Silicon-based composite particles with growth-ring resembling structure

Abstract

The present invention relates to a particulate silicon- and carbon-based composite particle and a method for manufacturing the particle, where the particle has a median volume-weighted diameter, D50, from 0.05 to 10 µm, comprises a bulk mass having a total amount of Si from 60 to 95 atom% and a total content of C from 5 to 5 40 atom%, and where the bulk mass further comprises one or more internal shell-resembling spatial regions having an increased carbon content with a peak in the elemental ratio carbon to silicon being in the range from 1 to 15 atom percentage points higher than the average elemental ratio of the bulk material.

Description

[0001] Silicon-based composite particles with growth-ring resembling structure

[0002] 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.

[0003] Background

[0004] 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.

[0005] 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.

[0006] 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.

[0007] Prior art

[0008] 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.

[0009] 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 SEI-layer 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.

[0010] 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.

[0011] 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%.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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 corresponded to a diameter of approx. 50, 100, and 150 nm for the spherical or pseudo- spherical components that are typically linked together in groups to form each particle in the three samples. 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 / cm2in 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 spherical “building blocks” in the particles size is were larger than 150 nm.

[0017] 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.

[0018] 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 were 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). 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 could 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.

[0019] 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. Objective of the invention

[0020] The main objective of the invention is to provide a particulate silicon- and carbonbased composite material having a high cycling capacity when applied as the active material in negative electrodes in rechargeable lithium-ion batteries.

[0021] A further objective of the invention is to provide a method for manufacturing said composite particles.

[0022] Description

[0023] The present invention is based on an observation that silicon and carbon-based particles, when applied as the active material of a negative electrode of a secondary LIB, obtain an enhanced cycling stability when the carbon content in the particles is inhomogeneously distributed in the silicon mass in such manner that it is formed one or more relatively carbon rich shell-resembling layers embedded in the bulk phase of the particles.

[0024] Each of these one or more relatively carbon rich embedded shell-resembling layers is often visible on a transmission electron microscope (TEM) photograph of a crosssection of the particle as a ring encompassing the core of the particle, similar to how growth-rings are made visible in a cross-section cut of a tree trunk. Figure 1 presents a TEM-photo of a single multilayer particle of an example embodiment, herein labelled as sample SI, of the particles according to the invention marked with reference number 1, and where the photograph shows a plurality of such rings of enhanced carbon content in the silicon-based matrix material marked with reference number 2. These rings need not be circular, but may, as shown on the photograph, have an irregular shape.

[0025] Without being bound by theory, it is believed that the reason for observing the increased cyclability of such multilayer particles may be that since the current of electrons in an anode goes through the solid phases only, lithium ions will pick up electrons and start diffusing into silicon-based particles primarily at a few locations on the particle surface (at the contact points) and leave at the same location. When this occurs, a flow of lithium ions will move in the same direction and may drag along silicon atoms out of the silicon-based particle leading to a silicon leakage into the electrolyte. However, having regions high in carbon inside the bulk mass of the silicon-based particle is believed to act as a partial barrier to this flow and thereby spread the lithium diffusion over a larger area inside the silicon-based particles. The carbon rich phases / regions in the silicon-based particles are also believed to be more reluctant to let the silicon pass and thus make the silicon-based particles more stable towards charging / discharging cycling. The one or more relatively carbon rich embedded shell-resembling layer may be seen as an “armouring” of the silicon based bulk material. On the other hand, if these carbon-rich rings are far too carbon- rich, the Lithium diffusion barriers may become too high, leading to a measurement of lower practically available charging capacity for a given charge / discharge rate.

[0026] The particle

[0027] In a first aspect, the invention relates to silicon based particles made of a bulk material comprising silicon and carbon, wherein

[0028] - the silicon-based particles have a median volume-weighted diameter, Dso, from 0.05 to 10 pm, as determined by laser diffraction analysis according to standard ISO 13320:2020, and

[0029] - the silicon-based particles have a chemical composition comprising:

[0030] - a total content of carbon, Ctotai being from 5 to 40 atom%, based on the total mass of the silicon-based composite particles, and

[0031] - the rest being silicon and unintentional impurities, characterised in that

[0032] - the particles contain one or more internal shell-resembling spatial regions within the bulk material of the particles having an increased carbon content with a peak in the elemental carbon content, Cpeak, being from 1 to 15 atom percentage points higher than an average carbon content, Caverage, and wherein

[0033] - the peak in the elemental carbon content, Cpeak, of each of the one or more internal shell-resembling spatial regions within the bulk material and the average carbon content, Caverage, are determined by electron energy loss spectroscopy elemental analysis taken along a straight line running from at least the particle core up to but not including the particle surface, and wherein variations in the less than 5 nm length scale, preferably less than 2 nm length scale, and most preferably less than 1 nm length scale are smoothed out.

[0034] The term “core” as used herein refers to the volumetric centre region of the particle while the term “outer surface region” refers to the particle mass being in close proximity of its outer surface, i.e. from the particle surface to 1 to 10 nm below the particle surface. The term does however not include any surface coating (if present) but refers strictly to the bulk particle mass at the surface region.

[0035] The term “internal shell-resembling spatial region within the bulk material” as used herein refers to a local spatial increase in the carbon content within the particle mass which is shaped as an internal layer encompassing the core of the particle. If there are two or more of these internal shell-resembling spatial region / layers, they should preferably be spaced apart from each other within the bulk material. The internal-shell resembling layers will usually be substantially concentric and attain a Russian doll resembling structure such as shown on the TEM-photo of figure 1 which displays several shell-resembling layers marked with reference number 2 in the particle of sample SI. The bulk material of the particle laying in-between the shell-resembling layers 2 is marked with reference number 3 in the figures. These one or more relatively carbon rich layers have a similar three-dimensional shape as a shell / coating deposited onto the outer surface of a particle, except that the layer(s) is / are located within the particle mass and not on its outer surface. A mapping of the elemental balance of a cross-section of a particle of sample SI shown in figure 1 taken along a straight line from the particle core to its outer surface would produce a graph where the C content is substantially horizontal close to the Caverage in areas representing the bulk material and where each of the shellresembling layers stands out as local maximums in the C content as shown in the elemental analysis diagram given in figure 2.

[0036] A mapping of the elemental balance by electron energy loss spectroscopy (EELS) in a straight line across and through the particle core of a particle having these internal shell-resembling layers will typically display a diagram as shown in figure 2. In this example embodiment, the particle is approx. 800 nm in diameter and contains four of these shell-resembling layers marked with reference number 2 on the figure. As seen in the diagram, the overall Si and C content in the particle is substantially homogenous in the sense that the curves for the Si-content and the C-content are horizontal. Thus, the (average) elemental ratio Si : C at the particle core substantially equal to the (average) elemental ratio Si : C at the outer region of the particle close to its surface. The EELS diagram of the particle from sample SI shown in figure 2 contains a sharp increase at the surface. This is due to this sample of the particles is given a surface coating of amorphous carbon.

[0037] Each of the internal shell-resembling layers 2 is displayed and marked in the EELS diagram shown in figure 2 by a vertical dotted line aligned over a descending peak in the Si content and a corresponding ascending peak in the carbon content. The figure also presents a cut-out section of a TEM-image of the same part of the particle being applied in the EELS analysis to show that the opposite directed peaks in the Si and C contents on the curves for Si and C content along the line are due to the internal shell-resembling layers 2 .

[0038] The graph for the carbon content shows a peak of increased carbon content at a distance of approx. 250, 380, 300, and 650 nm on both sides of the particle centre, and the graph for the silicon content show a corresponding but opposite directed peak of reduced silicon content at the same positions from the particle centre. The overlaid stapled vertical lines in figure 2 show that each of these corresponding peaks of increased carbon content and reduced silicon content at these positions from the particle centre coincides with an annular ring shown on the TEM-image. Each of these pairs of corresponding peaks of increased C and decreased Si on the graphs for the C and Si content, respectively. A Cpeak being at least 1 atom percent point higher than Caverage is thus an indication of the presence of one relatively carbon rich embedded shell-resembling layer according to the invention. In one embodiment, the determination of Cpeak content may advantageously comprise smoothing out variations on small length scales of < 2 nm length scale, and most preferably < 1 nm length scale by digital filtering of signal noise.

[0039] Digital filtering of signal noise in measurements is a standard procedure in most experimental procedures, and is thus well known to those skilled in the art. One way to do this is to let each datapoint be recalculated as the average of several neighbouring datapoints from the original data set. In a gaussian filter, the contribution of the neighbouring datapoints is weighted based on the distance from the datapoint to be calculated according to a gaussian distribution function. Thus, smoothening the measurements may in one embodiment be obtained by using a gaussian digital filter or by averaging the measurements made for a plurality of neighbouring spatial measuring-areas. The width of the digital filter should be < 5 nm , preferably < 2 nm, and most preferably < 1 nm, to avoid removing the information about the actual concentration variations.

[0040] In one embodiment, the determination of the Cpeak of the at least one shellresembling relatively carbon-rich layer and the determination of the Caverage by electron energy loss spectroscopy elemental analysis is obtained by:

[0041] - preparing a cross-sectional slice of less than 70 nm thickness including the particle centre of the particle by focused ion beam (FIB),

[0042] - performing the EELS elemental analysis along a linear line running through the cross-section from the centre to the outer surface of the slice, and then

[0043] - optionally, smoothening the measurements by using a gaussian digital filter or by averaging the measurements made for a plurality of neighbouring spatial measuring-areas, and

[0044] - applying the local maximum for the carbon content of a first peak in the carbon content along said line at a distance from the particle centre to define the Cpeak of the first peak and if present, make a similar determination of the Cpeak for each other corresponding pair of peaks of increased carbon content and decreased silicon content present on said line, and

[0045] - determining the Caverage by averaging the determined C-content of all measurement points along the line.

[0046] The observed effect of the one or more shell-resembling layers in the particles according to the invention indicates that there may be an upper limit to the carbon concentration. Without being bound by theory, this is believed to be due to the increased carbon concentration at the shell-resembling layers will contribute both to stabilisation and to a reduction of lithium mobility. At relatively low increase in the carbon concentration, the stabilisation effect is observed to be the dominating effect, while at higher carbon contents the effect of reduced lithium mobility becomes more significant than the increase in stability. The empirical observations made by the inventors indicate that the local increase in carbon at the one or more shell-resembling layers may advantageously be less than 15 atom percentage points. That is, if the overall (total) carbon content of the particles is 20 atom%, then the (local) carbon content at the peak of the one or more shell-resembling layers should preferably be no higher than 35 atom%.

[0047] In one embodiment, the Cpeak of the at least one shell-resembling relatively carbon- rich layer may advantageously be in the range from 1 to 12 atom percentage points, preferably from 1 to 10 atom percentage points, more preferably from 2 to 8 atom percentage points, more preferably from 3 to 6 atom percentage points, and most preferably from 4 to 5 atom percentage points higher than an average carbon Content, Caverage.

[0048] The term “total carbon content” or “total silicon content” as used herein refers to the overall content of carbon or silicon, respectively, in the bulk material and is a measure of the average content of carbon or silicon in said bulk material. E.g., if the total content of a carbon in a bulk material is 5 atom% (also denoted as at% in the literature), there will on average be 5 carbon atoms for every hundred atoms in the bulk material. Likewise, the term “total chemical composition of the particles” as used herein refers to the overall content of the present elements in the entire particle. However, the material / elements of an optional coating on the particle surface is not included. The total content refers only to the material of particle without an optional surface coating. The determination of the total content of an element, for example silicon, carbon, hydrogen, and / or oxygen in the bulk material 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). These are well known techniques mastered by persons skilled in the art. The total carbon content of a particle may also be determined by combusting a sample of the particles and measuring / determining the amount of carbon dioxide being formed. 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.

[0049] The term “bulk” as used herein, is applied in the meaning “most of something”. I.e., the “bulk material” of the claimed particle is the major constituent of the particle mass. Without being bound by theory, the bulk material of the particle according to the first aspect of the invention is believed to be a silicon-based chemical phase where carbon atoms substitute silicon atoms in the molecular structure (which may be mono or multi crystalline and / or amorphous) of the silicon-based chemical phase having a spatial variation in its elemental balance of Si and C forming the internal shells / layers. Anyhow, whether this assumption is correct or not, there is no observation or empirical evidence known to the inventors which indicates or suggests that the internal shells / layers in the bulk material of the particles according to the invention are separate chemical phases, which would make the particles to be composite particles. On the contrary, all evidence suggests the opposite, that these internal shells / layers are spatial variations in the element balance of the molecular structure of the silicon-based chemical phase constituting the bulk material of the particles, and thus being an integral part of the bulk material as would be expected when the particles are grown by CVD with temporary alterations of the reaction kinetics favouring more carbon.

[0050] The restriction of Cpeak to be from 1 to 15 atom percentage points higher than Caverage is introduced to avoid detecting signal noise and / or eventual arbitrary fluctuations in the elemental ratio as an embedded relatively carbon rich layer and further because such increase in the carbon content in the layer is believed to provide the effect of making the particles more robust towards the volume changes in lithiation / delithiation cycles when applied as active material in secondary lithium ion batteries. The lithium and silicon diffusivities will be strong functions of the local Si:C ratio, so even a small change in carbon content is expected to have a beneficial effect on the particle stability by making the expansion more homogeneous.

[0051] The determination of the elemental ratio in different parts of the particle is obtained by electron energy loss spectroscopy (EELS) elemental analysis. EELS passes an electron beam through a thin sample of the test specimen (typically less than 70 nm thick) and applies the scattering of the electrons to analyse the content of the sample. The EELS instrument is usually incorporated into a transmission electron microscope (TEM) and / or a scanning transmission electron microscope (STEM). STEM-based EELS with has a theoretical spatial resolution down to the atomic scale (0.1 to 1 nm) for samples with sharp boundaries, but somewhat less coarse resolution with samples having depth variations as the sample of the present particles. Anyway, STEM-based EELS is able to detect changes in the elemental content of the sample down to a few nm resolution. Thus, by applying EELS to scan and determine the elemental composition of a sliced sample of the particle according to the invention along a linear line crossing the cross-section of the particle including the centre point if the sliced sample, each of the at least one shellresembling relatively carbon-rich layer will stand out as a peak having a local maximum in a curve otherwise being representative for the carbon content of the bulk material (having an elemental ratio close to the average ratio determined by the total carbon content). Similarly, each of the at least one shell-resembling relatively carbon-rich layer will stand out as an opposite directed peak (compared to the carbon curve) having a local minimum in a curve otherwise being representative for the silicon content of the bulk material. An example of this is shown in figure 2.

[0052] The EELS elemental analysis method has been in use for decades and is therefore well-known to the person skilled in the art. Thus, even though it is known that EELS elemental analysis may be sensitive towards the instrument being applied and the method for preparing the sample which may affect the determination of the absolute values for the atomic composition of as sample, it is within the ordinary skills of the person skilled in this field to perform an analysis which provides a robust and correct answer of the elemental balance in the sample. This is especially the case when the measurements are aimed at finding relative changes in the content of elements (C and Si) as is the case for the present particles. Then the EELS elemental analysis is known to be especially robust and reliable making use of the EELS analysis highly suited for determining the Cpeak contents relative to the Caverage. Thus, there is no need to further specify how the carbon content of the at least one embedded shell-resembling relatively carbon rich layer is determined by electron energy loss spectroscopy elemental analysis. A skilled person in this field can reliably determine whether a sample of such particles have one or more embedded layers with a relative increase in the carbon content as specified above.

[0053] The EELS elemental analyses provided herein are however obtained as follows: EEELS Sample powders were dispersed on a clean, mirror-polished Si wafer. Transmission electron microscopy (TEM) specimens were made by focused ion beam (FIB) preparation, using a Helios G4 UX dual-beam instrument from Thermo Fisher Scientific. In the FIB, the particles chosen for TEM were first coated by a carbon or platinum protection layer. The first part of the protection layer was made by electron-beam assisted deposition to avoid possible Ga+ion-beam damage in the outer part of the particles. The upper and major part of the protection layer was made by ion-beam assisted deposition. After protection coating, the coated particles were cut out and transferred to dedicated Cu TEM half-grids by standard lift-out procedures, using a tungsten "Easylift" micromanipulator. All coarse thinning was done with 30 kV acceleration voltage for the Ga+ions. Final thinning was performed at 5 kV and then 2 kV on either side of the TEM specimens / lamellae to minimize surface damage. The TEM specimens had a final thickness in the range 30 - 60 nm.

[0054] TEM was performed with a double spherical aberration corrected cold field emission gun JEOL ARM 200FC, operated at 200 kV. This instrument is equipped with a Gatan Image Filter (GIF) with a Quantum ER spectrometer for electron energy loss spectroscopy (EELS) and a 100 mm2Centurio silicon drift detector, covering a solid angle of 0.98 sr, for energy dispersive X-ray spectroscopy (EDS). All spectroscopy was performed in scanning transmission electron microscopy (STEM) mode by doing both EDS and dual EELS simultaneously, meaning that an EDS spectrum and two EEL spectra were acquired in every pixel of the mapped region. For EELS, a low loss spectrum covering the zero loss peak, and a high loss spectrum covering the carbon, oxygen and silicon K-peaks, were acquired with a dispersion of 1 eV / channel on a 2048 x 2048 pixels CCD camera located at the end of the GIF. All quantifications of chemical composition were based on the EELS data. In every pixel, the low loss spectrum was used both to calibrate the energy and to deconvolute for plural scattering in the high loss spectrum. GMS3, version 3.4, was used to quantify the EELS data and provide the chemical composition in the mapped regions. The corresponding EDS data were primarily used to check if any other elements than silicon, carbon and oxygen were present in the particles. The graphs that shows line profiles of the chemical composition as a function of distance across the FIB-prepared cross-sections are based on rectangular maps running through the centre of the mapped particles. The width of these maps were typically in the range 10 - 40 nm. All pixels perpendicular to the line profile direction were summed and averaged to provide better statistics in the presented line profiles.

[0055] Alternatively, in one embodiment, the determination of the Cpeak of the at least one shell-resembling relatively carbon-rich layer by electron energy loss spectroscopy elemental analysis is obtained by the same procedure as given above, except that it is made across the entire cross-section of the particle sample. This latter alternative has the advantage of being a more robust identification of the presence of shellresembling relatively carbon-rich layer(s) within the bulk material of the particle, because, as seen on figure 2, the peaks on the carbon curve and similarly on the silicon curve are more or less identical and symmetrically distributed around the particle centre. Such configuration of the variations in the carbon (and corresponding opposite variations in the Si-content) content along the linear line is strong tell-sign of the embedded shell-resembling layer.

[0056] Due to the formation of the one or more relatively carbon rich embedded shellresembling layer by a temporary alteration of the reaction kinetics in the CVD- growth, the change in the elemental composition of the bulk mass associated with such a shell-resembling layer will be shaped as a peak / local maximum on a graph showing the elemental composition of the bulk mass as a function of distance from the particle centre to its outer surface as illustrated in figure 2. Thus, the term “a peak in the carbon content, Cpeak” as used herein, refers to the local maximum in carbon content associated with the shell-resembling relatively carbon-rich layer. In one embodiment, the particles according to the first aspect of the invention may in some embodiments further comprise an outer shell / layer / coating deposited onto their outer surface. The outer shell / layer / coating may be made of any material known to the person skilled in the art to be suited for silicon particles applied as the active material of a negative electrode of a lithium ion battery. An example of a suited outer shell / layer / coating is amorphous carbon.

[0057] In one embodiment, the total chemical composition of the particles comprises:

[0058] - a total content of carbon from 10 to 37 atom%, preferably from 15 to 33 atom%, more preferably from 20 to 30 atom%, and most preferably from 23 to 26 atom%, and

[0059] - the rest being silicon and unintentional impurities,

[0060] The bulk material of the particle according to the invention may further comprise any element in addition to carbon and silicon as long as the total contents of Si and C in the bulk material are in the above given ranges, i.e. Ctot is minimum 5 atom% and maximum 40 atom% and silicon constitutes the balance of the chemical composition.

[0061] 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. Thus, the produced particles will mainly contain silicon and carbon, and some small amount of hydrogen, and eventual traces of unavoidable 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.

[0062] 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 on 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%.

[0063] Furthermore, because the particle according to the invention in one embodiment may be made by condensation and chemical vapour deposition of hydrogencontaining precursor gases, the resulting particles are likely to 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 thus, in an embodiment, encompass as much as 30 atom% hydrogen but is preferably less than 1 atom%, depending on the temperature and residence applied in the formation of the particles. Thus, in one embodiment, the particle may contain 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%.

[0064] In one especially preferred embodiment, the elemental composition of the particle according to the invention may advantageously be:

[0065] - a total carbon content, Ctotai, of from 5 to 30 atom%, preferably from 6 to 25 atom%, preferably from 7 to 20 atom%, more preferably from 8 to 15 atom%, more preferably from 9 to 14 atom%, or most preferably from 10 to 12 atom%,

[0066] - a total amount of oxygen in the particle from 0.1 to 4 atom% O, preferably from 0.2 to 3 atom % O, more preferably from 0.3 to 2 atom % O, more preferably from 0.4 to 1 atom % O, or most preferably from 0.5 to 1 atom% O,

[0067] - a total amount of hydrogen in the particle of less than 5 atom%, preferably less than 1 atom%, and

[0068] - the rest being Si and unintentional impurities.

[0069] In one embodiment, the particles according to the invention may advantageously have a relatively large particle medium average particle size, Dso, because the first cycle loss of lithium is known due to solid electrolyte interface formation to increase with increased (total) particle surface of the active material. Furthermore, a relatively narrow particle size distribution, especially when the Dso diameter is relatively high, is beneficial because there will be fewer “oversized” particles and / or less of the undersized particles. The larger the particles become, the longer the diffusion distances become for the lithium atoms moving in and out of the particles, and the higher the risk of lithium being diffusively trapped inside the particles becomes. Too large particles are thus more or less “dead” as active material. On the other hand, if one sets a firm requirement to the maximum size of the largest particles, a narrow distribution will lead to less of the smaller particles. Too small particles will have a lot of surface area and lead to a low First Cycle Efficiency.

[0070] Thus, in one embodiment, the particle according to the invention may further have a median volume-weighted diameter, Dso, from 1 to 9 pm, preferably 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 to 5 pm, as determined by laser diffraction analysis according to the standard ISO 13320:2020.

[0071] In one embodiment, a plurality of the particles according to the invention has a D90 / D10 ratio in the range from 1 to 10, preferably from 2 to 8, and most preferably from 3 to 6, as determined by laser diffraction analysis according to standard ISO 13320:2020.

[0072] In one embodiment, the particle according to the invention may further comprise a plurality of nanoscale silicon domains embedded in the bulk material. An advantage of this embodiment of the particle is that the primary nanodomains 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.

[0073] In one embodiment, the average diameter of the nanoscale silicon domains may typically be 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 diffusion (XPD) data. Optionally, the particle may be subject to a heat treatment which crystallizes potentially amorphous nanoscale silicon domains being embedded therein before the Rietveld determination of the average particle size of the primary particles. The heat treatment may be at 900 °C or higher for at least a half hour or more and preferably with a ramp rate of 1 C per minute above 600 C.

[0074] The average particle size of the nanoscale silicon domains may be determined by Rietveld analysis of XPD data, which is well known and mastered by the person skilled in the art. The Rietveld analysis detects crystalline phases such that the determination of the primary particle sizes may advantageously comprise a prior heat treatment at around 900 °C for half an hour to ensure that eventual amorphous nanoscale silicon domains in the bulk material of the particle is crystallized before the determination. 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 secondary 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 7] or commercial software such as Topas [Ref 8], The instrumental contribution to the width of the Bragg peaks should either be calculated from the geometry of the instrument (“fundamental parameters approach” [Ref 9]) or be described by a Thomson-Cox-Hastings pseudo-Voigt function [Ref 10] 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 7T2where 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.

[0075] Furthermore, the particle according to the invention may advantageously further comprise a coating on its outer surface to protect the particle surface from oxidation when in contact with air and from reaction the electrolyte forming a SEI layer. Examples of outer coatings include amorphous or crystalline carbon allotropes, polymers, resins or crosslinked or pyrolyzed polymers or resins, oxides like LixSiyO, TixO, AkO, or any combination thereof. In one embodiment, the coating may have a thickness in the range 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 to enhance the surface properties, lower fire-risk and promote formation of a stable solid- electrolyte-interface (SEI). The coating may be applied using wet chemical methods, CVD, ALD or other techniques.

[0076] 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. In case of applying a surface coating of carbon on the particle, the carbon may advantageously be heat treated, afterwards to obtain good transport and adhesion properties by e.g.:

[0077] - Crosslinking / cyclization only, significant hydrogen remains in the particles and coating, more elastic carbon phase (-300 °C), less stable bulk power, low / medium capacity

[0078] - Low temperature annealing and CVD coating (leaving amorphous silicon, allowing faster first cycle charging) (600-800 °C),

[0079] - 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

[0080] - 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).

[0081] In one preferred embodiment, the particle according to the invention further comprises a surface coating of amorphous or crystalline carbon having a thickness in the range of from 0.5 to 10 nm, preferably from 2 to 5 nm, as determined by Auger spectroscopy.

[0082] 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.

[0083] In one embodiment, the first aspect of the invention relates to silicon based particles made of a bulk material comprising silicon and carbon, wherein

[0084] - the silicon-based particles have a median volume-weighted diameter, Dso, from 0.05 to 10 pm, as determined by laser diffraction analysis according to standard ISO 13320:2020, characterised in that

[0085] - the silicon-based particles have a chemical composition consisting of

[0086] - a total content of carbon, Ctotai being from 5 to 40 atom%, based on the total mass of the silicon-based composite particles, and

[0087] - the rest being silicon and unintentional impurities, and wherein

[0088] - the particles contain one or more internal shell-resembling spatial regions within the bulk material of the particles having an increased carbon content with a peak in the elemental carbon content, Cpeak, being from 1 to 15 atom percentage points higher than an average carbon content, Caverage, and

[0089] - the peak in the elemental carbon content, Cpeak, of each of the one or more internal shell-resembling spatial regions within the bulk material and the average carbon content, Caverage, are determined by electron energy loss spectroscopy elemental analysis taken along a straight line running from at least the particle core up to but not including the particle surface, and wherein variations in the less than 5 nm length scale, preferably less than 2 nm length scale, and most preferably less than 1 nm length scale are smoothed out.

[0090] Method of manufacturing the particle

[0091] The silicon- and carbon-based composite particles according to the invention are preferably produced by condensation and chemical vapour deposition (CVD) of a mixture of a silicon containing precursor gas and a carbon containing precursor gas in a protected atmosphere (i.e., contains no or only insignificant amounts of oxygen).

[0092] In a second aspect, the invention relates to a method for producing silicon-based particles according to the first aspect of the invention, wherein the method comprises the steps of

[0093] - 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 from 0.2 to 50,

[0094] - preheating the precursor gas mixture to a temperature of 300 to 350 °C

[0095] - applying a reactor having a decomposition chamber heated to a first reaction temperature at which the precursor gas mixture will condense and form particle seeds which subsequently grows by chemical vapour decomposition (CVD), and

[0096] - injecting the precursor gas mixture into the decomposition chamber, characterised in that the method further comprises:

[0097] - maintaining the injected precursor gas mixture inside the decomposition chamber for a residence time in the range from 1 to 300 seconds and, during the residence time, subjecting the injected precursor gas mixture to at least one temperature alteration from the first reaction temperature to a second reaction temperature and then back to the first reaction temperature, and then

[0098] - extracting the particles from the decomposition chamber, wherein

[0099] - the absolute temperature difference between the first and second reaction temperature is in the range from 1 to 100 °C, preferably from 3 to 75 °C, more preferably from 5 to 50 °C, more preferably from 10 to 35 °C, and most preferably from 20 to 25 °C.

[0100] The term “absolute temperature change of X °C” as applied herein, means that the temperature difference between the first and second reaction temperature may either be such that the second reaction temperature is either X °C lower than the first reaction temperature or X °C higher than the first reaction temperature.

[0101] In one embodiment, the first reaction temperature may be in the range from 450 to 900 °C, preferably from 500 to 800 °C, more preferably from 550 to 700 °C, and most preferably from 600 to 650 °C.

[0102] In one embodiment, the residence time may advantageously be in the range from 2 to 250 seconds, preferably from 3 to 200 seconds, more preferably from 5 to 150 seconds, more preferably from 10 to 120 seconds, more preferably from 15 to 90 seconds, and most preferably from 20 to 60 seconds.

[0103] In one embodiment, the atomic ratio between silicon and carbon, Si:C, in the precursor gas mixture may advantageously 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.

[0104] Without being bound by theory, it is believed that the cyclic temperature fluctuation during the particle growth alters the relative reaction kinetics of the CVD growth process such that there is deposited relatively high concentrations of silicon and correspondingly low carbon concentrations in the deposited material when the reaction temperature is low and that the carbon concentration in the deposited material increases when the reaction temperature increases. Thus, by making several successive cyclic temperature fluctuations spaced apart from each other in time during the CVD-growth of the particles, the particles will attain a series of shellresembling carbon-rich layers separated by intermediate layers of a bulk mass having a relatively high silicon content and correspondingly low carbon content.

[0105] In one embodiment, the method according to the invention may further comprise preloading the decomposition chamber with a first reactor gas having an initial pressure in the range from 5 • 103to 6- 105Pa at the first reaction temperature. This feature has the effect of rapidly heating the injected precursor gas mixture to the first reaction temperature by mixing it with a preheated first reaction gas phase inside the decomposition chamber forming a second reactor gas mixture. The first reactor gas phase may be a fully inert gas, like Ar, a gas inert at the relevant temperature, like N2, or it may be the exhaust gas (including unreacted species) of a previous production step, with hydrogen and some remaining silicon or carbon precursor species, but at a too low concentration to form particles at the relevant temperature. Without being bound by theory, it is believed that this feature enables formation of the embodiment of the particles containing a plurality of nanoscale silicon domains embedded in the bulk material described above. The mechanism is believed to be that when the particles are made by condensation and chemical vapour deposition of a mixture of a silicon containing precursor gas and a carbon containing precursor gas in a protected atmosphere, that the ratio of precursor gases and the reaction conditions applied causes the silicon containing precursor gas to begin decomposing and condense to a vast number of tiny nanodomains of more or less pure silicon before the carbon containing precursor gas begins to decompose and condense together with the remaining silicon containing precursor gas and thus grow by chemical vapour deposition a bulk phase of silicon and carbon onto the already formed tiny nanodomains of more or less pure silicon. The domain size may be determined both by the availability of the different precursor gases, and by the rate at which these particles form and meet. Because the chemical reactions in this early formation phase are strong functions of temperature, the domain size is also expected to be a strong function of heating rate and temperature.

[0106] Furthermore, if the silicon precursor gas is SiF , the initial reaction is assumed to be initiated from H2 leaving SiFh, forming highly reactive SiFte molecules. The SiH2 may react either with another SiF to form Si2He, or with a carbon precursor gas molecule, for example C2H4, forming SiC2He, or with C2H2, forming SiC2H4. The ratio of Si2He to SiC2H4 or SiC2He and also the reactivity of the carbon containing gas with the silicon nano domains will be a strong function of temperature. Thus, smaller variations in the temperature can affect the relative incorporation probability of Si and C in the final product.

[0107] Both of these explanations (which may both be true under various conditions) lead to the conclusion that small variations in temperature will lead to significant variations in the relative consumption rates for Si and C precursor gases. For at least some gas mixtures, increasing the reactor temperature will lead to a higher relative consumption of C, while decreasing the temperature will lead to a higher relative consumption of Si.

[0108] In one embodiment of the invention, the temperature variation is achieved by allowing a thermal convection current to transport the gas between different zones of the decomposition chamber heated to the first and second reaction temperature, respectively. The particles carried with the gas current can exchange energy through convection and / or radiation with the chamber walls, meaning that the temperature controlling the particle growth varies with the position in the decomposition chamber.

[0109] In one embodiment of the invention, the temperature variation is due to the turbulent flows created when the gas enters a hot-wall decomposition chamber. If the particle is near the nozzle feeding gas, the temperature will be lowered to a second reaction temperature, while if the particle is at a reactor wall far from the nozzle, the temperature will be the first reaction temperature.

[0110] In another embodiment of the invention, the gas is transported by a laminar or near plug-flow through a tubular hot-wall decomposition chamber with alternating temperature zones heated to the first and second reaction temperatures, respectively, again giving fluctuations in the decomposition temperature.

[0111] In one embodiment, the at least one temperature alteration from the first reaction temperature to a second reaction temperature is obtained by either:

[0112] - applying a reactor having a hot-wall decomposition chamber comprising at least one first zone and at least one second zone, where the at least one first zone has a wall temperature equal to the first reaction temperature, and the at least one second zone has a wall temperature equal to the second reaction temperature, and

[0113] - transporting the precursor gas mixture in the hot-wall decomposition by thermal convection currents between the at least one first and the at least one second zones, or:

[0114] - applying a reactor having a hot-wall decomposition chamber comprising an injection zone and a first decomposition zone having walls with a wall temperature equal to the first reaction temperature, and

[0115] - forming a turbulent flow in the injected precursor gas mixture inside the decomposition chamber transporting the second reactor gas mixture between the injection zone and the first decomposition zone of the of the how-wall decomposition chamber, or:

[0116] - applying a reactor having a tubular hot-wall decomposition chamber comprising a plurality of alternating first zone and second zones, where the first zones have a wall temperature equal to the first reaction temperature, and the second zone has a wall temperature equal to the second reaction temperature, and

[0117] - injecting the precursor gas mixture, optionally mixed with a first reactor gas to form a second reactor gas, at a first end of the tubular hot-wall decomposition chamber and passing the precursor gas mixture, optionally the second reactor gas mixture, through the tubular hot-wall decomposition chamber under laminar flow conditions with a Reynolds number of less than 2000.

[0118] The particle size will also be dependent on temperature. High temperature and low residence time will give small particles. Low temperature and long residence time will give larger particles. It is further possible to achieve smaller particles by dilution of the silane or the silane / hydrocarbon mix by for instance H2, argon or N2. It is further possible to run the process at low pressure <100 mbar, atmospheric pressure or higher pressure. Alternatively it is possible to alter the pressure during the growth of the particles. After the particles have grown for the chosen residencetime the particles may in one embodiment be extracted by either flushing or vacuum.

[0119] In order to get an average particle size >500 nm and without too much SiC formation, the reaction temperature can be relatively low (<640 °C), and the residence time long (>10 s). In many reactor types (fluidized bed or flow-through reactors) it is very difficult to control the reaction time, as some gas will find a short route through the decomposition chamber, while others will find a long route. This wide distribution will lead to production of small particles and low utilization of the reactant species for the gas that took the short route, or excessive particle growth and agglomeration in the gas that took the long route. It may therefore in one embodiment, be advantageous to control the residence time to obtain both size control and good reactant gas utilization. A long residence time means that a laminar flow field or near plug flow reactor will be more difficult to achieve due to the change in gas density as precursor gases decompose and produce light hydrogen gas.

[0120] One way to achieve a fairly homogeneous residence time, is by filling a closed decomposition chamber so that the pressure rises. As the pressure increases to a point where the reaction rate becomes significant, the reaction will accelerate at the same time in all parts of the chamber, giving fairly homogeneous growth conditions. The homogeneity of the growth conditions can also be enhanced by a medium filling rate. If the chamber furthermore has walls with a temperature difference, this may enable a thermal convection current mixing the gas and providing the desired temperature fluctuations to produce the ring structures. The reaction can be stopped by pumping or flushing the gas from the decomposition chamber and moving the particles and gas to a cold zone where the reaction stops.

[0121] As silane is consumed faster than most hydrocarbon precursors, this could lead to a gradient with increased carbon content to the surface of the particle which is likely to result in larger expansion in the core section of the particle as than at the periphery, resulting in cracking of the particle during cycling. By continuously feeding more gas into the chamber even after the reaction has started, it is possible to maintain similar conditions in the early and late phases of the particle production, so that the carbon content is mainly controlled by temperature, not primarily by the gas mixture. Also, allowing a little carbon precursor gas to be present at the start of the silane filling, can contribute to smaller differences between core and outside of the particle. The continuous filling of a fixed volume can also contribute to turbulent mixing of the gases, giving more homogeneous gas concentrations, and allowing the temperature variation required for forming the one or more shell-resembling layers inside the bulk material of the particle.

[0122] 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 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 also minor amounts of oxygen (remains of air) inside the reactor. Thus, the produced particles will mainly contain silicon and carbon, and probably some hydrogen and oxygen, and eventual traces of unavoidable impurities.

[0123] 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 (HCLSi), or a mixture thereof.

[0124] 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), propene (CsHe), butene (C4H8), pentene (C5H10), ethyne (C2H2), cyclohexane, cyclohexene, toluene, benzene or mixtures thereof.

[0125] 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 (SiEL) or disilane (Si2He) mixed with a hydrocarbon gas chosen from one of; methane (CEL), ethane (C2H6), propane (CsEE), ethene (C2H4), propene (CsEE), butene (C4H8), pentene (C5H10), hexene (CeHu), ethyne (C2H2), 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. One or more precursor gases comprising both C and Si, such as SiC2Hs can obviously also be used, as long as the gas mixture contains at least two different gases, with different Si:C ratios, and different conversion temperature dependency.

[0126] The manufacturing method according to the second aspect of the invention produces, as seen on the TEM image of a particle of sample SI in figure 1, dense particles with no or very limited measurable porosity after the hydrogen has been removed by a heat treatment. This is further confirmed by FIB-SEM or FIB-TEM cross sections of several example embodiments of the particles. There are no signs of porosity in the individual particles.

[0127] 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 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.

[0128] The term “reactor gas” as used herein, encompasses exhaust gas formed by a previous production of the secondary 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.

[0129] List of figures

[0130] Figure 1 is a high angle annular dark field scanning transmission electron microscopy photograph of an example embodiment, herein labelled SI, of the multilayer particle according to the invention having a plurality of shell-resembling relatively carbon-rich layers embedded in the bulk material.

[0131] Figure 2 presents a diagram of EELS-determined carbon and silicon content in a particle of sample SI taken along a line across the particle according to the invention, and a TEM-image of the particle showing the same section as being applied in the EELS analysis. As can be seen, the EELS average is lower than the total carbon measurement, but the variation is still systematic and significant.

[0132] Figure 3 is a high resolution TEM image of a particle of a sample, herein labelled as S2, according to the invention and subsequently heat treated to 900 °C for 2h. In the image, the individual atoms can be recognized. The image is further treated with a circular band pass filter highlighting areas where the lattice constant matches that of crystalline silicon (0.314 nm), so the brightest spots are atoms incorporated in silicon nanocrystals. The low part of the image is the graphite sample holder. The interlayer spacing in graphite is similar to the filter value. This may explain the somewhat lower intensity spots in the dark part, where the band pass filter dampens the signal, but does not fully remove it. The inset shows that the ring structure in this particle is maintained on the longer length scale after the heat treatment, while the <2 nm domains have undergone a significant reorganization. The production parameters of this powder are so similar to those of HM47B and HM52 that the image of the ring structure is expected to be representative also for those samples.

[0133] Figures 4 and 5 show two TEM images of different particles in a sample embodiment, herein labelled as S4, of the particles according to the invention. The two figures demonstrate that there is some variation in each batch, but that the average carbon content and the characteristic peak heights are very similar for the two particles, even though the rings indicate that they have spent different amounts of time in the different temperature zones.

[0134] Figure 6 is a high angle annular dark field scanning transmission electron microscopy image showing crystalline silicon nanodomains in secondary composite particles (sample S4) after heat treatment for 30 min at 900 °C.

[0135] Figure 7 shows cycling data for a powder sample, herein labelled S5, according to the invention being produced by the method according to the invention and further including 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 based powder in 90 wt% graphite, both are full cells with an LFP counter electrode. 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 taken from a separate cell with only S6, no graphite, to avoid uncertainty in the numbers. The cell maintains an almost unchanged capacity for >100 cycles.

[0136] Figure 8 shows cycling data for a powder sample, herein labelled S6, according to the invention being produced by the method according to the invention and further including 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 based powder in 85 wt% graphite, both are full cells with an LFP counter electrode. 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 taken from a separate cell with only S6, no graphite, to avoid uncertainty in the numbers. The cell maintains a capacity much higher than the reference for >150 cycles.

[0137] Figure 9 is a post mortem SEM image of bitumen coated SiC composite particle (sample S6) after extended cycling. The bright regions indicate silicon. The outer regions show some degradation, presumably from Si co-diffusing with Li out of the particle and forming SiO phases after interacting with the electrolyte. The shape of the particle is largely preserved, because the presence of the SiC internal boundaries prevent significant shape changes.

[0138] Verification of the invention

[0139] The invention will be described in further detail by way of example embodiments.

[0140] The following reactor concept was used to produce the particles used in the examples.

[0141] A roughly cylindrical steel chamber of approx. 30 litres is heated in one half to the highest reaction temperature. In the other end, the tank is connected to a feeding nozzle and a valve leading past a filter to a vacuum pump. The temperature of the chamber wall next to the nozzle and valve have been kept at a lower temperature, but here the given temperature is an estimate, in particular the inner wall temperature was not measured exactly and contains a strong gradient, while the high temperature is measured exactly. We assume no growth is occurring in the coldest part of the chamber. Optimization of the lowest temperature can be performed by any person skilled in the art by adding a sensor and varying the level of insulation on the cold wall.

[0142] The chamber was closed and filled with a gas mixture to a low pressure limit of 20 kPa. A mixture of silane and ethylene was fed into the chamber through the nozzle. The nozzle is somewhat heated from the wall and the gas in the chamber, the exact nozzle temperature has been difficult to determine. The filling time was varied by changing the filling rate. The filling continued until a ‘high pressure limit’ of 90 kPa was reached.

[0143] The gas mixture was allowed to remain in the reactor for a ‘holding time’ after filling was completed. There was some further pressure increase during the ‘holding time’. The gas was pumped from the chamber. Powder was harvested both from the chamber walls and from the filter. The applied parameters for the manufacturing of the samples are summarised in the table below.

[0144] Ring structures according to the invention have been discovered for all samples produced by the method described above, that were investigated with the EELS technique. The experiments with samples S4 and S3 illustrate that small details in the reactor may influence the total carbon consumption, but by varying the flow rates, the desired C content can be achieved through experimental tuning. In HD39 the locations of the hot sections were placed in a way that reduced convection, probably giving longer residence time in each temperature zone and therefore stronger rings. In S3, the reactor had somewhat thicker walls, affecting the heat distribution in the chamber - again influencing the average, but maintaining the features described in the invention.

[0145] Furthermore, the S2 sample was heat treated to 900 °C for 2h, resulting in local reorganization of the atoms. Figure 3 shows that this reorganization does not reach the length scales of the ring structures - the rings remain clearly visible after the heat treatment. To verify that the produced particles show good cyclability, the powders produced by the above method were post-treated in order to remove excess hydrogen, coated with a carbon precursor, and pyrolyzed in order to both make the carbon coating more conductive and improve the local ordering of the Si-C atoms. In this process, the carbon and silicon atoms do not move so much as to affect the observed ring structures (>5 nm movement), but some crystallization or reordering on the <lnm scale is expected to occur. Cross section images of heat-treated and non-treated samples show no systematic differences in the ring structures. For the S5 and S6 samples, the amount of carbon was chosen so that the added carbon made up 2 wt% of the sample total after heat treatment. S5 was degassed at 650 °C, then coated with Polyacrylonitrile and crosslinked at 500 °C, while S6 was coated with bitumen and pyrolyzed at 900 °C. Several different types of PAN and bitumen precursors have been tested, and the results were not substantially different from those presented here.

[0146] For the powders S5 and S6, 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.

[0147] References

[0148] 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.

[0149] 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.jallcom.2021.159315

[0150] 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.

[0151] 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.jmat.2019.03.005

[0152] 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

[0153] 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

[0154] 7 B.H. Toby, R.B. Von Dreele, GSAS-IE the genesis of a modern open-source all purpose crystallography software package, Journal of Applied Crystallography, 46 (2013) 544-549.

[0155] 8 A. A. Coelho, TOP AS and TOPAS-Academic: an optimization program integrating computer algebra and crystallographic objects written in C plus, Journal of Applied Crystallography, 51 (2018) 210-218.

[0156] 9 R.W. Cheary, A. A. Coelho, J.P. Cline, Fundamental parameters line profile fitting in laboratory diffractometers, Journal of Research of the National Institute of Standards and Technology, 109 (2004) 1-25.

[0157] 10 P. Thompson, E.D. Cox, J.B. Hastings, Rietveld Refinement of Debye- Scherrer Synchrotron X-ray Data from Al2O3, Journal of Applied Crystallography, 20 (1987) 79-83.

Claims

CLAIMS1. Silicon-based particles made of a bulk material comprising silicon and carbon, wherein- the silicon-based particles have a median volume-weighted diameter, Dso, from 0.05 to 10 pm, as determined by laser diffraction analysis according to standard ISO 13320:2020, and- the silicon-based particles have a chemical composition comprising:- a total content of carbon, Ctotai being from 5 to 40 atom%, based on the total mass of the silicon-based composite particles, and- the rest being silicon and unintentional impurities, characterised in that- the particles contain one or more internal shell-resembling spatial regions within the bulk material of the particles having an increased carbon content with a peak in the elemental carbon content, Cpeak, being from 1 to 15 atom percentage points higher than an average carbon content, Caverage, and wherein- the peak in the elemental carbon content, Cpeak, of each of the one or more internal shell-resembling spatial regions within the bulk material and the average carbon content, Caverage, are determined by electron energy loss spectroscopy elemental analysis taken along a straight line from a particle core up to but not including the particle surface and with smoothing out variations in the less than 5 nm length scale, preferably less than 2 nm length scale, and most preferably less than 1 nm length scale.

2. The particle according to claim 1, wherein the Cpeak is in the range from 1 to 12 atom percentage points, preferably from 1 to 10 atom percentage points, more preferably from 2 to 8 atom percentage points, more preferably from 3 to 6 atom percentage points, and most preferably from 4 to 5 atom percentage points higher than an average carbon content, Caverage.

3. The particle according to claim 1 or 2, wherein the total chemical composition of the particles comprises:- a total content of carbon from 10 to 37 atom%, preferably from 15 to 33 atom%, more preferably from 20 to 30 atom%, and most preferably from 23 to 26 atom%, and- the rest being silicon and unintentional impurities,4. The particle according to any of the preceding claims, wherein the total oxygen content of the particle 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%.

5. The particle according to any of the preceding claims, wherein the total hydrogen content of the particle is 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%.

6. The particle according to claim 1 or 2, wherein the total chemical composition of the particles comprises:- a total amount of carbon, Ctotai, in the particle from 5 to 30 atom%, preferably from 6 to 25 atom%, preferably from 7 to 20 atom%, more preferably from 8 to 15 atom%, more preferably from 9 to 14 atom%, or most preferably from 10 to 12 atom%,- a total amount of oxygen in the particle is from 0.1 to 4 atom% O, preferably from 0.2 to 3 atom % O, more preferably from 0.3 to 2 atom % O, more preferably from 0.4 to 1 atom % O, or most preferably from 0.5 to 1 atom% O,- a total amount of hydrogen in the particle is less than 5 atom%, preferably less than 1 atom%, and- the rest being Si and unintentional impurities.

7. The particle according to any of the preceding claims, wherein a plurality of the particle has a median volume-weighted diameter, Dso, from 1 to 9 pm, preferably 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 to 5 pm, as determined by laser diffraction analysis according to standard ISO 13320:2020.

8. The particle according to any of the preceding claims, wherein a plurality of the particle according to the invention has a D90 / D10 ratio in the range from 1 to 10, preferably from 2 to 8, and most preferably from 3 to 6, as determined by laser diffraction analysis according to standard ISO 13320:2020.

9. The particle according to any of the preceding claims, wherein the particle further comprises a plurality of nanoscale silicon domains embedded in the bulk material, and where the average diameter of the nanoscale silicon 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 diffusion (XPD) data, optionally after a heat treatment of 900 °C or higher for at least half an hour.

10. 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, TixO, or AkO, and- have a thickness in the range 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 10 nm, preferably from 2 to 5 nm, as determined by Auger spectroscopy.

11. The particle according to any of the preceding claims, wherein the determination of the Cpeak of the at least one shell-resembling relatively carbon-rich layer by electron energy loss spectroscopy elemental analysis is obtained by:- preparing a cross-sectional slice of less than 70 nm thickness including the particle centre of the particle by focused ion beam (FIB),- performing the EELS elemental analysis along a linear line running through the cross-section from the centre to the outer surface of the slice, and then- optionally, smoothening the measurements by using a gaussian digital filter or by averaging the measurements made for a plurality of neighbouring spatial measuring-areas, and- applying the local maximum for the carbon content of a first peak in the carbon content along said line at a distance from the particle centre to define the Cpeak of the first peak and if present, make a similar determination of the Cpeak for each other corresponding pair of peaks of increased carbon content and decreased silicon content present on said line, and- determining the Caverage by averaging the determined C-content of all measurement points along the line except for the outer surface region.

12. The particle according to any of the preceding claims, wherein the smoothening out of the variations of the electron energy loss spectroscopy elemental analysis is obtained by either:- using a gaussian digital filter having a width of the digital filter of < 5 nm , preferably < 2 nm, and most preferably < 1 nm, or: averaging the measurements made for a plurality of neighbouring spatial measuring-areas.

13. A method for producing silicon-based particle, wherein the method comprises the steps of:- 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 from 0.2 to 50,- preheating the precursor gas mixture to a temperature of 300 to 350 °C- applying a reactor having a decomposition chamber heated to a first reaction temperature at which the precursor gas mixture will condense and formparticle seeds which subsequently grows by chemical vapour decomposition (CVD), and- injecting the precursor gas mixture into the decomposition chamber, characterised in that the method further comprises:- maintaining the injected precursor gas mixture inside the decomposition chamber for a residence time in the range from 1 to 300 seconds and, during the residence time, subjecting the injected precursor gas mixture to at least one temperature alteration from the first reaction temperature to a second reaction temperature and then back to the first reaction temperature, and then- extracting the particles from the decomposition chamber, wherein- the absolute temperature difference between the first and second reaction temperature is in the range from 1 to 100 °C, preferably from 3 to 75 °C, more preferably from 5 to 50 °C, more preferably from 10 to 35 °C, and most preferably from 20 to 25 °C.

14. The method according to claim 13, wherein the first reaction temperature is in the range from 450 to 900 °C, preferably from 500 to 800 °C, more preferably from 550 to 700 °C, and most preferably from 600 to 650 °C.

15. The method according to claim 13 or 14, wherein the residence time is in the range from 2 to 250 seconds, preferably from 3 to 200 seconds, more preferably from 5 to 150 seconds, more preferably from 10 to 120 seconds, more preferably from 15 to 90 seconds, and most preferably from 20 to 60 seconds.

16. The method according to any of claims 13 to 15, wherein the atomic ratio between silicon and carbon, Si:C, 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.

17. The method according to anyone of claims 13 to 16, wherein:- the first precursor gas is one or more of: silane (Sifh) and disilane (Si2He), and- the second precursor gas is one or more of methane (CHf), ethane (C2H6), propane (CsHs), ethene (C2H4), hexene (CeHu), ethyne (C2H2), cyclohexane, cyclohexene, toluene, and benzene.

18. The method according to anyone of claims 13 to 17, wherein- the method further comprises preloading the decomposition chamber with a first reactor gas having an initial pressure in the range from 5- 103to 6- 105Pa at the first reaction temperature, and- the first reactor gas is one or more of argon, nitrogen, or an exhaust gas of a previous particle production.

19. The method according to claim 18, wherein the method:- applies a closed decomposition chamber preloaded with the first reactor gas, and- the precursor gas mixture is injected gradually over a time period of at least 50 % to 100 % of the residence time and, when the residence time is obtained, the decomposition chamber is opened and flushed with an inert gas to extract the formed particles.

20. The method according to anyone of claims 13 to 18, wherein the at least one temperature alteration from the first reaction temperature to a second reaction temperature is obtained by either:- applying a reactor having a hot-wall decomposition chamber comprising at least one first zone and at least one second zone, where the at least one first zone has a wall temperature equal to the first reaction temperature, and the at least one second zone has a wall temperature equal to the second reaction temperature, and- transporting the precursor gas mixture in the hot-wall decomposition by thermal convection currents between the at least one first and the at least one second zones, or:- applying a reactor having a hot-wall decomposition chamber comprising an injection zone and a first decomposition zone having walls with a wall temperature equal to the first reaction temperature, and- forming a turbulent flow in the injected precursor gas mixture inside the decomposition chamber transporting the second reactor gas mixture between the injection zone and the first decomposition zone of the of the how-wall decomposition chamber, or:- applying a reactor having a tubular hot-wall decomposition chamber comprising a plurality of alternating first zone and second zones, where the first zones have a wall temperature equal to the first reaction temperature, and the second zone has a wall temperature equal to the second reaction temperature, and- injecting the precursor gas mixture, optionally mixed with a first reactor gas to form a second reactor gas, at a first end of the tubular hot-wall decomposition chamber and passing the precursor gas mixture, optionally the second reactor gas mixture, through the tubular hot-wall decomposition chamber under laminar flow conditions with a Reynolds number of less than 2000.

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