This invention relates to composite particles that are suitable for use as anode active
materials in lithium ion batteries. The composite particles comprise a porous particle
5 framework and a plurality of silicon domains located within the pores of the porous
particle framework. The dimensions and the chemical composition of the silicon
domains are controlled for optimum electrochemical performance.
BACKGROUND TO THE INVENTION
Lithium-ion batteries (LIBs) comprise in general an anode, a cathode and a lithium-
1 0 containing electrolyte. The anode generally comprises a metal current collector
provided with a layer of an electroactive material, defined herein as a material which is
capable of inserting and releasing lithium ions during the charging and discharging of a
battery. When a LIB is charged, lithium ions are transported from the cathode via the
electrolyte to the anode and are inserted into the electroactive material of the anode as
15 intercalated lithium atoms. The terms "cathode" and "anode" are therefore used herein
in the sense that the battery is placed across a load, such that the anode is the negative
electrode. The term "battery" is used herein to refer both to devices containing a single
lithium-ion cell and to devices containing multiple connected lithium-ion cells.
LIBs were developed in the 1980s and 1990s and have since found wide application in
20 portable electronic devices. The development of electric or hybrid vehicles in recent
has created a significant new market for Ll Bs and renewable energy sources have
created further demand for on-grid energy storage which can be met at least in part by
LIB farms. Overall, global production of LIBs is projected to grow from around 290 GWh
in 2018 to over 2,000 GWh in 2028.
25 Alongside the growth in total storage capacity, there is significant interest in improving
the gravimetric and/or volumetric capacities of rechargeable metal-ion batteries such
that the same energy storage is achieved with less battery mass and/or less battery
volume. Conventional LIBs use graphite as the anode electroactive material. Graphite
anodes can accommodate a maximum of one lithium atom for every six carbon atoms
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resulting in a maximum theoretical specific capacity of 372 mAh/g in a lithium-ion
battery, with a practical capacity that is somewhat lower (ca. 340 to 360 mAh/g).
Silicon is a promising alternative to graphite because of its very high capacity for lithium
(see, for example, Insertion Electrode Materials for Rechargeable Lithium Batteries,
5 Winter, M. eta!. in Adv. Mater. 1998, 10, No. 1 0). Silicon has a theoretical maximum
specific capacity of about 3,600 mAh/g in a lithium-ion battery (based on Li1sSi4).
However, such a high ratio of intercalated lithium to silicon results in expansion of the
silicon material by up to 400% of its original volume. Repeated charging and
discharging cycles result in significant mechanical stress on the silicon material leading
10 to fracturing and structural failure. Furthermore, the charging of anodes in Ll Bs results
in the formation of a solid electrolyte interphase (SEI) layer. This SEI layer is an ionconductive
yet insulating layer that is formed by the reductive decomposition of
electrolytes on exposed electrode surfaces during the initial charge. In a graphite
anode, this SEI layer is relatively stable during subsequent charge/discharge cycles.
15 However, the expansion and contraction of a silicon anode results in fracturing and
delamination of the SEI layer and the exposure of fresh silicon surface, resulting in
further electrolyte decomposition, increased thickness of the SEI layer and irreversible
consumption of lithium. These failure mechanisms collectively result in an unacceptable
loss of electrochemical capacity over successive charging and discharging cycles.
20 The present inventors have previously reported the development of a class of
electroactive materials having a composite structure in which electroactive materials,
such as silicon, are deposited into the pore network of highly porous particles, e.g. a
porous carbon material, having a carefully controlled pore size distribution. For
example, WO 2020/095067 and WO 2020/128495 report that the improved
25 electrochemical performance of these materials can be attributed to the way in which
the electroactive materials form small domains with dimensions of the order of a few
nanometres or less within the pore network of the porous particles, which thus function
as a framework for the composite particles. The fine electroactive structures are
thought to have a lower resistance to elastic deformation and higher fracture resistance
30 than larger electroactive structures, and are therefore able to lithiate and delithiate
without excessive structural stress. As a result, the electroactive materials exhibit good
reversible capacity retention over multiple charge-discharge cycles. Secondly, by
controlling the loading of silicon within the porous particle framework such that only part
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of the pore volume is occupied by silicon in the uncharged state, the unoccupied pore
volume of the porous particle framework is able to accommodate a substantial amount
of silicon expansion internally. Excessive expansion is constrained by the particle
framework. Furthermore, only a small area of the electroactive material surface is
5 accessible to electrolyte and so SEI formation is substantially prevented.
In WO 2022/029422, the applicant has reported a further development in which control
of the distribution of electroactive silicon within the pore network of the particle
framework results in still a further improvement in the electrochemical performance of
the composite particles. Specifically, the applicant has shown that electrochemical
10 performance is optimised when the length scale of the individual silicon structures in
the composite particles is minimised such that a large proportion of the silicon atoms
are in a surface region of the silicon structures, with a relatively smaller proportion of
silicon atoms located inside bulky/coarse silicon structures. The applicant has identified
an optimised pore structure of the porous particle framework and a set of conditions for
15 the deposition of silicon into the porous particle framework that allows for an increased
proportion of this so-called "surface silicon" while also ensuring a large amount of silicon
in total is incorporated into the composite particles to meet overall volumetric energy
density requirements. The nanoscale silicon domains formed by thermal decomposition
of a silicon-containing precursor are thought to be in the form of nanoclusters of silicon
20 atoms that are substantially terminated by silicon-hydrogen bonds (Si-H).
There remains a need in the art for further improvements to electroactive composite
particles of the type described above to provide improvements in electrochemical
performance and longevity of the materials over multiple charge discharge cycles. It
has now been found the surface functionality of the nanoscale silicon domains may be
25 altered to obtain improved electrochemical properties.
30
SUMMARY OF THE INVENTION
In a first aspect, the invention provides a particulate material consisting of a plurality of
composite particles, wherein the composite particles comprise:
(a) a porous particle framework comprising micropores and mesopores,
wherein the total volume of micropores and mesopores in the porous
particle framework as measured by gas adsorption is from 0.5 to 1.8
cm3/g; and
5
10
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(b) a plurality of nanoscale elemental silicon domains located within the
pores of the porous particle framework,
wherein:
(i) the composite particles comprise from 30 to 70 wt% silicon;
(ii) at least 30 wt% of the silicon is surface silicon as determined by
thermogravimetric analysis (TGA);
(iii) the hydrogen content of the composite particles is no more than 1.2 wt%;
(iv) the weight ratio of oxygen to silicon in the composite particles is no more
than 0.15; and
(v) the BET surface area of the composite particles is no more than 40 m2/g.
The invention therefore relates in general terms to composite particles comprising a
plurality of nanoscale silicon domains into the pore network of porous particles
comprising micropores and mesopores. As used herein, the term "nanoscale silicon
domain" refers to a nanoscale body of elemental silicon having maximum dimensions
15 that are determined by the location of the silicon within the micropores and mesopores
of the porous particles.
The deposition of nanoscale silicon domains in mesoporous and microporous particles
is kinetically controlled such that thermal deposition takes place preferentially at the
internal pore surfaces of the porous particles. The applicant has previously
20 demonstrated in WO 2022/029422 how it is possible to obtain composite particles
comprising a large proportion of so-called "surface silicon", which refers herein to silicon
that is located in the surface region of a silicon microstructure. Silicon in a surface
region of a silicon microstructure can be quantified by TGA measurements and
therefore provides a measurement of the fineness of the silicon microstructures.
25 The nanoscale silicon domains formed by thermal decomposition of a silicon-containing
precursor are thought to be in the form of nanoclusters of silicon atoms that have a
range of different bonding interactions at their surfaces, between silicon atoms, between
silicon atoms and the porous particle framework, as well as silicon atoms that are
terminated by silicon-hydrogen bonds (Si-H). The surfaces of these silicon nanoclusters
30 are highly reactive as a result of these unbalanced bonding interactions of the silicon
atoms at the surfaces of the silicon nanoclusters. In particular, silicon surfaces that are
terminated by silicon-hydrogen bonds (Si-H) are highly reactive in Li-ion cells when
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lithium insertion takes places. Increased stability can therefore be achieved when the
composite particles have a high proportion of "surface silicon" but wherein Si-H bonding
is reduced and wherein Si-Si bonding is maximised. Accordingly, the present invention
provides a particulate material as defined above wherein at least 30 wt% of the silicon
5 is surface silicon and wherein the hydrogen content of the composite particles is less
than 1.2 wt%. In this way, the silicon maintains a desirable length scale for effective
electrochemical performance while the reducing surface reactivity of the silicon.
In preferred embodiments of the invention, the oxygen content of the composite
particles is no more than 10 wt%. When the oxygen content of the composite particles
10 is too high, too much silicon is sequestered as Si-0 bonding. This is more inert than
Si-H bonding but it increases the resistivity of the electroactive material and also
undergoes irreversible lithium insertion, with the potential to reduce the efficiency of the
electroactive material in Li-ion cells.
The particulate material therefore builds upon prior disclosures by the applicant that
15 have identified the performance benefit that is achieved by composite particles
comprising a high proportion of surface silicon by the identification of a further beneficial
structural feature of the composite particles. This is found to contribute to improved
stability of the electroactive material during charging and discharging and therefore to
an improvement in the cycle life of lithium ion batteries comprising the particulate
20 material as an anode active material.
In a second aspect, the invention provides a composition comprising the particulate
material of the first aspect and at least one other component.
In a third aspect, the invention provides an electrode comprising the particulate material
of the first aspect or the composition of the second aspect.
25 In a fourth aspect, the invention provides a rechargeable metal-ion battery comprising
the electrode of the third aspect.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 shows the TGA trace for a particulate material according to the invention,
comprising a high level of surface silicon and a low level of bulk coarse silicon.
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Figure 2 shows the TGA trace for a particulate material comprising a low level of surface
silicon and a high level of bulk coarse silicon.
DETAILED DESCRIPTION OF THE INVENTION
The composite particles of the invention comprise a plurality nanoscale elemental
5 silicon domains located within the pores of a porous particle framework. The length
scale of the nanoscale silicon domains in the particulate material of the invention is
quantified using TGA analysis. This method of analysis relies on the principle that a
weight gain is observed when silicon is oxidized to silicon dioxide (Si02) in air and at
elevated temperature. The mechanisms by which silicon oxidizes are dependent on
10 temperature. Silicon atoms at the surface of a silicon nanostructure are oxidized at a
lower temperature than silicon atoms in the bulk of a silicon nanostructure (reference:
Bardet et al., Phys. Chern. Chern. Phys. (2016), 18, 18201). By plotting the weight
gain against temperature, it is possible to differentiate and quantify the bulk and surface
silicon in the sample.
15 The determination of the amount of unoxidized surface silicon is derived from the
characteristic TGA trace for these materials, as shown in Figures 1 and 2. Following
an initial mass loss up to ca. 300 °C (shown in Figures 1 and 2 as the mass reduction
from (a) to (b)) a significant increase in mass is observed starting at ca. 400 °C and
peaking between 550 °C and 650 °C (shown in Figures 1 and 2 as the mass increase
20 from (b) to (c)). A reduction in mass is then observed as the porous particle framework
is oxidized to C02 gas (the mass reduction from (c)), then above ca. 800 °C a mass
increase is again observed corresponding to the continued conversion of silicon to Si02
which increases toward an asymptotic value above 1 000 °C as silicon oxidation goes
to completion (the mass increase from (d) to (e)). The temperature at which the weight
25 increase occurs is related to the structure of the silicon, with surface silicon oxidized at
low temperatures and bulk silicon oxidized at higher temperatures. Therefore, the more
coarse the silicon domains, the more oxidation is observed at higher temperatures.
Any native oxide that is already formed on silicon surfaces that are exposed to air does
not affect the TGA analysis, since silicon that is already oxidized does not give rise to a
30 mass increase in the TGA analysis. Therefore the more the silicon surfaces are able to
react with air to form a native oxide, the less surface silicon is observed by TGA. For
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avoidance of doubt, the calculation of "surface silicon" therefore takes into account only
silicon that is unoxidized at the start of the TGA analysis after the material has been
passivated by air or another surface passivating agent as described herein (i.e. the
particulate material is not kept under any special inert conditions prior to the TGA
5 analysis).
As defined herein, "surface silicon" is calculated from the initial mass increase in the
TGA trace from a minimum between 150 °C and 500 °C to the maximum mass
measured in the temperature range between 550 °C and 650 °C, wherein the TGA is
carried out in air with a temperature ramp rate of 10 °C/min. This mass increase is
10 assumed to result from the oxidation of surface silicon and therefore allows the
percentage of surface silicon as a proportion of the total amount of silicon to be
determined according to the following formula:
Y = 1.875 x [(Mmax- Mmin) I Mf] x1 00%
Wherein Y is the percentage of surface silicon as a proportion of the total silicon in the
15 sample, Mmax is the maximum mass of the sample measured in the temperature range
between 550 °C to 650 °C (mass (c) in Figures 1 and 2), Mmin is the minimum mass of
the sample above 150 °C and below 500 °C (mass (b) in Figures 1 and 2), and Mf is the
mass of the sample at completion of oxidation at 1400 °C (mass (e) in Figures 1 and
2). For completeness, it will be understood that 1.875 is the molar mass ratio of Si02
20 to 02 (i.e. the mass ratio of Si02 formed to the mass increase due to the addition of
oxygen). Typically, the TGA analysis is carried out using a sample size of 10 mg ±2
mg.
It has been found that optimum reversible capacity retention over multiple
charge/discharge cycles is obtained when the surface silicon as determined by the TGA
25 method described above is at least 30 wt% of the total amount of silicon in the material.
Preferably at least 32 wt%, or at least 35 wt%, or at least 38 wt%, at least 40 wt%, or at
least 42 wt%, or at least 45 wt%, or at least 48 wt%, or at least 50 wt% of the silicon is
surface silicon as determined by thermogravimetric analysis (TGA).
In addition to the surface silicon content, the particulate material of the invention
30 preferably has a low content of coarse bulk silicon as determined by TGA. Coarse bulk
silicon is defined herein as silicon that undergoes oxidation above 800 °C as determined
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by TGA, wherein the TGA is carried out in air with a temperature ramp rate of 10 °C/min.
This is shown in Figures 1 and 2 as the mass increase from (d) to (e). The coarse bulk
silicon content is therefore determined according to the following formula:
Z = 1.875 x [(Mt- Maoo) I Mt] x1 00%
5 Wherein Z is the percentage of unoxidized silicon at 800 °C, Maoo is the mass of the
sample at 800 °C (mass (d) in Figures 1 and 2), and Mt is the mass of ash at completion
of oxidation at 1400 °C (mass (e) in Figures 1 and 2). For the purposes of this analysis,
it is assumed that any mass increase above 800 °C corresponds to the oxidation of
silicon to Si02 and that the total mass at completion of oxidation is Si02.
10 Preferably, no more than 6 wt%, or no more than 5 wt%, or no more than 4 wt% of the
silicon, or no more than 3.5 wt%, or no more than 3 wt%, or no more than 2.5 wt%, or
no more than 2 wt% or no more than 1.5 wt% of the silicon is coarse bulk silicon as
determined by TGA.
Preferably at least 35 wt% of the silicon is surface silicon and no more than 6 wt% of
15 the silicon, more preferably no more than 5 wt% of the silicon is coarse bulk silicon,
wherein both are determined by TGA as defined herein. More preferably at least 40
wt% of the silicon is surface silicon and no more than 5 wt% of the silicon, more
preferably no more than 4 wt% of the silicon is coarse bulk silicon, wherein both are
determined by TGA as defined herein. More preferably at least 45 wt% of the silicon is
20 surface silicon and no more than 4 wt% of the silicon, more preferably no more than 3
wt% of the silicon is coarse bulk silicon, wherein both are determined by TGA as defined
herein. More preferably at least 50 wt% of the silicon is surface silicon and no more
than 3 wt% of the silicon, more preferably no more than 2 wt% of the silicon is coarse
bulk silicon, wherein both are determined by TGA as defined herein.
25 The total volume of micropores and mesopores (i.e. the total pore volume of pores
having a diameter in the range of 0 to 50 nm) in the porous particle framework is from
0.5 to 1.8 cm3/g. For the avoidance of doubt, references herein to the pore volume of
the porous particle framework relate (in the absence of any indication to the contrary)
to the pore volume of the porous particle framework taken in isolation, i.e. as measured
30 in the absence of any electroactive material (or any other material) occupying the pores
of the porous particle framework.
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Preferably, the total volume of micropores and mesopores in the porous particle
framework is from at least 0.55 cm3/g, or at least 0.6 cm3/g, or at least 0.65 cm3/g, or at
least 0.7 cm3/g, or at least 0.75 cm3/g.
A high pore volume may be advantageous since it allows a larger amount of silicon to
5 be accommodated within the pore structure without compromising the resistance of the
porous particle framework to fracturing under compressive stress during electrode
manufacture or expansion stress due to lithiation of the silicon. However, if the pore
volume is too high, then it is not possible to achieve the elevated levels of surface silicon
that characterize this invention. Accordingly, the total volume of micropores and
10 meso pores in the porous particle framework is preferably no more than 1.6 cm3/g, or no
more than 1.4 cm3/g, or no more than 1.3 cm3/g, or no more than 1.2 cm3/g, or no more
than 1.1 cm3/g.
For example, the total volume of micropores and mesopores in the porous particle
framework is preferably in the range from 0.55 to 1.6 cm3/g, or from 0.6 to 1.4 cm3/g, or
15 from 0.65 to 1.3 cm3/g, or from 0.7 to 1.2 cm3/g, or from 0.75 to 1.1 cm3/g.
The general term "POn pore diameter'' refers herein to the volume-based nth percentile
pore diameter of the porous particle framework, based on the total volume of micro pores
and mesopores. For instance, the term "POso pore diameter'' as used herein refers to
the pore diameter below which 50% of the total micropore and mesopore volume is
20 found. For the avoidance of doubt, any macropore volume (pore diameter greater than
50 nm) is not taken into account for the purpose of determining POn values.
The POgo pore diameter of the porous particle framework is preferably no more than 12
nm, or no more than 10 nm, or no more than 8 nm, or no more than 6 nm, or no more
than 4 nm. Preferably, the POgo pore diameter is at least 3 nm. If the POgo value is too
25 high, then excessive deposits of coarse silicon and/or excessive native oxide formation
result in a low content of surface silicon. However, if the POgo is too low then infiltration
of the pore volume by a silicon precursor is impeded and silicon instead deposits on the
external surfaces of the porous particle framework.