains elongate silicon containing elements these can be
selected from the group comprising fibres, rods, tubes, wires, nano-wire, ribbons and flakes.
The term "fibre" should be understood to include wires, nano-wires, threads, filaments, pillars
and rods as described herein below and these terms may be used interchangeably.
However, it should be appreciated that the use of the term "pillar" in the context of the
present invention is used to describe an elongate structure such as a fibre, wire, nano-wire,
thread, filament or rod which is attached at one end to a particular substrate. Fibres, wires,
nano-wires, threads and filaments may in one embodiment be obtained by detaching pillars
from the substrate to which they are attached. The term "fibre" should also be understood to
mean an element defined by two smaller dimensions and one larger dimension, the aspect
ratio of the larger dimension to the smallest dimension being in the range 5:1 to 1000:1 . As
indicated above, where the material according to the first aspect of the invention includes a
silicon containing fibre, this fibre preferably has a diameter in the range 0.05 to 2 m,
preferably 0.1 to 1 m and especially 0.1 to 0.5mpi. Silicon fibres having a diameter of 0.2 or
0.3 m are preferred. Silicon containing fibres of the first aspect of the invention suitably
have a length in the range 1pm to 400pm, preferably 2 m to 250pm. Silicon fibres having a
length of 20 m are preferred.
Branched structures may be referred to as bipods, tripods or tetrapods depending upon the
number of branches attached to a main stem.
In the context of the foregoing, the term "nano-wire" should be further understood to mean
an element having a diameter in the range 1nm to 500nm, a length in the range 0.1 m to
500 m and an aspect ratio that may be greater than 10, preferably greater than 50 and
especially greater than 00. Preferably the nano-wires have a diameter in the range 20nm to
400nm, more preferably 20nm to 200nm and especially 100nm. Examples of nano-wires that
can be included in the compositions of the present invention are disclosed in US
2010/0297502 and US 2010/0285358.
The term elongate element also includes a pillared particle having one or more pillars
provided on the surface thereof, where the pillars have a length in the range 1 to 100pm.
Such pillars may be formed integrally with the particle core or may be formed independently
of the particle core. These pillared particles provided as elongate elements should be
distinguished from pillared particles having a pillar length of less than 1 to 100pm.
Alternatively, where the silicon containing elongate elements comprise ribbons, tubes or
flakes, these are each suitably defined by three separate dimensions. The ribbon includes a
first dimension, which is smaller in size than the other two dimensions; a second dimension,
which is larger than the first dimension and a third dimension, which is larger than both the
first and second dimension. The flake includes a first dimension, which is smaller in size than
the other two dimensions; a second dimension, which is larger than the first dimension and a
third dimension, which is similar to or marginally larger than the second dimension. The tube
includes a first dimension, the tube wall thickness, which is smaller in size than the other two
dimensions, a second dimension, the outer diameter of the tube wall, which is larger than the
first dimension and a third dimension, the tube length, which is larger than both the first and
second dimension. For ribbons, tubes and flakes, the first dimension is suitably of the order
of 0.03mpi to 2m , typically 0.08pm to 2mih, preferably 0.1 m h to 0.5pm. The second
dimension is suitably at least two or three times larger than the first dimension for ribbons
and between 10 and 200 times the first dimension for flakes and between 2.5 and 100 times
the first dimension for tubes. The third dimension should be 10 to 200 times as large as the
first dimension for both ribbons and flakes and between 1 to 500 times as large as the first
dimension for tubes. The total length of the third dimension may be as large as 500pm, for
example.
Ribbons having a typical thickness of 0.125 to 0.5pm, a width of more than 0.5pm and a
length of 50pm may be used. Flakes having a thickness of 0.1 to 0.5mih , a width of 3 m and
a length of 50pm are also suitable. Tubes having a wall thickness of 0.08 to 0.5pm, an outer
diameter of 0.2 to 5pm and a length of at least five times the outer diameter are particularly
suitable.
The minimally porous silicon containing particles referred to above may be in the form of
native particles or pillared particles.
Native particles typically have a principle diameter in the range 0.5um to 15pm, preferably 1
to 15pm, more preferably 3 m to 10 m and especially 4pm to 6 m. By the term "Pillared
Particles" it is to be understood to mean particles comprising a core and a plurality of pillars
extending there from, where the pillars have a length in the range 0.5 to 10 m, preferably 1
to 5 m. Pillared particles can be prepared by etching silicon particles having dimensions in
the range 5 to 40 m, preferably 5 to 25pm using the procedure set out in WO
2009/010758. Such pillared particles include particles having a principle diameter in the
range 5 to 15 m, 5 to 25pm and 25 to 35 m. Particles having a principle diameter in the
range 5 to 15 m typically include pillars having heights in the range 0.5 to 3pm. Particles
having a principle diameter in the range 15 to 25pm typically include pillars having heights in
the range 1 to 5pm. Particles having a principle diameter in the range 25 to 35pm typically
include pillars having heights in the range 1 to 10 m, preferably 1 to 5pm. The pillared
particles can be directly applied to the current collector or can be included in a composite
electrode material and may be provided as discrete particles, in the form of a network n
which the pillars of one particle overlap or are directly connected to the pillars of another
particle in the network or as a mixture of both. The pillared particles are most preferably
provided in a composite electrode material in the form of discrete particles which, during the
charging and discharging cycles, are able to expand and contract without significantly
affecting or impinging upon the expansion and contraction of other pillared particles in the
electrode material and which are able to contribute to the continued electrical conductivity of
the electrode material over a significant number of charging and discharging cycles.
The silicon containing elongate elements referred to above may be prepared by any suitable
methods known to a person skilled in the art. The elongate elements are preferably prepared
from single crystalline wafers or from single crystalline, polycrystalline or amorphous silicon
particles having a dimension larger than 80 m.
Silgrain™ polycrystalline silicon particles having dimensions in the range dqmhi to 0.8mm
can be obtained by grinding and sieving any one of the Silgrain materials sold by Elkem of
Norway. Suitable Silgrain products that can be used in the preparation f elongate elements
(fibres) (and also pillared particles) include Silgrain™ Coarse having dimensions in the range
0.2 to 2mm, Silgrain™ HQ having dimensions in the range 0.2 to 0.8mm and Jetmilled
Silgrain™ having dimensions in the range 10 to 425pm. These Silgrain products typically
contain from 97.8 to 99.8% silicon and include impurities such as iron, Aluminium, Calcium
and Titanium.
The silicon containing native particles, elongate elements and porous particle fragments may
comprise pure or impure silicon as described herein, doped silicon or may be in the form of
an alloy if the doping exceeds 1wt% or a n intermetallic alloy. Typical dopants include boron,
nitrogen, phosphorous, aluminium and germanium.
Pillared particles may also be manufactured using growth techniques such a s high and low
temperature CVD, vapour solid liquid growth, molecular beam epitaxy, laser ablation and
silicon monoxide evaporation to grow fibres on particle cores. Such growth techniques are
well known to a skilled person and are set out in JP 2004-281317, US 2010/0285358 and
also in Chem. Rev. 2010, 110, 361-388.
By the term "scaffold" it should be understood to mean a three dimensional arrangement of
one or more structural elements selected from the group comprising fibres, wires, nanowires,
threads, pillars, rods, flakes, ribbons and tubes, which structures are bonded together
at their point of contact. The structural elements may be arranged randomly or non-randomly
in the three dimensional arrangement. The three dimensional scaffold may comprise coated
or uncoated structures having a core comprising a n electroactive material such as silicon,
tin, germanium or gallium. Alternatively, the scaffold may be a hetero-structure comprising a
three-dimensional arrangement of structures comprising a n electroactive or a nonelectroactive
base scaffold material onto which is deposited small islands, nano-wires or a
coating of an electroactive material having a composition different to that of an electroactive
material from which the scaffold is formed; preferred scaffolds of this type comprise a
network of carbon fibres, threads, wires, ribbons or nano-wires having small islands, nanowires
or a thin film coating of an electroactive material such as silicon, germanium, gallium,
tin or alloys or mixtures thereof applied thereto. Where the scaffold comprises a silicon
based coating, one or more additional coating layers may be applied thereto. A coating layer
may be continuous and extend over substantially the entire surface of the scaffold structure.
Alternatively, a coating layer may be discontinuous and may be characterised by an absence
of a coating layer over some regions of the surface of the scaffold structure. In one
embodiment, the coating material may be distributed randomly or in a set pattern over the
surface of the scaffold. Examples of scaffold structures that can be included in the binder
compositions of the present invention are disclosed in US 201 0/0297502.
Each of the particles, tubes, wires, nano-wires, fibres, rods, sheets and ribbons and
scaffolds that can be included in the composite electrode materials used in the manufacture
of the battery cells of the present invention may be crystalline, microcrystajljne,
polycrystalline or amorphous or may include crystalline or polycrystalline regions within an
amorphous structure. These structures may be fabricated using etching techniques such as
those outlined in WO 2009/01 0758 or electrospinning as described in US201 0/033041 9.
Alternatively, they can be manufactured using growth techniques such as a catalysed
Vapour-Liquid-Solid approach as described in US 2010/0297502. It will be apparent to a
skilled person that it is possible to grow nano-particles, nano-wires and nano-tubes on the
surface of a conductive substrate such as a carbon particulate substrate using the technique
set out in US 201 0/0297502.
The minimally porous silicon containing native particles, fibres, tubes, ribbons and/or flakes
comprising the material of the first aspect of the invention may also be provided with a
coating. Suitable coatings include lithium salts, amorphous carbon, graphitic carbon, hard
carbon and carbon based polymers. Suitable lithium salts include lithium fluoride, lithium
carbonate and complex salts of cyclic carbonate species such as ethylene ca rbonate,
propylene carbonate, diethylene carbonate and vinyl carbonate with lithium.
Coats are typically applied to the silicon structures to a thickness of between 1 and 30% of
the total weight of the silicon/carbon product. Methods of coating silicon particles and
elongate elements are known to a person skilled in the art and include mechanical
techniques, chemical vapour deposition, and pyrolysis techniques. Carbon coating of silicon
structures through the use of Chemical Vapour Deposition techniques is disclosed in US
2009/0239151 and US 2007/0212538. Pyrolysis methods are disclosed inWO 2005/011030,
JP 2008/186732, CN 101442124 and JP 04035760.
Where the composition according to the first aspect of the invention comprises one or more
components selected from elongate silicon elements, native silicon particles, substrate
particles, scaffolds, columnar bundles and pillared particles in addition to the silicon
containing porous particle fragments, these are preferably present in an amount comprising
0 to 60% by weight of the electroactive material, either alone or in combination.
3. Manufacture of Starting Material
As indicated above, the porous particles used to fabricate the silicon containing porous
particle fragments according to the first aspect of the invention can be readily manufactured
using techniques that are well known to a person skilled in the art. Silicon containing porous
particles are typically fabricated using techniques such as stain etching of silicon particles or
wafers or by etching particles of silicon alloy, such as an alloy of silicon with aluminium.
Methods of making such porous particles are well known and are disclosed, for example, in
US 2009/0186267, US 2004/0214085 and US 7,569,202. They can also be manufactured by
etching particles of a silicon metal alloy to remove the metal and precipitate the porous
silicon structure.
The particulate alloy material used to prepare the porous particles from which the porous
particle fragments of the present invention are derived are generally prepared using
techniques that rapidly quench samples of the molten alloy such as gas atomisation, melt
spinning, splat quenching, cold rolling, and laser surface modification, all of which are known
to a person skilled in the art. Such preparation techniques followed by etching are preferred
for making the parent porous particles but other methods of making the parent porous
particles, such as those described above can be used. The structure of the silicon within a
rapidly quenched alloy particle depends on factors such as the concentration of the silicon
(or other electroactive material) in the alloy, whether modifying additives such as Na, Sr or Ti
are present in the alloy and the processing conditions used to form the solid alloy particles
from its corresponding molten form. It has been found, for example, that for any particular
alloy composition, the morphology of the silicon (or electroactive material) component within
the alloy particle depends on factors such as the size of the alloy droplets and the rate of
cooling applied to the molten alloy during particle formation. The rate of cooling that can be
achieved depends on the techniques used. Where gas atomisation techniques are used to
form the alloy particles, the rate of cooling that can be achieved depends upon the nature of
the gas used and the velocity at which it impinges the alloy droplets within the reaction
chamber. Gas atomisation techniques are generally associated with cooling rates in the
range 1 3 to 105 K s or faster the use of cooling rates in this region results in the formation
of alloy structures including regions of silicon comprising finely branched silicon structures.
These finely branched silicon structures typically comprise tree like structures comprising
branched rods of silicon having a diameter in the range 50 to 10fJnm, the rods including
branches every 100 to 400nm. Where melt spinning techniques are used the rate of cooling
depends on the rotational velocity of the cooled disc onto which the molten alloy particles
impinge, the temperature of the disc, surrounding gas and its temperature and the alloy
droplet size. Melt spinning techniques are generally associated with a cooling rate in the
range 102 to 10 K/s. The use of cooling rates in this region results in the formation of alloy
structures including regions of silicon comprising both coarse and fine silicon structures.
Silicon structures having a minimum dimension of between 00nm and 500nm, preferably
between 100nm and 200nm and a length in the range 5 to 10 m have been observed. Alloy
additives can also affect the shape and form of the silicon structures. A sodium additive may
tend to spheroidise the silicon structures which is not preferred, whilst a combination of Ti
and Sr additives may reduce the size of fibre-like structures. .Alloys of silicon with
aluminium, in which the alloy comprises up to 30% silicon are preferred. Silicon aluminium
alloys comprising 2%, 26% and 30% silicon can be used in the fabrication of the porous
silicon particles from which the porous particle fragments are derived. The use of silicon
aluminium alloys comprising 12% silicon are preferred. However, silicon alloys containing
both 27% and 30% silicon as a constituent have also been observed to yield porous particle
fragments that can be used in the manufacture of half cells having a capacity retention of
almost 100% over more than 80 cycles when charged to 1200mAh/g or 1400mAh/g. As
indicated above, the precipitated porous silicon structure can be isolated from the bulk alloy
by etching away the some or all of the bulk metal, provided the etching method does not
etch the silicon structures but does etch the metal. Etchants may be liquid or gaseous phase
and may include additives or sub-processes to remove any by-product build up which slows
etching. Etching can be done chemically, e.g. (in the case of Al) using ferric chloride, or
electrochemically using copper sulphate/sodium chloride electrolytes. The vast majority of
known aluminium etchants/methods do not attack the fine Si structures , leaving them intact
after a sufficient amount of the aluminium (some or all) has been etched away. Any
aluminium or aluminium silicide intermetallics remaining after etching, for example adhering
to the crystalline silicon, can be tolerated when the silicon is used to form an anode as they
are themselves excellent Li-ion anode candidates, and so long as any aluminium and
intermetallic structures have comparable thickness to the silicon they can be expected to
survive Li insertion cycling. In fact, aluminium and intermetallics may also aid in making
electrical contact between the porous silicon particles and metal electrode. Similar
techniques known to a person skilled in the art can be used to manufacture porous particles
comprising germanium, gallium, lead or tin or a mixture thereof.
The most common commercially practised method of bulk aluminium etching involves
caustic etching using an etchant containing 10-20% NaOH. The etchant will be selected to
prevent substantial attack of the silicon by the etchant. Other etching solutions that can be
used to selectively remove the aluminium from the alloy sample include solutions comprising
a mixture of nitric acid, hydrochloric acid and hydrofluoric acid as well as solutions
comprising a mixture of phosphoric acid, nitric acid and acetic acid. Solutions comprising a
mixture of phosphoric acid, nitric acid and acetic acid are generally preferred
After partially or fully etching away the metal matrix, the porous silicon structures will be
released into the etchant. These will generally need cleaning to remove contaminants, by¬
products (e.g. aluminium hydroxide in caustic etching)) and remnants generated during
etching, which may be achieved using acids or other chemicals, followed by rinsing and
separating the porous silicon structures from the liquid, which may be achieved by filtering,
centrifuging or other separation method. The porous silicon structures may then be handled
in liquid suspension.
Once the porous silicon structures are released and isolated, they can be partially crushed
using any suitable technique to give silicon containing porous particle fragments. Suitable
techniques for partially crushing the porous silicon structures include ultrasound, a pestle
and mortar and ball milling, the use of ultrasound being preferred. Ultra-sonic crushing is
suitably carried out at or around 27KHz for 5 minutes using a suspension of silicon in a
solvent such as water, in aqueous solutions, in organic solvents such as Nmethylpyrrolidone
(NMP) or other solvents used in battery manufacture. Ball milling is
suitably carried out using a high energy ball mill, an epicyclic ball mill or a standard ball mill,
preferably using ceramic balls.
The fragments are then separated according to their size, using either centrifugation or
sieving. The fragments are then further cleaned and dried. The isolated particles can be
included into an electrode or anode mix and used in the fabrication of an electrode,
preferably an anode. The silicon containing porous particle fragments are typically mixed
with a binder, a solvent and optionally one or more additional ingredients selected from the
group comprising a conductive material, a further electro-active material, a viscosity adjuster,
a filler, a cross-linking accelerator, a coupling agent and an adhesive accelerator and coated
onto a substrate. The coated substrate is then dried to remove the solvent and calendared
and used to form an anode as set-out in WO2007/0831 55, WO2008/1 391 57,
WO2009/010758, WO2009/010759 and WO2009/01 0757, all incorporated herein by
reference.
Although aluminium is preferred as the main component of the silicon alloy from which the
silicon structures are precipitated, the skilled person will understand that other metals that
will precipitate silicon during alloy cooling and can be etched may be used. Furthermore the
skilled person will understand there are ways of foaming metals by injecting gases into the
cooling molten mass and this can be applied to Si to create a 'foamed' Si matrix. One such
method is described in "Fabrication of lotus-type porous silicon by unidirectional solidification
in hydrogen" in Materials Science and Engineering A 384 (2004) 373-376. Using techniques
such as this potentially allows a skilled person to avoid the need to etch away a sacrificial
material to obtain the Si mesh or matrix. Solgel formation methods such as those used with
silica to produce Aerogel may also be applied to silicon. The silicon containing components
or structures of the composition of the first aspect of the invention suitably comprise a high
purity polycrystalline silicon material as well as polycrystalline silicon materials comprising
either n-type or p-type dopants as impurities. Polycrystalline silicon materials comprising ntype
or p-type dopants are preferred because these materials exhibit a greater conductivity
compared to that of high purity polycrystalline silicon. Polycrystalline silicon materials
comprising p-type dopants are preferred and can be easily prepared from the aluminium
silicon alloys referred to herein or using methods (such as ion implantation) known to a
person skilled in the art; these materials suitably include one or more impurities selected
from aluminium, boron or gallium as dopants.
4. Methods of Making Porous Particle Fragments
A second aspect of the invention provides a method for fabricating a composition according
to the first aspect of the invention, the method comprising the steps of preparing a silicon
containing porous particle and partially crushing that particle to give a silicon containing
porous particle fragment. In a first preferred embodiment of the second aspect of the
invention porous particles having a diameter in the range 10 to 1500pm, preferably 10 to
lOOOpm, more preferably 10 to 200pm, especially 10 to 100 m and which are prepared by
etching a silicon aluminium alloy as specified above are partially crushed to give silicon
containing porous particle fragments having a diameter in the range 1 to 40 m. The silicon
containing porous particles used to prepare the silicon containing porous particle fragments
suitably have a porosity in the range 0.2 to 0.8 and an pore wall thickness in the range 20 to
200nm, preferably 50 to 200nm. The etched particles are cleaned and placed in an ultra
sound bath for between 30 and 90 seconds to give silicon containing porous particle
fragments having a diameter in the range 5 to 25pm.
In a second preferred embodiment of the second aspect of the invention, a method
comprises preparing a silicon-containing porous particle having a diameter of at least 60pm,
preferably at least 100 pm and especially at least 120 pm and having an average smallest
dimension (thickness) in the range 50nm to 2 pm, preferably 100nm to 1 pm are partially
crushed to give porous particle fragments having an overall diameter in the range 1 to 40pm,
preferably 1 to 20pm, more preferably 1 to 15pm and especially 1 to 10pm and an average
smallest dimension (or thickness) of the fragment's microstructure in the range 50nm to 2
p , preferably 100nm to 1 pm. A further preferred embodiment of the second aspect of the
invention Comprises partially crushing a silicon-containing porous particle having a diameter
in the range 60 to 1500pm, preferably 100 to 1000pm to give porous particle fragments
having a diameter and average smallest dimension as specified herein above.
As indicated above, the porous particle fragments of the first aspect of the invention are
prepared by partially crushing a porous particle fragment, which is preferably prepared, for
example, by etching a silicon aluminium alloy particle. The structure of the porous particle
fragment depends on the composition of the alloy particle from which the porous particles
are derived and the techniques used to fabricate the alloy particles, in a third particularly
preferred embodiment of the second aspect of the invention there is provided a method of
preparing porous particle fragments according to the second aspect of the invention, the
method comprising the steps:
a) Providing a molten silicon aluminium alloy composition comprising between 12 and
30wt% silicon;
b) Cooling the molten silicon aluminium alloy composition at between 102 and 10 K/s to
form a silicon aluminium particulate material having a diameter in the range 40 to
1500pm;
c) Etching the particulate material formed in (b) to form porous particles
d) Partially crushing the porous particles formed in (c) to give porous particle fragments
having a maximum diameter of less than 40pm
The molten alloy is suitably cooled in step (b) using either gas atomisation or melt spinning
techniques. The etchants used in step (c) are well known to a person skilled in the art and
are listed herein above. Where the alloy is a silicon aluminium alloy the amount of aluminium
that can be removed from the alloy will depend on the etching conditions used.
The invention also provides silicon containing porous particle fragments prepared according
to the method of the second aspect of the invention. A third aspect of the invention provides
a composition comprising a plurality of silicon containing porous particle fragments prepared
by partially crushing a plurality of silicon containing porous particles. The silicon containing
porous particles from which the silicon containing porous particle fragments are derived
suitably have a diameter of greater than 40pm, preferably greater than 60 pm, more
preferably greater than 100pm and especially greater than 20 m. Typically the silicon
containing porous particles from which the silicon containing porous particle fragments are
derived suitably have a diameter in the range 40 to 200pm, preferably 50 to 150pm and
especially 70 to 1 Om h . Preferably the porous particle fragments of the first aspect of the
invention are prepared from porous particles (including porous particles comprising a
network of elongate elements) having a diameter in the range 60 to 1500pm, more
preferably 150 to 1000pm. These "starting particles" may be prepared by either etching a
silicon aluminium alloy or by stain etching of a silicon particle using methods that are known
to a skilled person as set out above.
In a first particularly preferred embodiment of the third aspect of the invention there is
provided a composition comprising a plurality of silicon-containing porous particle fragments
having a diameter in the range 1 to 40 m, preferably 1 to 20 m, more preferably 1 to 15 m
and especially 1 to 10 m and an average smallest dimension (thickness) in the range 50nm
to 2 , preferably 0Onm to 1 pm, the porous particles being prepared by partially crushing
porous silicon-containing particles having a diameter of at least 60 m, preferably at least
00 pm and especially at least 120 pm and having an average smallest dimension in the
range 50nm to 2 pm, preferably 100nm to 1 pm. The silicon-containing whole porous
particles are suitably prepared by etching a silicon alloy, preferably a silicon/aluminium alloy
in a solution comprising a mixture of acetic acid, nitric acid and phosphoric acid to remove
the bulk metal with which the silicon material is alloyed, washing and drying the particle.
A second preferred embodiment of the third aspect of the invention provides a porous
particle fragment prepared by a method comprising the steps of:
a) Providing a molten silicon aluminium alloy composition comprising between 12 and
30wt% silicon;
b) Cooling the molten silicon aluminium alloy composition at between 102 and 105K s to
form a silicon aluminium particulate material having a diameter in the range 40 to
1500pm;
c) Etching the particulate material formed in (b) to form porous particles
) partially crushing the porous particles formed in (c) to give porous particle fragments
having a maximum diameter of less than 40 m
The molten alloy is suitably cooled in step (b) using either gas atomisation or melt spinning
techniques. The etchants used in step (c) are well known to a person skilled in the art and
are listed herein above. Where the alloy is a silicon aluminium alloy the amount of aluminium
that can be removed from the alloy will depend on the etching conditions used.
The porous particle fragments according to the third aspect of the invention can be
characterised by fractal or pore wall thicknesses in the range 50nm to 2 m. Porous particle
fragments having a fractal thickness of 50 to 100nm can be used to make batteries. Fractal
structures having a fractal or pore wall thickness in the range 0Onm to 200nm can be used
to prepare composite electrodes, which exhibit particularly good cycling behaviour.
5. Electrodes
The electroactive material according to the first aspect of the invention can be used in the
manufacture of an electrode. The electrode is typically an anode. The electrodes are
preferably used in the manufacture of a lithium secondary battery. A fourth aspect of the
invention therefore provides an electrode comprising a composition according to the first
aspect of the invention and a current collector. Preferably an electrode according to the
fourth aspect of the invention comprises a composition according to the first aspect of the
invention, a binder and a current collector. The electroactive material according to the first
aspect of the invention is suitably provided in the form of an electrode or anode material,
said electrode or anode material comprising in addition to the silicon containing porous
particle fragments, a binder and optionally one or more components selected from the group
comprising, a conductive material and a further electroactive material. Preferably the
electrode or anode material comprises fractals as described above. The anode material can
be in the form of a free-standing mat which can be connected or adhered to a current
collector. Alternatively the anode mix can be provided in the form of a coating, which can be
adhered to a current collector. The components of the anode mix from which the mat is
formed are typically randomly arranged within the anode structure to provide optimum
connectivity between the elements. The electrodes of the fourth aspect of the invention are
easily prepared and a fifth aspect of the invention provides a method for fabricating an
electrode comprising the steps of forming an electrode or anode mix, said electrode or
anode mix comprising a slurry of a composition according to the first aspect of the invention
and a solvent (herein after referred to as an electrode mix) and casting the electrode or
anode mix onto a substrate and drying the product to remove the solvent thereby to form the
electrode or anode material on the substrate. The dried product (electrode material) is in the
form of a cohesive mass which may be removed from the substrate, connected to a current
collector and used as an electrode. Alternatively, where the composition according to the
first aspect of the invention is adhered to the substrate as a result of casting and drying the
slurry (electrode mix), the resulting cohesive mass will be connected or bonded to a current
collector. In a preferred embodiment of the first aspect of the invention the composition is
cast onto a substrate, which is itself a current collector. One or more components selected
from the group comprising a conductive material, a viscosity adjuster, a filler, a cross-linking
accelerator, a coupling agent and an adhesive accelerator may also be included in the slurry
mixture (electrode mix). Examples of suitable conductive materials, viscosity adjusters,
fillers, cross-linking accelerators, coupling agents and adhesive accelerators are provided
above. Suitable solvents include N-methylpyrrolidone and water.
Suitable current collectors for use in electrodes according to the fourth aspect of the
invention include copper foil, aluminium, carbon (including graphite), conducting polymers
and any other conductive materials. The current collectors typically have a thickness in the
range 0 to 50 m, preferably 10 to 20pm.. Current collectors can be coated with the
electrode mix on one side or can be coated with the electrode mix on both sides. In a
preferred embodiment of the fifth aspect of the invention a composition of the first aspect of
the invention is preferably applied to one or both surfaces of the current collector to a
thickness of between 1mg/cm2 and 6mg/cm , preferably between 1mg/cm2 and 3mg/cm2
per surface such that the total thickness of the electrode (current collector and coating) is in
the range 25um to 1 m where only one surface of the current collector is coated or in the
range 50mih to 1mm where both surfaces of the current collector are coated. In a preferred
embodiment, the electrode or anode material is applied to a thickness of between 30 and
40 m onto one or both surfaces of a copper substrate having a thickness of between 10 and
5 m. The current collector may be in the form of a continuous sheet or a porous matrix or it
may be in the form of a patterned grid or mesh defining metallised regions and nonmetallised
regions. Where the current collector comprises a continuous sheet, the electrode
may be readily manufactured by applying a slurry of the anode mix directly to the current
collector. Where the current collector comprises a metallised grid, this metallised grid may
be formed onto a non-stick substrate such as PTFE to give a metallised non-stick surface
(such as metallised PTFE) and the slurry of the anode mix is applied to the metallised non¬
stick surface and dried to give a metallised mat or felt. Alternatively the mesh or grid can be
dipped into a solution or slurry to form a composite electrode.
In one embodiment of the fifth aspect of the invention, the electrode may be formed by
casting the silicon containing mixture onto a substrate thereby to form a self supporting
structure and connecting a current collector directly thereto.
The electrode of the fourth aspect of the invention can be used as an anode in the
manufacture or formation of a lithium secondary battery. A sixth aspect of the invention
provides a secondary battery comprising a cathode, an anode comprising a composition
according to the first aspect of the invention and an electrolyte.
6.Cathode
The cathode is typically prepared by applying a mixture of a cathode active material, a
conductive material and a binder to a cathode current collector and drying. Examples of
cathode active materials that can be used together with the anode active materials of the
present invention include, but are not limited to, layered compounds such as lithium cobalt
oxide, lithium nickel oxide or compounds substituted with one or more transition metals such
as lithium manganese oxides, lithium copper oxides and lithium vanadium oxides. Many
such materials can be defined by the generic formula Li1+ ( ibCocAl Mn -x0 ,where b, c, e, f
and x have values of between 0 and 1. Examples of suitable cathode materials include
LiCo0 2, LiCO0.99AI0.01O2, LiNi0 2, LiMn0 2, LiMn2O4.LiCoo.5Nio.5O2, LiCoo.7Nio.3O2,
LiCoo.8Nio.2O2, LiCoo.82Nio.i8O2, LiCoo.8Nio.15Alo.05O2, LiNio.4Coo.3Mno.3O2 ,
Lii + (Nio.333Coo.333Mn0 .333)i-x02, Li1+x( io.4Coo.2 no. i - 2 , and
Lii +x i0.8Coo.i5Alo.o50 2, phosphate-based cathodes such as LiFeP0 , non-lithiated cathode
materials like V60 3 , and sulphur / polysulphide cathodes. The cathode current collector is
generally of a thickness of between 3 to 500 mi . Examples of materials that can be used as
the cathode current collector include aluminium, stainless steel, nickel, titanium and sintered
carbon.
7. Electrolyte
The electrolyte is suitably a non-aqueous electrolyte containing a lithium salt arid may
include, without limitation, non-aqueous electrolytic solutions, solid electrolytes and inorganic
solid electrolytes. Examples of non-aqueous electrolyte solutions that can be used include
non-protic organic solvents such as N-methylpyrrolidone, propylene carbonate, ethylene
carbonate, butylenes carbonate, dimethyl carbonate, diethyl carbonate, gamma butyro
lactone, 1,2-dimethoxy ethane, 2-methyl tetrahydrofuran, dimethylsulphoxide, ,3-dipxolane,
formamide, dimethylformamide, acetonitrile, nitromethane, methylformate, methyl acetate,
phosphoric acid trimester, trimethoxy methane, sulpholane, methyl sulpholane and ,3-
dimethyI-2-imidazqlidione.
Electrolyte solutions comprising a mixture of cyclic and acyclic carbonate species are
preferred. Examples of cyclic carbonates that can be used as base solvents include, but are
not limited to, ethylene carbonate (EC), diethylene carbonate (DEC), propylene carbonate
(PC) and butylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate
(DFEC), Y-butyrolactone and g -valerolactone. Examples of chain or linear carbonates that
can be used as base solvents include, but are not limited to, dimethylcarbonate (D C),
diethylcarbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dibutyl
carbonate (DBC) and methyl octyl carbonate (MOC). Examples of halogenated cyclic
carbonates that can be used as electrolyte solvents include but are not limited to 4-fluoro- ,
3- dioxolane- 2-one, 4-chloro- 1, 3- dioxolane- 2-one, 4, 5- difluoro-1 , 3- dioxolane- 2-one,
tetrafluoro , 3- dioxolane- 2-one, 4-fluoro- 5-chloro- , 3- dioxolane- 2-one, 4, 5- dichloro- 1,
3- dioxolane- 2-one, tetrachloro- , 3- dioxolane- 2-one, 4, 5- bistrifluoromethyl-1, 3-
dioxolane- 2-one, 4-trifluoromethyl- , 3- dioxolane- 2-one, 4, 5- difluoro 4, 5- dimethyl- , 3-
dioxolane- 2-one, 4 -methyl- 5, 5- difluoro-1 , 3- dioxolane- 2-one, 4-ethyl- 5, 5- difluoro-1, 3-
dioxolane- 2-one, 4-trifluoromethyl- 5-fluoro- , 3- dioxolane- 2-one, 4-trifluoromethyl- 5-
methyl- 1, 3- dioxolane- 2-one, 4-fluoro- 4, 5- dimethyl- 1, 3- dioxolane- 2-one, 4, - difluoro
5-(1 , - difluoro ethyl) - 1,3- dioxolane- 2-one, 4, 5- dichloro- 4, 5- dimethyl- 1, 3- dioxolane-
2-one, 4-ethyl- 5-fluoro- 1, 3- dioxolane- 2-one, 4-ethyl- 4, 5- difluoro 1, 3- dioxolane- 2-one,
4-ethyl -4,5,5- trifluoro- , 3- dioxolane- 2-one, 4-fluoro- 4-trifluoromethyl- 1, 3- dioxolane- 2-
one. Fluorinated cyclic carbonates are preferred. Preferably the base cyclic carbonate is
ethylene carbonate (EC) or fluoroethylene carbonate (FEC). Preferably the chain (or linear)
carbonate is ethyl methyl carbonate or diethyl carbonate. In a particularly preferred third
embodiment the base solvent comprises a mixture of fluoroethylene carbonate (FEC) and
ethyl methyl carbonate (EMC). Electrolyte compositions suitably comprise a mixture of a
cyclic carbonate and a chain or linear carbonate in a ratio of between 30:70 to 70:30,
preferably between 30:70 and 1:1.
It is further preferred that the electrolyte solutions comprise a cyclic carbonate
including a vinyl group, examples of which include vinylene carbonate, methyl vinylene
carbonate, ethyl vinylene carbonate, propyl vinylene carbonate, phenyl vinylene carbonate,
dimethyl vinylene carbonate, diethyl vinylene carbonate, dipropyl vinylene carbonate,
diphenyl vinylene carbonate, vinyl ethylene carbonate and 4, 5- divinyl ethylene carbonate.
Vinyl ethylene carbonate, divinyl ethylene carbonate and vinylene carbonate are preferred.
In general the cyclic carbonate including a vinyl group will suitably comprise at least 1%, 2%,
3%, 5%, 10% or 15% by weight of the electrolyte solution. The concentration of the
halogenated cyclic carbonate will, in general, not exceed 70wt% of the electrolyte solution.
In a particularly preferred embodiment of the sixth aspect of the invention there is
provided a battery comprising a cathode, an anode comprising a composition according to
the first aspect of the invention and an electrolyte including an electrolyte solvent and an
electrolyte salt, wherein the electrolyte solvent comprises a mixture of fluoroethylene
carbonate (FEC) and ethylmethyl carbonate (EMC). It is particularly preferred that the
electrolyte solvent includes a cyclic carbonate including a vinyl carbonate as an additive. In a
most preferred embodiment of the sixth aspect of the invention the electrolyte solvent
comprises 40 to 60vol%, preferably 50vol% FEC, 40 to 48vol%, preferably 46vol% EMC and
2 to 10vol%, preferably 4vol% VC.
Examples of organic solid electrolytes include polyethylene derivatives polyethyleneoxide
derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, polyester
sulphide, polyvinyl alcohols, polyvinylidine fluoride and polymers containing ionic
dissociation groups.
Examples of inorganic solid electrolytes include nitrides, halides and sulphides of lithium
salts such as Li5NI2, Li3N, Lil, LiSi0 , Li2SiS3, Li4Si0 4 LiOH and Li3P0 .
The lithium salt is suitably soluble in the chosen solvent or mixture of solvents. Examples of
suitable lithium salts include LiCI, LiBr, Lil, LiCI0 , LiBF , LiB10C2o, LiPF6, LiCF3S0 3, LiAsF ,
LiSbF6, LiAICI , CH3S0 3Li and CF3SO3 , lithium bis(oxatlato)borate (LiBOB) or a mixture
thereof, dissolved in one or more cyclic and dialkyl carbonates referred to above. Examples
of other electrolyte salts that can be used are found in JP 2008234988, US 7659034, US
2007/0037063, US 7862933, US 2010/0124707, US 2006/0003226, US 7476469, US
2009/0053589 and US 2009/0053589. Preferably the electrolyte salt is LiPF6 or a mixture of
LiPF6 and lithium bisoxalate borate (LiBOB). A preferred electrolyte solution comprises 0.9 to
0.95M LiPFe and 0.05 to 0.1 M LiBOB. The concentration of the lithium salt in the electrolyte
solution is not limited but is preferably in the range of 0.5 to 1.5M. When larger amounts of
additives are used it is preferable to increase the concentration of the lithium salt to prevent
excessive depletion of lithium in the final electrolyte solution.
Where the electrolyte is a non-aqueous organic solution, the battery is provided wit a
separator interposed between the anode and the cathode. The separator is typically formed
of an insulating material having high ion permeability and high mechanical strength. The
separator typically has a pore diameter of 0.01 to 100 m and a thickness of 5 to 300pm.
The battery according to the sixth aspect of the invention can be used to drive a device,
which relies on battery power for its operation. Such devices include mobile phones, laptop
computers, GPS devices, motor vehicles and the like. A seventh aspect of the invention
therefore includes a device including a battery according to the sixth aspect of the invention.
The invention will now be described with reference to the following figures and non-limiting
examples. Variations on these falling within the scope of the invention will be evident to a
person skilled in the art.
8. Figures
Figure 1 shows a schematic view of a prior art battery.
Figure 2a shows an SEM image of a porous particle produced according to the procedure
set out in the examples below. These porous particles were prepared by etching particles of
an AISi alloy comprising 12wt% silicon and having a diameter of 10 to 63 m, the alloy
particles being formed through the use of a gas atomisation cooling technique. The silicon
structures are characterised by a network of fine silicon structures.
Figure 2b shows a schematic diagram of the fine silicon fractal structures formed by partially
crushing a porous particle illustrated in 2a.
Figure 2c shows an SEM of the silicon structures formed by partially crushing a porous
particle illustrated in 2a.
Figure 2d shows an SEM image of a porous particle produced according to the procedure
set out in the examples below. These porous particles were prepared from particles of an
AISi alloy comprising 27wt% silicon and having a diameter of 10 to 90 m, the alloy particles
being formed through the use of a gas atomisation cooling technique. The porous particle is
characterised by regions of particulate silicon interspersed within regions of finely branched
silicon fibres.
Figure 2e shows an SEM image of a porous particle fragment obtained by partially crushing
porous particles illustrated in Figure 2d.
Figure 2f shows an SEM of a porous particle fragment (fractal) produced by partially
crushing the porous particles produced from AISi(1 2wt%) alloy particles (formed using a gas
atomisation technique) and having a diameter of 60 to 90 m, the porous particles being
produced according to the procedure set out in the examples below.
Figure 3a shows an SEM image of a porous particle fragment produced according to the
procedure set out in the examples below. These porous particle fragments were prepared by
partially crushing porous particles obtained by etching the larger particles of an AISi alloy
comprising 2wt% silicon and having a diameter in the range 90 to 1000pm, the alloy
particles being the larger size fraction obtained using a gas atomisation technique. Melt
spinning appears to yield similar results. Note the relatively coarser nature of the silicon
structures compared to the structures illustrated in figures 2a, 2b and 2e.
Figure 3b shows a schematic diagram of the silicon structures formed in the AISi(1 2wt%)
alloy particle, which was used to form the porous particle fragments in Figure 3b above
(these structures also being present in the corresponding porous particle) using the gas
atomisation technique. Melt spinning appears to yield similar results.
Figure 3c is a plot of discharge capacity s the number of cycles for a battery cell produced
using the material of 3a and 3b and cycled at a constant capacity of 1200mAh/g
Figure 3d is a plot of discharge capacity vs the number of cycles for a half cell produced
using the material of 3a and 3b and cycled at a constant capacity of 500mAh/g
Figure 4 shows an SEM image of an electrode composite mix comprising porous particle
fragments produced according to the procedure set out in the examples below. These
porous particle fragments were prepared by partially crushing porous particles obtained by
etching 60-90pm particles of an AISi alloy comprising 12wt% silicon, alloy particles being
obtained using a gas atomisation technique.
Figure 5 shows a plot of discharge capacity and coulombic efficiency vs the n ber of
cycles for a battery cell produced and tested according the procedure set out in the
examples below, produced using the electrode of Figure 4 and cycled at a constant capacity
of 200mAh/g..
Figure 6a shows an SEM of the coarse silicon porous particle fragment comprising silicon
particulates as well as rod like and fibrous structures, formed by partially crushing a porous
particle obtained by etching particles of an AISi alloy comprising 30wt% silicon, the alloy
particles being of size 90-1 5 and obtained using a gas atomisation technique.
Figure 6b shows a schematic diagram of the structures in 6a.
Figures 7a shows an SEM image of an unetched silicon aluminium alloy particles containing
12wt% silicon produced using the melt spinning technique.
Figure 7b shows an SEM image of a fragment produced by partially crushing a porous
particle fabricated by etching a melt spun alloy particle illustrated in Figure 7a. The fragment
comprises a mixture of fine and coarse elongate elements formed into branched dendritic or
tree-like structures.
Examples
Example 1a - Preparation of Electrode Materials
General steps outlining the etching an aluminium silicon alloy to obtain silicon structures:
1. Particles of an Al-Si alloy comprising 12%, 27% and 30% silicon were obtained from
a foundry or obtained using the methods set out in; O.Uzun et al. Production and
Structure of Rapidly Solidified Al-Si Alloys, Turk J. Phys 25 (2001), 455-466;
S.P.Nakanorov et al. Structural and mechanical properties of Al-Si alloys obtained by
fast cooling of a levitated melt. Mat. Sci and Eng A 390 (2005) 63-69). The making of
such alloys is commonly applied in industry for making the 4XXX group of aluminium
casting alloys (see http://www.msm.cam.ac.uk/phase-trans/abstracts/M7-8.htmi An
example would be a commercially available Al-Si 2% alloy as cast that is cooled at
a rate of approx 100°Ks and that is subjected to no further post-solidification heat
treatment.
2. The aluminium matrix was etched away using a mixed acid reagent, which is
commonly practised in industrial processes. Keller's reagent (2m HF, 3ml HCI, 5 ml
HN03, 190 ml water) can be used as an acid etchant, The silicon structures were
separated from the liquor using centrifuging and/or filtration. The product was rinsed
with deionised water 1-5 times until etchants were removed by suspending the
structures in the aqueous solution.
3. The structures were isolated from the rinsing water by filtering and/or centrifuging to
the required maximum moisture level, which may involve a drying step.
4. Isolated structures having diameters falling within a predetermined size range were
partially crushed as follows: the structures were dispersed in deionised water and
placed in the reservoir of a sonic bath. The sample was ground with a pestle and
mortar for 15 minutes and then sonicated for 5 minutes at 27KHz to give samples
having a diameter of between 2 and 5 m. Alternatively the sample was sonicated at
27KHz for between 5 and 50 Minutes, preferably 10 to 20 minutes to give silicon
containing porous particle fragments having a maximum diameter in the range 1to
5 m.
Small amounts (less than 30grams) of etched silicon particles may also be broken up by
grinding in a mortar and pestle. Larger amounts can be ball milled
Example 1b - Preparation of a porous particle fragment silicon electrode material using a
Si/A! alloy. The steps outlined in the general method defined in Example 1a above were
followed. Specifically:
1. An Al-Si matrix material comprising particles of Argon or Nitrogen-fired 12wt% Si-AI
alloy having an initial particle size in the range 12-63 m h was used as a starting
material. A typical chemical analysis of this material shows 11.7% Si + 3.49% Bi,
0.45% Fe, 0.28% Mn.
2. The starting material was etched using an etch solution having a composition by
reactants of: 5 ml concentrated 70% Nitric Acid (15.8M); 3 ml concentrated 36%
hydrochloric acid ( 1 1.65M); 2 ml 40% hydrofluoric acid; and 00 ml water. The molar
composition of the etch solution was therefore: 0.72M nitric acid; 0.32M hydrochloric
acid; and 0. 1 hydrofluoric acid.
3. 1.4 grams of Al-Si alloy per 100 ml etchant was added to the etchant in an HDPE
container with a magnetic follower and stirrer at room temperature for 1-2 hours on a
slow setting. The stirrer was turned off and the reaction continued over 16 hours to
completion. The silicon particles settled at bottom of reaction vessel.
4. The spent etch was poured off and the silicon particles were rinsed with deionised
water until they are pH 5/7. Because the particles tended to separate under the
influence of gravity between rinses, a centrifuge was used to speed up the process.
Figure 2a shows an example of the porous particles so produced.
5. The isolated particles were further dispersed in water (5ml) in a beaker and were
subjected to ultra sonic agitation at 27KHz for 5 minutes to partially crush the silicon
containing porous particles to give particle fragments having a diameter of from1 to
2pm. Alternatively, the silicon containing porous particles could be added to a small
amount of water and crushed using a pestle and mortar for 5 minutes or a ball bill as
specified herein above. Figure 2c illustrates the nature of porous particle fragments
created from subjecting porous particles to 5 minutes ultra-sonication.
Example 1c
The methods of Examples a and 1b were followed, but were modified in that (a) the
composition of the etch solution (step 2) was: 5% concentrated nitric acid; 80% concentrated
phosphoric acid; and 5% glacial acetic acid; and (b) the loading level (step 3) is 50 ml
etchant to 1 gram alloy. During etching the reaction temperature was observed to rise by
between 10-15°C. Most of the reaction is completed in 1-2 hours and the temperature falls
back to room temperature.
Etching can be performed more vigorously by adding less water. This causes a
considerable increase in the etchant temperature. For example, a two-fold increase in
concentration leads to a temperature of 50-60°C.
EDX (energy-dispersive X-ray spectroscopy) analysis of a batch of 12% Si particles showed
that there was less than 1% Al retained in the bulk silicon. There may be traces of A left in
the very small pearls of Silicon. Aluminium is a good high capacity anode element in its own
right and aids electrical connectivity. It may therefore be preferable that some Al is retained
in the Si particles, or even connects one or more Si particles together
Example 1
The method of example 1b was followed except that the starting material was an Al-Si alloy
material comprising particles of Argon or Nitrogen-fired 30wt% Si-AI alloy having an initial
particle size in the range 10-90prn. Figure 2d shows an example of porous particles made by
this method.
Example 1e
The method of example 1b was followed except that the starting material was an Al-Si alloy
material comprising particles of Argon or Nitrogen-fired 1 wt% Si-AI alloy having an initial
particle size in the range 60-90 mih. Figure 2f shows porous particle fragments produced
using this method.
Example 1f
The method of example 1b was followed except that the starting material was an Al-Si alloy
material comprising particles of Argon or Nitrogen-fired 12wt% Si-AI alloy having an initial
particle size in the range 90-1 50 m. Figures 3a and 3b show porous particle fragments
produced using this method.
Example 1g
The method of example b was followed except that the starting material was an Al-Si alloy
material comprising particles of Argon or Nitrogen-fired 30wt% Si-AI alloy having an initial
particle size in the range 90-1 50 m.
Example 1h
The method of any one of examples 1a to 1c were followed except that the cleaned and
etched porous particles were formed into a slurry with the binder and optionally graphite
and/or a conductive carbon and treated to partially crush the porous particles to give a slurry
(optionally an electrode mix) comprising porous particle fragments.
Example 2 - Characterisation of Silicon containing Porous Particle Fragments
2a - porous particle fragments made using method 1b and 1d
The silicon containing porous particles used as the starting material for preparing the silicon
containing porous particle fragments as well as the silicon containing porous particle
fragments themselves were characterised using scanning electron microscopy (SEM).
Figures 2a and 2c are SEM images of a silicon containing porous particle and a porous
particle fragment of the present invention prepared from the porous particle using the
method described in Example b (prepared from 10-63 mhi AISi alloy particles with 12wt%
silicon). Figure 2b is a schematic representation of the structures observed in Figure 2c.The
structures illustrated in Figures 2a to 2c are characterised by very fine rod like branches or
fibrils of Si approx 50-1 OOnm diameter in fractal patterns throughout the particles. Branching
of these fibrils is approx every 200nm. The cooling rate was estimated to be approx 1 K/s.
The BET value of the porous particle fragments produced from these materials (Fig 2c) was
70m2/g.
The structures illustrated in Figure 2d and 2e prepared from 1 -90 m AISi alloy particles
with 27wt% silicon are also characterised by very fine rod like branches. However, distinct
islands of silicon are distributed amongst these rod like branches, reflecting the
hypereutectic nature of the alloy used to prepare the porous particles.
2b-porous particle fragments made using method 1e
Figure 2f is an SEM image of porous particle fragments of the present invention produced
using method e from AISi alloy particles of size 60-90pm and 12wt% silicon. The structures
are a bit more coarse compared to the fragments produced by method 1b though still quite
fine. The BET value of these fragments was found to be 40m2/g. This material was also
characterised using XRD as described herein and the 11 1 lattice spacing was measured as
3.15589 Angstroms and the crystallite size was calculated to be 5 1nm.
2c -porous particle fragments made using methods 1fand 1g
Figure 3a is an SEM image of the porous particle fragments of the present invention
produced using method 1f (from alloy particles of size 90-1 50 m and 12wt% silicon). Figure
3b is a schematic representation of the structures observed in 3a It can be seen that the
larger alloy particle size obtained from the gas atomisation technique is characterised by a
much coarser structure, which comprises a network of elongate plates, fibres and flakes
many of which appear to be layered or fused together. Melt spinning appears to yield similar
results. The BET value of these porous particle fragments was found to vary between 7 and
20m /g. This material was also characterised using XRD as described herein and the 111
lattice spacing was measured as 3.145 Angstroms and the crystallite size was calculated to
be 46nm. The Seebeck coefficient, S, of these porous particle fragments at room
temperature was measured as 57pV/K. Using this value of S, the resistivity of the porous
particle fragments was estimated using the procedure described herein to be in the range
0.0001 to 0.001 W-cm. The tap density of a sample of the fragments was 0.1 5 g/cm3.
Porous particle fragments produced using method 1g of the present invention from
hypereutectic alloy particles of size 90-1 50 m and 30wt% silicon were also characterised.
The BET value of such porous particle fragments was found to be between 12 and 14m /g,
the 1 lattice spacing was measured as 3. 2 Angstroms and the crystallite size was
calculated to be 49nm. The Seebeck coefficient, S, of these porous particle fragments at
room temperature was measured as 53pV/K. Using this value of S, the resistivity of the
porous particle fragments was estimated using the procedure described herein to be in the
range 0.0001 to 0.001 W-cm. The tap density of a sample of these fragments was 0.49
g/cm3.
Example 3 - Preparation of Anode Mix and Electrode
10g of the porous particle fragments from etched Si-AI material prepared as described above
and containing less than 1% Al.
A composite electrode mix was prepared by mixing the etched porous particle fragments
with a sodium polyacrylic acid binder and carbon black in the proportions 76 : 12 : 12 (Si :
Polyacrylic acid : Carbon Black). The Si material and the Carbon black were high shear
stirred as an aqueous solution for several hours.
The polyacrylic acid binder was added (as a 10wt% solution in water) and the resulting
composite was further mixed by a dual asymmetric centrifugation technique for 0 minutes
and then cast onto electrodeposited Cu foil. Coat Weights of 15 - 30 g/m2 are typically used
for electrochemical testing in a Soft Pack Pair cell.
Figure 4 is an SE image of a composite anode mix prepared in the way described above
using porous particle fragments prepared using method 1e. It was not possible to make a
uniform composite mix using porous particle fragments prepared using method 1 with very
high BET values of 70m /g.
Example 4 - Preparation of Batteries
Electrode pieces were cut to the required size, and then dried overnight in a vacuum oven at
0 °C, under dynamic vacuum. Slightly smaller pieces of standard lithium ion cathode
material were prepared in a similar manner (active component either lithium cobalt oxide or a
mixed metal oxide (MMO) i.e. LiNio.80Coo.15Alo.05O2). Tags were ultrasonically welded to
exposed areas of copper and aluminium on the two electrode pieces. Then the electrodes
were wrapped between a continuous layer of porous polyethylene separator (Tonen), so that
there was one layer of separator between the two electrodes. The winding was placed in an
aluminium laminate bag, and the tags were thermally sealed along one edge. The cell was
filled with the required quantity of electrolyte under partial vacuum, and the electrolyte was
allowed to disperse into the pores. The bag was then vacuum sealed, and the cells were
allowed to soak for a further thirty minutes before the start of cycle testing.
Example 5 - Performance data on cells
Cells produced as described in Example 4 were cycled using Arbin battery cycling units
using a constant capacity charge/discharge method. Discharge capacities close to either
1 00 mAhr per gram of silicon or 500mAh/g was maintained over more than 00 cycles.
Figures 3c and 3d show discharge capacities for a cell comprising an MMO cathode and
produced using porous particle fragments prepared using method 1f and cycled at a
constant capacity of 1200mAh/g (Fig 3c) or 1500mAh/g (Fig 3d) until the cell fails after 230
or 160 cycles respectively. Figure 5 shows discharge capacities and coulombic efficiencies
for a cell comprising a lithium cobalt oxide cathode and produced using porous particle
fragments prepared using method 1e, cycled at a constant capacity of 1200mAh/per gram of
silicon until the cell started to fade around 110 cycles. Cells made using the coarser porous
particle fragments prepared using method 1f with BET values of 7-20m 2/g cycled for more
cycles and at a higher capacity than the cells made with porous particle fragments with a
BET value of 40m /g.
Example 6 SEM Characterisation of Particles produced using the Gas Atomisation
and Melt Spinning Techniques
Scanning Electron Microscopy was carried out on fragment samples comprising 12wt% or
30wt% silicon and which had been fabricated using the gas atomisation and melt spinning
cooling techniques. An alloy particle comprising I2wt% Si produced by melt spinning was
also characterised.
Figure 6a illustrates that fragments produced from silicon aluminium alloys comprising
30wt% silicon using a gas atomisation technique are characterised by a plurality of plate-like
structures interspersed between a plurality of silicon nodules.
Figure 7a illustrates that melt spinning a silicon aluminium alloy comprising 12wt% silicon
results in the formation of irregularly shaped disc and tube shaped structures.
Figure 7b illustrates that porous particle fragments produced by etching and then partially
crushing the structures illustrated in Figure 7a produces a fragment structure comprising a
mixture of fine and coarse elongate elements formed into branched dendritic or tree-like
structures. The fragments were found to have a BET value of 10.51 to 15.97m2/g.

Claims
1. A composition comprising a silicon containing electroactive porous particle
fragments characterised in that the porous particle fragments are selected from
the group comprising pore containing porous particle fragments and fractals.
2. A composition according to claim , wherein the porous particle fragments have
an average thickness in the range 50nm to 2 m.
3. A composition according to claim 1 or claim 2, wherein the silicon containing
porous particle fragments have a maximum diameter of less than 40 m.
4. A composition according to any one of claims 1 to 3, wherein the silicon
containing porous particle fragments have a maximum diameter in the range 1 to
20 m, preferably 1 to 5 m and especially 1 to 10m .
5. A composition according to any one of claims to 4, wherein the silicon
containing porous particle fragment has an aspect ratio (length (largest diameter)
to width (smallest diameter) of particle) in the range 2:1 to 5:1 .
6. A composition according to any one of claims 1 to 5, which comprises 10 to
100vol% of porous particle fragments having a maximum diameter of less than
4 m.
7. A composition according to any one of the preceding claims, wherein the
fragment is a fractal and the fractal structure comprises a substantially irregular
shape and/or surface morphology, these fractal structures being derived from:
a . the silicon material originally defining or bounding the pores or network of
pores within the porous particle from which the fractal fragment structures
are derived, without themselves comprising pores, channels or a network
of pores or channels; or
8. (b) the silicon material originally defining a random or ordered network of linear,
branched or layered elongate elements, wherein one or more discrete or
interconnected void spaces or channels are defined between the elongate
elements of the network.A composition according to claim 7, wherein the fractal
structure comprises at least one peak or ridge disposed over the surface thereof.
9. A composition according to claim 7 or claim 8, wherein the fractal structure has a
spiky appearance.
0.A composition according to claim 7, claim 8 or claim 9, wherein the fractal
structure has a ridged appearance.
11.A composition according to any one of claims 7 to 10, wherein the elongate
elements comprising a random or ordered network have a thickness of from 0.05
to 2 m, preferably 0.1 to 1 m.
12. A composition according to any one of claims 1 to 6, wherein the pore containing
porous particle fragments comprise a three dimensional network of pores,
cavities and channels, the pores, channels and cavities being separated and
defined by silicon containing walls within the particle structure.
13. A composition according to claim 12, wherein the silicon containing walls have a
thickness in the range 0.05 to 2 m, preferably 0.1 to m.
14. A composition according to any one of claims 1 to 6 or claim 12 or 13,
characterised in that each pore has a width in the range 100nm to 10 m,
preferably 150nm to 5 m.
15. A composition according to claim 12, 13 or 14 wherein the ratio of pore width to
wall thickness is greater than 2.5:1 and is preferably in the range 3:1 to 25:1.
16. A composition according to any one of claims 12 to 15, wherein the ratio of the
total volume of the pores in the fragment to the total volume of the fragment is in
the range 0.2 to 0.9, preferably 0.2 to 0.8, more preferably 0.25 to 0.75 and
especially 0.3 to 0.7.
17. A composition according to any one of the preceding claims, wherein the particle
fragments have a BET surface area greater than 4m2/g preferably greater than
5m2/g.
18. A composition according to any one of the preceding claims, wherein the particle
fragments have a BET surface area of less than 50m2/g, preferably less than
40m2/g, suitably less than 30m /g.
19.A composition according to any one of the preceding claims, which is an
electroactive material.
20. A composition according to any one of the preceding claims, which further
comprises one or more components selected from a binder, a conductive
material and optionally a non-silicon containing electroactive material, such as
graphite.
2 1.A composition according to any one of the preceding claims, which comprises 50
to 90% of an electroactive material by weight.
22. A composition according to claim 21, in which the electroactive material
comprises from 10 to 100% silicon containing porous particle fragments by
weight.
23. A composition according to any one of the preceding claims, comprising 5 to
40wt% of a silicon-containing electroactive material including a silicon-containing
porous particle fragment and 60 to 95wt% of an electroactive carbon material.
24. A composition according to claim 23, which comprises 20wt% of a siliconcontaining
electroactive material including silicon-containing porous particle
fragments and 80wt% of graphite.
25. A composition according to claim 23 or claim 24, in which the porous particle
fragments comprise 10 to 100% of the silicon-containing electroactive material.
26. A composition according to any one of claims 1 to 22, comprising 60 to 80wt% of
silicon-containing porous particle fragments and especially 20 to 40wt% of an
electroactive carbon material.
27. A composition according to claim 26, comprising 70wt% silicon-containing porous
particles and 30wt% of an electroactive carbon material.
28. A composition according to any one of claims 20 to 27, wherein the binder is
selected from the group comprising a polyacrylic acid, a modified polyacrylic acid,
a polyethylene maleic anhydride, an alkali metal salt of a polyacrylic acid, an
alkali salt of a modified polyacrylic acid, an alkali metal salt of a polyethylene
maleic anhydride and mixtures thereof.
29. A composition according to any one of the preceding claims, which further
comprises one or more silicon further containing components selected from the
group comprising native silicon containing particles having a minimal or negligible
porosity; silicon containing wires, nano-wires, fibres, rods, tubes, sheets,
elongate bundles, substrate particles, scaffolds, ribbons and silicon containing
pillared particles.
30. A composition according to claim 29, wherein the further silicon containing
components are electro-active.
31. A composition according to any one of the preceding claims, wherein one or
more of the silicon containing porous particle fragments and one or more silicon
containing components include a coating.
32. Acomposition according to claim 3 1 , wherein the coating is a carbon coating or a
coating comprising a lithium salt.
33. A composition according to any one of the preceding claims, which further
comprises one or more components selected from a viscosity adjuster, a crosslinking
accelerator, a coupling agent and a n adhesive accelerator
34. A composition according to any one of the preceding claims, wherein the
electrode material is a n anode material.
35. A composition according to any one of the preceding claims, which is a n
electrode material.
36. A composition according to any one of the preceding claims, which is in the form
of a mat.
37. A method of making a composition according to any one of claims 1 to 36, which
comprises partially crushing a silicon containing porous particle and isolating the
silicon containing porous particle fragments.
38. A method according to claim 37, wherein the silicon containing fragments are
obtained by partially crushing silicon containing whole porous particles having a
diameter greater than 40 m, preferably greater than 20m i.
39. A method according to claim 37 or claim 38, which comprises the steps of:
a . forming a molten silicon aluminium alloy composition;
b. cooling the molten composition to give alloy particles;
c. etching the alloy particles to give silicon-containing porous particles;
d. partially crushing the silicon-containing porous particles from (c) to give
silicon-containing porous particle fragments.
40. A method according to claim 39, wherein the molten alloy composition comprises
from to 35wt% silicon.
41. A method according to claim 40, wherein the molten alloy composition comprises
12wt% silicon.
42. A method according to claim 40, wherein the molten alloy composition comprises
25wt% silicon.
43. A method according to claim 40, wherein the molten alloy composition comprises
27wt% silicon.
44. A method according to claim 40, wherein the molten alloy composition comprises
30wt% silicon.
45. A method according to any one of claims 39 to 44, wherein the molten
composition is cooled at a rate of between 102 and 10 K/s.
46. A method according to claim 45, wherein the molten composition is cooled at a
rate of 02 to 03 s using a gas atomisation technique or a melt spinning
technique.
47. A method according to any one of claims 39 to 46, wherein the cooled alloy
particles have a diameter of from 40 to 1500pm.
48. A method according to claim 47, wherein the cooled alloy particle has a diameter
of from 60 to 1500m .
49. A porous particle fragment prepared according to any one of claims 37 to 48.
50. An electrode comprising a current collector and a composition according to any
one of claims 1 to 36.
5 1.An electrode according to claim 50, which is an anode.
52. An electrode according to claim 50 or claim 5i, wherein the current collector is
selected from the group comprising copper foil, aluminium, graphite and nickel.
53. An electrode according to any one of claims 50 to 53, wherein the composition is
in the form of a mat or felt connected, adhered or applied to a current collector.
54. An electrode according to claim 53, wherein the composition is in the form of a
free standing felt or mat to which a current collector is connected.
55. An electrode according to any one of claims 50 to 54, wherein the composition is
in the form of a felt or mat having a mass per surface area of between 1mg/cm 2
and 6mg/cm 2 thereby to give a composite electrode having a thickness of froml O
tol OO m
56. A method of manufacturing an electrode according to any one of claims 50 to 55
comprising the steps of forming a slurry of the composition according to any one
of claims 1 to 36 in a solvent, applying the slurry to a current collector and drying
the product to remove the solvent.
57. A method according to claim 56, wherein the solvent is selected from the group
comprising N-methylpyrrolidone, propylene carbonate, ethylene carbonate,
butylene carbonate, propylene carbonate, fluoroethylene carbonate,
difluoroethylene carabonate, vinylene carbonate, dimethyl carbonate, diethyl
carbonate, gamma butyro lactone, 1,2-dimethoxy ethane, 2-methyl
tetrahydrofuran, dimethylsulphoxide, ,3-dioxolane, formamide,
dimethylformamide, acetonitrile, nitromethane, methylformate, methyl acetate,
phosphoric acid trimester, trimethoxy methane, sulpholane, methyl sulpholane
and 1,3-dimethyl-2-imidazolidione.
58. A battery comprising a cathode, an anode comprising a composition accord ing to
any one of claims 1 to 36 and an electrolyte.
59. A battery according to claim 58, wherein the cathode is selected from the group
comprising LiCo0 2, LiCoo.99Alo.01O2, LiNi0 2 > LiMn02, LiCoo.5Nio.5O2, LiCoo.7Nio.3O2,
LiCoo.8Nio.2O2, LiCoo.82Nio.i8O2, LiCoo8Nio.15Alo.05O2, LiNio.4Coo.3Mno.3O2 and
LiNio.33Coo.33Mno.34O2
60. A battery according to claim 58 or claim 59, wherein the electrolyte is selected
from the group comprising a non-aqueous electrolytic solution, a solid electrolyte
and an inorganic solid electrolyte.
61. A battery according to claim 60, wherein the electrolyte further comprises one o r
more lithium salts selected from the group comprising LiCI, LiBr, Lil, LiCI0 ,
L1BF4, L1B10C20, LiPF 6 LiCF 3S0 3, LiAsF , LiSbF , LiBoB, LiAICI CH3S0 3Li and
CF3S0 3Li.
62. A device including a battery according to claims 58 to 6 1.