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1 Silicon-based micro-sized particles and use of them in secondary LIBs The present invention relates to silicon-based particles suitable for use as the active material in negative electrodes in secondary lithium-ion batteries (LIBs) and a method of manufacturing the particles. 5 Background Carbon-containing fossil fuels covers currently around 80 % of the global energy demand. The major part of the fossil fuels are combusted with varying degree of cleaning of the produced exhaust/combustion gases before venting them to the atmosphere. These emissions amount to a major pollution problem, a global 10 warming problem, and an ocean acidification problem. There is therefore a growing desire and interest in the society for developing and implementing climate neutral and non-polluting alternatives. Electric power is a versatile form of energy which hardly pollutes when used to provide heat, drive electric engines, run electronics, etc. Furthermore, some sectors 15 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 20 available LIBs may provide to fully take advantage of the electrification option. The major energy storing constraint of present LIBs is their graphite-based negative electrodes since graphite has a relatively limited capacity to store lithium. There is therefore a desire in the battery sector to find other and better suited materials than graphite as the active material of the negative electrode of LIBs. 25 Prior art Silicon is known to have a relatively strong capability for taking up lithium and form a silicon and lithium alloy. At typical ambient temperatures, the most lithiated phase of silicon is Li3.75Si 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 30 industry has therefore for more than a decade sought to find a solution to apply silicon as the active material in the negative electrode of secondary LIBs. However, the lithium uptake causes a significant volume fluctuation of the silicon material. At is most lithiated state of Li3.75Si, the silicon material has a volume of around 320 % higher as compared to its non-lithiated state. Also, electrolyte 35 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 2 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. 5 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 10 applying the silicon in the form of nanoscaled particles, typically less than 200 nm, preferably with a surface coating. Sourice et al. (2016) [Ref 1] discloses producing amorphous silicon core particles of 30 nm diameter by laser-driven chemical vapour pyrolysis (LCVP) of silane gas diluted in helium. The particles are given a 1 nm thick carbon coating made by a 15 second LCVP stage of ethylene gas. The particles are reported to, after 500 charge/discharge cycles, retaining a capacity of 1250 mAh.g−1 at a C/5 rate and 800 mAh.g−1 at 2C, with an outstanding coulombic efficiency of 99.95%. It is further known that nanoscaled silicon-based particles containing other elements may be manufactured in industrial scale by thermally induced decomposition of a 20 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 25 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 30 nitrogen. The residence time was approx. 1 second. The particles are described to have a homogenous structure. Orthner et al. 2021 [Ref 2] reports a study on formation of amorphous silicon-based particles by flowing a mixture of silane and ethylene gas diluted in nitrogen at atmospheric pressure through a tubular hot-wall reactor at 640, 690, and 1100 °C. 35 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 homo- geneous 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 3 200 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, 5 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 10 was found to be 70 nm. Orthner informs further that the formation of SiC was found to be disadvantageous due to particles exhibiting significantly lower first cycle efficiency and specific capacity (of 917 mAh/g) compared to pure Si. WO 2022/200606 discloses that a heat treatment at relatively high temperature and long endurance may transform amorphous structures into crystalline structures. The 15 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 20 nanosized crystallites of 1 to 15 nm in diameter embedded therein. It is further known that baking nanoscaled silicon particles in a carbon matrix may provide stable particles and reducing the formation of SEI-layers. Wang et al. (2013) [Ref 3] discloses composite particles made by pyrolyzing a mixture of nanoscaled silicon particles of 50 – 100 nm in a coal tar pitch followed by 25 comminuting the pyrolyzed mixture to form a composite of Si-particles embedded in an amorphous carbon matrix (Si/αC). 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 30 wrapped by amorphous SiOx and amorphous carbon in the (Si/αC) composite which can supply sufficient conductivity and strong elasticity to suppress the stress resulting from the reaction of Si with Li during charge/discharge process. Zhu et al. (2018) [Ref 4] reports a study investigating the correlations between key physical parameters and electrochemical properties of silicon particles when used in 35 the anode of LIBs. The investigations included three samples of crystalline silicon particles denoted as S1, S2, and S3 which had a BET specific surface area of 41.4, 36.11, and 7.33 m2/g, respectively. This corresponds to a D50 particle size of approx. 50, 100, and 150 nm. Each of the three particle samples were mixed with conductive carbon and sodium alginate binder and then applied on a copper 40 conductor to form three anode samples with the S1, S2, and S3 particles, 4 respectively. The amount of active material loading was ca. 0.5 mg/cm2 in each anode sample. The anode samples were assembled in CR2032-type coin half-cells having the same electrolyte and cathode to investigate the effect of the silicon particle size of the anode on the electrochemical properties of the cells. The 5 investigations show that all three cells with S1, S2, and S3, respectively had a reversible capacity of approximately 2500 mAh/g while the first cycle coulombic efficiencies (FCE) obtained with S1, 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 10 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 15 sizes. At a rate of 20C, the delivered capacities for S1, 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 S1, S2 and S3 are 96.12 %, 93.98 % and 76.73 %, respectively. This result was attributed 20 to the large Si particles being more prone to pulverize and crack during the repeated charge-discharge cycles, especially when the particle size is larger than 150 nm, as reported in the literature. Another factor which speaks for applying small silicon particles, reported by Rhenlund et al. (2017) [Ref 5], is diffusion controlled trapping of lithium in the 25 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. 30 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 35 synthetized films was grown on either a planar amorphous carbon nanoparticle substrate or on spherical graphite particles and then coated with 5 wt% pitch based carbon and annealed at 900 °C. The films were made to have a thickness of 20-25 nm or 60-70 nm which took around 45 minutes or around 78 minutes to grow, respectively. The particle sizes of the graphite particles at which the films were 40 deposited is not disclosed explicitly in the document but is seems from figure 3 of 5 Sung et al. (2021) that the graphite particles where substantially spherical and had a diameter around 10 µm. 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 5 and C between the silicon crystallites as shown schematically in figure 1 of Sung et al. (2021), replicated as figure 1 herein. The calculation showed further that the lower silane to ethylene ratio, the smaller the silicon crystallites in the film became. This result was confirmed by analysis of the synthesized films which found that the silicon crystallites in the film had a size ranging from 40 nm for a pure Si-film 10 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 (corre- 15 sponds to 20.1 wt% C) and was found to exhibit a specific capacity of approx. 2000 mAh/g. The silicon crystallites in this film had an average particle size of 0.97 nm. The document further reports - as expected – that the specific capacity and FCE of the synthesized films decreased significantly with increasing carbon content in the films. However, the capacity did not fall with increased film thickness as would be 20 expected. This result is described in Sung et al. as an indication that it is possible to increase the Si amount without any side effects of the growth of Si size, which has been a severe limitation for high specific capacity via a chemical vapour deposition process. Document CN 115 881 931 A1 discloses a composite material for a secondary 25 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 30 (XRD); a solid nuclear magnetic resonance (NMR) detection 29Si 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 D50 of the novel composite material is 35 disclosed to be from 1 nm to 50 µm, and the mass of the carbon atoms accounts for 0.5 – 50 % of the mass of the novel composite material. Objective of the invention The main objective of the invention is provision of silicon-based particles suited for being used as active material in the negative electrodes of secondary lithium ion 40 batteries. 6 A further objective of the invention is to provide a method suited for large scale industrial manufacturing of these silicon-based particles. Description of the invention The present invention relates to silicon and carbon based particles suitable for use 5 as an active material in the negative electrode of secondary lithium-ion batteries and may be considered being an improvement of the secondary particles described in European patent application No. EP22158616.7. The particles described in EP22158616.7 are manufactured by injecting a homo- geneous gas mixture comprising a first silicon containing precursor gas and a 10 second carbon containing precursor gas at an atomic ratio Si : C in the range from near zero up to 10 into a reactor space holding a reaction temperature in the range of 500 to 1200 °C, most preferably 700 to 900 °C. The gas mixture of precursor gases may be preheated to a temperature in the range of less than 300 up to 500 °C. XRD analysis of the resulting particles finds that they are a composite of an 15 amorphous silicon and carbon matrix having nanoscaled domains of amorphous silicon embedded therein. The particle sizes are in the range from 10 n...