WO2025040331A1 - Silicon and carbon based composite particles - Google Patents

Abstract

The present invention relates to silicon- and carbon-based composite particles, wherein each of the silicon-based composite particles is composed of a bulk material having a plurality of nanoscaled silicon domains embedded therein, and where the bulk material comprises carbon in a total amount of from 1 to 40 wt%, 5 based on the total mass of the silicon-based composite particles, the rest being silicon and unintentional impurities, and wherein the silicon- and carbon-based composite particles have a median volume-weighted diameter, D₅₀, in the range from 1 to 9 µm, and a volume weighted diameter ratio D₉₀/D₁₀ in the range from 1 to 6, as determined by laser diffraction analysis according to standard ISO 10 13320:2020.

Description

Silicon and carbon based composite particles

The present invention relates to a particulate silicon- and carbon-based composite material having a high charge/discharge capacity and first cycle efficiency when applied as the active material in the negative electrode of secondary lithium ion batteries, and a method for manufacturing said particles.

Background

Carbon-containing fossil fuels covers currently around 80 % of the global energy demand. The major part of the fossil fuels are combusted with varying degree of cleaning of the produced exhaust/combustion gases before venting them to the atmosphere. These emissions amount to a major pollution problem, a global warming problem, and an ocean acidification problem. There is therefore a growing desire and interest in the society for developing and implementing climate neutral and non-polluting alternatives.

Electric power is a versatile form of energy which hardly pollutes when used to provide heat, drive electric engines, run electronics, etc. Furthermore, some sectors in the society require portable storage of electric energy to enable electrification. Secondary lithium-ion batteries (LIBs) are presently the best commercially available battery type for applications needing high volumetric and gravimetric energy storage densities and high effect delivering capacities. However, there is a need for batteries having higher storage densities than what presently commercially available LIBs may provide to fully take advantage of the electrification option.

The major energy storing constraint of present LIBs is their graphite-based negative electrodes since graphite has a relatively limited capacity to store lithium. There is therefore a desire in the battery sector to find other and better suited materials than graphite as the active material of the negative electrode of LIBs.

Prior art

Silicon is known to have a relatively strong capability for taking up lithium and form a silicon and lithium alloy. At typical ambient temperatures, the most lithiated phase of silicon is Li3.7sSi which has a theoretic specific capacity of 3579 mAh/g, as opposed to the graphite’s theoretical specific energy of 372 mAh/g. The battery industry has therefore for more than a decade sought to find a solution to apply silicon as the active material in the negative electrode of secondary LIBs.

However, the lithium uptake causes a significant volume fluctuation of the silicon material. At is most lithiated state of Lis.vsSi, the silicon material has a volume of around 320 % higher as compared to its non-lithiated state. Also, electrolyte contacting the surface of the active material usually reacts and forms a lithium- containing solid phase known as the solid electrolyte interface layer (SEI). This SELlayer represents an irreversible loss of lithium in the electrochemical cell which correspondingly reduces its energy storage capacity. Since the formation of the SEI- layer occurs mainly during the first charge-discharge cycle, the magnitude of irreversible loss of lithium associated with the SEI-layer formation is often represented by a first cycle efficiency (FCE) measure.

Furthermore, the volume changes of the silicon material over a lithiation and de- lithiation (charge/discharge) cycle has shown to cause severe problems both with structural degradation/disintegration of the silicon material and instability of the SEI-layer, leading to unacceptably low cyclabilities and large capacity losses of the LIBs. This integrity problem of the silicon material has been suggested solved by applying the silicon in the form of nanoscaled particles, typically less than 200 nm, preferably with a surface coating.

Sourice et al. (2016) [Ref 1] discloses producing amorphous silicon core particles of 30 nm diameter by laser-driven chemical vapour pyrolysis (LCVP) of silane gas diluted in helium. The particles are given a 1 nm thick carbon coating made by a second LCVP stage of ethylene gas. The particles are reported to, after 500 charge/discharge cycles, retaining a capacity of 1250 mAh.g-1 at a C/5 rate and 800 mAh.g-1 at 2C, with an outstanding coulombic efficiency of 99.95%.

It is further known that nanoscaled silicon-based particles containing other elements may be manufactured in industrial scale by thermally induced decomposition of a mixture of precursor gases. An example of this is known from WO 2021/160824 which discloses manufacturing amorphous particles with a diameter of from 10 to 200 nm of silicon alloyed with from 0.05 to 2 atom% of C and/or N by simultaneous thermally induced decomposition of silicon and carbon containing gases. In an example embodiment, Si0.98C0.02 particles are shown made by passing a homogeneous gaseous mixture of silane and ethene preheated to 400 °C and then passing the mixture into a reactor where it becomes mixed with heated nitrogen gas to a temperature giving a temperature in the resulting gas mixture of 810 °C. The relative amounts of the gases in the final gas mixture in the reactor were approximately 28 mole% silane, 1.5 mole% ethene and the rest (~70 mole%) was nitrogen. The residence time was approx. 1 second. The particles are described to have a homogenous structure.

Orthner et al. 2021 [Ref 2] reports a study on formation of amorphous silicon-based particles by flowing a mixture of silane and ethylene gas diluted in nitrogen at atmospheric pressure through a tubular hot-wall reactor at 640, 690, and 1100 °C. The residence time was from 1 to 5 seconds. The gas mixture had a concentration of silane gas of from 10 to 30 viol% and ethylene gas of from 0 to 11.3 vol%. The particles made at 640 and 690 °C were found to be both amorphous and homogeneous with no or only some partial crystallization for those made at 640 and 690 °C, respectively. The particle size varied from 80 to 300 nm, with an average size of 200 nm. XRD analysis showed no formation of SiC. An XPS analysis showed that the carbon content in the amorphous particles decreased almost linearly from the particle surface and into the bulk of the particle. The initial capacity of the particles was found to be 3070 mAh/g which dropped to 2200 mAh/g after the second cycle, but the Coulombic efficiency levelled out at above 99.5 %. The high Coulombic efficiency was attributed to low SEI-layer formation due to the relatively high presence of carbon at the particle surface. However, the particles made at 1100 °C were found to consist of a mixture of crystalline Si (about 15 wt%), amorphous Si (about 14 wt%) and amorphous SiC (about 71 wt%). The crystallite size of the Si was found to be 70 nm. Orthner informs further that the formation of SiC was found to be disadvantageous due to particles exhibiting significantly lower first cycle efficiency and specific capacity (of 917 mAh/g) compared to pure Si.

WO 2022/200606 discloses that a heat treatment at relatively high temperature and long endurance may transform amorphous structures into crystalline structures. The document discloses forming carbon alloyed silicon particles by a thermally induced decomposition of a mixture of precursor gases as in WO 2021/160824 above, and then heat treat them at 800 to 900 °C for 10 to 240 minutes. The heat treated particles are disclosed to have a BET from 25 to 180 m²/g (approx. 15 to 110 nm in diameter), a total content of from 0.05 to 20 atom% C and/or N and contain nanosized crystallites of 1 to 15 nm in diameter embedded therein.

It is further known that baking nanoscaled silicon particles in a carbon matrix may provide stable particles and reducing the formation of SEI-layers. Wang et al. (2013) [Ref 3] discloses composite particles made by pyrolyzing a mixture of nanoscaled silicon particles of 50 - 100 nm in a coal tar pitch followed by comminuting the pyrolyzed mixture to form a composite of Si-particles embedded in an amorphous carbon matrix (Si/aC). Composites with 20 wt% Si were found to exhibit stable lithium storage ability for prolonged cycling. The composite anode delivered a capacity of 400.3 mAh/g with a high capacity retention of 71.3% after 1000 cycles. This was explained being due to the silicon nanoparticles being wrapped by amorphous SiOx and amorphous carbon in the (Si/aC) composite which can supply sufficient conductivity and strong elasticity to suppress the stress resulting from the reaction of Si with Li during charge/discharge process.

Zhu et al. (2018) [Ref 4] reports a study investigating the correlations between key physical parameters and electrochemical properties of silicon particles when used in the anode of LIBs. The investigations included three samples of crystalline silicon particles denoted as SI, S2, and S3 which had a BET specific surface area of 41.4, 36.11, and 7.33 m²/g, respectively. This corresponds to a D50 particle size of approx. 50, 100, and 150 nm. Each of the three particle samples were mixed with conductive carbon and sodium alginate binder and then applied on a copper conductor to form three anode samples with the SI, S2, and S3 particles, respectively. The amount of active material loading was ca. 0.5 mg/cm² in each anode sample. The anode samples were assembled in CR2032-type coin half-cells having the same electrolyte and cathode to investigate the effect of the silicon particle size of the anode on the electrochemical properties of the cells. The investigations show that all three cells with SI, S2, and S3, respectively had a reversible capacity of approximately 2500 mAh/g while the first cycle coulombic efficiencies (FCE) obtained with SI, S2, and S3 are 78.51%, 83.12% and 89.26%, respectively. The strong positive correlation between particle size and the first cycle efficiency is attributed to the difference of the specific area. The increased specific area of the Si anode with smaller particle size will inevitably aggravate the SEI formation reaction at the interface of the electrode and electrolyte, which results in a high irreversible capacity for the SEI formation. However, the rate capability of the Si anode was found to be enhanced by reducing the particle size. This is attributed to the short Li diffusion distance for the Si anode with smaller particle sizes. At a rate of 20C, the delivered capacities for SI, S2 and S3 were found to be 992.23, 323.17 and 233.43 mAh/g, respectively. Also, the cycling performance of the Si anode with different particle sizes were found to exhibit superior cycling stability with small particle sizes. After 300 cycles, the capacity retentions of SI, S2 and S3 are 96.12 %, 93.98 % and 76.73 %, respectively. This result was attributed to the large Si particles being more prone to pulverize and crack during the repeated charge-discharge cycles, especially when the particle size is larger than 150 nm, as reported in the literature.

Another factor which speaks for applying small silicon particles, reported by Rhenlund et al. (2017) [Ref 5], is diffusion controlled trapping of lithium in the electrodes. Their investigations show that during the cycling, small amounts of elemental lithium are trapped within the active electrode material due to a two-way diffusion causing lithium to move into the bulk of the active material, which makes the lithium extraction process significantly more time consuming. This Li trapping mechanism was demonstrated with silicon particles with a D50 of 50 nm.

Sung et al. (2021) [Ref. 6] reports a study of the nucleation and growth mechanisms of a silicon and carbon containing film on a carbon substrate. The study included computer simulations based on density functional theory (DFT) and synthesis of films by thermal decomposition of a mixture of silane and ethylene gas at 475 °C at various silane to ethylene ratios ranging from only silane to a ratio of 10 : 7. The synthetized films was grown on either a planar amorphous carbon nanoparticle substrate or on spherical graphite particles and then coated with 5 wt% pitch based carbon and annealed at 900 °C. The films were made to have a thickness of 20-25 nm or 60-70 nm which took around 45 minutes or around 78 minutes to grow, respectively. The particle sizes of the graphite particles at which the films were deposited is not disclosed explicitly in the document but is seems from figure 3 of Sung et al. (2021) that the graphite particles where substantially spherical and had a diameter around 10 pm. The DFT calculations suggested that the carbon atoms released by the simultaneous decomposition of silane and ethylene function as crystal growth inhibitors for the silicon atoms by forming interposed layers of SiC and C between the silicon crystallites as shown schematically in figure 1 of Sung et al. (2021), replicated as figure 1 herein. The calculation showed further that the lower silane to ethylene ratio, the smaller the silicon crystallites in the film became. This result was confirmed by analysis of the synthesized films which found that the silicon crystallites in the film had a size ranging from 40 nm for a pure Si-film while silicon and carbon films had silicon crystallites of 3.8 nm for the film with smallest carbon content and 0.85 nm for the film with the highest carbon content. The film found to exhibit the best cycling stability, comparable to the cycling stability of graphite and thus acceptable for commercial use, where synthesized with a ratio silane to ethylene of 10 : 5 and consisted of silicon with 36.8 at% C (corresponds to 20.1 wt% C) and was found to exhibit a specific capacity of 1974.3 mAh/g. The silicon crystallites in this film had an average particle size of 0.97 nm. The document further reports - as expected - that the specific capacity and FCE of the synthesized films decreased significantly with increasing carbon content in the films. However, the capacity did not fall with increased film thickness as would be expected. This result is described in Sung et al. as an indication that it is possible to increase the Si amount without any side effects of the growth of Si size, which has been a severe limitation for high specific capacity via a chemical vapour deposition process.

Document CN 115 881 931 Al discloses a composite material for a secondary lithium battery as well as a preparation method and application of the novel composite material. The novel composite material comprises nano silicon and carbon atoms, and the carbon atoms are uniformly distributed in the nano silicon at an atomic level; carbon atoms and silicon atoms are combined to form amorphous Si-C bonds, and no SiC crystallization peak exists in X-ray diffraction spectroscopy (XRD); a solid nuclear magnetic resonance (NMR) detection 29 Si NMR spectrum of the novel composite material shows that when the peak of silicon is located between - 70 ppm and - 130 ppm, a resonance peak of Si-C is located between 20 ppm and - 20 ppm, the resonance peak of Si-C and the area ratio of the silicon peak is (0.1, 5.0). The average particle size Dso of the novel composite material is disclosed to be from 1 nm to 50 pm, and the mass of the carbon atoms accounts for 0.5 - 50 % of the mass of the novel composite material.

It is well known (see for example Nilsen and Kleiv (2020), [Ref 8]) that crystalline silicon can be milled in order to make particles smaller and with a fairly narrow size distribution when measured as D90/D10. Such particles, however, will get a very different and much less attractive shape as can be seen in Figure 16. In the milling processes of crystalline silicon in [Ref 8], they could achieve D90/D10 at 5 for a material with D50 at 1 micron, but the BET surface area was as high as 32.6 sqm/gram - a parameter that will lead to very low (and non-suitable) first cycle efficiencies in a Lithium anode.

Objective of the invention

The main objective of the invention is to provide a particulate silicon- and carbonbased composite material having a relatively narrow particle size distribution and a relatively low surface area, and which have a favourable charge/discharge capacity and first lithiation efficiency when used as the active material in negative electrodes in rechargeable lithium-ion batteries.

A further objective of the invention is to provide a method for manufacturing said secondary composite particles.

Description

The present invention may be considered being an improvement of the secondary particles described in co-pending European patent application No. EP22158616.7.

The particles described in EP22158616.7 are manufactured by injecting a homogeneous gas mixture comprising a first silicon containing precursor gas and a second carbon containing precursor gas at an atomic ratio Si : C in the range from near zero up to 10 into a reactor space holding a reaction temperature in the range of 500 to 1200 °C, most preferably 700 to 900 °C. The gas mixture of precursor gases may be preheated to a temperature in the range of less than 300 up to 500 °C. XRD analysis of the resulting particles finds that they are a composite of an amorphous silicon and carbon matrix having nanoscaled domains of amorphous silicon embedded therein. The particle sizes are in the range from 10 nm to 1 pm and the particles are described to have a Si : C ratio in the range of [0.2, 7], most preferably [1, 4], This corresponds to a total total carbon content in the particles ranging from 12.5 to 83.3 at% (5.8 to 68.1 wt%).

In one example embodiment of EP22158616.7, the precursor gases were silane and ethene in a molar ratio Si : C of 1 : 1 preheated to about 400 °C. The gas mixture was injected into a reactor space containing nitrogen gas heated to a temperature at which the resultant gas mixture inside the reactor had a temperature of 810 °C. The residence time was less than 0.5 seconds. The resulting particles are described to be a composite of an amorphous silicon and carbon matrix of particle size in the range from 10 nm to 1 pm and which have a plurality of nanosized domains of amorphous silicon having an average crystallite size of 1.7 nm embedded therein. The overall carbon content of the particles was 35 atom%. Table 1 of EP22158616.7 lists measured first cycle capacities and cyclabilities for samples of the particles having a total carbon content ranging from 14 to 35 atom%. The table shows that the particles with the highest carbon content (of 35 atom%) had the best cyclability of 621 cycles until 20 % decreased capacity but the poorest first cycle capacity of 1000 mAh/g, while the particles with the lowest total carbon content of 14 atom% had the poorest cyclability of only 36 cycles and the best first cycle capacity of 2200 mAh/g.

Note on terminology

In EP 22158616.7, there were applied three adjectives “primary”, “secondary” and “tertiary” when referring to the particles. These adjectives refer to the level in the assembly of the particles, and not to any number of elements composing the particle. A primary particle is a particle that may later be embedded in a secondary particle but does not contain any particle other than itself. A secondary particle is a particle comprising a plurality of primary particles. A tertiary particle is a particle comprising a plurality of secondary particles.

Since the present application focuses only on the “secondary” level of assembly, it is for clarity reasons preferred herein to apply the terms “nanoscaled domain” instead of “primary particle” and “silicon-based composite particle” instead of “secondary particle”. Thus, the present term “nanoscaled domain” corresponds to the “primary particle” of EP 22158616.7, and the present term “silicon-based composite particle” corresponds to the “secondary particle” of EP 22158616.7.

This does not, however, imply that the present nanoscaled domain is necessarily identical to the prior primary particle, and similar that the present silicon-based composite particle is necessarily identical to the prior secondary particle. There may in certain embodiments of the present particles be differences in chemical composition and/or structure as compared to the primary and secondary particles of EP 22158616.7.

Silicon-based composite particles

In a first aspect, the invention relates to silicon-based composite particles, wherein each of the silicon-based composite particles:

- is made of a bulk material comprising silicon and carbon, and

- has a plurality of nanoscaled silicon domains embedded in the bulk material, characterised in that the silicon-based composite particles have a total chemical composition comprising:

- from 2.3 to 60.9 atom% (approx. 1 to 40 wt%) carbon, based on the total mass of the silicon-based composite particles, and

- the rest being silicon and unintentional impurities, and wherein the silicon-based composite particles: - have a median volume-weighted diameter, Dso, in the range from 1 to

9 gm, and

- a volume weighted diameter ratio D90/D10 in the range from 1 to 9, and wherein

-the Dio, D50, and D90 volume weighted particle diameters are determined by laser diffraction analysis according to standard ISO 13320:2020.

The term “total chemical composition comprising an element” and/or the term “in a total amount of an element” as used herein encompasses the entire content of that element in all constituents and parts of the present silicon-based composite particle from its core centre to its surface, but not included an eventual surface coating laid onto the particles. An eventual content of that element in a surface coating laid onto the particle is excluded from the total content of the particle.

A technical distinction common to all embodiments of the present silicon-based composite particles in view of the prior secondary particles of EP 22158616.7, is that due to an improved control with the temperature and flow conditions during the nucleation and growth of the particles, that they may be made larger and still have a relatively narrow size distribution by being specified to have a median volume- weighted diameter, D50, in the range from 1 to 9 gm and a D90/D10 ratio of from 1 to 9. The BET surface area of the particles are measured to be in the range from 0.4 to 5 m²/g, as determined by standard ISO 9277:2010.

In one embodiment, the silicon-based composite particles of the invention may advantageously have a median volume-weighted diameter, D50, from 1.1 to 8 pm, more preferably from 1.2 to 7 pm, more preferably from 1.6 to 6 gm, and most preferably from 2.0 to 5 gm, as determined by laser diffraction analysis according to the standard ISO 13320:2020.

The term “comprising silicon and carbon” as used herein refers to the

Laser diffraction analysis according to the standard ISO 13320:2020 may also be applied to provide the volume-weighted diameter particle size distribution Dio and D90 of the silicon-based composite particles and be applied to determine the D90/D10 ratios.

In one embodiment, the silicon-based composite particles of the invention may advantageously have a D90/D10 ratio in the range from 1.5 to 8, preferably from 2 to 7, more preferably from 2 to 6, and most preferably from 3 to 5, as determined by standard ISO 13320:2020.

In one embodiment, the total chemical composition of the silicon-based composite particle may advantageously comprise carbon in a total amount of from 2.3 to 50.1 atom% (approx. 1 to 30 wt%), preferably from 4.5 to 47 atom% (approx. 2 to 27.5 wt%), more preferably from 6.7 to 43.8 atom% (approx. 3 to 25 wt%), more preferably from 11 to 36.9 atom% (approx. 5 to 20 wt%), more preferably from 15 to 33.9 atom% (approx. 7 to 18 wt%), and most preferably from 18.8 to 29.2 atom% (approx. 9 to 15 wt%), and where the rest is silicon and unintentional impurities.

Higher carbon contents give higher stability advantageous for some user applications of the particles while lower carbon contents give higher capacity - as preferred for other user applications.

The relatively large particle size of the present silicon-based composite particles is advantageous because it enhances the first cycle efficiency. After fabrication and assembling of the secondary lithium ion battery it needs to go through a first charge/discharge cycle (often labelled as the formation). During the formation, lithium ions are released during the charging stage by the active material of the positive electrode, migrating through the electrolyte and are absorbed by the active material of the negative electrode. Then, during the discharge state, the opposite is taking place. However, it turns out that there are less lithium ions returning to the positive electrode during the discharging stage than which left the positive electrode during the charging stage. This loss of (available) lithium in the battery during the first cycle is known to increase linearly with the total surface area of the active material, and may typically, depending on type of electrode being applied, constitute a reduction in the energy storing capacity of the battery of between 2 and 20 %. Lee et al. (2016) [Ref 7] has estimated this loss of lithium due to the formation of the solid electrolyte interface layer during the first cycle to be about 10 mAh/m² surface area.

The relative narrow particle size distribution of the present silicon-based composite particles provides the advantage, especially when the Dso diameter is relatively high, that there will be relatively few “oversized” particles, i.e. that the D99 fraction of particles does not become so large to significantly reduce the capacity and cyclability of the material. The larger the particles become, the longer the diffusion distances become for the lithium atoms moving in and out of the particles, while the current density over the surface increases and thereby adds charge transfer resistance, both mechanisms increasing the risk of lithium being diffusively trapped inside the particles. Too large particles are thus partially inactive as active material. The largest particles also risk having too large absolute expansion, which may lead to delamination or other damages to the cell. Furthermore, a narrow size distribution also means that there are few very small particles, i.e. particles with much higher surface area per weight that thereby contribute to lithium losses during the formation cycles.

Thus, the relatively low D90/D10 ratio combined with the relatively large D50 particle size of the silicon-based particles of the invention provides the combined effect of providing an active material with relatively few of the smallest size fraction that lead to relatively high FCE losses and also relatively few of the largest size fraction being partially “dead” as active material, and enables thus employing particles with a relatively large average diameter giving a favourable first cycle efficiency without compromising on the charging/discharging rate nor compromising on the stability.

The present silicon-based composite particles are, as the prior secondary particles of EP 22158616.7, believed to be composite particles where each particles has a “raisins in a bun dough”-resembling structure with a plurality of tiny nanoscaled silicon domains (the “raisins”) being embedded in the bulk material of silicon and carbon (the “bun dough”). The term “composite particle” as used herein refers thus to the plurality of nanoscaled silicon domains, each constituting a first chemical phase, being distributed in the bulk material comprising silicon and carbon, which constitutes at least one second chemical phase of the particle. While the local variations in the silicon and carbon in the matrix may vary with production history and subsequent heat treatments, the nanoscaled silicon domains are expected to be the main contributors to the lithium storage capacity.

Experimental experience and testing of the particle formation indicates that the reaction temperature may alter the reaction kinetics and mechanisms during growth of the silicon-based composite particles. This is supported by e.g. the temperature dependency of the silanes and of hydrocarbons, more precisely the fact that Si-H bond vibrations of the silanes are activated at lower temperature than the C-H bonds of hydrocarbons.

Without being bound by theory it is believed that the early stage of homogeneous nucleation, where the nanoscaled domains of silicon are formed, results in liquidlike or semisolid droplets that are somewhat fluid at the reaction temperature. When carbon-containing precursor gas becomes energetically available for the reaction, it is assumed to form a layer of amorphous SiC and/or C around these droplets and thereby prevent the silicon nanoscaled domains to combine and/or grow to larger silicon domains. The silicon domains are forced by the carbon layers to remain small in the nanoscale, such as shown in Sung el al. (2021) [Ref 6],

These droplets of liquid or semiliquid silicon having a carbon layer then experience two competing processes - a droplet agglomeration (driven by Brownian motion) and a solidification (gradual hydrogen removal from the Si or C precursor molecules). Droplets that meet before solidification (of at least one of the droplets) will agglomerate into a near spherical silicon-based composite particle. The agglomeration process is only weakly dependent on temperature, but the droplet formation, gas consumption and solidification processes (hydrogen removal) are all exponential functions of temperature. Thus, at relatively high reaction temperatures above 700 - 750 °C, it is believed that initially formed droplets solidify to nanoscale silicon domains and release hydrogen so rapidly that they only have time for a limited agglomeration before the agglomerates become less fluid and particle growth becomes limited. Also, the high temperature can initiate a high number of particles, so there is little gas left around the particles to continue particle growth after the initial droplet formation. The CVD growth of carbon and silicon on the already formed droplets is also occurring much faster at higher temperatures. This is believed to be why it is observed that particles made at high temperatures (of at least 700 - 750 °C) and otherwise identical conditions tend to produce small particles (down to a few tenths of nm in diameter) and which have higher carbon contents as compared to particles formed at decomposition temperatures below 700 °C given the same gas mixture. The “bun dough” in this case (with high temperature) comprises a silicon and carbon alloy.

At decomposition temperatures below 700 °C, down to about 450 °C, the reduced temperature delays the solidification of the droplets. The reaction time will thus be increased, and more nanoscale domains can gather into a relatively large more, or less spherical agglomerates. At the same time, the gas consumption rate is also reduced, so a longer residence time is required to obtain commercially relevant precursor conversion rates. If the reaction temperature and time can both be controlled, it is thereby possible to grow relatively large particles and control the particle size. The silicon-based composite particle is this case believed to mainly consist of densely packed “raisins” of nanoscaled silicon domains having an amorphous SiC and/or C layer along the boundaries in-between the “raisins”. The “dough” (bulk material) comprises in this case an amorphous SiCx and/or a few atoms thin, amorphous C layer lying at a boundary in-between a densely packed agglomerate of nanoscaled silicon domains. This provides a relatively high volumetric silicon density in the particle mass in the form of a relatively large plurality of nanoscaled silicon domains. This structure is believed to be somewhat similar to the structure described by the silicon and carbon containing film disclosed in Sung el al. (2021) [Ref 6] to be laid onto a micrometre sized carbon particle, however with the essential distinction that the present particles has this composition throughout the entire particle from its core to the outer surface.

Thus, in one embodiment, the bulk material comprising silicon and carbon is believed to be a network of amorphous SiCx surrounding the nanoscaled silicon domains. This amorphous SiCx may include Si atoms covalently bonded to both C and Si atoms, as well as C atoms bonded to both C and Si atoms. Most of the carbon atoms in the bulk material are expected to be bonded to at least one silicon atom. This clearly differentiates the product from prior art powders produced by the deposition of silicon particles into a porous carbon matrix. In another embodiment, said amorphous SiCx may be partially crystallized into nanoscaled SiCx domains in addition to the nanoscaled silicon domains. Thus, the term “bulk material comprising silicon and carbon” as used herein means that the bulk material may encompass C, Si, and/or SiCx phases/domains in the bulk material.

In one embodiment, the bulk material of the silicon-based composite particles according to the first aspect of the invention consists of silicon and carbon present in the form of C and Si, and/or SiCx phases/domains.

Particles that meet after solidification remain separate particles. Particles that meet at the intermediate stage between ‘fluid’ and solid, can agglomerate together into large clusters of small particles, where laser diffraction or DLS may give a large value, while BET indicates a much smaller particle size.

Furthermore, when the particles are made by decomposition of silicon and carbon containing gases and subsequent seed formation and particle growth by agglomeration and/or chemical vapour deposition, the relatively large particle sizes and narrow size distribution of the present composite particles may be obtained by stimulating rapid heating to the (relatively low) target process temperature to initiate condensation combined with applying sufficiently long residence times, low turbulence in the gas phase, and controlling the flow conditions in the reactor such that at least the major part of the particles attain the same or nearly the same residence time.

An advantage of applying condensation and chemical vapour deposition of a mixture of a silicon containing precursor gas and a carbon containing precursor gas in a protected atmosphere is that the method gives excellent control with which elements are being introduced into the reactor and thus which elements that will be present in the produced particles. For example, if the silicon-containing precursor gas is a silane and the carbon-containing precursor gas is a hydrocarbon, it will mainly be the elements hydrogen, carbon, silicon and usually an inert gas element (nitrogen, argon, etc.) and only very minor amounts of oxygen (remains of air) inside the reactor.

Furthermore, since the particle according to the invention may in one embodiment be made by condensation and chemical vapour deposition of hydrogen-containing precursor gases, the resulting particles may contain remains/traces of hydrogen also in the final product. Theoretically, the particle can be expected to become solid when the hydrogen content falls to < 1 atom per silicon or carbon atom. The hydrogen bonded to silicon is likely to leave at lower temperature than that bonded to carbon, so a hydrogen content < 1 atom per carbon should be easily achievable. Ideally, the hydrogen should be removed, as it may contribute to HF formation when interacting with the LiPFe salt in the electrolyte of a Li-ion battery. The term “unintentional impurities” as used herein may, in an embodiment, thus encompass as much as 30 atom% hydrogen but is preferably less than 1 atom%. In one embodiment, the unintentional impurities may encompass less than 30 atom % H, preferably less than 20 atom% H, more preferably less than 15 atom%, more preferably less than 10 atom%, more preferably less than 5 atom%, and most preferably less than 1 atom%.

Ideally, there should be no oxygen in the particles because the presence of oxygen causes loss of (irreversible bonding) of lithium atoms and reduces the first cycle efficiency of a LIB applying the particles as the active material in the negative electrode. However, in the practical life, it may be difficult to shield the particles from coming into any contact with ambient oxygen such that the term “unintentional impurities” as used herein may in an embodiment encompass as much as 4 atom% oxygen but is preferably less than 0.5 atom%. Thus, in one embodiment, the total oxygen content of the particle according to the invention is less than 4 atom%, preferably less than 3 atom%, more preferably less than 2 atom%, more preferably less than 1 atom%, and most preferably less than 0.5 atom%.

Thus, the produced particles may in an embodiment mainly consist of silicon and carbon, and unintended impurities comprising oxygen and/or hydrogen, some small amount of hydrogen, and eventual traces of unintentional impurities and oxygen. When measuring a practical product, however, one will frequently measure 0.3 - 4 wt% oxygen, but this is due to air exposure occurring after the production is completed, for example in the preparation of the characterization sample.

In one especially preferred embodiment, the total chemical composition of the silicon-based composite particles comprises:

- from 2.3 to 50.1 atom% (approx. 1 to 30 wt%) carbon, preferably from 6.7 to 43.8 atom% (approx. 3 to 25 wt%) carbon, more preferably from 11 to 36.9 atom% (approx. 5 to 20 wt%) carbon, more preferably from 15 to 33.9 atom% (approx. 7 to 18 wt%) carbon, or most preferably from 18.8 to 29.2 atom% (approx. 9 to 15 wt%) carbon, based on the total mass of the silicon-based composite particles , from 0.1 to 4 atom% oxygen, preferably from 0.2 to 3 atom% oxygen, more preferably from 0.3 to 2 atom% oxygen, more preferably from 0.4 to 1 atom% oxygen, or most preferably from 0.5 to 1 atom% oxygen,

- less than 30 atom% hydrogen, preferably less than 20 atom% hydrogen, more preferably less than 15 atom% hydrogen, more preferably less than 10 atom% hydrogen, more preferably less than 5 atom% hydrogen, and most preferably less than 1 atom% hydrogen, and

- the rest being Si and unintentional impurities.

The determination of the total silicon, and/or carbon, and/or hydrogen and/or oxygen content of the silicon-based composite particles may be obtained by e.g. atomic absorption (AA), inductively coupled plasma-mass spectrometry ICP-MS (ICP-MS), ICP-optical emission spectroscopy (ICP-OES), or X-ray fluorescence analysis (XRF). The total carbon content of a secondary particle may also be determined by combusting a sample of the particles and measuring/determining the amount of carbon dioxide being formed. For coated particles, the determination of the elemental composition of the particle inside the coating can be determined by using a Focussed Ion Beam Scanning Electron Microscopy or Tunnelling Electron Microscopy (FIB-SEM or FIB/TEM) cross section combined with Electron dispersive Spectroscopy (EDS) and/or Electron Energy Loss Spectroscopy (EELS) elemental analysis techniques. These are also well known techniques mastered by persons skilled in the art. The hydrogen content can be estimated by pyrolyzing the sample under inert atmosphere and measuring the emitted hydrogen gas with a mass spectrometer. These are well known techniques mastered by persons skilled in the art.

The silicon-based composite particles may be used directly in an anode. In a preferred embodiment, the silicon-based composite particles may further comprise an outer coating for protection against oxidation in air or reaction with the electrolyte and may be used directly in an anode. In a preferred embodiment, the anode may further comprise battery grade graphite or other carbon allotropes, so that the silicon-based composite material of the invention and the graphite/carbon both contribute to storage of lithium. Battery grade graphite is commercially available from many vendors, using various production methods.

In an embodiment, the silicon-based composite particle may further comprise an outer coating. The outer coating may for example be intended to enhance stability against oxidation, to enhance electron conductivity, or to provide good strong chemical bonding between matrix and particle, or to improve dispersion properties. The outer coating may further be optimized to facilitate electron transport, lithium transport, and/or the charge transfer reaction. The outer coating may further be optimized to obtain a thin and stable SEI layer. The outer coating material may comprise carbon, metal-organic frameworks, organic molecules, oxides like LixSiyO, TixO, AkO, or a metal-organic framework, or any combination thereof. The coating may have a thickness in the range of from 1 to 100 nm, preferably from 2 to 60 nm, more preferably from 3 to 20 nm, and most preferably from 3 to 10 nm. The coating may be applied using wet chemical methods, CVD, ALD or other techniques.

In one preferred embodiment, the particle according to the invention further comprises a surface coating of an amorphous or crystalline carbon layer having a thickness in the range of from 0.5 to 10 nm, preferably from 1 to 5 nm, and most preferably from 2 to 3 nm as determined by Auger spectroscopy. In particular, carbon coatings in the range 1-10 nm can be fairly accurately characterized using Auger spectroscopy. Also, FIB-TEM cross section images will clearly show the carbon coating as separate from the particle, with a clear contrast between the heavy silicon atoms and lighter carbon atoms. This characterization can be performed by any person skilled in the art. Similarly, any oxide coating can be clearly separated from a silicon based bulk by the same methods.

In one embodiment, the coating applied to the silicon-based composite particles is made by exposing the silicon-based composite particles to a carbon containing gas and heating to a coating temperature where said gas reacts with the silicon-based composite particles . In one embodiment, the coating temperature is from 30 °C to 1200 °C, preferably from 300 °C to 1000 °C, and most preferably from 600 °C to 900 °C.

The above described silicon-based composite particles are typically shaped spherical or near spherical. However, the particles may agglomerate into singlebranched or multi-directional chains of particles. These chains may be broken by a gentle milling process, i.e. a milling process with forces (feeding and grinding pressure) that are sufficient to break the chains so that the resulting particle size distribution becomes more homogenous, but not so large that the majority of the particle surfaces afterwards have been altered by the milling process.

Thus, in one embodiment the silicon-based composite particles according to the first aspect of the invention are subject to a gentle milling which remains their largely spherical shape of the particles.

As used herein, the term “largely spherical” means that the particles are shaped such that their geometry satisfies the following criteria:

- at least 50 %, preferably at least 60%, more preferably at least 70 %, more preferably at least 80 %, more preferably at least 90 %, and most preferably at least 95 % of all subsections of particle circumference lines apparent in a SEM picture of the material are circular arcs having an arc length, s, being determined by the relation s = r-7t/180-9, where r is the radius of a circle having the same curvature of its circumference line as said circular arc, 7t is a mathematical constant equal to the ratio of a circle’s circumference line to its diameter, and 0 is the angle in degrees which said circular arc subtends at the centre of the said circle, and

- said angle 9 is in the range from 10 to 360°.

In one embodiment, the silicon based composite particles exhibit a BET surface area, as determined by standard ISO 9277:2010, from 0.2 to 10 m²/g, preferably from 0.3 to 8 m²/g, more preferably from 0.4 to 6 m²/g, and most preferably from 0.5 to 4 m²/g. Nanoscaled domains

A further distinction over the secondary particles of EP 22158616.7 is that the nanoscaled silicon domains (labelled as primary particles in EP 22158616.7) of the present silicon-based composite particles is substantially pure silicon phases. Due to the presence of carbon in the precursor gas mixture, it may be that some carbon may enter these nanoscale silicon domains, however probably in minor amounts. Determination of crystal lattice constants by XRD analysis (after a rapid crystallization at e.g. 900 °C for 30 min) matches the crystal lattice constants of pure silicon.

Also, based on experience from for example carbon gettering in solar silicon, the inventors are not aware of reports of high carbon mobility at these temperatures. This is an indication that the carbon content, if present at all in the nanosized domains, is very low. XRD analysis of the particles before crystallization matches that of amorphous silicon fairly well, while electron energy loss spectroscopy (EELS), averaging over slightly larger areas that contain both nanosized domains and the binding matrix, documents the presence of the carbon. This may indicate that there is some carbon present in the nanosized domains. The depth variation of the C:Si ratio in the volume contributing to a single data point, will also contribute to some uncertainty in the EELS data. Furthermore, if the silicon originates from SiH4 or other hydrogen-containing precursor gas, there may also be a remaining fraction of H in the nanoscale silicon domains.

Thus, the term “nanoscale silicon domains” as used herein refers to nanoscaled silicon particles which may contain minor amounts of carbon and/or hydrogen and/or oxygen. The silicon content is believed to be at least 90 atom% and may be as high as 98 to 100 atom%, based on the mass of the nanoscale silicon domains.

The average diameter of the nanoscale silicon domains is typically in the range from 0.5 to 10 nm. In one embodiment, the nanodomains of predominantly silicon constituting the nanosized domains have an average diameter between 1 and 8 nm, more preferably between 2 and 7 nm, more preferably between 3 and 6 nm, and most preferably between 4 and 5 nm as determined by Rietveld refinement of X-ray powder diffraction (XPD) data after, if necessary, exposing the silicon-based composite particles to a heat treatment which crystallizes the nanosized silicon domains therein.

An advantage of the secondary particles according to the invention is that they may be manufactured at temperatures at which the predominantly silicon nanodomains constituting the primary particles are amorphous. Lithium batteries made from amorphous silicon have an improved tolerance for the volume changes associated with lithiation cycles as compared to crystalline silicon. However, the predominantly silicon nanodomains constituting the primary particles may alternatively be crystalline or a mixture of amorphous and crystalline nanodomains.

An advantage of nanoscaled silicon domains are that they are believed to increase the lithiation capacity without compromising the cyclability when being applied as the active material in a secondary lithium ion battery. The lithiation capacity is increased by the silicon domains providing a high capacity storage volume for lithium atoms and the cyclability is increased due to nanoscaled silicon being more robust and endures the volumetric fluctuations associated with lithiation/delithiation cycles much better than larger silicon domains. Furthermore, since the lithiation of the silicon domains implies a substantial opening/breaking of Si-Si bonds, it is also regarded as being highly attractive that the C-C bonds remain intact in the lithiated state and thereby keep the particle structure in place.

X-ray diffraction (XRD) (when XRD is applied on particulate material it may also be denoted as powder X-ray diffraction (PXD) or X-ray powder diffraction (XPD) in the literature) give different diffraction patterns for crystalline and amorphous materials, respectively. Crystalline materials, due to their high degree of ordering and symmetry in their atomic structure, tend to give sharp peaks, Bragg peaks, in XRD-measurements. For crystalline silicon materials, the XRD-analysis typically gives sharp peaks at 28.4°, 47.4°, and at 56.1° in the measured diffraction patterns. In comparison, amorphous materials which lack the long-range order characteristic of crystalline atomic structures, typically gives broader peaks being significantly more “smeared-out” in the measured diffraction patterns. Amorphous silicon typically gives rounded peaks at 28° and 52°. These rounded peaks can be fitted with a Gaussian fit to reduce noise, and to get a well-defined value for the maximum and the width of the peak. Such a fit can be performed by any skilled XRD operator.

Also, the “sharpness” of a peak may be applied to distinguish between crystalline and amorphous materials. The typical Full Width at Half Maximum (FWHM) of an XRD-peak for crystalline silicon is typically less than 2° - 4°, while the FHWM for amorphous silicon is typically larger than 4° when measured with a diffractometer applying unmonochromated CuKa radiation and using a Gaussian fit. Full width at half maximum (FWHM) is the width of the peak curve measured between those points on the -axis which are half the maximum amplitude of the peak curve (after subtracting the background signal and/or signal from the sample holder). Samples containing both amorphous and crystalline silicon will obtain a diffraction pattern in XRD-analysis showing both sharp Bragg-peaks typical of the crystalline phase and the broader, more Gaussian peaks typical for the amorphous phase. The diffraction pattern may be applied to estimate the crystalline fraction of the sample from the ratio of area under the Bragg peak(s) above an amorphous broad peak and the total area of the broad peak and the Bragg peaks. A linear background should be subtracted from the calculation prior to the calculations.

The angles and angle tolerances in the XRD analysis as applied herein refer to use of a diffractometer applying unmonochromated CuKa radiation since the radiation has high intensity and a wavelength of 1.5406 A which corresponds well with the interatomic distances in crystalline solids making the analysis sensitive to presence of crystalline phases in the silicon particles. XRD analysis applying diffractometers with CuKa radiation is for the same reason the natural choice and thus the most widely used method in XRD analysis and is well known and mastered by the skilled person. Other diffractometers applying radiation with other wavelengths which may give different angles and angle tolerances. However, the skilled person will know how to convert these values from one radiation source to another.

Furthermore, crystalline nanodomains of silicon (less than a few hundred nanometres) gives a characteristic peak broadening of Bragg peaks obtained by X- ray powder diffraction (XPD). This peak broadening may be applied to determine the average diameter of the nanosized domains (embedded in the silicon-based composite particles ) by e.g. a Rietveld refinement of the XPD data from an XPD- analysis of the silicon-based composite particles in a single-modal or multimodal distribution.

If some or all of the nanodomains constituting the nanosized domains are amorphous, the particle size determination should include an initial heat treatment to crystallise the nanodomains constituting the nanosized domains, such as e.g. a heat treatment at 900 °C for at least 30 minutes.

Rietveld analysis of XPD data to determine average particle sizes is well known and mastered by the person skilled in the art. An example of such analysis may e.g. involve fitting calculated XPD data from a model of crystalline Si to experimental data obtained by a XPD measurement of a sample of the silicon-based composite particles with the least-squares method; a so-called Rietveld refinement. The Rietveld refinement can be performed with freely available software such as GSAS- II [Ref 5] or commercial software such as Topas [Ref 6], The instrumental contribution to the width of the Bragg peaks should either be calculated from the geometry of the instrument (“fundamental parameters approach” [Ref 7]) or be described by a Thomson-Cox-Hastings pseudo-Voigt function [Ref 8] determined experimentally from a highly crystalline standard material such as NIST SRM 640f silicon. The instrumental contribution to Bragg peaks is kept fixed during the Rietveld refinement. All additional broadening of the observed Bragg peaks is assumed to be due to small crystallite size and to have Lorentzian shape. This crystallite size broadening is modelled by refining an additional contribution, P, to the calculated Bragg peak widths which varies with the scattering angle as: 360

7T² where X is the X-ray wavelength used in the XPD measurement. P is the additional full-width-at-half-maximum (FWHM) of a Bragg peak at scattering angle 29, i.e. the width in degrees halfway between the top and the bottom of the peak. The value of T, the average crystallite diameter/size, is allowed to vary freely during the Rietveld refinement and converge to the value that gives the best agreement between the experimental and calculated XPD data.

Product-by-process

In a second aspect, the invention relates to particles produced by the method according to the third aspect of the invention.

Method of manufacturing

In a second aspect, the invention relates to a method for manufacturing a silicon- based composite particle according to the first aspect of the invention, wherein the method comprises the steps of

- applying a reactor having a decomposition compartment containing a first reactor gas having an initial pressure in the range from 5 - 10³ to 6 - 10⁵ Pa and a first reactor temperature in the range from 450 to 650 °C,

- forming a precursor gas mixture comprising a first precursor gas of a silicon containing compound and a second precursor gas of a carbon containing compound, wherein the atomic ratio between silicon and carbon, Si:C, in the precursor gas mixture is in the range 0.2 to 50,

- injecting the precursor gas mixture into the decomposition compartment and mixing it with the first reactor gas to form a second reactor gas mixture, characterised in that the method further comprises:

- maintaining the second reactor gas mixture inside the decomposition compartment for a residence time in the range from 10 to 300 seconds while maintaining the temperature of the second reactor gas mixture to be within ±20 °C, preferably within ±10 °C, more preferably within ±5 °C, and most preferably within ±1 °C from the first reactor temperature, and

- extracting the particles from the decomposition compartment.

An essential feature of the present particles is that they shall have a relatively large Dso of from 1 to 9 pm and a relatively narrow particle size distribution of a D90/D10 ratio in the range from 1 to 9. This is obtained by controlling the flow conditions such that the major part, preferably all particles, in the gas phase inside the reactor are exposed to as close as practically possible the same average temperature conditions and endurance (residence time) before being extracted and cooled.

In a regular continuous flow reactor, thermal convection flows will often become dominant when the reaction time goes above seconds, making it difficult to obtain laminar flow. The residence time for any single atom or particle in the reactor can vary greatly depending on the path taken. This means that more precursor gas can leave the reactor without being consumed, or that many particles exit the reactor while still being very small. At the same time, there is no obvious upper limit to particle size, as some particles may be cycled around in the reactor for a long time.

Thus, the reactor may be a tubular hot-wall reactor where a constant stream of preheated mixture of precursor gases and the preheated first gas are simultaneously injected in one end, and where the flow conditions through the reactor are laminar to ensure a predictable travel time before the resulting particles are extracted at the opposite end.

Another way of ensuring more or less equal processing conditions is to inject the mixture of preheated precursor gases into a closed hot wall reactor and either extract the entire batch when obtaining the intended residence time by flushing or vacuum suction, or to allow the particles to grow to be so heavy that they settle out of the gas phase and then be collected and extracted. In particular, rapid acceleration at a corner may lead to heavier particles settling out, as in a cyclone particle separator.

The latter solution of letting the particles grow until they fall out of the gas phase may have a profound sorting effect on the produced particles. This sorting effect may be sufficiently strong to obtain a narrow particle size distribution having the intended D90/D10 ratio also when including the smaller size fraction particles which remain entrained in the gas phase and that exit the reactor together with the gas they are entrained in when the particles are collected.

The term “first precursor gas of a silicon containing compound” as used herein means any silicon containing chemical compound being in the gaseous state and which reacts to form Si-particles at the intended reaction temperatures. Examples of suited first precursor gases include, but are not limited to, silane (SiEL), disilane (Si2He), and trichlorosilane (HCESi), or a mixture thereof.

Likewise, the term “second precursor gas of a carbon containing compound, ” as used herein means any chemical compound containing carbon that causes C-atoms to be incorporated into the matrix surrounding the Si-particles being formed when heated to the intended reaction temperatures. Examples of suited second precursor gases of a carbon containing compound include, but are not limited to alkanes, alkenes, alkynes, aromatic compounds, and mixtures thereof. In example embodiments, the second precursor gas of a carbon containing compound may be is at least an organosilane or a hydrocarbon, preferably methane (CEL), ethane (C2H6), propane (CsHs), ethene (C2H4), ethyne (C2H2), cyclohexane, cyclohexene, toluene, benzene or mixtures thereof. This gas phase may also contain smaller or larger amounts of hydrogen. Especially preferred example embodiments of precursor gas, i.e. the homogeneous gas mixture of a gaseous silicon and hydrogen compound and a gaseous substitution element C and hydrogen compound, are either silane (SiEU) or disilane (Si2He) mixed with a hydrocarbon gas chosen from one of; methane (CEE), ethane (C2H6), propane (CsHs), butane (C4HIO), ethene (C2H4), ethyne (C2H2), propene (Cs E), butene (C4 E), pentene (C5H10), cyclohexane, cyclohexene, toluene, benzene and mixtures thereof. The partial use of larger and stable ring structures are likely to be preferable as this will likely enhance the ratio of C-C to Si-C bonds in the first matrix.

The production yield in gas phase reaction processes, defined as the ratio of the mass of produced particles over the mass of precursor gas being fed to the reactor, is shown to be dependent on process parameters such as the concentration of the precursor gases in the reaction zone, the reaction temperature, and/or the residence time of the precursor gases in the reaction zone. In general, the higher reaction temperature, the higher dissociation degree of the precursor gases and thus higher production yields. Furthermore, the residence time may also have a profound effect on the production yield. In general, long residence times enables more of the injected gases to react and form particles. The feature of using relatively long residence times to gain large particles is therefore beneficial from a production yield point of view. The present method has thus the advantage of obtaining high production yields which gives the method a significant economic advantage since silicon hydride gases such as silane, disilane etc. are comparatively expensive.

In one embodiment, the method according to the invention may advantageously obtain the maintaining of the temperature of the second reactor gas mixture inside the decomposition compartment by: either

- applying a reactor having a tubular hot-wall decomposition compartment having a first end and a second, and

- injecting a constant volume flow of preheated mixture of precursor gases and a constant volume flow of preheated first reactor gas in the first end forming a constant flow volume of second reactor gas mixture inside the tubular hot-wall decomposition compartment,

- passing the second reactor gas mixture under laminar flow conditions with a Reynolds number of less than 2000 through the tubular hot-wall decomposition compartment from its first to its second end, and

- extracting the second reactor gas mixture including formed particles at the second end, or

- applying a reactor having a closed hot-wall decomposition compartment containing a first reactor gas, and - injecting the preheated mixture of precursor gases into the hot-wall decomposition compartment at a first moment in time forming a second reactor gas mixture, and

- when the residence time is obtained, counting from the first moment in time, extracting both the second reactor gas mixture and formed particles from the hot-wall decomposition compartment by either vacuum suction or flushing, or

- applying a reactor having a closed hot-wall decomposition compartment containing a first reactor gas and a cooled collecting chamber at the bottom of the decomposition compartment,

- injecting the preheated mixture of precursor gases into the hot-wall decomposition compartment forming a second reactor gas mixture, and

- maintaining the second reactor gas mixture in the hot-wall decomposition compartment until the formed particles grow to a size where they settle out of the second reactor gas mixture and falls by gravity into the collection chamber.

The relatively large span in the defined residence time reflects the relatively large span in decomposition temperatures of 450 to 650 °C and the span on particle sizes from 1 to 9 pm which may be applied to form the particles of the invention. From Table 2 below, we have two examples where a residence time of 45 seconds and reaction temperature of 570 and 620 °C gave particles with a Dso of 3.8 and 3.9 pm. A person skilled in the art is able on basis of the present disclosure and common general knowledge to figure out which residence times and temperatures to be applied to make particles of these sizes, and to obtain the intended low particle size distribution by controlling the flow of gases and temperature in the reactor by simple trial and error attempts if need be.

In an embodiment of the invention, the first reactor temperature is in the range from 475 to 630 °C, preferably from 500 to 620 °C, more preferably from 525 to 600 °C, and most preferably from 550 to 580 °C.

In an embodiment of the invention, the residence time may advantageously be in the range from 12.5 to 250 seconds, preferably from 15 to 200 seconds, more preferably from 17.5 to 150 seconds, more preferably from 20 to 100 seconds, more preferably from 25 to 75 seconds, and most preferably from 30 to 50 seconds.

The reaction kinetics in the gas reactions forming the particles from the precursor gases may vary significantly depending on which gases are applied as the first reactor gas, the first and/or the second precursor gas, and the reaction temperature at which the particles are formed such that the atomic ratio C : Si in the precursor gases may deviate significantly from the overall (average) atomic ratio C : Si in the produced particles. Thus, the term “the relative amounts of the first and the second precursor gases are adapted such that the formed particles obtain an atomic ratio C : Si in the range of’ as used herein means that the relative amounts of the first and the second precursor gas being mixed and homogenised is adjusted such that the resulting particles obtain the intended atomic ratio when the precursor gas mixture is heated to the intended reaction temperature and reacts to form the particles.

In an embodiment of the invention the atomic ratio Si : C in the precursor gas mixture may be in the range from 0.3 to 45, preferably from 0.4 to 40, more preferably from 0.5 to 30, more preferably from 0.6 to 25, more preferably from 0.8 to 15, more preferably from 1.0 to 8, more preferably from 1.2 to 5, and most preferably from 1.5 to 2.0.

In an embodiment of the invention, the precursor gas mixture may be preheated to a temperature in the range from 250 to 450 °, preferably from 300 to 390 °C, more preferably from 325 to 380 °C, or most preferably from 350 to 370 °C before injecting the homogeneous gas mixture into the reactor space.

The term “reactor gas” as used herein, encompasses exhaust gas formed by a previous production of the silicon-based composite particles and/or any inert gas at the applied reaction temperature. Inert in this context means being chemically inactive towards the condensed particles. Examples of suited inert gases includes hydrogen, nitrogen, a noble gas like helium, neon, argon, or any other gas that will not chemically react with the precursor gases at the reaction temperature.

After or before the surface coating, the particles may be heat treated to obtain good transport and adhesion properties as well as enhance cycling stability. One can opt for:

- Crosslinking/cyclization only, significant hydrogen remains in the particles and coating, more elastic carbon phase (-300 °C), less stable bulk power, low/medium capacity

- Low temperature annealing and CVD coating (leaving amorphous silicon, allowing faster first cycle charging) (600-800 °C),

- Medium temperature annealing and CVD coating (better Lithium conductivity, better CE, higher capacity, but crystallizing parts or all of the Si nano domains) (800-1000 °C), or

- High temperature carbonization (high carbon diffusion, risk of forming excess SiC at Si-C interfaces, risk of lower capacity, but possibly even better coating quality) (>1000 °C).

In one embodiment, the method according to the invention may comprise depositing an amorphous or crystalline carbon layer onto the outer surface of the particles by chemical vapour deposition of a carbon containing precursor gas, preferably an alkene gas, at a temperature in the range from 650 to 750 °C. The amorphous or crystalline carbon layer may advantageously have a thickness in the range of from 0.5 to 20 nm, preferably from 1 to 10 nm, and most preferably from 2 to 7 nm (as determined by Auger spectroscopy) to enhance the surface properties, electron conductivity, lower fire-risk and promote formation of a stable solid-electrolyte- interface (SEI). The invention is not tied to any particular coating material or method of coating the particles but may apply any coating and coating method known to the skilled person for coating silicon particles.

List of figures

Figure 1 is a diagram showing an XRD analysis of 1,7 nm silicon nano particles according to EP 22158616.7 embedded in a carbon-rich matrix (sample SI or S2) after heat treatment for 30 min at 900 °C. The composite material has an average atomic content of 35% C and 65% Silicon.

Figure 2 is a diagram showing an XRD analysis of 3,4 nm silicon nano particles according to EP 22158616.7 embedded in a carbon-rich matrix (sample S3) after heat treatment for 30 min at 900 °C using a single-modal distribution of crystallite sizes. The composite material has an average atomic content of 16% C and 84% Silicon.

Figure 3 is a TEM picture showing crystalline silicon nanodomains in silicon-based composite particles (sample SI) according to EP 22158616.7 after heat treatment for 30 min at 900 °C.

Figure 4 is a series of TEM pictures showing secondary composite particles according to EP 22158616.7.

Figure 5 is a TEM pictures showing pure silicon nano particles of similar external size as in Figure 4 that did not undergo any heat treatment.

Figure 6 is a diagram showing an XRD analysis particles as produced according to the present invention. The particle has a total carbon content of 14 wt%.

Figure 7a) shows a TEM photograph of an example embodiment of the invention and figure 7b) shows an EELS elemental analysis across the particle.

Figure 8 a) shows a TEM photograph of an example embodiment of the invention and figure 8b) shows an EELS elemental analysis across the particle.

Figures 9 and 10 shows a Fourier transform with band pass-masks centered at 0.314 and 0.252 nm, respectively to highlight crystalline silicon and silicon carbide of the example embodiment shown in the TEM-photo of figure 7a).

Figures I la) and 1 lb) show the size distribution of two powders made with similar temperature and gas mixture. In the S4 powder (grey line), the residence time and temperature in the reactor has been inhomogeneous. In the S5 powder (black line), the residence time and temperature have been according to the present invention. Both size distributions have been measured directly after production of the silicon particles, before coating. Figure I la) is the volume weighted distribution, figure 1 lb) is the area weighted distribution. The volume weighted figure shown that the small particles in the left tail do not contribute much to the capacity of the cell. The area weighted picture shows that the same tail contributes significantly to the SEI formation and thereby reduction of FCE.

Figure 12 shows cycling data for the S6 powder, produced by the method described in this document, with a subsequent 650 °C heat treatment, PAN coating and 500 °C crosslinking of the polymer. The reference is a pure graphite electrode, while the upper line shows a cell with 10wt% silicon in 90 wt% graphite. The cycling program was 4xC/20 - 3xC/10 - 3xC/5 -3xC/3 - 3xC/2 - 3xlC - 3x2C - lxC/20, followed by repeated sets of 2xC/10+20xC/2. The capacity and FCE are measured in a separate cell without graphite to reduce uncertainty, while the graphite helps to avoid electrode delamination, so the silicon degradation can be properly measured.

Figure 13 shows cycling data for the S5 powder, produced by the method described in this document, with a subsequent bitumen coating and 900 °C heat treatment. The reference is a pure graphite electrode, while the upper line shows a cell with 15 wt% silicon in 85 wt% graphite. The cycling program was 4xC/20 - 3xC/10 - 3xC/5 - 3xC/3 - 3xC/2 - 3xlC - 3x2C - lxC/20, followed by repeated sets of 2xC/10+20xC/2. Again, FCE and capacity is measured in separate cells without graphite.

Figure 14 shows cycling data for the S4 powder in three versions:

S4-1 : No heat treatment

S4-2: Heat treatment to 820 °C for 2h

S4-3: Coating with sugar, then heat treatment to 820 °C for 2h

In all cases the powder is mixed 50/50 with graphite

Formation - Rate test (3 cycles each of C/10, C/5, C/3, C/2, C,2C)

Further testing for S4-2: 20x(C/20+10xC/3), after approx. 250 cycles: New rate test, then back to 25x(C/10+10xlC/3)

For S4-3 20x(C/20 + 10xC/2) after approx. 250 cycles (C/10+20xC/2)

The highest point of a slow cycle has been used as the ‘max capacity’ for S4-3.

Figures 15a) and 15b) show SEM pictures of a particle material according to the invention before (figure 15 a)) and after (figure 15 b)) a gentle milling process. In the non-milled material the circumference of all particles can be seen as agglomerates of substantially circular circumferences, while in the gently milled material parts of the circumference of some of the particles are non-circular as indicated by the white arrows. Figure 16 shows SEM images of different milled metallurgical grade crystalline silicon particles [Ref 8] as a function of different milling conditions. As silicon is a brittle material, the surface edges are initially very sharp, while after more extensive milling the particles gradually tend to agglomerate to each other leading to more odd shapes.

Figure 17 shows a subsection of the SEM photograph in figure 15b) and illustrates examples of circumference lines being circular arcs. The circumference parts which are the result of breakage due to the milling process and does not form part of an arc/circle are marked with a dotted line.

Verification of the invention

The invention will be described in further detail by way of example embodiments.

Comparison experiments

This chapter gives a comparison of obtained particle sizes and particle size distributions according to the present invention with particle distributions and particle sizes for samples made by the method described in EP 22158616.7 and further in comparison to prior art silicon particles disclosed in US 2014/225030 Al.

All samples of silicon-based composite particles employed in this comparison were made by condensation and chemical vapour deposition of a mixture of a silicon containing precursor gas and a carbon containing precursor gas (except for US 2014/225030 Al which only applied a silicon containing precursor gas) in a protected atmosphere in a decomposition reactor.

A typical manufacturing process for manufacturing the particles in Cl, C2, C3 presented below may be as follows: A homogeneous first mixture of silane gas and ethene gas in a particular molar ratio was heated to a particular preheating temperature and thereafter introduced into a reactor chamber where the homogeneous precursor gas mixture was rapidly heated to a particular reaction temperature by being further mixed with about 10 times more of a preheated inert gas (nitrogen) to form a second gas mixture. The second gas mixture passed through the reactor for a controlled residence time before the second gas mixture could exit and cooled to stop the gas condensation/chemical vapour deposition reactions. The inhomogeneity in how quickly the different fractions of the gas were diluted/heated led to differences in the size distribution.

The S4 powder was produced in a reactor where there were some differences in the thermal history of different gas fractions, leading to a fraction with a high temperature getting a small size, while the lower temperature fraction got a large size. A typical manufacturing process for manufacturing the particles in the present invention (covering the powders S6, S5, S7 and S8 below) may be as follows: A homogeneous first mixture of silane gas and ethene gas in a particular molar ratio was heated to a particular preheating temperature and thereafter introduced into a reactor chamber where the homogeneous precursor gas mixture was fed into a closed chamber with some preheated gas (remains from previous production batch) to form a second gas mixture. The low reaction temperature ensures that the variations in dilution/heating become less important for the particle production. The second gas mixture is maintained for a given residence time, during which the chemistry is allowed to consume reactant species and increase particle sizes. Finally, the gas is pumped from the chamber, stopping the reaction. This ensures a very similar history for all particles, and a very homogeneous size distribution, while maintaining good silane and carbon precursor yields.

The applied molar ratio of the silane and ethene precursor gases, preheating temperature (of the precursor gas mixture), the reaction temperature (of the second gas mixture), and residence times are given in table 1 or 2 below. Table 1 presents the applied production parameters for “old” secondary particles according to the disclosure of EP 22158616.7, while table 2 presents the applied production parameters for particles according to the present invention. The tables further show the resulting BET of the collected particles and their D90/D10 ratios after an annealing and CVD based coating process to make the particles tolerant to air. The high D90/D10 for sample C2 and C3 can probably be explained by quite uneven gas mixtures due to the short residence time as well as turbulent flows giving variations in residence time.

The collected particles include both the heavy settled fraction (collected from the reactor bottom) and the smaller entrained particles collected by a downstream filtration of the gas flow exiting the reactor. The BET was measured according to ISO 9277:2010. The D90/D10 ratios given in table 1 were determined by laser diffraction according to ISO 13320:2020 (using a Malvern Mastersizer 3000 instrument). A low BET surface area of the particles is vital to obtain a high First Cycle Efficiency (FCE) in the formation cycle of a Lithium battery. A high FCE is important to lower the manufacturing costs of LIB s, Table 1 Applied process parameters and resulting properties of comparison silicon-based composite particles

1) Prior art examples from US 2014/0225030 Al Table 2 Applied process parameters and resulting properties of samples of silicon-based composite particles according to the invention

Example embodiment

A mixture of silane gas and ethene gas was mixed and preheated to 350 °C and fed into a closed reactor containing a first gas having a pressure of about 200 kPa and holding a temperature of 560 °C. The reactor was fed with a flow volume of 8.8 slm (standard litre per minute - equals 0.00073386 mol/second) silane gas and 3.2 slm ethene gas for about one minute. Then the pressure inside the reactor had increased to about 0.9 kPa. The gases were then held inside the reactor for additional 45 seconds before the reactor was opened, and the particles where flushed out, collected, and applied a coating to make them stable in ambient air.

The particles was then analysed by XRD and found to be fully amorphous, as seen by the XRD diagram of figure 6. The Dio, Dso and D90 of a nearly identical later reproduction of this product was measured to be 3.0, 6.9 and 14.3.

The particles where thereafter heat treated at 900 °C for 2 hours to crystallize their domain to determine the domains sizes by XRD analysis and Rietveld refinement. The resulting Si crystallites were estimated at 1.9 nm while the SiC crystallites were estimated at 1.5 nm.

Figures 7a) and 8a) are TEM photographs of two of the particles, of particle size of about 3 and about 0.8 pm, respectively. The photographs show that the particles are very dense, with no discernible porosity, and without any discernible phases. This character is confirmed by an electron energy loss spectroscopy (EELS) based elemental composition analysis taken across the particles along the section marked by two white lines in figures 7a) and 8a), respectively.

The expected nanosized crystalline domains of Si and SiC expected to be found in the heat treated particles are too small to be seen directly on these TEM photographs. The crystal phases may be made visible by a Fourier transformation with band pass-masks centered at 0.314 and 0.252 nm, corresponding to the highest distances between crystal lattice planes in crystalline silicon and silicon carbide, respectively.

Figure 9 is a photograph showing the result with the transformation with the band pass-masks centered at 0.314 nm of the sample shown in the TEM-photo in figure 7a). The transformation was taken at a section of the particle which includes the graphite substrate at which the particle sample were fixed during the analysis. This graphite is barely visible as a somewhat darker area at the lower part of the photograph covering around 10 % of the area. Above this relatively dark area, there are numerous lighter dots which mark the presence of crystalline silicon. Note the relatively even and homogenous distribution. This indicates that the nanosized domains of silicon are evenly distributed all over the particle. The same impression if found in figure 10 which shows the result for the transformation at the same part of the particle with the band pass-masks centered at 0.258 nm and thus marks the presence of nanoscaled domains of crystalline silicon carbide.

These photographs were overlaid each other with the crystalline phases marked in one colour and the crystalline silicon carbide phases marked in a contrasting colour. This result is not included in the application because it would be impossible to discern the phases from each other in a black and white reproduction. However, the overlayed photographs showed that the silicon crystals and silicon carbide crystals are spatially separated and intertwined as would be expected by a composite structure consisting of a densely packed agglomerate of silicon domains having a layer of amorphous carbon and/or amorphous silicon carbide at the grain boundaries between them, and which was crystallized and collected in crystalline silicon carbide regions in-between the crystalline silicon domains by the heat treatment.

These results are therefore considered a confirmation that this example embodiment of the particles, manufactured at the relatively low temperature of 560 °C and pressure below one atmosphere, have a similar composite structure as the film of Sung et al. (2021), but that this composition is homogeneous throughout the entire particle from its core to its outer surface. This homogeneity of nanoscale silicon domains, relatively large particle sizes, and the relatively low particle size distribution makes this particles particularly suited for use as the active material in negative electrodes of secondary LIBs by having a size which gives a high first cycle efficiency, has an even distribution of carbon phases providing excellent electrical conductivity and silicon phases providing excellent lithium diffusivity (conductivity), and at the same time a relatively high silicon loading giving a high volumetric lithium storage capacity by numerous tiny nanoscaled domains which have excellent cyclability.

While there is solid theoretical evidence for the claim that a narrow size distribution is an advantage, the experimental evidence from cell data supports the claim. Because the different powders included as references were not available for rigorous testing, some quantities have been estimated from the available data, others are directly measured. All cell data in the following are from coin cells. We have used a standard CMC binder with a pH controlled aqueous buffer, and a standard carbonate electrolyte with 2% fluoroethylene carbonate as the only additive. This tends to underestimate the FCE, as there are many other surfaces giving losses. The good FCE results demonstrated in this document are therefore expected to become even better in an optimized, commercial battery.

To improve the electron transport properties of the material, samples were coated with a small amount of conductive carbon. In each case, the material was coated with a wet-chemical coating with a solvent and a polymer. Subsequently, the polymer was cross-linked or carbonized to form a conductive carbon network around the particle. For the S6 and S5 samples, the amount of carbon was chosen so that the added carbon made up 2%wt of the sample total after heat treatment. S6 was degassed at 650 °C, then coated with Polyacrylonitrile and crosslinked at 500 °C, while S5 was coated with bitumen and pyrolyzed at 900 °C. As a reference we used data from the smaller and more diverse powder of S4, sugar coated and pyrolyzed to 820 °C for 2h. Several different types of PAN and bitumen precursors have been tested, and the results were not substantially different from those presented here.

For the powders S6 and S5, two cell types were prepared. First, an electrode with only the silicon-based active material was cycled as anode in a full cell, to test capacity and first cycle efficiency of the powder. Second, a similar cell was made, where the electrode contained a fraction of the silicon based material, mixed with a flaky graphite to ensure that delamination or loss of contact in the electrode should not influence results. This cell was used for rate testing and long term cycling stability tests. The FCE of these cells is dominated by the high-BET flaky graphite. The motivation for this type of testing was to obtain reliable data without going through rigorous optimization of electrode processing recipes which would have required very large test batch sizes. It is expected that any commercial battery manufacturer will be able to combine the material with a commercial grade graphite, and through regular optimization efforts will be able to combine the demonstrated cyclability with the demonstrated FCE and capacity.

For the S4 powders we only have access to cells where the powder was mixed 50/50 with a flaky graphite. To assess the capacity and FCE of these cells, we have therefore used a graphite reference cell to evaluate the first cycle lithiation and delithiation capacity of the graphite fraction. These numbers are then used to calculate what capacity and FCE the silicon must have to give the measured total. The results for the S4 calculations are in the table below. Furthermore, because the sample seems to need a couple of cycles to fully activate all the powder, we have recalculated the FCE based on the maximum capacity measured, while assuming all the FCE losses occurred in the first cycle. Thus we are clearly overestimating the FCE of the reference, but we use this best number in the later comparisons.

We can now compare the FCE of the powder with the narrow size distributions with the results of the S4 reference, which was made using the same binder systems, and the same electrolyte.

As can be seen, the powders produced by the disclosed method show astonishingly high FCE numbers, in particular taking into account that they are measured in coin cells. At the same time, the long term cycling shows that the same powders can deliver high capacity for many cycles when incorporated in a suitable electrode. Materials made according to a method similar to sample S5, but in a chamber with a more homogenous temperature control resulted in slightly smaller particles. These have afterwards been exposed to a relatively gentle jet milling with the use of Spiral Jet Mill SSM 100 from Schedio.

An example of the resulting particles can be seen in Figure 15. Most of the surfaces are completely untouched, but the longest chains of particles are broken into more homogenously sized subsections. Using a feed-rate of 1.9 kg/hr a grinding pressure of 3.5 bar and a feeding pressure of 6 bar resulted in the D90/D10 ratio declining from 4.5 to 2.6, and the BET surface area was still only 2 m²/gram. This improvement is probably valuable because it will make it possible to distribute the silicon composite material more evenly in the anode (the largest trolls are gone) when mixed with graphite. The material can also easily be recognized as it consists largely of spherical particles, but among which some of the surfaces are the result of a broken chain or sphere.

Figure 17, which zooms in on a few particles from the SEM photograph shown in figure 15b), illustrates how the majority of the circumference lines of the particles are circular arcs. The circumference parts which are the result of breakage due to the milling process and does not form part of an arc/circle are marked with a dotted line in figure 17.

References

1 Sourice et al. (2016), “Core-shell amorphous silicon-carbon nanoparticles for high performance anodes in lithium-ion batteries”, Journal of Power Sources, vol. 328, pp. 527-535.

2 Orthner et al. (2021), “Direct gas phase synthesis of amorphous Si/C nanoparticles as anode material for lithium ion battery”, Journal of Alloys and Compounds, 870 (2021), 159315, https://doi.org/10.1016/j.j allcom.2021.159315

3 Wang Y.K., Chou S. L., Kim J. H., Liu H. K. and Dou S. X., ‘"Nano-composites of silicon and carbon derived from coal tar pitch: Cheap anode materials for lithium-ion batteries with long cycle life and enhanced capacity“ Electrochim. Acta, 2013, 93 , 213 — 221.

4 Zhu et al. (2018), “Correlation between the physical parameters and the electrochemical performance of a silicon anode in lithium-ion batteries”, Journal ofMateriomics, 5, (2019), pp. 164 - 175, https://doi.org/10.1016/j.j mat.2019.03.005

5 Rhenlund et al. (2017), “Lithium trapping in alloy forming electrodes and current collectors for lithium based batteries”, Energy Environ. Sci., 10, pp. 1350 - 1357,

DOI: 10.1039/c7ee00244k

6 Sung et al. (2021), “Subnano-sized silicon anode via crystal growth inhibition mechanism and its application in a prototype battery pack”, Nature energy, VOL 6, DECEMBER 2021, pp. 1164-1175, https://doi.org/10.1038/s41560-021-00945-z

7 Lee et al. (2016), “Insights from Studying the Origins of Reversible and Irreversible Capacities on Silicon Electrodes”, Journal of The Electrochemical Society, 164 (1) A6206-A6212

8 Nilsen, B.E. and Kleiv, R.A., “Silicon Powder Properties Produced in a Planetary Ball Mill as a Function of Grinding Time, Grinding Bead Size and Rotational Speed" , Silicon, October 2020, Volume 12, Issue 10, pp 2413 - 2423.

Claims

1. Silicon-based composite particles, wherein each of the silicon-based composite particles:

- is made of a bulk material comprising silicon and carbon, and

- has a plurality of nanoscaled silicon domains embedded in the bulk material, characterised in that the silicon-based composite particles have a total chemical composition comprising:

- from 2.3 to 60.9 atom% (approx. 1 to 40 wt%) carbon, based on the total mass of the silicon-based composite particles, and

- the rest being silicon and unintentional impurities, and wherein the silicon-based composite particles:

- have a median volume-weighted diameter, Dso, in the range from 1 to

9 pm, and

- a volume weighted diameter ratio D90/D10 in the range from 1 to 9, and wherein

-the Dio, D50, and D90 volume weighted particle diameters are determined by laser diffraction analysis according to standard ISO 13320:2020.

2. The silicon-based composite particles according to claim 1, wherein the silicon-based composite particles have a median volume-weighted diameter, D50, from 1.1 to 8 pm, more preferably from 1.2 to 7 pm, more preferably from 1.6 to 6 pm, and most preferably from 2.0 to 5 pm, as determined by laser diffraction analysis according to the standard ISO 13320:2020.

3. The silicon-based composite particles according to claim 1 or 2, wherein the silicon-based composite particles have a volume weighted diameter ratio D90/D10 in the range from 1.5 to 8, preferably from 2 to 7, more preferably from 2 to 6, and most preferably from 3 to 5, as determined by standard ISO 13320:2020.

4. The silicon-based composite particles according to any of the preceding claims, wherein the average diameter of the nanosized domains is in the range from 0.5 to 10 nm, preferably from 1 to 8 nm, more preferably from 2 to 7 nm, more preferably from 3 to 6 nm, and most preferably from 4 to 5 nm, as determined by Rietveld refinement of X-ray powder diffraction (XPD) data after, if necessary, including exposing the silicon-based composite particles to a heat treatment which crystallizes the nanosized domains therein before the Rietveld refinement of X-ray powder diffraction (XPD) data.

5. The silicon-based composite particles according to any of the preceding claims, wherein the total chemical composition of the silicon-based composite particles comprises: - carbon in a total amount of from 2.3 to 50.1 atom% (approx. 1 to 30 wt%), preferably from 4.5 to 47 atom% (approx. 2 to 27.5 wt%), more preferably from 6.7 to 43.8 atom% (approx. 3 to 25 wt%), more preferably from 11 to 36.9 atom% (approx. 5 to 20 wt%), more preferably from 15 to 33.9 atom% (approx. 7 to 18 wt%), and most preferably from 18.8 to 29.2 atom% (approx. 9 to 15 wt%),

- where the rest is silicon and unintentional impurities.

6. The silicon-based composite particles according to claim 5, wherein the total chemical composition of the silicon-based composite particles further comprises:

- oxygen in a total amount of less than 1.5 wt%, preferably less than 1.25 wt%, more preferably less than 1 wt%, more preferably less than 0.75 wt%, and most preferably less than 0.5 wt%, based on the total mass of the silicon-based composite particles , and

- hydrogen in a total amount of less than 1.5 wt%, preferably less than 1.25 wt%, more preferably less than 1 wt%, more preferably less than 0.75 wt%, and most preferably less than 0.5 wt%, based on the total mass of the silicon-based composite particles.

7. The particle according to any of the preceding claims, wherein the particle further comprises an outer coating on its outer surface, where the coating is: either:

- one or more of: an amorphous or crystalline carbon allotrope, an oxide chosen from LixSiyO, Ti_xO, or AkO, or a metal-organic framework and

- have a thickness in the range of from 1 to 100 nm, preferably from 2 to 60 nm, more preferably from 3 to 20 nm, and most preferably from 3 to 10 nm. or:

- an amorphous or crystalline carbon layer having a thickness in the range of from 0.5 to 20 nm, preferably from 1 to 10 nm, and most preferably from 2 to 7 nm as determined by Auger spectroscopy.

8. The silicon-based composite particles according to any one of claims 1 to 7, wherein the bulk material comprises an alloy of silicon and carbon.

9. The silicon-based composite particles according to any one of claims 1 to 7, wherein the bulk material comprises an amorphous SiC and/or amorphous C layer lying at a boundary in-between a densely packed agglomerate of nanoscaled silicon domains.

10. The silicon-based composite particles according to any one of claims 1 to 9, wherein the particles are subject to a milling.

11. The silicon based composite particles according to any one of claims 1 to 10, wherein the particles are largely spherical as determined by the following criteria:

- at least 50 %, preferably at least 60%, more preferably at least 70 %, more preferably at least 80 %, more preferably at least 90 %, and most preferably at least 95 % of all subsections of particle circumference lines apparent in a SEM picture of the material are circular arcs having an arc length, s, being determined by the relation s = r-7t/180-9, where r is the radius of a circle having the same curvature of its circumference line as said circular arc, 7t is a mathematical constant equal to the ratio of a circle’s circumference line to its diameter, and 0 is the angle in degrees which said circular arc subtends at the centre of the said circle, and

- said angle 9 is in the range from 10 to 360°.

12. The silicon based composite particles according to any one of claims 1 to 11, wherein the BET surface area, as determined by standard ISO 9277:2010, is from 0.2 to 10 m²/g, preferably from 0.3 to 8 m²/g, more preferably from 0.4 to 6 m²/g, and most preferably from 0.5 to 4 m²/g.

13. A method for manufacturing a silicon-based composite particle according to any of claims 1 to 12, wherein the method comprises the steps of:

- applying a reactor having a decomposition compartment containing a first reactor gas having an initial pressure in the range from 5 - 10³ to 6 - 10⁵ Pa and a first reactor temperature in the range from 450 to 650 °C,

- forming a precursor gas mixture comprising a first precursor gas of a silicon containing compound and a second precursor gas of a carbon containing compound, wherein the atomic ratio between silicon and carbon, Si:C, in the precursor gas mixture is in the range 0.2 to 50,

- injecting the precursor gas mixture into the decomposition compartment and mixing it with the first reactor gas to form a second reactor gas mixture, characterised in that the method further comprises:

- maintaining the second reactor gas mixture inside the decomposition compartment for a residence time in the range from 10 to 300 seconds while maintaining the temperature of the second reactor gas mixture to be within ±20 °C, preferably within ±10 °C, more preferably within ±5 °C, and most preferably within ±1 °C from the first reactor temperature, and

- extracting the particles from the decomposition compartment.

14. The method according to claim 13, wherein the residence time is in the range from 12.5 to 250 seconds, preferably from 15 to 200 seconds, more preferably from 17.5 to 150 seconds, more preferably from 20 to 100 seconds, more preferably from 25 to 75 seconds, and most preferably from 30 to 50 seconds.

15. The method according to claim 13 or 14, wherein the precursor gas mixture is preheated to a temperature in the range from 250 to 450 °C, preferably from 300 to 390 °C, more preferably of from 325 to 380 °C, or most preferably from 350 to 370 °C, before injecting the homogeneous gas mixture into the reactor space.

16. The method according to anyone of claims 13 to 15, wherein:

- the first precursor gas is one of: silane (Sifh), disilane (Si2He), trichlorosilane (HChSi), an organosilane, or a mixture thereof, and

- the second precursor gas is one of an organosilane or a hydrocarbon, preferably one of methane (CHT), ethane (C2H6), propane (CsHs), butane (C4HIO), ethene (C2H4), ethyne (C2H2), propene (CsHe), butene (C4H8), pentene (C5H10), cyclohexane, cyclohexene, toluene, benzene, or mixtures thereof.

17. The method according to anyone of claims 13 to 16, wherein the first reactor temperature is in the range from 475 to 630 °C, preferably from 500 to 620 °C, more preferably from 525 to 600 °C, and most preferably from 550 to 580 °C.

18. The method according to anyone of claims 13 to 17, wherein the atomic ratio between silicon and carbon in the precursor gas mixture is in the range from 0.3 to 45, preferably from 0.4 to 40, more preferably from 0.5 to 30, more preferably from 0.6 to 25, more preferably from 0.8 to 15, more preferably from 1.0 to 8, more preferably from 1.2 to 5, and most preferably from 1.5 to 2.0.

19. The method according to anyone of claims 13 to 18, wherein the temperature of the second reactor gas mixture is obtained by: either

- applying a reactor having a tubular hot-wall decomposition compartment having a first end and a second, and

- injecting a constant volume flow of preheated mixture of precursor gases and a constant volume flow of preheated first reactor gas in the first end forming a constant flow volume of second reactor gas mixture inside the tubular hot-wall decomposition compartment,

- passing the second reactor gas mixture under laminar flow conditions with a Reynolds number of less than 2000 through the tubular hot-wall decomposition compartment from its first to its second end, and

- extracting the second reactor gas mixture including formed particles at the second end, or

- applying a reactor having a closed hot-wall decomposition compartment containing a first reactor gas, and

- injecting the preheated mixture of precursor gases into the hot-wall decomposition compartment at a first moment in time forming a second reactor gas mixture, and

- when the residence time is obtained, counting from the first moment in time, extracting both the second reactor gas mixture and formed particles from the hot-wall decomposition compartment by either vacuum suction or flushing, or - applying a reactor having a closed hot-wall decomposition compartment containing a first reactor gas and a cooled collecting chamber at the bottom of the decomposition compartment,

- injecting the preheated mixture of precursor gases into the hot-wall decomposition compartment forming a second reactor gas mixture, and

- maintaining the second reactor gas mixture in the hot-wall decomposition compartment until the formed particles grow to a size where they settle out of the second reactor gas mixture and falls by gravity into the collection chamber.

20. The method according to anyone of claims 13 to 19, wherein the method further comprises depositing an amorphous or crystalline carbon layer onto the outer surface of the particles by chemical vapour deposition of a carbon containing precursor gas, preferably an alkene gas, at a temperature in the range from 650 to 750 °C.