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.
The use of silicon as an active anode material in secondary batteries such as lithium ion
batteries is well known. Early forms of these lithium ion batteries were prepared using bulk
silicon electrodes, thin film silicon electrodes and silicon powder electrodes. Subsequently,
silicon comprising electrodes including silicon comprising pillars, rods, fibres and wires have
been prepared. Silicon comprising particles having a surface array of pillars have also been
used in the fabrication of lithium ion batteries. US 2008/0241 647discloses a cylindrical
lithium battery comprising silicon or silicon alloy particles having dimensions in the range
5 m to 5 m. According to US 2008/0241647, batteries including silicon particles having
dimensions outside this range exhibit inadequate performance; particles with diameters of
less than 5 m give batteries with inadequate capacity, whereas batteries including particles
having diameters greater than 15 m exhibit inadequate mechanical properties due to
stresses arising from the expansion and contraction of the silicon material during the charge
and discharge cycles of the battery. The particles used in the cylindrical batteries of US
2008/0241647 are prepared by chemical vapour deposition of silane onto seeded
polycrystalline silicon. These particles are characterised by the presence of crystallites with
dimensions of between 30 and 100nm. T e polycrystalline nature of these particles means
that each particle includes a large number of grain boundaries.
US 2009/0253033 discloses an anode active material suitable for use in lithium ion
secondary batteries. The anode material comprises silicon or silicon alloy particles with
dimensions of between 500nm and 20 m. These particles are manufactured using
techniques such as vapour deposition, liquid phase deposition or spraying techniques.
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, rods or wires having nanoscale dimensions. Rods or wires having
diameters in the range 1 to 50nm and lengths in range 500nm to 10 m can be prepared by
laser ablation using an iron catalyst. Other techniques such as solution synthesis and
chemical vapour deposition are also disclosed as being useful. Iron Germanium alloy
nanostaictures comprising a mixture of particles having a dimension in the range 1 to 50 nm
and micron length rods having a diameter in the range 5 to 30nm can also be prepared using
laser ablation. Similar nanostaictures 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.
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 particles having a primary particle size in
the range 20 to 200nm and a specific surface area of 1m /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 1000mA/g over 50 cycles are illustrated.
The life of the battery is significantly increased if the battery is operated at a limited voltage
level.
US 2010/0143798 discloses a solid nano-composite particulate composition for use in
lithium ion batteries. The nano-composite particles comprise an electroactive material in the
form of fine particles, rods, wires, fibres and tubes; nano-graphene platelets and a protective
matrix. The electro-active material includes silicon comprising nano-structures having a
typical diameter of 60nm. Solid nano-composite particles were prepared by dispersing a
mixture of the electro-active material and nano-graphene platelets having an average
diameter of 60nm in a polymeric matrix and spray drying the resulting mixture to produce
nano-composite particles having an average diameter in the range miti to 5m .
US 2008/0261112 discloses an electrode material including an electroactive material
comprising a mixture of silicon comprising particles and nano-wires. The nano-wires are
entangled to form a network, which is in contact with the particles. The electroactive
composition is prepared by placing a silicon comprising material in a thermal plasma at a
temperature of between 600 and 1500°C to give a composition comprising a mixture of
silicon comprising particles having a diameter of about 5m i and silicon comprising fibres
having a diameter in the range 30 to 50nm. The components of the compositions become
both entangled with and fused to adjacent components during the fabrication process.
Further, the fabrication process means that the composition as a whole is fused to the
current collector and anodes prepared in the way do not include additional components such
as a binder or a conductive additive. Although it is alleged that these compositions are able
to both accommodate silicon expansion during lithium intercalation and reduce the build up
of an irreversible capacity over the lifetime of the cell, the diameter of the nano-wires present
in the composition mean that they have a relatively high surface area and it is expected that
the compositions of US 2008/0261 1 2 will exhibit a relatively high first cycle loss as a result
of SEI (Surface Electrolyte Interphase) formation. In addition the fused nature of the
composition is expected to result in the build up of heave over the lifetime of the cell, leading
to delamination of the electrode material, an undesirable increase in cell volume and a
potentially hazardous build up of pressure.
US 7,767,346 discloses a n electroactive particulate material comprising a porous composite
of carbon and silicon prepared by pulverising a mixture of silicon metal, a carbon source
such a s polyvinyl alcohol and a pore forming agent such as oxalic acid in a ball mill and then
sintering the pulverised mixture at a temperature of between 700 and 1000°C for 10 hours to
give a composite structure comprising a network of carbon coated silicon fibres and
powders. This networked structure is then further pulverised to give the electroactive
particulate material, which can be combined with graphite and a binder to prepare anodes
for inclusion in a lithium ion battery. It will be appreciated that electrode structures
comprising these materials do not comprise a network of silicon fibres and particles
extending over the entirety of the electrode structure; the electrode comprises islands of
network like composite particles distributed within a matrix of binder and graphite.
US 2009/0269677 discloses an electrode material comprising a three dimensional structure
of metal fibres having a plurality of anode active particles distributed therein. The metal
fibres are selected from the group comprising titanium, iron, copper, silver, aluminium, zinc,
cobalt, nickel and chromium and typically have a diameter in the range 500nm to 50mh and
an aspect ratio (ratio of the length to the diameter of the fibre) of greater than 2. The anode
active particles typically comprise silicon comprising particles having a diameter in the range
0.1 to 30 m. Electrode structures having a porosity in the range 20 to 95% and a tensile
strength in the range 0.1N/mm to 168N/mm are prepared by dipping a structure comprising a
network of metal fibres in a solution of electro-active particles. The metal network structure
may be in the form of a free standing network or may be applied to a substrate such as a
copper current collector. The fibre/powder electrode thus produced can be used in the
fabrication of lithium ion batteries.
The silicon structures described above have been prepared using a variety of techniques.
For example, the use of epitaxial and non-epitaxial vapour growth techniques in the
production of silicon nano-wires having cross-sectional diameters in the range 20 to 500nm
and aspect ratios of greater than 10, 50 or 100 is disclosed in US 7,273,732.
An alternative approach to using the epitaxial, solution and chemical vapour deposition
techniques taught in the above-mentioned documents for the production of silicon fibres,
pillars or rods is disclosed in US 7,402,829; WO 2007/083155; WO 2007/083152; WO
2009/010758 and WO 2010/040985. US 7,402,829 discloses the use of island lithography
and etching for fabricating silicon pillars on a silicon substrate and using the integrated
structure in a n anode. Such pillars have diameters in the range 0.1 to 1 m and lengths in the
range 1 to 10mhi.WO 2007/083155 discloses how to prepare silicon fibres for an anode that
have been detached from a silicon substrate etched using a similar technique. This produces
fibres with a diameter in the range 0.05 to 0.5pm and a length in the range 20 to 300pm.
WO 2007/083152 uses an alternative nucleation and etching process for the preparation of
silicon based pillars and fibres having a diameter in the range 0.2 to 0.6pm. The length of
the pillars or fibres depends upon whether the etching step is carried out in the same
solution a s the solution in which nucleation occurred. If the etching step is carried out in a
separate solution, fibre lengths of 70 to 75 m are observed. If the etching step is carried out
in the same solution, fibre lengths of 20 to 100pm, typically 85 to 100 m are observed.
WO 2009/010758 discloses a method for preparing silicon fibres or pillars detached from
etched metallurgical grade silicon granules having diameters in the range 10 m to 1mm as
starting materials. The silicon granules are etched to produce particles bearing pillars or
fibres having a diameter in the range 0.1 to 0.5 m and a length in the range 4 to 100pm. The
pillars or fibres are then detached from the granule substrate and can be used in the
fabrication of lithium ion batteries.
An additional suitable etching method is disclosed in WO201 0/040985. It should be noted
that the fibres or pillars produced using the etching techniques described above are typically
single crystal structures, which are devoid of individual crystallites and therefore grain
boundaries, or they are polycrystalline with only a few grains.
WO 2009/010758 and WO 2009/010757 describe methods of fabricating anode active
materials using detached fibres of the type disclosed above. These fibres can be used as the
electrochemically active material in a n anode of a lithium ion secondary battery. Often these
fibres or wires form part of a composite material (usually known a s a n anode mix) used in
the fabrication of a n anode. The anode mix may include other components such a s a binder,
a conductive carbon material and optionally graphite (or other electroactive forms of carbon).
This anode mix is typically mixed with a solvent and/or water to create a slurry, which is
applied to a thin metal foil, such a s copper foil, to a predetermined layer thickness and then
allowed to dry. This process produces an entangled 'felt" or "mat" of silicon fibres, which 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. Anodes fabricated according to the methods described above can
be incorporated into lithium ion batteries. During the first charge-discharge cycle of a battery
including a n anode of the type described above, the silicon fibres will tend to fuse together
where they touch each other, strengthening the connectivity of the felt. By the term
connected it should be understood to mean, in relation to the present invention, that each of
the silicon comprising elements from which the felt or mat are constructed are in electrical
contact, either via physical connections or interfaces, with the electrolyte and optionally with
one or more other electroactive elements and/or one or more conductive elements that may
be present in the mix as well as the current collector. During operation of the cell, it is
essential that all the electroactive elements in the anode are connected to at least one other
electroactive element and/or to at least one conductive element such that they form a
network with a low resistance to the movement of both electrons and ions and provide an
efficient interface between the electrolyte and the current collector.
As disclosed in WO 2009/010757, silicon fibres can also be formed into a felt or a mat and
bonded together either through the application of heat and pressure or by providing the
fibres with a metallic bridging element, prior to the first charge-discharge cycle of the anode.
WO 2009/010758 further discloses that a felt comprising silicon fibres can be formed by
directly bonding the silicon to the current collector. Felts comprising bonded fibres exhibit
improved conductivity compared to non-bonded materials because of the increased
connectivity between the fibres.
It is well known that a random arrangement of spheres has a maximum packing density of
64% (the Bernal sphere packing factor); in other words the randomly arranged spheres
cannot fill more than 64% of a fixed volume. In fact every particle shape has its own unique,
size invariant maximum random packing density. As disclosed in "Improving the Density of
Jammed Disordered Packings Using Ellipsoids" by A.Donev, . Cisse, D. Sachs, E.A.
Variano, F.H. Stillinger, R. Connelly, S. Torquato and P.M. Chaikin, Science February 2004,
pp990-993, particles such as spheroids and ellipsoids with low aspect ratios can have higher
random packing densities, in excess of 70%, but for high aspect ratio particles the maximum
random packing density decreases. As disclosed in "Random packings of spheres and
spherocylinders simulated by mechanical contraction" by S.R. Williams and A.P. Philipse,
Phys. Rev. E, 67, 051301, 2003, the maximum random packing density of stiff rods with high
aspect ratios (e.g. >10) appears to vary approximately as 5 divided by the aspect ratio. For
example, this predicts that stiff rods of diameter 100nm and length 10 m would theoretically
have a maximum random packing density of approximately 5 divided by the aspect ratio of
100, or approximately 5%.
It has been observed that the initial felt structures (both bonded or unbonded) produced
using the silicon based fibre products obtained from the etching techniques described above
have an inherent porosity, (that is they contain voids or spaces between the fibres) which
arises as a result of the maximum attainable packing density that can be obtained for a
random arrangement of fibres within a defined volume. However, the silicon fibres as
described above are able to flex or bend to a limited extent. This flexibility together with the
aspect ratio of the silicon fibres produces electrode or anode materials having a higher
packing density compared to electrode or anode materials prepared from rigid silicon rods
described in the prior art, for example. It will be appreciated, therefore, that the porosity of an
anode material comprising nano-structured silicon will depend, to a large extent, on the
shape and relative proportions of the silicon nano-structures from which the material is
formed.
The inherent porosity (pores or voids) in the electrode structure provides the silicon fibres
with space into which they can expand in response to the intercalation or insertion of lithium
that occurs during the charging cycle of the battery. These pores or voids also provide a
route for the electrolyte to penetrate the whole of the electrode structure, which means that
the electrolyte will be in contact with as much of the surface of the silicon material as
possible during charging and discharging of the anode. This porosity is important as it
provides a path by which the lithium can be intercalated into the bulk of the silicon material
so that the lithiation of the silicon is as uniform as possible throughout the anode mass.
However, the presence of an excessive number of pores within the anode structure means
that the mass of anode active material per unit area is generally low compared to bulk silicon
anodes or anode materials prepared using more closely packed particulate silicon, for
example. This means that the inherent capacity of the anode is also correspondingly less.
A further problem that has been observed for anode structures comprising silicon fibres, rods
and wires is an effect known as "heave" in which the silicon fibres making up the bulk of the
silicon electrode material expand as an aggregate body away from the surface of the current
collector during lithium intercalation rather than expanding substantially independently into
the pores or voids present in the uncharged composite. Heave causes a temporary increase
in the thickness of the electrode, which may increase the internal stresses within the battery.
Although 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 initially, over
time de-lamination of the anode material may occur. Further, the methods and costs
associated with the manufacture of these silicon structures are generally involved and not
cost effective and further refinement of these production methods with a view, in particular,
to a reduction in the costs associated therewith is needed. There is a need, therefore, for a
silicon-based electroactive material that addresses the problems of the prior art outlined
above. In particular there is a need for a silicon based electroactive material that is at least
able to accommodate one or more of the stresses arising from the expansion and
contraction of the material during the charging and discharging phases of the battery and
which also has an improved capacity performance, a longer cycle life and a more costefficient
method of manufacture compared to the fibre-containing anode materials of the
prior art. The present invention addresses that need.
A first aspect of the invention provides a composition comprising a plurality of elongate
elements and a plurality of particles, the elongate elements and particles each comprising a
metal or semi-metal selected from o e or more of the group comprising silicon, tin,
germanium and aluminium or mixtures thereof but excluding compositions in which (i)
silicon-comprising elongate elements having a diameter of 30 to 50nm and (ii) aluminiumcomprising
elongate elements having a diameter in the range 500nm to 50 m and an aspect
ratio (ratio of the length to the diameter of the fibre) of greater than 2. Compositions of the
type described herein are electroactive and can be used in the fabrication of electrodes for
use in batteries such as lithium ion batteries, sodium ion batteries or magnesium ion
batteries, for example. It should be understood that although the invention relates, in
general, to compositions including elongate elements and particles comprising a metal or
semi-metal selected from one or more of silicon, tin, germanium and aluminium, it will be
specifically described herein with reference to compositions comprising elongate elements
and particles in which the metal or semi-metal is a silicon comprising material selected from
the group comprising substantially pure silicon, a silicon alloy or a material selected from the
group comprising silicon oxide, silicon nitride and silicon boride providing the silicon
comprising material is electroactive.
The term "silicon comprising material" as used herein should be understood to mean that the
material consists, comprises or includes silicon within its structure. Further, it should be
appreciated, therefore, that the scope of the invention is not limited to silicon comprising
materials as defined above but extends to compositions comprising elongate elements and
particles comprising, consisting or including a metal or semi-metal selected from one or
more of the group comprising tin, germanium and aluminium and mixtures thereof. Such
compositions may also be referred to as compositions comprising metal or semi-metal
comprising elongate elements and metal or semi-metal comprising particles respectively. It
will be apprecited that, as specified herein above, the metal or semi-metal may suitably be
provided i a substantially pure form, in the form of an alloy or in the form of an oxide, nitride
or boride as described above. In this respect all references to "silicon comprising elongate
elements" and "silicon comprising particles" should be interpreted to include elongate
elements consisting, comprising and including a metal or a semi-metal respectively.
Although the invention will be specifically described in relation to "silicon comprising elongate
elements" and "silicon comprising particles" it should be understood that the scope of the
invention extends to include elongate elements and/or particles consisting, comprising or
including elements other than silicon. In this respect, the elongate elements and particles of
the first aspect of the invention will hereafter be specifically referred to as "silicon comprising
elongate elements" and "silicon comprising particles" respectively.
For the avoidance of doubt, it should be appreciated that the silicon comprising elongate
elements and silicon comprising particles included in the composition according to the first
aspect of the invention may include solid elongate elements, solid particles, hollow tubes
and porous and hollow particles respectively formed from a single silicon-comprising
material, solid elongate elements, solid particles, tubes and porous and hollow particles
having a silicon comprising coating provided on a core other than silicon and solid elongate
elements, solid particles, tubes and porous and hollow particles having a core comprising a
first silicon comprising material and a coating comprising a second silicon comprising
material. Where the silicon comprising elongate elements and particles comprise a silicon
coating, the cores of these coated elements can be selected from materials such as carbon,
a suitably conductive metal such as copper, nickel, aluminium or gold; a conductive ceramic
or a silicon comprising material having a different composition to the silicon comprising
material used for the coating. Preferred cores include carbon based cores such as hard
carbon or graphite or a suitable metal. The silicon comprising materials used to form the
elongate elements, tubes and particles of the composition according to the first aspect of the
invention can include a substantially pure silicon, a silicon-alloy or a ceramic type silicon
material selected from the group comprising silicon oxide, silicon boride and silicon nitride. A
substantially pure silicon will suitably have a purity of from 90% to 99.999%, preferably 90%
to 99.99%, more preferably 90% to 99.95% and especially 95% to 99.95% and will include
high purity silicon used in the manufacture of semi-conductors as well as metallurgical grade
silicon such as the Silgrain® material produced by Elkem of Norway. A substantially pure
silicon may include impurities to further improve the conductivity of the material. Suitable
desirable impurities include boron, nitrogen, tin, phosphorous, aluminium and germanium.
The impurities are preferably present in an amount up to 1% by weight of the silicon , which
provides a balance between cost and performance. Suitable silicon-alloys comprise 50 to
90wt% silicon. The composition of the first aspect of the invention is an eiectroactive material
that is able to form an alloy with lithium and which can also be used in the fabrication of
electrodes, preferably anodes for use in lithium ion secondary batteries or batteries based
around alternative ions as the charge carrier, for example sodium ion or magnesium ion
batteries. By the term "eiectroactive material" it should be understood to mean that the
material is able to accommodate and release lithium or other alkali ions, or magnesium ions
from its structure during the charging and discharging cycles of a battery. The silicon
comprising elongate elements may comprise discrete elongate elements only or may include
structures in which the elongate element includes a silicon comprising particle in its
structure.
The silicon comprising particles and elongate elements of the first aspect of the invention are
preferably formed into a felt-like structure or mat in which the fibres and particles are either
randomly entangled or are in the form an ordered arrangement within the composition.
Preferably the elongate elements and particles are randomly entangled. Such entanglement
results in a structure in which the elongate silicon comprising elements and silicon
comprising particles are randomly connected with each other, either directly or indirectly
through any other components present in the composition. By the term connected it should
be understood to mean, in relation to the present invention, that each of the silicon
comprising elements from which the felt or mat are constructed are in electrical contact,
either via physical connections or interfaces, with the electrolyte and optionally with one or
more other electroactive elements and/or one or more conductive elements that may be
present in the mix as well as the current collector. It should be understood that the inclusion
of the elongate elements of high aspect ratio significantly increases the potential number of
connection points between elements in the mix, whilst the inclusion of structurally simpler
particles can reduce the overall manufacturing cost per unit mass. The inclusion of particles
also increases the mass of silicon present in the electrode structure, thereby increasing the
capacity of an electrode comprising this mixture relative to that of an electrode comprising
elongate elements only. The felt like structure or mat can be formed as a layer on a current
collector such as copper foil or can be in the form of a free standing felt or mat and can be
used in the fabrication of electrodes, preferably anodes for use in lithium ion batteries. It
should be appreciated that the entanglement of the elongate elements and particles of the
first aspect of the invention results in the formation of a network of elongate elements and
particles that extends across the entirety of a substrate, such as a copper current collector.
Without wishing to be constrained by theory, it is believed that the formation of an extended
network of elongate elements and particles over the surface of the current collector improves
both the connectivity within an electrode structure compared to known electrodes and the
cycle life. In a preferred embodiment of the first aspect of the invention, the composition may
optionally include, in addition to the silicon elements, one or more additional components
selected from the group comprising a binder, a conductive material and a non-silicon
comprising electroactive material, such as graphite. It is particularly preferred that the
compositions of the first aspect of the invention include a binder since it is the binder, which
binds, adheres or connects the elongate elements and particles of the composition to the
current collector. In an especially preferred embodiment of the first aspect of the invention
there is provided a composition comprising a plurality of silicon comprising elongate
elements, a plurality of silicon comprising particles and a binder; such a composition is also
known as a composite electrode or anode material, since it is this material that is connected
to a current collector during the fabrication of composite electrodes, preferably composite
anodes. Additional 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 optionally be added to the especially preferred embodiment according to the
first aspect of the invention. These additional components are generally referred to as nonsilicon
comprising components. These non-silicon comprising components generally
comprise carbon as a major constituent, but may comprise silicon as a minor constituent.
By the term "electrode material" it should be understood to mean a material comprising an
electroactive material, which can be applied, bonded 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 filler, a cross-linking accelerator, a
coupling agent and an adhesive accelerator. T e 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.
It will be appreciated, therefore, that the total volume of the electrode or anode material, V
(either in the form of a freestanding felt or mat or in the form of a layer applied to a current
collector) 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 Vc + V + VP
Where V is the total volume of the anode material; V FFL is the total volume of electro-active
silicon elements in the anode material; V B is the total volume of the binder; V C is the total
volume of conductive material (where present, V G is the total volume of additional
electroactive material (such as graphite, where present) and V is the total volume occupied
by the pores or voids within the anode material. The total pore volume, V P L of a material is
otherwise known as the porosity and can be expressed as a percentage of the total volume
It will be further appreciated that because the volume of the silicon-comprising material
expands by a factor of up to approximately 400% when the material is charged, the porosity
of the electrode decreases. Without being constrained by theory, it is believed that the total
porosity of the electrode in the charged state at first cycle should be in the range 20 to 30%,
preferably 25% to ensure that access of the electrolyte to the components of the material is
not inhibited in this charged state. The porosity of the electrode may decrease over the
lifetime of a cell including the electrode due to the build up of SEI layers on the surface of the
silicon particles and elongate elements and loss of cohesiveness within the composite
structure.
Without wishing to be further constrained by theory, the porosity of the uncharged material
will depend, in part, on the nature of the 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 20 and 30% when the material is in the charged
state. The material will typically have a porosity of between 35 and 80% in the uncharged
state, preferably between 40 and 75%.
The anode porosity, V Sig, of an uncharged anode mix comprising an electroactive material
consisting of both silicon comprising and a further non-silicon comprising electroactive
material can be reduced relative to the anode porosity, V Si, of an uncharged anode mix of
equivalent volume comprising an electroactive material comprising only silicon comprising
material, so that the porosity in the charged state is the same in both cases when the silicon
is lithiated to the same capacity value. This reduction in porosity in the uncharged state can
be expressed as follows:
where V Sig,is the volume occupied by pores in an uncharged material comprising an
electroactive material comprising an electroactive material comprising silicon and a further
non-silicon comprising electroactive material, V Si is the volume occupied by pores in an
uncharged material comprising an electroactive material comprising silicon only, V G is the
volume of the additional electroactive material, and a is the average volume expansion
factor of the silicon-comprising electroactive material (in other words, the volume V of the
silicon comprising electroactive material increases to aV at the end of the charge cycle with
the insertion of lithium ions). This calculation assumes that the silicon comprising
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 mix in the final charged state is the same.
Without wishing to be still further constrained by theory, it is believed that the overall
structure of the material of the first aspect of the invention and hence its electrical and
mechanical properties will depend upon the relative dimensions, volumes and shapes of all
the components (silicon and non-silicon comprising components) from which the material is
formed as well as the proportions in which they are present. Where the material comprises a
plurality of elongate elements and a plurality of particles having diameters that are no more
than three times larger than the diameter of the elongate elements, it is believed that the
particles will tend to be dispersed within the voids created by the random entanglement of
the elongate elements and the resulting structure will have a relatively high capacity. It will
be appreciated that it is not desirable for a particle to occupy the entire space of a void within
the felt structure, since this would otherwise inhibit the expansion of the silicon comprising
elongate elements and particles within the structure and lead to stresses within and buckling
of the electrode material. A particle will most preferably occupy between 5 and 50% of the
volume of each pore, so that expansion of the silicon material due to intercalation of lithium
ions can be accommodated. However it is advantageous for the particles to stay
conductively connected to the elongate elements and therefore they should not be too small.
Furthermore as the particle diameter decreases, the surface area to volume ratio of the
particle increases causing a higher amount of Solid Electrolyte Interphase (SEI) material to
be formed during charging which reduces the cycling efficiency. Therefore the diameter of
the particles is preferably at least as large as the diameter of the elongate elements.
Without wishing to be constrained by theory, it is believed that where the material includes
particles having diameters that are significantly larger, for example more than a factor of
three larger, than those of the elongate elements, then the dispersal of particles and
elongate fibres will depend upon the length of the elongate elements relative to the diameter
of the fibres. If the average length of the elongate elements is less than half the particle
diameter, these elongate elements will tend to be dispersed within the voids created by the
particles and the elongate element volume should exceed the inter-particle volume that
would exist in a packed particle-only mix to maintain good connectivity.
Without wishing to be further constrained by theory it is believed that where the length of the
elongate element is greater than half the particle diameter, the elongate elements will tend to
occupy space between adjacent particles rather than the inter-void space created by
particles and result in a structure in which the inter-particle contact is minimised. The actual
structure will depend upon the relative ratio of the volume of elongate elements to the
volume of particles present in the structure. Where the volume of elongate elements
exceeds that of the particles, the structure of composition comprises a network in which
islands of particles are distributed within a matrix of elongate elements.
Compositions in which either the elongate elements partially fill the voids between particles
or which comprise a network in which islands of particles are distributed within a matrix of
elongate elements results in the formation of anodes, which exhibit better cycle-ability
compared to an anode comprising silicon comprising particles only; it is believed that this is
because the composition of the present invention is better able to accommodate the
stresses arising from the intercalation of lithium, whilst maintaining a good connectivity
between all the elements in the mix compared to an anode mix comprising silicon comprising
particles only. The partial filling of the pores or voids in the electrode structure means that it
also exhibits good capacity characteristics, higher than that attained with anode mixes
comprising only elongate elements with a limit on the maximum achievable packing density.
The materials of the first aspect of the invention are therefore able to exhibit good capacity
characteristics over a prolonged period of time.
As disclosed above, the compositions of the present invention comprising a plurality of
silicon comprising elongate elements and a plurality of silicon comprising particles can be
used to fabricate electrode or anode materials. The electrodes or anodes so prepared are
characterised by good connectivity both within the material itself and between the material
and the electrolyte and current collector respectively, good capacity performance over a
prolonged number of cycles and a reduced manufacturing cost due to the low cost of the
materials used. An electrode or anode mix or material (composition) according to the first
preferred embodiment of the first aspect of the invention will suitably comprise 50 to 90% of
a n electroactive material by weight, preferably 60 to 80% and especially 70 to 80%. The
electroactive material suitably comprises the silicon comprising elongate elements and
silicon comprising particles according to the first aspect of the invention and optionally a
further material that is also electroactive. Examples of further electroactive materials are
provided herein.
The elongate silicon comprising electroactive elements can be selected from one or more
structures selected from the group comprising fibres, tubes, ribbons and flakes. By the term
"fibre" it should be understood to mean a n 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 . Elongate elements having a n aspect ratio of 40:1 to 100:1
are preferred. In this respect the term "fibre" may be used interchangeably with the terms
pillars, threads and wires. As indicated above, where the material according to the first
aspect of the invention includes a silicon comprising fibre, this fibre preferably has a
diameter in the range 50 to 2000nm, preferably 100nm to 500nm, more preferably 150nm to
200nm and especially 100 to 350nm. Silicon fibres or elongate elements having a diameter
of from 150nm to 200nm are especially preferred. Silicon fibres having a diameter of 50nm
or less are not preferred as their small diameter means that they have a large surface area
to volume ratio, which results in capacity loss due to the build up of a n SEI layer during the
charging phases of the battery. Silicon comprising fibres of the first aspect of the invention
suitably have a length in the range O. mh to 00 m, preferably 1 m to 50pm, more
preferably 2 m to 40 m and especially 10 to 15 m. A first embodiment of the first aspect of
the invention the elongate element has a diameter of from 150nm to 200nm and a length of
from 10 to 15 m. The term elongate element also includes a particle having one or more
pillars provided on the surface thereof, where the pillars have a length in the range 1 to
1OO m. Such pillars may be formed integrally with the particle core in or may be formed
independently of the particle core. Silicon-comprising pillared particles having an overall
diameter of 20 to 30pm, preferably 25 m and pillar lengths of up to 10 m, preferably 4 to
6 m are preferred.
Alternatively, where the silicon comprising 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 O.Od mh to 2pm, preferably 0.1 m to 0.5mh 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 0 to 500 times as large as the first dimension for tubes. The total
length of the third dimension may be as large as dqqmiti , for example.
Ribbons having a thickness of 0.25mh , a width of 0.5mih and a length of 50mih are
particularly preferred. Flakes having a thickness of 0.25pm, a width of 3m h and a length of
50 m are particularly preferred. Tubes having a wall thickness of 0.08 to 0.5mh , an outer
diameter of 0.2 to 5 m and a length of at least five times the outer diameter are particularly
preferred.
The silicon comprising electroactive particles of the material of the first aspect of the
invention may be in the form of native particles, pillared particles, porous particles, porous
particle fragments or porous pillared particles.
By the term "native particle" it is to be understood to mean particles that have not been
subjected to an etching step. Such particles typically have a principle diameter in the range
1mh to 15 m, preferably 3 m to 10 m and especially 4mhi to 6m i and are obtained by
milling bulk or particulate silicon, preferably metallurgical grade silicon or high purity waste
silicon produced during semi-conductor manufacture to the size required. 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 5pm. Pillared particles can be prepared by etching silicon particles having
dimensions in the range 5 to 40m h , preferably 15 to 25 m using the procedure set out in
WO 2009/010758. Such pillared particles include particles having a principle diameter in the
range 5 to 15pm, 15 to 25 m and 25 to 35mih . Particles having a principle diameter in the
range 5 to 15pm 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 10pm, preferably 1 to 5pm. Pillared particles
having a core diameter of 14 to 16pm and a pillar length of 4 to 6 m are particularly
preferred.
A second preferred embodiment of the first aspect of the invention provides a composition
comprising elongate elements having a diameter of from 150 to 200nm and a length of 10 to
15 m and particles having a diameter in the range 1 to 8 m with a D50 diameter of 4 to 6 m,
preferably 4 m. (The Dso diameter is the diameter at which 50% of the volume of particles
present in the sample have a diameter of this value or less). The elongate elements and
particles are both suitably silicon-comprising elongate elements and particles as described
herein above. The silicon-comprising elongate elements suitably comprise 5 to 95% by
weight of the composition of the first aspect of the invention and the silicon-comprising
particles suitably comprise 95 to 5% by weight of the composition of the first aspect of the
invention. Compositions comprising 90wt% silicon fibres having a diameter of 0 to 200nm
and a length of 10 to 5m i and 10wt% silicon particles having a diameter in the range 1 to
8 m with a D5 of 4 m were observed to maintain a charging capacity of 1900mAh/g at a
coat weight of 9.7g/m2 using a constant current charging regime for between 130 and 170
cycles. Compositions comprising 0wt% silicon fibres having a diameter of 150 to 200nm
and a length of 10 to 15 m and 90wt% silicon particles having a diameter in the range 1 to
8 m with a D5 of 4pm were observed to maintain a charging capacity of 1200mAh/g at a
coat weight of 16.5g/m2 using a constant current charging regime for more between 175 and
185 cycles.A third preferred embodiment according to the first aspect of the invention
provides a composition comprising silicon-comprising pillared particles having an overall
diameter of from 14 to 40pm, with a D50 of 24 m and silicon particles having a diameter in
the range 1 to 8 m with a D50 of 4 m. Preferably the composition comprises 30 to 70wt%
each of the silicon-comprising pillared particle and the silicon-comprising particles as
specified above. Compositions comprising 50wt% of each of the pillared particles and
particles having a diameter in the range 1 to 8 m with a D50 of 4pm are especially preferred
a s these were observed to maintain a charging capacity of 110OmAh/g at a coat weight of
13g/m2 for more than 330 cycles using a constant current charging regime.
By the term "Porous particle" it should be understood to mean particles having a network of
voids or channels extending there through. These voids or channels include voids or
channels that are enclosed or partially enclosed within the total volume of the particle as well
a s particles having channels extending into the interior of the particle from its surface. The
porous particles are generally characterised by a substantially spherical shape and a
relatively smooth surface morphology. 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 branched elongate elements typically have a diameter in
the range 50 to 100nm with branches every 100 to 400nm. By the term porous particle
fragment it should be understood to include all fragments derived from silicon comprising
porous particles as defined herein above. Such fragments include structures having a
substantially irregular shape and surface morphology, these structures being derived from
the silicon material originally defining or bounding 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. These fragments will
hereafter be referred to as fractals. The surface morphology of these fractal structures
(which are devoid of pores or channels or a network of pores or channels) may include
indentations or irregularities arising from the pores or channels or network of pores or
channels originally bounded by the silicon structure. 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 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 term silicon comprising porous particle fragment also includes
porous particle fragments comprising a network of pores and/or channels defined and
separated by silicon comprising walls. These fragments will herein after be referred to as
pore containing fragments. By the term "pore" or "channel" as defined in relation to porous
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. 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 of the electroactive material
separating two adjacent pores within a pore containing porous particle.
These pore and/or channel comprising porous particle fragments are also characterised by
an irregular shape and surface morphology. In contrast, the porous particles from which the
fragments are derived are characterised by a substantially spherical shape and a relatively
smooth surface morphology. Where the fractals and pore containing porous particle
fragments are described together hereinafter they will collectively be referred to as silicon
comprising porous particle fragments. 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. Porous
particles typically have a principle diameter in the range 1 to 15 m, preferably 3 to 5m p
and contain pores having diameters in the range 1nm to 1500nm, preferably 3.5 to 750nm
and especially 50nm to 500nm. Such 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 2009/01 86267, US 2004/0214085 and US 7,569,202.
The term "particle" in relation to the particles referred to herein includes essentially spherical
and non-spherical particles. Non-spherical particles include cubic, prismatic and decahedric
shaped particles having a principle diameter and a minor diameter. It is preferred that the
aspect ratio of the principle diameter to the minor diameter is in the range 3:1, preferably 2:1
and especially 1:1.
The silicon comprising elongate elements of the present invention may be prepared by any
suitable methods known to a person skilled in the art, for example using the methods
disclosed in WO 2009/010758, WO 2009/010757 and WO 2007/083155. The elongate
elements are preferably prepared from single crystalline wafers or from single crystalline or
polycrystalline silicon particles having a dimension in the range 80 to 800pm. Silgrain™
silicon particles having dimensions in the range 80m i to 0.8mm that can be used in the
manufacture of elongate elements 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 of 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 15 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 comprising elongate elements and particles may include a coating, preferably a
coating made with carbon, such as amorphous carbon, graphite, electroactive hard carbon,
conductive carbon, carbon based polymers or carbon black. 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 2009/0239151 and US 2007/0212538. Pyrolysis
methods are disclosed in WO 2005/011030, JP 2008/186732, CN 101442124 and JP
04035760. Carbon coatings are able to assist in controlling the formation and stability of SEI
layers on the surface of the anode. As indicated above coatings other than carbon based
coatings can be used. Examples of suitable alternative coatings include compounds such as
lithium fluoride or lithium salts of cyclic organic carbonate species or suitable metals such as
aluminium, copper, gold and tin as well as conductive ceramic materials. 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.
As indicated above, the silicon component of the electroactive material according to the first
aspect of the invention preferably comprises 5 to 95% by weight of elongate elements,
preferably 10 to 90wt%, more preferably 15 to 90% by weight, most preferably from 20 to
70% and especially from 30 to 50%. As indicated above, the elongate elements may be
selected from one or more of the group comprising fibres, ribbons, pillared particles or
flakes.
The silicon comprising component of the material according to the first aspect of the
invention preferably comprises from 5 to 95% by weight of silicon comprising particles,
preferably 10 to 85%, more preferably from 30 to 80% and especially from 50 to 70% of
silicon comprising particles by weight of the silicon component. As indicated above, the
silicon comprising particles may be selected from one or more of the group comprising
native particles, pillared particles and porous particles.
Compositions comprising fibres and/or ribbons as elongate elements and pillared particles
are preferred because this provides the most efficient connectivity between the silicon
comprising components of the electroactive material of the first aspect of the invention,
whilst minimising the stresses that occur as a result of the volume changes that occur during
the charging phase of the battery cycle. Native particles and/or porous particles may also be
preferably added to the composition, since this has been found to improve the homogeneity
of the composition. A fourth embodiment of the first aspect of the invention therefore
provides a composition in which the silicon comprising component comprises one or more
components selected from the group silicon comprising fibres, silicon comprising ribbons,
pillared particles and optionally native particles and/or porous particles. The pillared particles
preferably have a dimension in the range 5 to 35 m and comprise pillars having a width in
the range 80nm to 250nm and a length in the range 0.5 to 5 m. The elongate elements
preferably have a diameter in the range 80 to 250 nm and a length in the range 0.8 to
100 m. The native particles, where present, preferably have a diameter in the range 1 to
8 m with a D 0 of 4 m. Without wishing to be constrained by theory, it is believed that the
selection of pillared particles characterised by the dimensions given ensures that the
particles tend to occupy the voids or pores created by the entanglement of the fibres or
ribbons upon formation of the felt structure rather than contribute to the creation of additional
pores or voids. The entanglement of the particle pillars with the fibres or ribbons of the mat
ensures that the particles are retained within the felt structure during the charge and
discharge phase of the battery, which maximises the connectivity between the silicon
comprising components of the electroactive material perse and also between the silicon
comprising components and any other conductive materials present therein, thereby
improving the capacity of an anode prepared using the material according to the first aspect
of the invention. Further, because the elongate elements and the particles are retained
within the structure through entanglement rather than through close packing of particles,
there is sufficient space to accommodate the inherent volume changes of the material that
occur during the charging and discharging phases of a battery cell, for example. This
entangled structure therefore improves both the capacity and cycle-ability of a material
including this structure and hence its long term performance. Further, because the silicon
structures used in the manufacture of the electrode or anode materials according to the
invention are themselves relatively easy and inexpensive to produce, the associated costs of
fabricating anodes or electrodes from such materials is consequently low.
In a fifth embodiment of the first aspect of the invention there is provided a composition
comprising silicon fibres having a diameter (d) in the range 0.1 to 0.8 m and silicon particles
having a diameter (D) in the range 0.2 to 2.5pm. The ratio of the diameter of the particles "D"
to the diameter of the fibres d" will depend, in part, on the packing density of the fibres and
the relative volume of the fibres and particles within the material. Where the packing density
of the fibres is in the range 3 to 30%, preferably 5 to 25%, it is preferred to use particles
having a diameter that is not more than 2 to 3 times that of the diameter of the fibres. The
volume ratio of the particle component of the electroactive material to the fibre component is
preferably in the range 2:1 to 0.5:1 . Without wishing to be constrained by theory, it is
believed that the material of the third embodiment of the first aspect of the invention provides
for good contact between the silicon components after many charging and discharging
cycles of a battery.
A sixth embodiment of the first aspect of the invention provides a composition comprising
silicon fibres having a diameter (d) in the range 0.08 to 0.3 m and silicon particles having a
diameter (D) in the range 0.4 to 10pm and where D>3d. The weight ratio of silicon particles
to silicon fibres is in the range 9:1 to 1:9, suitably 4:1 to 0.6:1 and preferably, for example,
3:1 to 2:1 . Preferably the composition according to the sixth embodiment of the first aspect
of the invention comprises silicon fibres having a diameter in the range 100 to 200nm and a
length in the range 10 to 15pm and native silicon particles having a diameter in the range 1
to 8 m with a D 0 of 4 m. The ratio of fibres to native particles for compositions comprising
fibres having a diameter in the range 100nm to 200nm and a length 0 to 15 m and native
silicon particles having a diameter in the range 1 to 8 m with a D 0 of 4 m is preferably 9:1
since electrodes prepared using such compositions have a good stability and reduced
delamination compared to electrodes of the prior art; batteries prepared with this 9:1 ratio of
fibres and native particles also exhibit good capacity retention when charged to 1200mAh/g
at constant current conditions over more than 150 cycles. Compositions in which the ratio of
fibres to native particles is 1:9 also exhibit good capacity retention when charged to
1200mAh/g at constant current conditions over more than 150 cycles. However, some
delamination of the electrode material was observed.
A seventh embodiment of the first aspect of the invention provides a composition comprising
pillared particles of silicon and native silicon particles. The pillared particles of silicon provide
both an elongate silicon comprising component and a particulate component and comprise a
core having a diameter or thickness of less than 20 m, preferably 5-1 5 m and pillars having
diameters around 50nm-0.2pm (for example 70nm to 0.2pm) and heights of 1-5pm attached
to the core with a packing density (or fractional coverage of the core surface area) of less
than 50%, preferably in the region of 25-30%. Pillared particles having an overall diameter in
the range 20 to 30pm, preferably 25pm and pillar lengths of up to 10pm, preferably 4 to 6 m
are preferred. The native silicon particles suitably have a diameter of 30% to 100% of the
diameter of the pillared particle core (e.g. 2-1 5pm) and may comprise between 30 and 80%
of the total weight of silicon present in the material. Preferably the native silicon particles
have an overall diameter in the range 1 to 8pm and a D50 of 4 m. It will be appreciated that
the relative proportion of pillared particles to native silicon particles in the material will
depend, in part, on the relative diameters of the components. The weight ratio of pillared
particles to native particles in the silicon-comprising component of the composition is suitably
in the range 90:10 to 10:90, preferably 70:30 to 30:70 and especially 50:50. Native particles
having a relative diameter in the ranges stated above will generally fill any inter-particle
spaces created by the pillared particles. A particular example would be native particles
having a diameter of 5pm and comprising 30% by weight of the silicon component mixed
with pillared particles having a core diameter of 10pm. Alternatively, in a particularly
preferred embodiment, there is provided a composition comprising 50wt% of the silicon
component of pillared particles having an overall diameter in the range 20 to 30pm,
preferably 25pm and pillar lengths of up to 10 m, preferably 4 to 6pm and 50wt% native
silicon particles having an overall diameter in the range 1 to 8pm and a D5oof 4pm. Batteries
prepared using these compositions and charged and discharged under constant current
conditions exhibit a capacity retention of 1200mAh/g over more than 300 cycles. An
electrode or anode material according to any of the preferred embodiments of the first
aspect of the invention will suitably comprise 50 to 90% of an electraactive material by
weight of the electrode or anode material, preferably 60 to 80% and especially 70 to 80%.
The electroactive material suitably comprises from 40 to 100% by weight silicon comprising
elongate elements and particles, preferably 50 to 90% and especially 60 to 80%. Electrode
materials comprising 70wt% of a silicon comprising electroactive material are especially
preferred. The electraactive material may include additional components selected from the
group comprising non-silicon comprising electroactive materials; silicon powders; elongate
silicon comprising elements such as silicon rods, fibres, wires, ribbons and sheets; and
silicon comprising pillared particles. Examples of further electroactive materials that may be
present include graphite and transition metal oxides or chalcogenides such as Mo0 2,W0 2,
MnV20 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 comprise 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 comprising elongate elements and particles, an electroactive carbon
material. These electroactive carbons may be present in an amount comprising 2 to 50%,
preferably 4 to 50%, for example 8 to 50% of the total weight of the electroacive material.
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 30m . 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.
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 the anode mix
adhere to the current collector. The binder can be added in an amount of 0 to 30%,
preferably 6 to 20%, more preferably 6 to 14% and especially 12% by weight based on the
weight of the anode mix. 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, polyacrylic acid, styrene 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 hydroxyl 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 polymerizable 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. Suitably
the polyacrylic add binder has a molecular weight in the range 150,000 to 700,000,
preferably 250,000 to 550,000, especially 450,000. Polyacrylic acid binders and sodium
polyacrylic acid binders are able to bind to silicon materials containing impurities and are an
ionically conductive component within the assembled cell. Suitably the silicon materials used
will have a silicon purity of 90% to 99.999%, preferably 90% to 99.99%, more preferably
90% to 99.95% and especially 95% to 99.95% and will include high purity silicon used in the
manufacture of semi-conductors as well as metallurgical grade silicon such as the Silgrain ®
material produced by Elkem of Norway. Silicon materials having a purity of less than 99.95%
may be advantageous because these materials can be cheaper and the impurities can
improve conductivity. However if the level of impurities is too high the performance of the
active material in the cell can be reduced and a purity in the range 90% to 99.95% is
preferred, for example, 95% to 99.9%. It will be appreciated therefore, that the silicon
comprising elongate elements, particles and other silicon comprising components used in
the preparation of compositions according to the first aspect of the invention may be derived
from metallurgical grade silicon which can reduce the materials cost compared to
compositions containing higher purity grades of 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 and longer cycling.
A particularly preferred eighth embodiment of the first aspect of the invention provides a
composition comprising 0 to 95% by weight of silicon comprising components, including
silicon comprising elongate elements and particles, 5 to 85% by weight of non-silicon
comprising 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 comprising components have a purity in
the range 90 to 99.95% or in the range 95 to 99.9%.
A further preferred ninth embodiment of the first aspect of the invention provides a
composition in the form of an electrode material, the composition comprising 60 to 80wt%
and preferably 70wt% of an electroactive silicon-comprising material, 10 to 15wt%,
preferably 12 to 14wt% of a binder, 0 to 4wt% graphite and 6 to18wt%, preferably 6 to
1 wt% and especially 6wt% of a conductive carbon. Especially preferred electrode
compositions comprise 70wt% of a silicon-comprising material comprising a mixture of
silicon-comprising fibres and silicon-comprising native particles in a ratio of from 90:10 to
10:90, 14wt% of a binder comprising polyacrylic acid or an alkali metal salt thereof, 4wt% of
graphite and 12wt% of a conductive carbon. Compositions comprising 70wt% of a siliconcomprising
material comprising a 50:50 mixture of native particles and pillared particles,
12wt% of a binder comprising polyacrylic acid or an alkali metal salt thereof, 12wt% graphite
and 6wt% of a conductive carbon. The native silicon particles typically have a diameter in the
range 1 to 8 m with a D5oof 4 m. The silicon fibres typically have a diameter in the range
100 to 200nm, preferably 150nm to 200nm and a length in the range 10 to 15 m. The silicon
pillared particles typically have an overall diameter in the range 14 to 40 m, preferably
25 m.
A viscosity adjuster may be present and 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 (N P) 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.
A conductive material may also be present and is a component used to further improve the
conductivity of the electrode or anode mix or 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; metellic 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 4 to 85% of the total electroactive material by weight.
A filler is a further ingredient that may be present and can be used to inhibit anode
expansion. There is no particular limit to the filler so long as it does not cause chemical
changes in the fabricated battery and is a fibrous material. As examples of filler there may be
used olefin polymers such as polyethylene and polypropylene and fibrous materials such as
glass fibre and carbon fibres.
A coupling agent, if present, 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 hydroxyl 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.
An 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. The silicon comprising
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 n-type 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; these materials suitably include
one or more impurities selected from aluminium, boron or gallium as dopants.
The electrode materials can be characterised in relation to their density and their porosity.
The electrode materials of the first aspect of the invention typically have density in the range
0.3 to 0.9g/cm2
, preferably 0.4 to 0.8g/cm2. Electrode materials comprising a mixture of
silicon-comprising fibres and silicon-comprising native particles in a ratio of 10:90 are
characterised by a density of 0.79g/cm2. Electrode materials comprising a mixture of siliconcomprising
fibres and silicon-comprising native particles in a ratio of 90:10 are characterised
by a density of 0.43g/cm2. Further, the electrode materials of the first aspect of the invention
typically have a porosity in the range 65 to 95%, preferably 65 to 85%. Electrode materials
comprising a mixture of silicon-comprising fibres and silicon-comprising native particles in a
ratio of 0:90 are characterised by a porosity of 69%. Electrode materials comprising a
mixture of silicon-comprising fibres and silicon-comprising native particles in a ratio of 90:10
are characterised by a porosity of 83%.
The composition of the first aspect of the invention can be easily manufactured and a
second aspect of the invention provides a method of preparing an electroactive material
according to the first aspect of the invention, the method comprising mixing a plurality of
elongate silicon comprising elements with a plurality of silicon comprising particles.
Additional components may be used in the preparation of the material according to the first
aspect of the invention. In a first embodiment of the second aspect of the invention there is
provided a method of preparing a composition according to the first aspect of the invention,
the method comprising mixing a plurality of elongate silicon comprising elements with a
plurality of silicon comprising particles and adding thereto one or more components selected
from the group comprising a binder, a conductive material, a viscosity adjuster, a filler, a
cross-linking accelerator, a coupling agent and an adhesive accelerator. The material
prepared according to this first embodiment can be used in the manufacture of electrodes,
preferably anodes for use in lithium ion batteries. In a preferred embodiment of the second
aspect of the invention, the method comprises the steps of mixing a plurality of elongate
silicon comprising elements with a plurality of silicon comprising particles and a binder.
As discussed above, the composition 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 third aspect of the
invention therefore provides an electrode comprising a composition according to the first
aspect of the invention and a current collector. The composition according to the first aspect
of the invention is suitably provided in the form of an electrode or anode mix or material, said
mix or material comprising a plurality of elongate silicon comprising elements, a plurality of
silicon comprising particles, a binder and optionally one or more components selected from
the group comprising a conductive material and optionally a further electroactive material.
The anode mix can be provided in the form of a free-standing felt or mat for connection to a
current collector. Alternatively the anode mix can be in the form of a layer, which is adhered
to a substrate and connected to a current collector. In a particularly preferred embodiment,
the substrate is a current collector and the electrode or anode mix or material is in the form
of a layer applied thereto. The components of the anode mix from which the felt or mat is
formed are preferably randomly entangled to provide optimum connectivity between the
elements. The electrodes of the third aspect of the invention are easily prepared and a fourth
aspect of the invention provides a method for fabricating an electrode comprising the steps
of forming a slurry from a mixture comprising a plurality of silicon comprising elongate
elements, a plurality of silicon comprising particles, a binder and a solvent; casting the slurry
onto a substrate and drying the product to remove the solvent. The dried product is in the
form of a cohesjve 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 current collector as a result of casting and
drying the slurry, the resulting cohesive mass will be connected to a current collector. In a
preferred embodiment of the first aspect of the invention the anode mix is cast as a layer
onto a substrate, which is itself a current collector. Additional 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. Examples of suitable conductive materials, viscosity adjusters, fillers, cross-linking
accelerators, coupling agents and adhesive accelerators are provided above. Suitable
solvents include N-methylpyrrolidone. Other suitable solvents known to a person skilled in
the art of electrode design may also be used. The relative proportions of each of the
components of the anode mix and the solvent, which are used in the manufacture of the
electrode will depend, in part, on the dimensions of the elongate silicon comprising elements
used in the mixture.
Suitable current collectors for use in electrodes according to the fourth aspect of the
invention include copper foil, aluminium, carbon, conducting polymers and any other
conductive materials. The current collectors typically have a thickness in the range 0 to
50 m. 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 compositions 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/cm2 per
surface such that the total thickness of the electrode (current collector and coating) is in the
range 40mih to 1mm where only one surface of the current collector is coated or in the range
70mhh to 1mm where both surfaces of the current collector are coated. In a preferred
embodiment, the electrode or anode mix or 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 15 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 defining within the area prescribed by the grid
metallised regions and non-metallised 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.
In one embodiment of the fourth aspect of the invention, the electrode may be formed by
casting the composition according to the first aspect of the invention onto a substrate
thereby to form a self supporting structure and connecting a current collector directly thereto.
In a preferred embodiment of the fourth aspect of the invention, a mixture of silicon
comprising elongate elements, a plurality of silicon comprising particles, a binder and
optionally 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 in a solvent is applied to a substrate and dried to remove the solvent.
The resulting product can be removed from the substrate and used as a self supporting
electrode structure. Alternatively, in a further embodiment, the composition according to the
first aspect of the invention is cast onto a current collector and dried to form an electrode
including a first layer comprising a composition according to the first aspect of the invention
applied to a current collector.
The electrode of the third aspect of the invention can be used as an anode in the formation
of a lithium secondary battery. A fifth aspect of the invention provides a secondary battery
comprising a cathode, an anode comprising an electroactive material according to the first
aspect of the invention and an electrolyte.
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. Examples
of suitable cathode materials include LiCo0 2, LiCoo.99Alo.onO2, LiNi0 2, LiMn0 2, LiCo 0.5Nio.s0 2,
LiCoo.7Nio.3O;>, LiCo . Nio.202 LiCo 0 .82Nio.i80 2, LiCoo.eNio.15Alo.05O2, Li i . oo.3 no.3O2 and
LiNio.33Coo.33Mno.34O2. The cathode current collector is generally of a thickness of between 3
to 50 m i . Examples of materials that can be used as the cathode current collector include
aluminium, stainless steel, nickel, titanium and sintered carbon.
The electrolyte is suitably a non-aqueous electrolyte containing a lithium salt and 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-methylpyrroIidone, propylene carbonate, ethylene
carbonate, butylenes carbonate, dimethyl carbonate, diethyl carbonate, gamma butyro
lactone, 1,2-dimethoxy ethane, 2-methyl tetrahydrofuran, dimethylsulphoxide, 1,3-dioxolane,
formamide, dimethylformamide, acetonitrile, nitromethane, methylformate, methyl acetate,
phosphoric acid trimester, trimethoxy methane, sulpholane, methyl sulpholane and 1,3-
dimethyl-2-imidazoiidione.
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, Li Si0 , LiOH and Li3P0 4.
The lithium salt is suitably soluble in the chosen solvent or mixture of solvents. Examples of
suitable lithium salts include LiCI, LiBr, Lil, LiCI0 4, LiBF , LiB 0C20, LiPF , LiCF 3S0 3, LiAsF 6,
LiSbFe, LiAICI , CH3S0 3Li and CF3S0 3Li.
Where the electrolyte is a non-aqueous organic solution, the battery is provided with 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 100mih and a thickness of 5 to 300 m .
Examples of suitable separators include microporous polyethylene films.
The battery according to the fifth 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 sixth aspect of the invention
therefore includes a device including a battery according to the fifth aspect of the invention.
It will also be appreciated that the invention can also be used in the manufacture of solar
cells, fuel cells and the like.
The invention will now be described with reference to the following non-limiting examples.
Variations on these falling within the scope of the invention will be evident to a person skilled
in the art.
Examples
Example 1 - Preparation of Electrode Materials
Example 1a - Fibres
The fibres were prepared by etching either p or n-type silicon wafers to produce pillars of
around 10mi high and diameters of from 100-200nm, using a method set out in US7402829
or US2010/0 151 324 and then removing pillars from the wafer ultrasonically to produce
fibres. The silicon fibres produced are characterised by a BET value of around 10-1 1 m2/g.
Alternatively, the fibres can be produced by etching native silicon particles having an overall
diameter in the range 40 to 200mih using the method set out in WO201 0040985 or
EP2204868 and removing the pillars from the etched surface.
Example 1b - Native Silicon Particles
These were used as supplied. Specifically silicon particles comprising p-type doped
metallurgical grade silicon having a purity of 99.8%, a diameter in the range 1 to 8 m, with a
50 of 4 m and a BET value of around 5m /g were used in the preparation of compositions
according to the first aspect of the invention. The particles were sold as Silgrain® J230 and
were supplied by Elkem of Norway
Example 1c - Pillared Particles
Pillared particles were prepared by etching p-type metallurgical grade silicon powder
particles having a purity of 99.8% and a diameter in the range 14 to 40 m, with a D5oof
24pm in accordance with the methods set out in US 201 1/0067228, WO2010040985 or
WO20 10040986. The silicon powder particles used as the starting materials in the
preparation of pillared particles were obtained from Elkem of Norway and were sold as
Silgrain® J320. The pillared particles produced were characterised by an overall diameter in
the range 14 to 40pm, with a D oof 24 m.
Example 2 - Preparation of Anodes
Example 2a - Anode 1
A silicon mix ( 1) was prepared by mixing 90wt% of native silicon particles as described
above with 10wt% silicon fibres prepared in accordance with the procedure set out in
Example 1a above.
A composite electrode mix was prepared by mixing the silicon mix ( ) with a sodium
polyacrylic acid binder, graphite and carbon black in the proportions 70 : 4 : 4: 12 (Si :
Polyacrylic acid : Graphite: 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 10 minutes
to give Anode Mix 1, which was then cast onto electrodeposited Cu foil to a coat weight of
approximately 16.5g/cm2. Coat weights of 15 - 30 g/m2 are typically used for electrochemical
testing in a Soft Pack Pair cell. The coat was characterised by a density of 0.79g/cm 3 and a
porosity of 69%.
Example 2a - Anode 2
A silicon mix (2) was prepared by mixing 10wt% of native silicon particles as described in
Example 1b above with 90wt% silicon fibres prepared in accordance with the procedure set
out in Example 1a above.
A composite electrode mix was prepared by mixing the silicon mix (2) with a sodium
polyacrylic acid binder, graphite and carbon black in the proportions 70 : 4 : 4: 12 (Si :
Polyacrylic acid : Graphite: 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 10 minutes
to give Anode Mix 2, which was then cast onto electrodeposited Cu foil to a coat weight of
approximately 9.7g/cm2 and were used in the manufacture of a Soft Pack Pair cell. The
composite coat was characterised by a density of 0.43g/cm3 and a porosity of 83%.
Example 2c - Anode 3
A silicon mix (3) was prepared by mixing 50wt% native silicon particles as set out above with
50wt% of pillared particles as described in Example 1c above with 50wt% native silicon
particles as described in Example 1b above.
A composite electrode mix was prepared by mixing the silicon mix (3) with a sodium
polyacrylic acid binder, graphite and carbon black in the proportions 70 : 12 : 12: 6 (Si :
Polyacrylic acid : Graphite: 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
to give Anode Mix 3, which was then cast onto electrodeposited Cu foil to a coat weight of
approximately 13g/cm2 and were used in the manufacture of a Soft Pack Pair cell. The
composite coat was characterised by a density of 0.43g/cm 3 and a porosity of 83%.
Example 3 - Preparation of Batteries (Cells)
Negative electrodes (anodes) , 2 and 3 were prepared as described above. Electrode
pieces were cut to the required size, and then dried overnight in a vacuum oven at 120 °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. Cells 1, 2 and 3 prepared
from the negative electrodes 1, 2 and 3 were tested as set out below.
Example 4 - Performance data on cells
Cells produced as described in Example 3 were cycled using Arbin battery cycling units
using a constant capacity charge/discharge method. Discharge capacities close to either
1200 mAh/g (Cell 1) , 1900mAh/g (cell 2) and 10OOmAh/g (Cell 3) was maintained over more
than 130 cycles. Table 1 lists discharge capacities for cells 1, 2 and 3 (4 repeats) comprising
an MMO cathode, prepared as described above and cycled at a constant capacity of
1200mAh/g (Cell 1), 1900mAh/g (Cell 2) and 1000mAh/g (Cell 3) until the cell fails (number
of cycles until the discharge capacity falls below 80% of the first cycle discharge capacity).
From Table 1 it can be seen that Cell 1 (90:10 native particles:fibres) failed after 175 to 185
cycles. Some delamination of the electrode material was observed on failure.
Cell 2 (10:90 native particles:fibres) failed after 130 to 170 cycles. However, the non
delamination of the composite anode was observed.
Cell 3 (50:50 native particles:pillared particles) failed after more than 300 cycles. The
integrity of the composite anode was maintained throughout.
Table 1
1 Cells were continuously charged and discharged to and from the capacity indicated using constant current conditions until the cell capacity
dropped to
below 80% of its initial value. The number of cycles indicates the cycle number at which this drop in capacity occurred.

Claims
1. A composition comprising a plurality of elongate elements and a plurality of particles,
the elongate elements and particles each comprising a metal or semi-metal selected
from one or more of the group comprising silicon, tin, germanium and aluminium or
mixtures thereof but excluding compositions in which (i) silicon-comprising elongate
elements having a diameter of 30 to 50nm and (ii) aluminium-comprising elongate
elements having a diameter in the range 500nm to 50pm and an aspect ratio (ratio of
the length to the diameter of the fibre) of greater than 2.
2. A composition according to claim 1, wherein the metal or semi-metal is a silicon
comprising material selected from substantially pure silicon, silicon alloy and a
material selected from silicon oxide, silicon nitride and silicon boride or mixtures
thereof.
3. A composition according to claim 2, wherein the silicon comprising elongate
elements are selected from one or more of elongate elements having a core
comprising silicon; elongate elements having a silicon comprising coating
surrounding a core comprising a material other than silicon and elongate elements
comprising a core comprising a first type of silicon comprising material and a coating
comprising a second type of silicon comprising material.
4. A composition according to claim 2 or claim 3, wherein the silicon comprising
particles comprise particles selected from particles having a core comprising silicon;
particles having a silicon comprising coating surrounding a core comprising a
material other than silicon and particles having a core comprising a first type of
silicon comprising material and a coating comprising a second type of silicon
comprising material.
5. A composition according to any one of claims 2 to 4, wherein the elongate elements
are selected from one or more of the group comprising tubes, threads, ribbons and
flakes.
6. A composition according to any one of claims 1 to 5, which is an electro-active
material.
7. A composition according to any one of the preceding claims, which is in the form of a
felt or a mat.
8. 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 comprising electroactive material.
9. A composition according to claim 8, which further comprises one or more
components selected from a viscosity adjuster, a filler, a cross-linking accelerator, a
coupling agent and an adhesive accelerator.
10. A composition according to any one of the preceding claims, which is an electrode
mix or material.
11. A composition according to claim 10, which is an anode material.
12. A composition according to any one of the preceding claims, which further comprises
one or more silicon comprising components selected from the group comprising
pillared silicon particles, silicon comprising porous particles and silicon comprising
porous particle fragments.
13. A composition according to claim 9, wherein the silicon comprising components are
electroactive.
14. A composition according to any one of the preceding claims, wherein one or more of
the silicon comprising elongate elements, the silicon comprising particles and the
further silicon comprising components include a coating.
15. A composition according to claim 14, wherein the coating is a carbon coating.
16. A composition according to claim 15, wherein the coating comprises 5 to 40% by
weight of the coated silicon material.
17. A composition according to any one of the preceding claims, wherein the elongate
elements include threads having a diameter in the range 50nm to 1000nm, a length
in the range 0.8 to 100pm and an aspect ratio in the range 5:1 to 1000:1.
18. A composition according to any one of the preceding claims, wherein the elongate
elements have a diameter in the range 150nm to 200nm and a length in the range 10
to 15 m.
19. A composition according to any one of claims 1 to 16, wherein the elongate elements
include ribbons having a thickness in the range 0.08pm to m, a width in the range
240nm to 300nm, a length in the range 0.8 m to 20 m and an aspect ratio in the
range 10:1 to 200:1.
20. A composition according to claim 17, which comprises ribbons having a thickness of
0.25pm, a width of 0.5pm and a length of 50pm.
2 1.A composition according to any one of claims 1 to 16, wherein the elongate elements
include flakes having a thickness in the range 80nm to 10Onm, a width in the range
0.8pm to 10 m, a length in the range 0.8pm to 20pm and an aspect ratio in the range
10:1 to 200:1.
22. A composition according to claim 2 1, which comprises flakes having a thickness of
0.25pm, a width of 3 m and a length of 50 m.
23. A composition according to any one of claims 1 to 17, wherein the elongate elements
include tubes having a wall thickness in the range 0.08pm to 2mih, an outer wall
diameter of between 2.5 and 100 times larger than the wall thickness and a length of
between 10 and 500 times as large as the wall thickness.
24. A composition according to claim 23, which comprises tubes having a wall thickness
in the range 0.08pm to 5 m and a length that is at least five times the outer diameter.
25. A composition according to any one of the preceding claims, wherein the
particleshave an average diameter in the range 80nm to 15mi .
26. A composition according to claim 25, wherein the particles have an average diameter in
the range 1 to 8mh , with a D5oof 4 m.
27. A composition according to any one of the preceding claims wherein at least some of
the elongate elements or particles comprise silicon with a purity in the range 90% to
99.95%.
28. A composition according to claim 27 wherein at least some of the elongate elements
or particles are metallurgical grade silicon.
29. A composition according to any one of claims 1 to 9, wherein the particles are
selected from the group comprising native particles, pillared particles, porous
particles and porous particle fragments.
30. A composition according to any one of the preceding claims, which comprises 50 to
90% by weight of an electroactive material.
3 1.A composition according to claim 28, wherein the electroactive material comprises 40
to 100% by weight silicon comprising elongate elements, silicon comprising particles
and further silicon comprising components.
32. A composition according to claim 3 1, wherein the electroactive material comprises 40
to 90wt% silicon comprising elongate elements, silicon comprising particles and
further silicon comprising components.
33. A composition according to any one of the preceding claims, wherein the silicon
comprising component of the composition comprises 10 to 90% by weight elongate
elements.
34. A composition according to claim 33, wherein the silicon comprising component of
the composition comprises 15 to 60wt% by weight elongate elements.
35. A composition according to any one of the preceding claims wherein the silicon
component of the composition comprises 0 to 90% by weight silicon comprising
particles.
36. A composition according to claim 35, wherein the silicon comprising component of
the composition comprises 40 to 85wt% silicon comprising particles.
37. A composition according to any one of the preceding claims, which further comprises
0 to 30 % by weight of a binder.
38. A composition according to claim 37, wherein the binder is selected from the group
comprising PVdF, PAA, CMC, modified CMC, modified PAA and alkali metal salts
thereof.
39. A composition according to any one of the preceding claims, which further comprises
4 to 85% by weight of carbon.
40. A composition according to any one of the preceding claims, wherein the carbon is
selected from one or more of graphite, hard carbon, graphene and a conductive
carbon.
4 .A composition according to any one of the preceding claims comprising 70wt% of a
silicon-comprising material comprising a mixture of silicon-comprising fibres and
silicon-comprising native particles in a ratio of from 90:10 to 10:90, 14wt% of a binder
comprising polyacrylic acid or an alkali metal salt thereof, 4wt% of graphite and
12wt% of a conductive carbon.
42. A composition according to any one of claims 1 to 40, comprising 70wt% of a siliconcomprising
material comprising a 50:50 mixture of native particles and pillared
particles, 12wt% of a binder comprising polyacrylic acid or an alkali metal salt
thereof, 12wt% graphite and 6wt% of a conductive carbon
43. A method of manufacturing a composition according to any one of claims 1 to 42,
comprising the steps of mixing a plurality of metal or semi-metal comprising elongate
elements with a plurality of metal or semi-metal comprising particles.
44. A method according to claim 43 wherein the metal or semi-metal comprising elongate
elements or particles are silicon comprising elongate elements or particles.
45. A method according to claim 44, which further comprises the steps of combining with
the silicon comprising elongate elements and the silicon comprising particles a binder
and 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.
46. Use of a composition according to any one of claims 1 to 46, in the manufacture of a
secondary battery.
47. An electrode comprising a current collector and a composition according to any one
of claims 1 to 42.
48. An electrode according to claim 47, which is an anode.
49. An electrode according to claim 47 or 48, wherein the composition is in the form of a
layer or coating applied to the current collector.
50. An electrode according to claim 47, 4 8 or 49, wherein the thickness of the layer or
coating is in the range 1mg/cm 2 to 6 mg m2.
51. An electrode according to any one of claims 4 7 to 50, wherein the current collector is
selected from the group comprising copper foil, aluminium foil and nickel foil.
52. A method of manufacturing an electrode according to any one of claims 4 7 to 5 1 ,
comprising the steps of preparing a slurry of a composition according to any one of
claims 1 to 4 2 in a solvent; applying the slurry to a substrate and drying the product
to remove the solvent, thereby forming a coating or layer of the composition on the
substrate and connecting the coated substrate to a current collector.
53. A method according to claim 52, wherein the solvent is N-methylpyrrolidone.
54. A method according to claim 52 or claim 53, wherein the substrate is a current
collector.
55. A method according to claim 52 or 53, which further comprises the step of removing
the coating or layer from the substrate to form a free standing mat or felt and
connecting the free standing mat or felt to a current collector.
56. A battery comprising a cathode, an anode comprising a material according to any
one of claims 1 to 4 2 and an electrolyte.
57. A battery according to claim 56, wherein the cathode is selected from the group
comprising LiCo0 2 LiCoo.99Alo.01O2, LiNi0 2, Li n0 2, LiCoo.5Nio.5O2, LiCoo.7Nio.3O2,
LiCoo.8Nio.20 2 LiCoo.82Nio.i8O2, LiCo0 8Ni0.15AI0.05O2, LiNio.4Coo.3Mno.3O2 and
L iN i .33 o o.33 n .34 .
58. A battery according to claim 56 or claim 57, wherein the electrolyte is selected from
the group comprising a non-aqueous electrolytic solution, a solid electrolyte and an
inorganic solid electrolyte.
59. A battery according to claim 58, wherein the electrolyte further comprises one or
more lithium salts selected from the group comprising LiCI, LiBr, Lil, LiCI0 4, LiBF ,
LiB C2 o, LiPFe, LiCF 3S0 3, LiAsF 6, LiSbF 6 LiAICI 4, CH3S0 3Li and CF3S0 3Li.
60. A device employing a battery according to claims 56 to 59.