Process for the preparation of silicon-containing composite particles

Abstract

A process for preparing composite particles, the process comprises the steps of: (a) providing a plurality of porous particles in a reactor, the reactor having an outer wall defining an internal volume for containing the plurality of porous particles; (b) stirring the plurality of porous particles with a stirring element located in the internal volume, the stirring element having one or more stirring surfaces, each stirring surface being an area of the stirring element defining an oblique angle to its velocity vector during stirring and arranged such that movement of the stirring surface through the plurality of porous particles urges the plurality of porous particles along the outer wall at an angle to the velocity vector and / or away from the outer wall, wherein either: the outer wall is movable for causing the stirring of the plurality of particles and the one or more stirring surfaces of the stirring element protrude from an inner surface of the movable outer wall; or the stirring element defines a clearance between the stirring element and an inner surface of the outer wall of less than 10% of an internal dimension of the reactor measured along the direction of the clearance; herein the mass of the plurality of porous particles contained in the internal volume during stirring is dependent on the volume of the internal volume, being no more than 500 kg m-3, and is dependent on the surface area of the one or more stirring surfaces, being no more than 1000 kg m-2, and wherein the one or more stirring surfaces pass through a volume per second during stirring that depends on the mass of the plurality of porous particles contained in the internal volume, being between 0.001 and 0.1 m3 s-1 kg-1; and; and (c) during stirring the plurality of porous particles, contacting the plurality of porous particles with a silicon precursor gas at conditions effective to cause deposition of silicon in the pores of the porous particles to provide composite particles comprising a porous particle framework and elemental silicon within the pores of the porous particle framework.

Description

[0001]PROCESS FOR THE PREPARATION OF SILICON-CONTAINING COMPOSITE PARTICLES FIELD OF THE INVENTIONThis invention relates to processes for the production of silicon-containingcomposite particles that are suitable for use as anode active materials in rechargeable lithium-ion batteries. BACKGROUND A typical lithium-ion battery (LIB) comprises an anode, a cathode and a lithium- containing electrolyte. The anode generally comprises a metal current collector provided with a layer of an electroactive material, defined herein as a material which is capable of inserting and releasing lithium ions during the charging and discharging of a battery. The terms “cathode” and “anode” are used herein in the sense that the battery is placed across a load, such that the anode is the negative electrode. When a LIB is charged, lithium ions are transported from the cathode via the electrolyte to the anode and are inserted into the electroactive material of the anode as intercalated lithium atoms. The term “battery” is used herein to refer both to devices containing a single lithium-ion cell and to devices containing multiple connected lithium-ion cells. LIBs were developed in the 1980s and 1990s and have since found wide application in portable electronic devices. The development of electric or hybrid vehicles in recent has created a significant new market for LIBs and renewable energy sources have created further demand for on-grid energy storage which can be met at least in part by LIB farms. Overall, global production of LIBs is projected to grow from around 290 GWh in 2018 to over 2,000 GWh in 2028. Alongside the growth in total storage capacity, there is significant interest in improving the gravimetric and / or volumetric capacities of rechargeable metal-ion batteries such that the same energy storage is achieved with less battery mass and / or less battery volume. Conventional LIBs use graphite as the anode electroactive material. Graphite anodes can accommodate a maximum of one lithium atom for every six carbon atoms resulting in a maximum theoretical specific capacity of 372 mAh / g in a lithium-ion battery, with a practical capacity that is somewhat lower (ca. 340 to 360 mAh / g). Silicon is a promising alternative to graphite because of its very high capacity for lithium (see, for example, Insertion Electrode Materials for Rechargeable Lithium Batteries, Winter, M. et al. in Adv. Mater. 1998, 10, No. 10). Silicon has a theoretical maximum specific capacity of about 3,600 mAh / g in a lithium-ion battery (based on Li15Si4). However, the intercalation of lithium into bulk silicon results in expansion of the silicon material by up to 400% of its original volume which can lead to failure of the battery. Repeated charge-discharge cycles cause significant mechanical stress, resulting in fracturing and delamination of the silicon. The formation of a solid electrolyte interphase (SEI) layer on the silicon surface consumes the electrolyte and newly exposed silicon surfaces on fracture surfaces results in further electrolyte decomposition and increased thickness of the SEI layer and irreversible consumption of lithium. These failure mechanisms collectively result in an unacceptable loss of electrochemical capacity over successive charging and discharging cycles. The applicant has previously reported the development of a class of electroactive materials having a composite structure in which electroactive materials, such as silicon, are deposited into the pore network of a highly porous conductiveparticulate material, e.g. a porous carbon material (see WO 2020 / 095067 andWO 2020 / 128495). The silicon in these materials is finely divided with individualsilicon structures having dimensions of the order of a few nanometres or less which therefore undergo minimal stress and strain during charging and discharging. As the silicon is confined to the pore volume of a porous material, exposure of the silicon surfaces to electrolyte is minimised, effectively limiting the extent of SEI formation. As a result, these materials exhibit good reversible capacity retention over multiple charge-discharge cycles.The materials described in WO 2020 / 095067 and WO 2020 / 128495 have beensynthesized by chemical vapour infiltration (CVI) in different reactor systems (static, rotary and fluidised bed reactors). More recently, close-clearance stirredbed reactors have been developed, as described in WO 2023 / 117047, toovercome problems present with each type of reactor system that lead to inhomogeneities in the final product. A problem with close-clearance stirredreactors of the sort disclosed in WO 2023 / 117047 is that they tend to producecompact regions of particles along the outer wall of the reactor that move tangentially along the stirring direction. These close-clearance stirred reactors therefore tend to achieve poor aeration of the bed and poor vertical circulation of particles within the reactor. These phenomena lead to inhomogeneities in the final composite particles. There is therefore a need for improved processes for producing composite particles using stirred bed reactors that overcome the above problems. SUMMARY OF INVENTIONIn accordance with a first aspect of the invention, there is provided a process fortreating particles, the process comprising the steps of: (a) providing a plurality ofporous particles in a reactor, the reactor having an outer wall defining an internal volume for containing the plurality of porous particles; (b) stirring the plurality of porous particles with a stirring element located in the internal volume, the stirring element having one or more stirring surfaces, each stirring surface being an area of the stirring element defining an oblique angle to its velocity vector during stirring and arranged such that movement of the stirring surface through the plurality of porous particles urges the plurality of porous particles along the outer wall at an angle to the velocity vector and / or away from the outer wall, wherein either: the outer wall is movable for causing the stirring of the plurality of particles and the one or more stirring surfaces of the stirring element protrude from an inner surface of the movable outer wall; or the stirring element defines a clearance between the stirring element and an inner surface of the outer wall of less than 10% of an internal dimension of the reactor measured along the direction of the clearance; wherein the mass of the plurality of porous particles contained in the internal volume during stirring is dependent on the volume of the internal volume, being no more than 500 kg m-3, and is dependent on the surface area of the one or more stirring surfaces, being no more than 1000 kg m-2, and wherein the one or more stirring surfaces pass through a volume per second during stirring that depends on the mass of the plurality of porous particles contained in the internal volume,being between 0.001 and 0.1 m3 s-1 kg-1; and (c) during stirring the plurality ofporous particles, contacting the plurality of porous particles with a gas at conditions effective to cause a reaction between the plurality of porous particles and the gas. The present inventors realised that there are several phenomena leading to the poor operating efficiencies of known close-clearance stirred bed reactors, which generally contribute to poor aeration of the stirred particle bed. During operation, these reactors may be considered as comprising a concentrated phase, which isa region of the reactor in which there is a relatively high concentration of porousparticles and a relatively low concentration of gas, and a dilute phase, which is a region in which there is a relatively low concentration of porous particles and a relatively high concentration of gas. The aim of a stirred bed reactor, i.e. goodaeration of the particle bed, may be considered in terms of the transition betweenthese phases within the reactor, and to achieve good aeration it is desirable tominimise the difference between these phases and ensure there is a gradual transition between these phases within the reactor. A first issue affecting this aim is the loading amount of the porous particles compared to the volume of the reactor. Relatively high loading amounts mean that there is little space for the particles to be moved into during stirring. A highloading amount will therefore mean that a region of the bed may be aerated by astirring process, but there may be large parts of the bed that cannot sufficientlyaerated and remain in a highly concentrated phase. The present inventors havefound that, to ensure it is possible to evenly aerate the particle bed, the mass of the plurality of porous particles contained in the internal volume should be no more than 500 kg m-3of the internal volume. A second issue relates to the area of the stirring element compared to the loading amount of the porous particles compared to the volume of the reactor. A relatively low loading amount, as discussed above, may ensure that there is sufficient space in the reactor to suitably aerate substantially the whole particle bed. However, in order to practically be able to induce this aeration of the bed using the stirring element, there must be sufficient surface area of the stirring surfaces compared to the loading amount of the porous particles. The surface area of the stirringsurfaces affects the rate at which the stirrer may displace particles as it movesthrough the bed in order to move particles from the concentrated phase towardsthe dilute phase. As the surface area of the stirring surfaces decreases for a fixedamount of the particles, it becomes necessary to move the stirring surfacesthrough the particle bed faster in order to achieve the same amount of aeration of the bed. However, when stirring speeds become too high, as mentioned above, this tends to produce compact regions of particles along the outer wall of the reactor that move tangentially along the stirring direction. Therefore, in order to ensure that sufficient particle agitation can be achieved at lower speeds, it has been found that the mass of the plurality of porous particles contained in the internal volume should also depend upon the surface area of the one or more stirring surfaces, specifically being no more than 1000 kg m-2of stirrer surface area. Finally, with a suitable loading amount of the particles, dependant on both thereactor volume and the stirrer surface area, it is necessary to ensure that sufficientvolume of the internal volume of the reactor is swept by the stirring surfaces and the stirring speed is in a good window for sufficiently aerating the particle bed without inducing the particle bed to compact along the outer wall or restrict movement to overwhelmingly be along the stirring direction. With the above loading amounts, good bed aeration has been observed when the one or more stirring surfaces pass through a volume per second that depends on the mass ofthe plurality of porous particles contained in the internal volume, being between0.001 and 0.1 m3 s-1 kg-1 during stirring. In other words, each kilogram of particleswill require a certain volume to be swept per second by the stirring surfaces in order to appropriately aerate the particle bed. This parameter therefore definesthe total volume of the internal volume of the reactor which is moved through byat least one of the stirring surfaces, in dependence on the mass of particles in thereactor and in dependence of the frequency of the stirring action. An examplecalculation of this is provided below in the detailed examples.In other terms, the present arrangement defines an area of operation which notonly provides an improvement of the heat and mass transfer but also provides an optimum range of operation that avoids overloading of the volume of the reactor where the plurality of porous particles are contained to allow for a proper aeration and avoiding highly concentrated areas and also avoids potential entrainment ofthe porous particles due to any traversed gas flow through the reactor. The presentarrangement also provides for an area of operation where there is sufficient stirringsurface area as to provide for relatively low difference in porous particlesconcentration. If the area was too low, the process would generate segregatedareas of concentrated phase and diluted phase due to the cohesive nature of the porous particles or could even create areas where the powder is not moving at all as the momentum cannot be effectively transmitted from the aerated areas. If the one or more stirring surfaces that pass through a volume per second during stirring of the mass of the plurality of porous particles contained in the internal volumewas too high then the process would be inducing the particle bed to compact alongthe outer wall, which can reduce heat transfer from the reactor walls, or restrictmovement to overwhelmingly be along the stirring direction. As set out above, the present process uses a stirring element having one or more stirring surfaces, which are areas of the stirring element defining an oblique angle to its velocity vector during stirring and which are arranged such that movement of the stirring surface through the plurality of porous particles urges the plurality of porous particles along the outer wall at an angle to the velocity vector and / or away from the outer wall. Thus, the stirring surfaces here refer to certain front-facing surfaces of the stirring element, i.e. which generally face at least partially in the direction of movement during stirring. It will be appreciated that there may be, for example, opposite faces to the stirring surfaces, e.g. rear-facing surfaces;however, as these do not face the stirring direction they are not arranged to urgethe plurality of porous particles along the outer wall at an angle to the velocity vector and / or away from the outer wall during stirring. Any such faces aretherefore not to be considered as part of the stirring surfaces, and as such theirarea is not accounted for when determining particle loading amount in line with the above. It will be appreciated that the stirring element may also have other portions that do not define an oblique angle to its velocity vector during stirring. Finally, it should be noted that the stirring surfaces may be arranged to induce motion along the outer wall or away from the outer wall, which both tend to move particles in the direction of a dilute phase of the reactor. A surface that defines an angle about an axis perpendicular to the outer wall will urge particles along the outer wall. A surface that defines an angle about an axis parallel to the outer walland perpendicular to its velocity vector will urge particles either away from theouter wall if its leading edge is closer to the outer wall than a trailing edge, for example. The stirring surfaces may also define an angle with components about both these directions to urge particles both along and away from the outer wall. The invention is concerned with so-called close-clearance stirred reactors. This includes both reactors in which a stirring element is separated from an outer wall of the reactor by a relatively small distance and reactors in which the stirring surfaces of the stirring element protrude from an inner surface of a movable outer wall, i.e. zero clearance, although the former is preferred. As indicated above, a small clearance is considered to be a clearance of less than 10% of an internal dimension of the reactor measured along the direction of the clearance. This clearance direction is generally a direction perpendicular to the velocity vector of the stirring element and, where the stirring element is rotated above an axis, is generally the direction perpendicular to the axis of rotation. It will be appreciated that this clearance refers to the closest clearance of the stirring element to the outer wall, and other parts of the stirring element may have other clearances to the outer wall. Preferably, the stirring element is provided with close-clearance(i.e. less than 10% of an internal dimension of the reactor) to the outer wall alongat least 30% of the length of the reactor wall along a direction perpendicular to the velocity vector of the stirring element, preferably at least 40%, more preferably at least 50%, more preferably at least 60%, most preferably at least 70%. The porous particles used in the present process may be of many different types. The porous particles may comprise open and / or closed pores. The pores of the porous particles may make up between 10% and 95% of the particle volume. Theporous particles will typically comprise porous carbon particles, but could also comprise porous particles comprising silicon carbide, silica, alumina, titania, zirconia, metal nitrides (e.g., Si, Al, Ti, etc.), mixed metal oxynitrides (e.g., SiAlON), mixed metal oxides of Si, Al, Ti, etc., and metal oxycarbides, as well as others.The above process is typically a process of preparing composite particles in whichstep (c) comprises during stirring the plurality of porous particles, contacting theplurality of porous particles with a gas at conditions effective to cause depositionin the pores of the porous particles. Preferably, the process is a process ofpreparing composite particles in which step (c) comprises during stirring theplurality of porous particles, contacting the plurality of porous particles with asilicon precursor gas at conditions effective to cause deposition of silicon in thepores of the porous particles to provide composite particles comprising a porousparticle framework and elemental silicon within the pores of thee porous particle framework. Such a process forms composite particles of the sort described inWO 2020 / 095067 and WO 2020 / 128495, which are found to be particularly usefulfor forming the electrode of a rechargeable metal-ion battery. The resulting composite particles may still be porous, with open and / or closed pores remaining, or the pores may be substantially filled in by the deposition process. The porosity of the composite particles may also be between 0.5% and 80% of the particlevolume (including open and closed pore volume), but will more typically be at thelower end of this range, and typically between 5% and 60% due to deposition ofmaterial in the pores. While deposition using a silicon precursor gas is preferred, the present process can be applied to other heterogeneous solid-gas reactions that contact a plurality of porous particles with gas in order to induce some reaction between the particles and the gas. Other suitable gases include metal halides having the generic formula MXn, where M may be Al, Zn, Ti, Mn, Co, Ni, etc., and X is preferably Cl, but could also be F or Br. In the notation MXn, n is an integer value based on oxidation state of M. Other suitable gases include organometallics, such as metalalkoxides and metal acetylacetonates, in which the metal may again be Al, Zn, Ti,Mn, Co, Ni, etc. Reactive gases, such as ammonia and hydrogen sulphide mayalso be used, for example, to produce nitrides and sulphides within the pores.Similarly, boron containing gases may be used, such as, diborane,trimethylborate, etc. Other possible coating processes including coatings generating layers with TiO2or Al2O3, including ALD type reactions, and in which case the case the temperature of step (c) will typically be between 100°C to 700°C.As well as deposition processes, the present method may include other reactionprocesses, such as passivation processes, including passivation processes thattake place during the preparation of composite particles of the sort discussedabove. For example, such a process may involve, after deposition of the silicon, a passivation process, in which, for both the deposition and passivation processes, the particles are processed within the disclosed parameter ranges. The advantages described herein with respect to aeration of the bed and mixing of the concentrated and dilute phases will also provide the same advantages in the context of a passivation process, for example, ensuring that the particles are more evenly passivated. A passivation process may involve the particles being passivated in a low oxygen concentration environment such as where the oxygen concentration is less than 10 vol% oxygen, or an inert gas such as nitrogen may be used to passivate the composite particles. A low-oxygen concentration gas mixture may also be used. In more detail, in a passivation process, the gas used in step (c) may be oxygen containing gases (such as oxygen, air, steam, preferably at a temperature in therange 15°C to 500°C), carbon dioxide (preferably at a temperature in the range15°C to 500°C), ammonia, phosphine, hydrocarbons such as alkenes, alkynes orcarbonyl functional groups, preferably terminal alkene, terminal alkyne, aldehydeor ketone group (preferably at a temperature in the range 15°C to 700°C) ethylene,propylene, 1-butene, butadiene, 1-pentene, 1,4-pentadiene, 1-hexene, 1-octene, styrene, divinylbenzene, acetylene, phenylacetylene, norbornene, norbonadiene and bicyclo[2.2.2]oct-2-ene. Mixtures of passivating gases may also be used.This passivation step, if taking place after a silicon deposition process, may takeplace in the stirred reactor while stirring with the same parameters discussed herein. Passivating the intermediate particles may remove the reactive Si-H bonds. The particles may also be cooled, optionally in combination with thepassivating step. The cooling may be to a temperature of below 100 °C, or below50 °C, or to ambient temperature, e.g. to 20 °C.Another example process is the deposition of a conductive carbon coating onto the particles by a carbon precursor gas. This may involve the thermal decomposition of a volatile carbon-containing gas (e.g. ethylene, acetylene,propane, propylene, ethane and other alkanes, alkenes or alkynes, C2 to C10hydrocarbons, cycloalkanes, cycloalkenes, arenes, polycyclic hydrocarbons andtheir derivations) onto the surface of the particulate material. The temperature of step (c) will typically be between 350°C to 700°C. By performing this process while stirring the particles in line with the presently disclosed parameter ranges, the same advantages of aeration and phase mixing are achieved in this process as well. Other suitable processes include functionalisation processes, in which a functionalgroup precursor is contacted with the porous particles and then evaporated toallow for the reaction providing the surface functional group. Precursors include:silanes with epoxy end-functional group (e.g., amino- or glycidyl-alkyl-trialkoxy-silane where, the alkyl group could be C1 – C6 and alkoxy group could be methoxyor ethoxy); imide surface functional group for silicon surface such as, poly(ether imide) or PEI; and functionalisation of silicon surface of composite material with electrolyte. Additionally, the process may be doping or addition of dopants in gas phase topretreat the initial porous particles or to treat the composite particles afterdeposition. Suitable dopants include nitrogen, phosphorus, sulphur, boron, which can significantly increase electrical conductivity and improve the structural stabilityof silicon-carbon composite particles, fluorine, which can create a stable solidelectrolyte interphase (SEI), and transition metals, such as Ti, Co, Ni, Mn, Mo, W, which can enhance conductivity and stabilises the structure of composite particles. It is generally preferred to have lower upper loading limits for the plurality of porous particles relative to both the reactor volume and the stirrer surface area. Reducing the upper loading limit brings further improvements by allowing more space for the particle bed to be displaced into and increasing the ratio of the stirrer area to the particle bed. Therefore, preferably, the mass of the plurality of porous particles contained in the internal volume during stirring is dependent on the volume of theinternal volume, being no more than 400 kg m-3, preferably no more than300 kg m-3, more preferably no more than 250 kg m-3. Similarly, preferably themass of the plurality of porous particles contained in the internal volume during stirring is dependent on the surface area of the one or more stirring surfaces, beingno more than no more than 750 kg m-2, preferably no more than 500 kg m-2, morepreferably no more than 400 kg m-2, most preferably no more than 300 kg m-2.Particularly preferable combinations include a mass of no more than 400 kg m-3relative to reactor volume and no more than 1000 kg m-2 relative to stirrer area, ora mass of no more than 400 kg m-3 relative to reactor volume and no more than750 kg m-2 relative to stirrer area, or a mass of no more than 400 kg m-3 relativeto reactor volume and no more than 300 kg m-2 relative to stirrer area, or and amass of no more than 300 kg m-3 relative to reactor volume and no more than750 kg m-2 relative to stirrer area, or and a mass of no more than 300 kg m-3relative to reactor volume and no more than 500 kg m-2relative to stirrer area.While the upper loading limit for the porous particles has been discussed abovein relation to the volume of the reactor and the surface area of the stirrer, it is also preferable to set a lower loading limit. In general, decreasing loading below theabove upper limit will further improve aeration, but with diminishing returns andvery low loading amounts are generally inefficient. It is therefore preferable that the mass of the plurality of porous particles contained in the internal volume during stirring is dependent on the volume of the internal volume, being no less than15 kg m-3, preferably no less than 50 kg m-3, more preferably no less than100 kg m-3. Similarly, it is preferable that the mass of the plurality of porousparticles contained in the internal volume during stirring is dependent on thesurface area of the one or more stirring surfaces, being no less than 1 kg m-2,preferably no less than 3 kg m-2, more preferably no less than 5 kg m-2, morepreferably no less than 10 kg m-2, most preferably no less than 15 kg m-2.Preferably, the mass of the plurality of porous particles contained in the internalvolume during stirring is between 15 kg m-3 and 500 kg m-3 of the volume of theinternal volume, preferably between 50 kg m-3 and 500 kg m-3, more preferablybetween 50 kg m-3 and 400 kg m-3, more preferably between 100 kg m-3 and400 kg m-3, more preferably between 100 kg m-3 and 300 kg m-3, most preferablybetween 100 kg m-3 and 200 kg m-3. Preferably, the mass of the plurality of porousparticles contained in the internal volume during stirring is between 1 kg m-2 and750 kg m-2 of the surface area of the one or more stirring surfaces, preferablybetween 3 kg m-2 and 500 kg m-2, more preferably between 10 kg m-2 and400 kg m-2, more preferably between 30 kg m-2 and 300 kg m-2, more preferablybetween 40 kg m-2 and 200 kg m-2.In combination with the above loading amounts, the porous particles may have atap density of at least 0.25 g / cm3 and no more than 1.5 g / cm3. Preferably theporous particles have a tap density of at least 0.3 g / cm3, or at least 0.35 g / cm3orat least 0.4 g / cm3, or at least 0.5 g / cm3. Preferably the porous particles have a tapdensity no more than 1 g / cm3, or no more than 0.8 g / cm3, or no more than 0.7g / cm3. The tap density is measured in accordance with ISO 3953:2011 and ISO 787- 11:1981 (Determination of tamped volume and apparent density after tamping), for example using a QuantachromeTMAutotap, using a minimum sample volume of at least 8ml. Drop height of the instrument is 3mm and tapping frequency of the instrument is fixed at 250-265 taps / min. The sample is tapped at least 5,000 times. If the sample volume is still observed to be changing after 5,000 taps then additional increments of 1,250 taps are applied until no further volume change is observed. In combination with the above loading amounts, the porous particles may have aHausner ratio of at most 4.0, preferably at most 3.0, more preferably at most 2.0.The Hausner ratio may also be least 1.2, preferably at least 1.4, preferably at least 1.6, preferably at least 1.8. Preferably, the porous particles may have a Hausnerratio in the range 1.2 to 4.0, preferably 1.2 to 3.0, preferably 1.2 to 2.0, preferably1.2 to 1.8, preferably 1.2 to 1.6, most preferably 1.2 to 1.4. The Hausner ratio, ^,is calculated as ^ =^^^^. The parameter ^^ is the tapped bulk density of the pluralityof porous particles, determined in the manner described above. The parameter^^ is the freely settled bulk density of the plurality of porous particles, i.e.measured during the tap density measurement process described above, before tapping. Particles with a high Hausner ratio tend to mix poorly due to high cohesivity, meaning it is much preferred to load particles having a relatively lowHausner ratio, within the above ranges. Since cohesivity depends on Hausnerratio, it will also be preferable to adapt the stirring speed dependent on the Hausner ratio of the particles. In particular, for porous particles with a Hausner ratio of no more than 1.4, preferably between 1.2 and 1.4, the one or more stirring surfaces may pass through a volume per second during stirring that depends onthe mass of the plurality of particles contained in the internal volume of between0.001 and 0.005 m3 s-1 kg-1, which is found to achieve especially good aeration ofthe particle bed. On the other hand, for porous particles with a Hausner ratio greater than 1.4, the one or more stirring surfaces may pass through a volume per second during stirring that depends on the mass of the plurality of particlescontained in the internal volume of between 0.005 and 0.1 m3 s-1 kg-1, such thatthe higher stirring speed compensates for the high cohesivity.Likewise, to further improve the balance between displacement of the particle bedto cause aeration and the tendency to induce tangential movement andcompaction of the particles at higher speeds, preferably the one or more stirringsurfaces pass through a volume per second dependent on the mass of the plurality of porous particles contained in the internal volume of between 0.002 and0.08 m3 s-1 kg-1 during stirring, preferably between 0.003 and 0.06 m3 s-1 kg-1,more preferably between 0.004 and 0.04 m3s-1kg-1, most preferably between 0.005 and 0.02 m3s-1kg-1. Particularly preferred combinations of the mass of the plurality of porous particlescontained in the internal volume during stirring and the volume swept by thestirring surfaces would be: a mass of no more than 400 kg m-3 relative to reactorvolume and no more than 750 kg m-2 relative to stirrer area and the one or morestirring surfaces pass through a volume per second dependent on the mass of theplurality of porous particles contained in the internal volume of between 0.002 and0.08 m3 s-1 kg-1 during stirring; or a mass of no more than 400 kg m-3 relative toreactor volume and no more than 500 kg m-2 relative to stirrer area and the one ormore stirring surfaces pass through a volume per second dependent on the mass of the plurality of porous particles contained in the internal volume of between0.002 and 0.08 m3 s-1 kg-1 during stirring; or a mass of no more than 400 kg m-3relative to reactor volume and no more than 750 kg m-2 relative to stirrer area andthe one or more stirring surfaces pass through a volume per second dependent on the mass of the plurality of porous particles contained in the internal volume ofbetween 0.003 and 0.06 m3 s-1 kg-1 during stirring; or a mass of no more than 400kg m-3 relative to reactor volume and no more than 500 kg m-2 relative to stirrerarea and the one or more stirring surfaces pass through a volume per seconddependent on the mass of the plurality of porous particles contained in the internalvolume of between 0.003 and 0.06 m3 s-1 kg-1; or and a mass of no more than 400kg m-3 relative to reactor volume and no more than 500 kg m-2 relative to stirrerarea and the one or more stirring surfaces pass through a volume per seconddependent on the mass of the plurality of porous particles contained in the internal volume of between 0.004 and 0.04 m3s-1kg-1; or a mass of no more than 300kg m-3 relative to reactor volume and no more than 400 kg m-2 relative to stirrerarea and the one or more stirring surfaces pass through a volume per seconddependent on the mass of the plurality of porous particles contained in the internal volume of between 0.004 and 0.04 m3s-1kg-1; or a mass of no more than 250kg m-3 relative to reactor volume and no more than 300 kg m-2 relative to stirrerarea and the one or more stirring surfaces pass through a volume per seconddependent on the mass of the plurality of porous particles contained in the internalvolume of between 0.005 and 0.02 m3 s-1 kg-1. Each one of these preferredcombinations is also preferably further provided with stirring elements that movethrough between 5% and 90% of the internal volume during stirring, preferably between 10% and 80%, more preferably between 20% and 70%, most preferably between 30% and 60% The geometry of the stirring surfaces may be varied within the above parametersto affect its arrangement relative to the particle bed. In some embodiments, thestirring element may be configured such that the one or more stirring surfaces extend through a full height of the particle bed, and possibly beyond the full height of the particle bed, prior to stirring. In this way, the stirring element may pass through a space corresponding to full height of the particle bed during stirring. In other embodiments, the stirring element may be configured such that the one or more stirring surfaces pass through only part of the height of the particle bed, prior to stirring. In some embodiments, the stirring element defines a clearance between the stirring element and an inner surface of the outer wall of less than 5% of an internal dimension of the reactor measured along the direction of the clearance, preferably less than 2%, most preferably less than 1%. As indicated above, these clearances may be provided along at least 30% the length of the reactor wall along a direction perpendicular to the velocity vector of the stirring element, preferably at least 40%, more preferably at least 50%, more preferably at least 60%, most preferably at least 70%. Lower clearance is generally preferred as it prevents agglomeration of the particles on the outer wall of the reactor. In particularly preferable embodiments, the clearance of the reactor is selected based on the size of the particles. Smaller particles have a greater tendency to agglomerate on the outer wall and will also contain more particles in a given thickness of build-up on the outer wall. Therefore, preferably, the clearance between the stirring element and an inner surface of the outer wall is dependent on the sizes of the plurality of porous particles provided in the reactor, wherein the clearance between the stirring element and an inner surface of the outer wall is less than 10% of an internal dimension of the reactor measured along the direction of the clearance for porous particles having a D50 particle diameter of more than 200 µm, wherein the clearance between the stirring element and an inner surface of the outer wall is less than 3% of an internal dimension of the reactor measured along the direction of the clearance for porous particles having a D50particle diameter in the range 30 µm to 200 µm, or wherein the clearance between the stirring element and an inner surface of the outer wall is less than 2% of an internal dimension of the reactor measured along the direction of the clearance for porous particleshaving a D50 particle diameter less than 30 µm. More preferably, the clearancebetween the stirring element and an inner surface of the outer wall is less than 10% of an internal dimension of the reactor measured along the direction of the clearance for porous particles having a D98particle diameter of more than 200 µm, wherein the clearance between the stirring element and an inner surface of the outer wall is less than 3% of an internal dimension of the reactor measured along the direction of the clearance for porous particles having a D98 particle diameter in the range 30 µm to 200 µm, or wherein the clearance between the stirring element and an inner surface of the outer wall is less than 2% of an internal dimension of the reactor measured along the direction of the clearance for porousparticles having a D98 particle diameter less than 30 µm. Preferably, theseclearances based on D50 or D98 particle diameter are provided in combination with the plurality of porous particles having a span of less than 5, preferably less than 3, preferably less than 2. Span is discussed in more detail below. For porousparticles below 30 µm, a clearance lower than 3% is recommended due to thehigh cohesiveness of the particles under this size to prevent a significant number of particles becoming attached to the wall of the reactor thus interfering with the heat transfer, then leading to a higher temperature differential across this porous particles layer attached to the wall, then leading to a very high temperature differential, then leading to a non-homogeneous infiltration and formation of“crusts” around the reactor wall. The relevance of clearance not only extends toporous particles below 30 µm but also the porous particles up to 200 µm, as notedabove. Although, they may behave differently to particles below 30 µm, particularly with regard to Geldart type, discussed in more detail below, theseparticles still need to be properly aerated, although the clearance can be relaxedup to the higher values discussed above. The term “particle diameter” as used herein refers to the equivalent spherical diameter (esd), i.e. the diameter of a sphere having the same volume as a given particle, wherein the particle volume is understood to include the volume of intra- particle pores. The terms “D50” and “D50 particle diameter” as used herein refer to the volume-based median particle diameter, i.e. the diameter below which 50% byvolume of the particle population is found. More generally, terms “DX” and “DXparticle diameter” refer to the diameter below which X% by volume of the particle population is found. Particle diameters and particle size distributions can be determined by standard laser diffraction techniques in accordance with ISO 13320:2020. Laser diffraction relies on the principle that a particle will scatter light at an angle that varies depending on the size the particle and a collection of particles will produce a pattern of scattered light defined by intensity and angle that can be correlated to a particle size distribution. A number of laser diffraction instruments are commercially available for the rapid and reliable determination of particle size distributions. Unless stated otherwise, particle size distribution measurements as specified or reported herein are as measured by the conventional Malvern MastersizerTM3000 particle size analyzer from Malvern InstrumentsTM. The Malvern MastersizerTM3000 particle size analyzer operates by projecting a helium-neon gas laser beam through a transparent cell containing the particles of interest suspended in an aqueous solution. Light rays which strike the particles are scattered through angles which are inversely proportional to the particle size and a photodetector array measures the intensity of light at several predetermined angles and the measured intensities at different angles are processed by a computer using standard theoretical principles to determine the particle size distribution. Laser diffraction values as reported herein are obtained using a wet dispersion of the particles in 2-propanol with a 5 vol% addition of the surfactant SPANTM-40 (sorbitan monopalmitate). The particle refractive index is taken to be 2.68 for porous particles and 3.50 for composite particles and the dispersant index is taken to be 1.378. Particle size distributions are calculated using the Mie scattering model. The above clearance values are stated as percentages of the reactor size. However, it can also be preferable to ensure that certain absolute clearance sizes are met. Preferably, the clearance between the stirring element and an inner surface of the outer wall is no more than 5 cm, preferably no more than 2 cm, more preferably no more than 1 cm, more preferably no more than 5 mm, most preferably no more than 2 mm. In general, due to differences in tolerances at different scales, reactors with smaller internal volumes are capable of sustaining smaller clearances during stirring than larger reactors. Preferably, the clearancebetween the stirring element and an inner surface of the outer wall is no more than10% or 5 cm, whichever is smaller, more preferably no more than 5% or 2 cm, whichever is smaller, most preferably no more than 2% or 1 cm, whichever is smaller. It may also be beneficial to select the particle sizes based on the absolute clearance between the stirring element and an inner surface of the outer wall. Preferably, the clearance between the stirring element and an inner surface of the outer wall is dependent on the sizes of the plurality of particles provided in the reactor, wherein the clearance between the stirring element and an inner surface of the outer wall is less than 150 mm for particles having a D50 particle diameter of more than 200 µm, wherein the clearance between the stirring element and an inner surface of the outer wall is less than 50 mm for particles having a D50particle diameter in the range 30 µm to 200 µm, or wherein the clearance between the stirring element and an inner surface of the outer wall is less than 10 mm forparticles having a D50 particle diameter less than 30 µm. Preferably, the clearancebetween the stirring element and an inner surface of the outer wall is less than 150 mm for porous particles having a D98 particle diameter of more than 200 µm, wherein the clearance between the stirring element and an inner surface of the outer wall is less than 50 mm for porous particles having a D98particle diameter in the range 30 µm to 200 µm, or wherein the clearance between the stirring element and an inner surface of the outer wall is less than 10 mm for porousparticles having a D98 particle diameter less than 30 µm . Alternatively, the pluralityof porous particles may have a D50 or D98 particle diameter of no less than 0.01% of the closest clearance between the stirring element and an inner surface of the outer wall, more preferably the D50or D98particle diameter is no less than 0.05%, more preferably no less than 0.1%, more preferably no less than 0.5%, more preferably no less than 1%, most preferably no less than 2% of the clearance between the stirring element and an inner surface of the outer wall. This limits the number of particles that may fit between the stirring element and the inner surfaceof the outer wall and so limits build-up of particles on the outer wall. Again,preferably these clearances based on D50 or D98 particle diameter are provided in combination with the plurality of porous particles having a span of less than 5, preferably less than 3, preferably less than 2. The invention is particularly useful for porous particles having a D50particle diameter of less than 200 µm, preferably less than 100 µm, more preferably lessthan 50 µm. The plurality of porous particles may have a D50 particle diameter inthe range 0.5 µm to 200 µm, preferably in the range 1 µm to 30 µm, morepreferably in the range 2 µm to 10 µm. The porous particles may alternatively oradditionally have a D1 particle diameter of at least 0.1 µm, preferably at least0.5 µm, more preferably at least 1 µm, more preferably at least 1.5 µm. Theporous particles may alternatively or additionally have a D10 particle diameter ofat least 0.2 µm, preferably at least 0.5 µm, more preferably at least 1 µm, morepreferably at least 1.5 µm, more preferably at least 2 µm, more preferably at least3 µm. The porous particles may alternatively or additionally have a D90particlediameter of no more than 250 µm, preferably no more than 150 µm, morepreferably no more than 100 µm, more preferably no more than 50 µm, morepreferably no more than 30 µm. The porous particles may alternatively oradditionally have a D98 particle diameter of no more than 300 µm, preferably nomore than 200 µm, more preferably no more than 100 µm, more preferably no more than 50 µm, more preferably no more than 30 µm. Particles of these sizesare considered particularly cohesive and so have a high tendency to agglomerate,but are still capable of being sufficiently aerated by stirring in the mannerdescribed. The use of a process including a stirred bed reactor with good aeration,of the sort provided here, is particularly useful in this context to prevent cohesion of the particles.The invention is also particularly applicable where the plurality of porous particleshave a span, defined as^^^^^^^^^^, of no more than 5, preferably no more than 4, more preferably no more than 3, more preferably no more than 2, more preferably no more than 1.5. In particular, a plurality of porous particles with a low span have relatively consistent particle diameters. This ensures that the particle bed responds relatively uniformly to stirring in the manner described herein. The invention may also be used with porous particles in which the plurality ofparticles preferably have a positive skew in the volume-based particle diameterdistribution. Preferably, the D50 diameter is less than the volume-based meanparticle diameter. Preferably, the skew of the porous particle diameter distribution (as measured by a Malvern MastersizerTM 3000 analyzer) is no more than 4, or no more than 3, or no more than 2, or no more than 1.5. Preferably, the skew isat least 0.2, or at least 0.3, or at least 0.4. The particle diameter distribution ofthe porous particles may be monomodal, bimodal or multimodal. The present process may advantageously be used for processing porous particlesclassified as of Geldart Group A or of Geldart Group C, as defined in Geldart."Types of gas fluidization." Powder technology 7.5 (1973): 285-292. Group Cparticles are characterised as cohesive and so are difficult to fluidise, meaning stirring is particularly important. However, the present process is alsoadvantageous in the context of processing Group A particles. These particles arecharacterised as aeratable, but the present process allows these particle types to be processed without also requiring the relatively high gas velocities needed for fluidisation. In some embodiments, the plurality of porous particles are of Geldart Group A and,for example, have a D50 particle diameter in the range 30 µm to 200 µm. Sinceparticles of Geldart Group A require relatively less energy to aerate, the one or more stirring surfaces may pass through a volume per second during stirring thatis between 0.001 and 0.005 m3 s-1 kg-1. Further preferably, since Geldart Group Aparticles will move more freely through the reactor, the total stirred volume, i.e. as a percentage of the total internal volume, for these embodiments may be lower, which helps reduce the weight of the stirrer and the torque required during stirring. In particular, this is preferably in the range 5% to 85%, preferably 10% to 80%, preferably 20% to 80%, preferably 20% to 70%, preferably 30% to 70%, preferably 30% to 60%, preferably 30% to 50%. In other embodiments, the plurality of porousparticles are of Geldart Group C and, for example, have a D50 particle diameter of less than 30 µm. Since Geldart Group C particles will require relatively more energy to aerate, the one or more stirring surfaces may pass through a volumeper second during stirring that is between 0.005 and 0.1 m3 s-1 kg-1. These typesof particles will be more prone to dead spots as the particles move less freely, andso the total stirred volume, i.e. as a percentage of the total internal volume, for these embodiments should be higher. In particular, this is preferably in the range 20% to 90%, preferably 30% to 90%, preferably 30% to 85%, preferably 40% to 80%, preferably 60% to 80%.Whether the particles are of Geldart Group A, and whether the above preferredlower stirring speeds are used, or whether Geldart Group C, and whether theabove preferred higher stirring speeds are used, the preferred loading limitsdetailed above may be observed. However, it is also particularly preferred toadjust loading amounts of the particles differently for different Geldart Group particles. In particular, in some embodiments, the plurality of porous particles are of Geldart Group A and the mass of the plurality of porous particles contained inthe internal volume during stirring is dependent on the volume of the internalvolume, being between 100 kg m-3 and 500 kg m-3, preferably between 150 kg m-3and 500 kg m-3, preferably between 200 kg m-3 and 500 kg m-3, preferablybetween 250 kg m-3 and 500 kg m-3, and optionally the one or more stirringsurfaces may pass through a volume per second during stirring that is between0.001 and 0.005 m3 s-1 kg-1. Likewise, alternatively or additionally, in someembodiments, the plurality of porous particles are of Geldart Group A, and the mass of the plurality of porous particles contained in the internal volume duringstirring is dependent on the surface area of the one or more stirring surfaces, beingbetween 200 kg m-2 and 1000 kg m-2, preferably between 300 kg m-2 and1000 kg m-2, preferably between 400 kg m-2 and 1000 kg m-2, and optionally theone or more stirring surfaces may pass through a volume per second duringstirring that is between 0.001 and 0.005 m3 s-1 kg-1. In contrast, in otherembodiments, the plurality of porous particles are of Geldart Group C, and the mass of the plurality of porous particles contained in the internal volume duringstirring is dependent on the volume of the internal volume, being less than 300kg m-3, preferably less than 200 kg m-3, preferably less than 150 kg m-3, preferablyless than 100 kg m-3, and optionally the one or more stirring surfaces may passthrough a volume per second during stirring that is between 0.005 and0.1 m3 s-1 kg-1. Likewise, alternatively or additionally, in some embodiments, theplurality of porous particles are of Geldart Group C and the mass of the pluralityof porous particles contained in the internal volume during stirring is dependenton the surface area of the one or more stirring surfaces, being less than 100 kg m-2, preferably less than 80 kg m-2, preferably less than 50 kg m-2, and optionally theone or more stirring surfaces may pass through a volume per second duringstirring that is between 0.005 and 0.1 m3 s-1 kg-1. The above combinations arepreferred for the following reasons. The use of relatively high loading masses of particles compared to reactor volume would tend to require higher stirrer area andhigher speeds of the stirrer in order to stir the required volume per second perkilogram of particles. As noted above, Geldart Group C particles already tend torequire higher speeds due to their higher cohesivity, meaning a high loading mass of Geldart Group C particles would require stirring speeds that are too high and bring about the negative consequences of high stirring speed discussed herein.Therefore, lower loading when using Geldart Group C particles makes stirringspeeds more feasible, which also helps when scaling up the size of the reactor to achieve higher output, as high stirring speeds are more difficult in larger reactors due to the increase in size and weight of the mechanical components. Geldart Group A particles are less cohesive, meaning the higher loading amounts can be used without requiring larger stirrers and speeds that are too high, meaning output can be increased without running into the negative consequences noted above. The invention may also be used for processes in which relatively large porous particles are used in steps (a) to (c) and in which a later step is provided in which the particles undergo comminution. In particular, in these processes, preferablythe method further comprises, in step (d), after step (c), comminuting the pluralityof particles, preferably by mechanical action, so as to reduce the D50particle diameter. In these embodiments, the porous particles provided in step (a) may have a D50particle diameter of at least 20 µm, preferably at least 30 µm, more preferably at least 40 µm, more preferably at least 50 µm, more preferably at least100 µm. Preferably, the porous particles have a D50 particle diameter of between20 µm to 200 µm, preferably between 30 µm to 200 µm, more preferably between40 µm to 200 µm, more preferably between 50 µm to 150 µm. The porous particlesprovided in step (a) may also have a D1 particle diameter of at least 1.5 µm,preferably at least 2 µm, more preferably at least 2.5 µm, more preferably at least3 µm. The porous particles provided in step (a) may also have a D10 particlediameter of at least 3 µm, preferably at least 5 µm, more preferably at least 10µm, more preferably at least 15 µm, more preferably at least 20 µm, morepreferably at least 30 µm. The porous particles provided in step (a) may also havea D90 particle diameter of no more than 500 µm, preferably no more than 400 µm,more preferably no more than 300 µm, more preferably no more than 250 µm,more preferably no more than 200 µm. The porous particles provided in step (a)may also have a D98 particle diameter of no more than 750 µm, preferably no morethan 500 µm, more preferably no more than 400 µm, more preferably no morethan 300 µm, more preferably no more than 250 µm. These larger particles maybe more readily aerated by processing in accordance with the parameters discussed above, leading to a more homogeneous reaction process across theparticles. However, it is believed that composite particles with a D50 particlediameter of less than 20 µm is beneficial for the desired end-use of thesecomposite materials in a metal-ion battery. Therefore, preferably step (d) comprises comminuting the plurality of particles so as to reduce the D50 particlediameter to less than 20 µm, preferably less than 15 µm, preferably less than 10µm, most preferably less than 5 µm, or less than 3 µm. In particular, compositeparticles less than 5 µm are more difficult to produce by deposition onto particlesof those sizes without milling. In the embodiments in which the particles undergo comminution, preferably theplurality of porous particles provided in step (a) have a span of no more than 5,preferably no more than 4, more preferably no more than 3, more preferably no more than 2, more preferably no more than 1.5, more preferably no more than 1. The porous particles provided in step (a) may have a positive skew in the volume-based particle diameter distribution. Preferably, the D50 diameter is less than thevolume-based mean particle diameter. Preferably, the skew of the porous particle diameter distribution (as measured by a Malvern MastersizerTM 3000 analyzer) is no more than 4, or no more than 3, or no more than 2, or no more than 1.5. Preferably, the skew is at least 0.2, or at least 0.3, or at least 0.4. In the embodiments in which the particles undergo comminution, after completion of deposition on elemental silicon in the pores of the porous particles, the particles may undergo passivation, as discussed above, and cooling, possibly at the same time. Cooling the particles has the advantage of facilitating the transfer of the intermediate particles to the comminuting device. With the particles optionally passivated and cooled, the particles may be removed from the reactor and transferred to a comminuting device. The comminuting canbe done by different types of comminuting device such as mill, wet mill, ball milling,jet milling, high-shear stirring, ultrasound etc. Preferably, the comminuting isperformed in a dry mill, especially where there has been no passivation step, dueto the reactivity of the particles immediately after deposition has finished, sincethe electroactive material deposited by the CVI process may be reactive. For example, silicon deposited from silane contains a significant amount of Si-H bonds. These bonds are reactive towards organic molecules and water. Accordingly, the presence of oxygen or an organic solvent may result in an exothermic reaction leading to partial destruction of the Si / C composite material and / or degradation below the quality required for commercial metal-ion battery material.Among the dry mills, jet mills are preferred because of capability to grind to lowersizes. Jet mills use a high-speed jet of compressed air or inert gas to impactparticles with each other. Jet mills can be used with starting material with a size up to about 1 mm and are known to readily achieve sizes of the order of 1 μm with relatively little energy input. Preferably, the plurality of porous particles each comprise a porous framework,typically a carbon framework, comprising micropores and / or mesopores.Preferably, the total pore volume of micropores and mesopores as measured bygas adsorption is in the range from 0.1 to 4.0 cm3 / g, preferably from 0.2 to 3.0cm3 / g, preferably from 0.4 to 2.2 cm3 / g. Preferably, the PD50 pore diameter asmeasured by gas adsorption is no more than 20 nm, preferably no more than 10 nm, more preferably no more than 5 nm. Preferably, the volumetric ratio ofmicropores to mesopores is from 90:10 to 30:70, preferably from 90:10 to 40:60,or from 90:10 to 50:50, or from 90:10 to 55:45, or from 90:10 to 60:40, or from 85:15 to 65:35. References herein to the volume of micropores, mesopores and macropores in the porous particles, and also any references to the distribution of pore volume within the porous particles, relate to the internal pore volume of the porous particles used as the starting material in the claimed processes, i.e. prior to deposition of silicon. The term “PD50pore diameter” as used herein refers to the volume-based 50th percentile pore diameter, based on the total volume of micropores and mesopores (i.e. the pore diameter below which 50% of the total micropore and mesopore volume is found). Pore diameter is also described in the art by the term “porewidth” and the terms are understood to be equivalent for the purposes of thisapplication. The total volume of micropores and mesopores and the pore size distribution of micropores and mesopores are determined using nitrogen gas adsorption at 77 K down to a relative pressure p / p0of 10-7using quenched solid density functional theory (QSDFT) in accordance with standard methodology as set out in ISO 15901-2:2022. Nitrogen gas adsorption is a technique that characterises the porosity and pore diameter distributions of a material by allowing a gas to condense in the pores of a solid. As pressure increases, the gas condenses first in the pores of smallest diameter and the pressure is increased until a saturation point is reached at which all of the pores are filled with liquid. The nitrogen gas pressure is then reduced incrementally, to allow the liquid to evaporate from the system. Analysis of the adsorption and desorption isotherms, and the hysteresis between them, allows the pore volume and pore size distribution to be determined. Suitable instruments for the measurement of pore volume and pore sizedistributions by nitrogen gas adsorption include the ASAP 2020 Plus porosityanalyzers, which are available from Micromeritics Instrument Corporation, USA, and the Autosorb IQ porosity analyzers, which are available from Quantachrome Instruments. The porous particles preferably have a BET surface area of at least 500 m2g-1, orat least 750 m2 g-1, or at least 1,000 m2 g-1, or at least 1,250 m2 g-1, or at least1,500 m2 g-1. The term “BET surface area” as used herein should be taken to referto the surface area per unit mass calculated from a measurement of the physical adsorption of gas molecules on a solid surface, using the Brunauer–Emmett– Teller theory, in accordance with ISO 9277:2022. Again, the measurement of the BET surface area of the porous particles refers to the porous particles used as the starting material in the claimed processes, i.e. prior to deposition of silicon Preferably, the BET surface area of the porous particles is no more than4,000 m2 g-1, or no more than 3,500 m2 g-1, or no more than 3,250 m2 g-1, or nomore than 3,000 m2g-1 or no more than 2,500 m2 g-1, or no more than 2,000 m2 g-1^.For example, the porous particles may have a BET surface area in the range from750 m2 g-1^ to 4,000 m2 g-1^, or from 1,000 m2 g-1^ to 3,500 m2 g-1^, or from1,250 m2 g-1^ to 3,250 m2 g-1^, or from 1,500 m2 g-^1 to 3,000 m2 g-1^.Another factor that must be balanced is the volume of the reactor that is swept through by the stirring element during stirring. On the one hand, it is desirable to ensure that a significant fraction of the reactor volume is swept through to directly act on a significant fraction of the particle bed. On the other hand, this fraction should not be too high, or else this can inhibit particles from fully circulating within the reactor. It is therefore preferable that the stirring element moves through between 5% and 90% of the internal volume during stirring, preferably between 10% and 80%, more preferably between 20% and 70%, most preferably between 30% and 60%. The volume moved through during stirring can be adjusted independently of the area of the stirring surfaces. For example, in a rotational stirring element comprising ribbon flight stirring elements, additional ribbons can be added at the same radius to increase the surface area without changing thevolume moved through during stirring. Adding more ribbons will also increase themomentum that can be effectively transmitted to a mass of particles withoutchanging the volume of the reactor that is swept through in each repeat of thestirring action. In this way, the amount of energy being imparted into aeration of the particle bed can be adjusted independently of the proportional swept volume by increasing stirring speed, or increasing number of stirring elements moving through the same volume. The invention is particularly applicable to reactors in which stirring the plurality of porous particles comprises rotating a stirring element located in the internal volume about a central axis of the internal volume. While this is preferred, it is also envisaged that the present principles could be applied to reactors with other stirrer types, such as reciprocating stirrers. In embodiments involving rotating a stirring element located in the internal volume about a central axis, preferably the internal volume has substantially continuous rotational symmetry about the central axis. While this is preferred, it is also envisaged that the internal volume may have a more complex shape, e.g.including a contoured outer surface about the periphery to induce favourablemovement patterns of the stirred particles. It has also been found to be particularly preferable to provide that the rotational stirring element defines an open centre around the central axis. This has been found to enable good circulation of the particles through the reactor. For example, in a vertical reactor design in which the stirring element lifts the particles against gravity, an open centre around the central axis can allow the particles to fall back down through the reactor to ensure good vertical mixing. This synergises especially well with the above particle loading parameters and stirring speed, which achieve a good balance. Alternatively, in a horizontal reactor, the open centre may allow good circulation of particles to and away from the central axis during stirring. Preferably, a radius of the open centre of the stirring element(measured from the central axis) is between 10% and 90% of a distance betweenthe central axis and the outer wall (e.g. a radius of the internal volume), preferably between 15% and 80%, further preferably between 20% and 70%, more preferably between 25% and 60%, most preferably between 30% and 50%. Preferably the open centre extends along substantially the full length of the stirring element along the direction of the central axis. Preferably, each stirring surface is an area of the stirring element further defining an angle of between 10° and 70° to its velocity vector during stirring, preferably between 20° and 60°, more preferably between 30° and 50°, and / or defining an angle to its velocity vector during stirring of less than 45°. These correspond to surfaces inclined relatively far back from their velocity vector, which will tend to produce less movement in the particle bed along the direction of the velocity vector. Also, it has been found to be preferable to provide the one or more stirring surfaces along between 50% and 100% of a length of the outer wall, preferably along between least 60% and 90% of the length of the outer wall, more preferably between 60% and 80% of the length of the outer wall. In particular, it is preferable to provide the one or more stirring surfaces substantially contiguously along this proportion of the length of the outer wall. In a vertical reactor, it is particularly preferable to provide the one or more stirring surfaces such that they extendsubstantially contiguously from a bottom of the internal volume to between 50%and 95% of the length of the outer wall, preferably between 60% and 90%, more preferably between 60% and 80%. Providing open headspace over the stirring element in the internal volume can help promote vertical circulation of the particle bed by allowing particles to fall back down from an upper portion of the internal volume. In a horizontal reactor, it is not as beneficial to provide the one or more stirring surfaces substantially contiguously along the outer wall as it is to ensure a good spread along the outer wall. In a horizontal reactor, preferably the one or more stirring surfaces are provided along between 50% and 100% of a length of the outer wall and substantially evenly distributed along a length of the outer wall, more preferably the one or more stirring surfaces are provided along between 60% and 100%, more preferably between 70% and 100%, most preferablybetween 80% and 100% of a length of the outer wall.As indicated above, the present process is preferably performed in a substantially vertical reactor, i.e. wherein vertical is the direction in which gravity acts. Substantially vertical may be considered to be within 45° of a vertical direction,preferably within 40°, more preferably within 30°, more preferably within 20°, morepreferably within 10°, most preferably within 5°. In a vertical reactor that uses a rotating stirring element, the central axis about which the stirring element is rotated will also generally be substantially vertical. In a vertical reactor, the outer wall will generally extend substantially vertically, e.g. a cylindrical outer wall, but the outer wall could also be frustoconical with asubstantially vertical central axis. The stirring surfaces of a vertical reactor maytypically be arranged such that movement of the stirring surface through the plurality of porous particles urges the plurality of porous particles along the outer wall against gravity, i.e. lifting the particles up the vertical reactor, and / or away from the outer wall, i.e. towards the centre. A substantially vertical reactor may have a floor at a base of the outer wall that further contributes to defining the internal volume. The floor may be flat, or alternatively could be generally curved, e.g. hemispherical or conical, or some combination of flat and curved. In a verticalreactor, preferably the stirring surfaces extends to within at most 5 cm of the floor,preferably within 1 cm, more preferably within 5 mm, more preferably within 2 mm. Alternatively, or additionally, the stirring surfaces may extend to within a distance corresponding to at most 2% of the height of the reactor, preferably at most 1%, more preferably at most 0.5%, most preferably at most 0.2%. This allows the stirring surfaces to act to displace particles close to the floor of the reactor and ensure good circulation. In a substantially vertical reactor, each stirring surface may be an area of the stirring element further defining an oblique angle to a gravity vector during stirring and arranged such that movement of the stirring surface through the plurality of porous particles lifts the plurality of particles against the gravity vector direction. This is in addition to each stirring surface being an area of the stirring element defining an oblique angle to its velocity vector during stirring. Generally, thedirection of the velocity vector will be substantially perpendicular to the directionof the gravity vector. Preferably, each stirring surface is an area of the stirring element further defining an angle of between 20° and 80° to the gravity vector during stirring, preferably between 30° and 70°, more preferably between 40° and 60°, and / or defining an angle to the gravity vector during stirring of more than 45°. The angle of the stirring surface may be selected to balance the tangential movement achieved during stirring with the lift and aeration of the particle bed. A higher angle to the gravity vector also has the benefit of decreasing the torque requirement for rotation of the stirring element. Various geometries of stirring surface may be used with the present techniques. However, preferably, the stirring element comprises a helical stirring surface, preferably at least two separate helical stirring surfaces, further preferably at least three helical stirring surfaces. A helical or ribbon stirring surface is particularly useful for rotational stirring so as to achieve good circulation of the particles. Helical stirring surfaces may be used in both vertical and horizontal reactors. Where multiple separate helical stirring surfaces are used, theses will generally be rotated in the same direction about the same central axis, and generally the two helical stirring surfaces will be rigidly connected to one another. With regard to the manner in which gas is introduced into the reactor, there are several distinct modes of operation. A first mode of operation is a full batch mode.In a full batch mode, the plurality of porous particles or charge of porous particlesare provided in the reactor and enclosed in the reactor. Then gas or charge ofgas is introduced into the reactor before the supply of gas is stopped and thereactor is sealed, and the particle bed is stirred while the inside of the reactor isheld at conditions effective to cause a reaction between the plurality of porousparticles and the gas. Effluent gas may be withdrawn without the addition of any new reactive gas into the reactor. A second mode of operation is a continuous or semi-continuous operation, in which introduction of the reactive gas is continuous or semi-continuous. In such an operation, the plurality of porous particles could be handled in a batch-wisemanner as a charge of porous particles, being provided as a single batch in thereactor and enclosed in the reactor. The continuous or semi-continuous operation involves gas being periodically or continuously introduced into the reactor whilethe porous particles or porous particles charge are stirred and the inside of thereactor is held at conditions effective to cause a reaction between the plurality ofporous particles and the gas, and optionally there is the withdrawal of effluent gas. In principle, continuous operation does not exclude the possibility of deviations inthe rate of flow of reactive gas to or effluent gas from the reactor. For instance, acontinuous reactor may operate in a pulsed mode. For example, the flow rate ofthe reactive gas into the pressure reactor may be reduced to aid the withdrawal ofeffluent gas from the pressure reactor. Alternatively, reactive gas may beintroduced into the pressure reactor at a constant pressure. Withdrawal of effluentgases may be operated continuously, such that both the supply of the reactive gasand the withdrawal of effluent gas from the reactor occur continuously and simultaneously with the reaction in progress. Alternatively, withdrawal of the effluent gas from the pressure reactor may be operated semi-continuously. As used herein, semi-continuously means that the effluent gas is removed intermittently. While the present technique aims to improve uniformity of the particle bed during stirring, in many embodiments there may still be a reasonably significant variation in the density of the particle bed across the internal volume during stirring, such as increase in density towards the outer wall. In particular, in reactors that use rotated stirring surfaces, there will generally be a higher density of particles close to the outer wall due to centrifugal forces during stirring. Therefore, in order to further combat possible inhomogeneities in the final product as a result of this variation in density of the particles during stirring, preferably the gas is introduced into the internal volume of the reactor through a plurality of inlets into the internalvolume, wherein the plurality of inlets are arranged to produce a gas flow rate intothe internal volume per unit area in a plane perpendicular to a central axis (e.g. the rotation axis of the stirring element) of the internal volume that increases from the central axis towards the outer wall. Generally, this will be achieved by controlling the size, number and / or density of inlets located across a floor of a vertical reactor, and / or by varying the flow rate of gas through each inlet (i.e. the size, number and density of the inlets across the floor may be constant per unit area). Another factor in vertical reactors in which the stirring element is acting to lift the particles against gravity is that the particle density will tend to be higher lower down in the reactor. In some embodiments, the gas is introduced into the internal volume of the reactor through a plurality of inlets through the outer wall, whereinthe plurality of inlets through the outer wall are arranged to produce a gas flowrate into the internal volume per unit area of the outer wall that decreases along the outer wall along a direction against the gravity vector direction. Since the density of the particles is generally at a maximum lower down in a vertical reactor, and gas introduced through the outer wall will tend to rise through the particle bed,this can make it difficult to ensure that the top and bottom of the reactor bed areevenly exposed to the gas when the gas is introduced through the outer wall. Thepresent arrangement allows the gas flow rate to be controlled to suitably matchthe characteristics of the stirred particle bed. Again, this variation in the gas flowrate into the internal volume may be achieved by controlling the size, number and / or density of inlets located along the outer wall, and / or by the flow rate of gas through each inlets. As a further option, the gas may be introduced into the internal volume of the reactor through a plurality of inlets located on the stirring element, whereinpreferably the plurality of inlets are arranged to produce a gas flow rate into theinternal volume that decreases along the stirring element, most preferably that decreases along a direction against the gravity vector direction. In some embodiments, the above techniques may be combined to provide for further control of the gas flow rate into the internal volume.The present process may be used at substantially atmospheric pressure, e.g. atapproximately 100 kPa, but is also suited to use at high pressures, and sopreferably the pressure inside the internal volume during step (c) is at least 100kPa, preferably at least 200 kPa, more preferably at least 300 kPa, morepreferably at least 500 kPa, more preferably at least 650 kPa, more preferably atleast 750 kPa, more preferably at least 1000 kPa, more preferably at least 1500kPa, more preferably at least 2000 kPa, more preferably at least 2500 kPa, more preferably at least 3000 kPa. It is noted that higher pressures may promote compaction of the particles, and so stirring in the manner described above may improve the handling of the particles at these higher pressures. Higher processes are more difficult to generate and maintain, and so preferablythe pressure inside the internal volume during step (c) is no more than 6000 kPa,more preferably no more than 5000 kPa, more preferably no more than 4000 kPa,more preferably no more than 3000 kPa, more preferably no more than 2000 kPa,more preferably no more than 1600 kPa, more preferably no more than 1500 kPa,more preferably no more than 1200 kPa, more preferably no more than 1000 kPa,more preferably no more than 900 kPa, more preferably no more than 800 kPa,more preferably no more than 700 kPa, more preferably no more than 650 kPa,more preferably no more than 600 kPa, more preferably no more than 300 kPa.In particular, preferably, the pressure inside the internal volume during step (c) isbetween 200 kPa and 6000 kPa, preferably between 300 kPa and 5000 kPa, morepreferably between 400 kPa and 4000 kPa, preferably between 500 kPa and 3000 kPa. Preferably, the temperature inside the internal volume during step (c) is in the range from 350 to 500 °C, for example from 350 to 450 °C, or 360 to 430 °C, or370 to 420 °C, or 370 to 400 °C.The present process may be performed in modes in which the plurality of porousparticles are introduced and removed in batches, but may also be performed inmodes in which a feedstock of porous particles is provided and the porousparticles are continuously introduced into a reaction zone of the reactor. In these latter embodiments, for example, the stirring element may be used to move the particles through and out of the reaction zone as part of a continuous process.The process may also be performed in modes in which the gas is introduced intothe internal volume substantially continuously or semi-continuously during step (c), as well as modes with phased or staggered introduction of the gas.A full batch mode, as discussed above, may involve the pressure inside theinternal volume during step (c) being at least 300 kPa, preferably at least 500 kPa,more preferably at least 650 kPa, more preferably at least 750 kPa, morepreferably at least 1000 kPa, more preferably at least 1500 kPa, more preferably at least 2000 kPa, more preferably at least 2500 kPa, more preferably at least3000 kPa. A full batch mode may also involve the pressure inside the internalvolume during step (c) being no more than 6000 kPa, preferably no more than5000 kPa, more preferably no more than 4000 kPa, more preferably no more than3000 kPa. In particular, the pressure may be between 300 kPa and 6000 kPa,preferably between 400 kPa and 5500 kPa, more preferably between 500 kPa and5000 kPa, more preferably between 750 kPa and 4500 kPa, more preferably between 1000 kPa and 4000 kPa, more preferably between 2000 and 3000 kPa. On the other hand, a continuous or semi-continuous mode may involve thepressure inside the internal volume during step (c) being at least 100 kPa,preferably at least 200 kPa, more preferably at least 300 kPa, more preferablyleast, 500 kPa, more preferably at least 600 kPa, more preferably at least 650kPa, more preferably at least 700 kPa, more preferably at least 750 kPa, morepreferably at least 800 kPa. A continuous or semi-continuous batch mode mayalso involve the pressure inside the internal volume during step (c) being no morethan 2000 kPa, preferably no more than 1500 kPa, more preferably no more than1000 kPa, more preferably no more than 800 KPa, more preferably no more than750 kPa, more preferably no more than 700 kPa, more preferably no more than650 kPa, more preferably no more than 600 kPa, more preferably no more than600 kPa, more preferably no more than 500 kPa, more preferably no more than300 kPa, more preferably no more than 200 kPa. In particular, the pressure maybe between 100 kPa and 2000 kPa, preferably between 100 kPa and 1000 kPa,more preferably between 100 kPa and 500 kPa, more preferably between 100 kPaand 200 kPa or less than 200 kPa. The pressures in a continuous or semi-continuous mode are focused on supporting a higher time of contact of a continuous gas flow taking advantage of the higher pressure. The pressures need not be as high as in a full-batch mode as there is generally the ability to evacuate the gas out of the reactor at the same time as supplying more gas, which is notthe case in a full-batch mode, where a higher pressure translates to a higherquantity of the desired reactive component of the gas. The continuous or semi- continuous mode can be used to reduce the synthesis time in comparison to thebatch gas dose and doesn’t need to use such high pressures, which isadvantageous in terms of equipment costs and safety. Preferably, the process also includes during step (c), withdrawing an effluent gas from the internal volume, preferably substantially continuously or semi- continuously, as noted above. Preferably, the gas is a silicon precursor gas and the space time of the silicon precursor gas in contact with the porous particles in step (c) is maintained in the range from 1 to 60 min, or from 2 to 45 min, or from 3 to 30 min, or from 4 to 25 min, or from 5 to 20 min. Preferably, the silicon precursor is selected from silane (SiH4), disilane (Si2H6), trisilane (Si3H8), methylsilane, dimethylsilane and chlorosilanes.In a second aspect of the invention, there is provided a composition comprising orconsisting silicon-containing composite particles obtainable by a processaccording to the first aspect of the invention. Preferably wherein 25 to 70 wt% ofthe composite particles is elemental silicon.In a third aspect, the invention provides an electrode comprising silicon-containingcomposite particles obtainable by a process according to the first aspect of the invention. In a fourth aspect, the present invention provides a rechargeable metal-ion batterycomprising an electrode according to the third aspect of the invention.In accordance with a fifth aspect of the invention, there is provided a process fortreating particles, the process comprising the steps of: (a) providing a plurality ofporous particles in a reactor, the reactor having an outer wall defining an internalvolume for containing the plurality of porous particles; (b) stirring the plurality ofporous particles with a stirring element located in the internal volume, the stirring element having one or more stirring surfaces, each stirring surface being an area of the stirring element defining an oblique angle to its velocity vector during stirring and arranged such that movement of the stirring surface through the plurality of porous particles urges the plurality of porous particles along the outer wall at anangle to the velocity vector and / or away from the outer wall, wherein either: theouter wall is movable for causing the stirring of the plurality of particles and the one or more stirring surfaces of the stirring element protrude from an inner surface of the movable outer wall; or the stirring element defines a clearance between the stirring element and an inner surface of the outer wall of less than 10% of an internal dimension of the reactor measured along the direction of the clearance; wherein the mass of the plurality of porous particles contained in the internal volume during stirring is dependent on the volume of the internal volume, beingno more than 500 kg m-3, and is dependent on the surface area of the one or morestirring surfaces, being no more than 1000 kg m-2, and wherein the stirringsurfaces move through between 5% and 90% of the internal volume during eachstirring cycle of a stirring action, and complete between 0.1 and 10 stirring cyclesper second; and (c) during stirring the plurality of porous particles, contacting theplurality of porous particles with a gas at conditions effective to cause a reaction between the plurality of porous particles and the gas. This aspect of the invention configures the process by considering the proportion of the internal volume that is moved through by at least one stirring surface during a stirring cycle, and considering how regularly the stirring surfaces complete astirring cycle. As with the above embodiments, stirring is preferably provided bya stirring element that is rotated about a rotation axis, but stirring cycles could equally be provided by a reciprocating stirring action. All of the optional features described above with regard to the first aspect of the invention apply equally to this aspect of the invention. In particular, the preferred masses of the particles dependent on the internal volume and the surface area ofthe one or more stirring surfaces apply here.It is particularly preferred that the stirring surfaces move through between 10%and 90% of the internal volume during each stirring cycle of a stirring action,preferably between 20% and 85%, more preferably between 30% and 80%, more preferably between 40% and 70%. It is also preferred that the stirring surfaces complete between 0.5 and 8 stirring cycles per second, preferably between 0.7 and 5, preferably between 0.8 and 4.5,most preferably between 1 and 4 stirring cycles per second.Preferred combinations would include: stirring surfaces that move through between 10% and 90% of the internal volume during each stirring cycle and complete between 0.5 and 8 stirring cycles per second; or stirring surfaces that move through between 20% and 85% of the internal volume during each stirring cycle and complete between 0.7 and 5 stirring cycles per second; or stirring surfaces that move through between 30% and 80% of the internal volume during each stirring cycle and complete between 0.7 and 5 stirring cycles per second; or stirring surfaces that move through between 30% and 80% of the internal volume during each stirring cycle and complete between 0.8 and 4.5 stirring cycles per second; or stirring surfaces that move through between 40% and 70% of the internal volume during each stirring cycle and complete between 1 and 4 stirring cycles per second. These combinations narrow in on a particularly preferred operating window for these processes. In some embodiments, the plurality of porous particles are of Geldart Group A and,for example, have a D50 particle diameter in the range 30 µm to 200 µm. Sinceparticles of Geldart Group A require relatively less energy to aerate, the one ormore stirring surfaces may complete between 0.1 and 5 stirring cycles per second,preferably between 0.5 and 4 stirring cycles per second, preferably between 0.7and 3 stirring cycles per second, most preferably between 0.8 and 2 stirring cyclesper second. This is preferably combined with stirring surfaces that move throughbetween 5% and 85% of the internal volume during each stirring cycle, or between 10% and 80%, or between 20% and 80%, or between 30% and 70%. In other embodiments, the plurality of porous particles are of Geldart Group C and,for example, have a D50 particle diameter of less than 30 µm. Since Geldart GroupC particles will require relatively more energy to aerate, the one or more stirringsurfaces may complete between 0.7 and 10 stirring cycles per second, preferablybetween 0.8 and 8, preferably between 1 and 6, preferably between 1.5 and 5.This is preferably combined with stirring surfaces that move through between 20% and 90% of the internal volume during each stirring cycle, preferably between 30% and 85%, preferably between 40% and 80%. BRIEF DESCRIPTION OF DRAWINGS The invention will now be described with reference to the accompanying drawings, of which: Figure 1 schematically shows a system suitable for performing a process according to an embodiment; Figure 2 shows the stirring element of the system of Figure 1;Figure 3 is a cross-section through the reactor and stirring element of the systemof Figures 1 and 2;Figure 4 is a cross-section through the reactor and stirring element of the systemof Figures 1 and 2 illustrating an embodiment of gas inlet arrangement; Figure 5 is a cross-section through the reactor and stirring element of the system of Figures 1 and 2 illustrating another embodiment of gas inlet arrangement; Figure 6 is a cross-section through the reactor and stirring element of the system of Figures 1 and 2 illustrating another embodiment of gas inlet arrangement; Figure 7 shows a system suitable for performing a process according to an embodiment; and Figure 8 shows a cross-section through the reactor of Figure 7. DETAILED DESCRIPTION Figure 1 schematically shows a system 100 suitable for performing a process according to the invention. The system 100 comprises a 30 L (0.03 m3) vertical reactor 1. As shown in Figure 3, the reactor 1 comprises a cylindrical outer wall 2 that extends vertically to define an internal volume in which the process may take place. The reactor also comprises a substantially flat floor 6 at a lower end of the cylindrical outer wall 2 and is closed at the upper end by a removable top portion 7. The cylindrical outer wall 2, floor 6 and top portion 7, together, define acylindrical closed 30 L (0.03 m3) internal volume within which the processaccording to the invention may take place. The system further comprises a stirring element 3, shown in Figure 2. The stirring element 3 comprises a central axis member 5 about which the stirring element 3is configured to rotate in use. Extending radially out from the central axis member5 are a plurality of horizontal support rods that connect to and support two helicalstirring surfaces 4, which extend outward from the horizontal support rods. Eachhelical stirring surface 4 is a ribbon shaped stirring surface that wraps around thecentral axis member 5 to form a double helix shape. The shape and configurationof the stirring surfaces 4 will be described further below.The stirring element of Figure 2 is for inserting inside the reactor 1 such that thecentral axis member 5 extends vertically, parallel to and concentric with the cylindrical outer wall 2, as shown in Figure 3. Once inserted inside the reactor, a top portion 7 is arranged over the reactor 1 to close the upper opening to the reactor through which the stirring element is inserted. The upper end of the central axis member 5 engages a drive connection of the top portion 7, which supportedby a gas-tight bearing, not shown, so that the motor 9 can drive the drive system10 to cause the stirring element 3 to rotate inside the reactor by transmitting therotational drive through the top portion 7. A peripheral flange of the top portion 7 engages an upper peripheral flange of the outer wall 2, and the top 7 may be secured to the outer wall by any suitable means, such as by bolting the top 7 to the outer wall to form a gas-tight seal.As shown in Figure 3, once inserted in the reactor 1, the two helical stirringsurfaces 4 extend away from the central axis member 5 towards the outer wall 2of the reactor 1. The outer wall 2 may have a radius from the central axis member 5 of approximately 12.1 cm. The helical stirring surfaces 4 may have an outerradius, i.e. to the outer edge, proximate the outer wall 2, of 11.9 cm, so as to definea close clearance between the helical stirring surfaces 4 and the outer wall. Thehelical stirring surfaces may also have a width, i.e. along the radius direction, ofapproximately 5.5 cm. Each helical stirring surface 4 performs approximately onefull turn over its height and extends substantially from the floor 6 of the reactoralong approximately 65% of the total internal height of the internal volume, such that an upper 35% of the internal volume does not contain any stirring surface 4. In this embodiment, each helical stirring surface 4 defines an angle to the vertical direction of approximately 60°. In this embodiment, each helical stirring surface is substantially flat along the radial direction of the stirring element. Thus, since the helical stirring surface 4 defines an angle to the vertical direction of approximately 60°, for example, rotation of the stirring element within the internal volume will tend to urge particles upwards, along the outer wall 2, tending to lift and aerate the particle bed. While in this embodiment, each helical stirring surface is substantially flat along the radial direction of the stirring element, it is alsopossible to provide the helical stirring surface defining an angle along the radialdirection so that rotation of the stirring element within the internal volume will alsotend to urge particles away from the outer wall 2 and towards the centre of the internal volume. It will be appreciated that the above dimensions relating to the volume of the reactor and the surface area of the stirrer are only examples. The volume can be changed by increasing the height and diameter of the reactor shown in the Figure, for example, and the surface area of the stirring surfaces can be changed by adjusting the parameters relating to the stirring surfaces, particularly the number of helical screws, the angle they make to the vertical direction, the inner and outer radii of the helical screws, and the height of the screws. As will be discussed further below, the process performed using this reactor will involve injecting a silicon precursor gas into the internal volume during stirring. This will generally be done using a series of gas inlets through the outer wall 2, floor 6 and / or stirring element 3, as will be described below. Effluent gas will also typically be withdrawn from the reactor as silicone precursor gas is injected. This may be done through gas outlets (not shown) which may be arranged in the top portion 7, for example. Figure 4 schematically illustrates a process performed using the reactor describedabove and particularly shows the way a silicon precursor gas may be injected intothe reactor. As illustrated in Figure 4, rotation of the stirring element 3 inside the internal volume of the reactor 1 causes the particle bed 11 to be rotated and lifted and to form a vortex shape. As a result of this vortex produced during stirring, particle density remains highest in the lower portion of the internal volume, and particularly towards the periphery of the internal volume. Figure 4 shows how gasmay be injected into the internal volume through gas inlets 12 arranged on thecylindrical outer wall 2. While gas inlets 12 could be arranged evenly along the cylindrical outer wall, due to the shape of the stirred particle bed 11 and the tendency of the injected gas to rise up through the reactor, this would not be conducive to a homogeneous end product. Therefore, in this embodiment, the gas inlets 12 are arranged along the cylindrical outer wall 2 such that the gas flow rate into the internal volume decreases up the cylindrical outer wall 2. This variation in gas flow rate may be achieved in several ways. More or larger inlets may be provided towards the lower end of the outer wall to increase the area of the cylindrical outer wall 2 through which the silicon precursor gas is being injected. Alternatively, the arrangement of inlets 12 may be uniform along the outer wall, with gas being injected with a higher flow rate through those inlets closer to the bottom of the reactor. These techniques may also be combined. The precise gradient to the flow rate along the outer wall may be varied as desired for a particular intended process to be performed in the reactor. Figure 5 schematically illustrates another process performed using the reactor described above and particularly shows an alternative way a silicon precursor gas may be injected into the reactor. As described above with respect to Figure 4, rotation of the stirring element 3 inside the internal volume of the reactor 1 causes the particle bed 11 to be rotated and lifted and to form a vortex shape. In this embodiment, rather than gas being injected through the cylindrical outer wall 2, the silicon precursor gas is injected through inlets 12 in the floor 6 of the reactor 1. Due to the vortex shape of the stirred particle bed 11, there is generally a higher density of particles located over any unit area of the floor 6 towards the outer wall 2 of the reactor 1. Therefore, while the inlets 12 could be arranged to produce aneven flow rate per unit area of the floor, in this embodiment, the inlets are arrangedsuch that there is a higher flow rate per unit area of the floor 6 towards the outer wall 2. Again, this may be achieved by providing more or larger inlets towards the outer wall, and / or by gas being injected with a higher flow rate through those inlets closer to the outer wall 2. Figure 6 schematically illustrates another process performed using the reactor described above and particularly shows another alternative way a silicon precursor gas may be injected into the reactor. In this embodiment, the silicon precursor gas is injected into the internal volume of the reactor 1 using inlets 12 arranged on the stirring element 3. This may be achieved by providing gas conduits extending through the central axis member 5 of the stirring element 3,through the horizontal support rods and through the insides of the helical ribbonsto inlets located along any face or edge of the ribbons. Again, in order to accommodate the varying particle density across the stirred particle bed 11, the inlets 12 are arranged so that the that there is a higher flow rate into the internal volume from the stirring element towards the bottom of the reactor. Again, this may be achieved by more or larger inlets on the stirring element towards the bottom of the reactor. Alternatively, gas may be injected with a higher flow rate through those inlets closer to bottom of the reactor. This may require several parallel conduits through the stirring element, through which the flow rate can be independently controlled. It will be appreciated that the inlet arrangement of Figures 4 to 6 can also be combined to allow further control of the gas injection profile of the silicon precursor gas. While the above-described reactors have been exclusively vertical reactors, the present principles are also applicable to horizontal reactors, and an example of this will now be described below. Figure 7 shows an example of a system 100 comprising a horizontal reactor 1. In this embodiment, the outer wall 2 of the reactor 1 is again a cylindrical outer wall 2, but this time the cylinder is arranged substantially horizontally. The reactor is supported by a central axis 50 extending concentrically along the centre of the cylindrical outer wall 2. The cylindrical outer wall 2 is closed at the two ends by opposing end surfaces 60, 70 and the central axis 50 further extends away from these end surfaces, where it is rotatably supported on a support structure of the system. A motor 9 and drive system 10 is provided to engage one end of the central axis 50 to drive rotation of the axis, which also causes rotation of the entire cylindrical outer wall 2. At the opposite end of the central axis 50, an opening is provided to feed a silicon precursor gas along the central axis, where it may be injected into the reactor 1 through inlets 12 arranged on the central axis 50 within the reactor.Figure 8 shows a cross-section through the cylindrical outer wall 2 perpendicularto the central axis 50. As shown in this Figure, a plurality of stirring surfaces 4project outwards from the inside surface of the outer wall 2, into the internalvolume of the reactor. Each stirring surface is inclined by an angle θ to the direction of its velocity vector during stirring. In this horizontal embodiment, this inclination causes the stirring surface to lift the particles towards the central axis during stirring. These stirring surfaces may, additionally, define an angle relative to the central axis direction during stirring, so that the stirring surfaces additionally urge the particles along the outer wall during stirring. For example, different stirring surfaces may define angles in opposite directions relative to the central axis direction, so that together the stirring surfaces act to lift the particle bed during stirring and also promote longitudinal mixing along the direction of the central axis. Some example processes will now be described in more detail. Example Process 1 A first example process uses the reactor described above with respect to Figures 1 to 3. This reactor, having a volume of 30 L (0.03 m3), is loaded with 4.5 kg ofporous particles, each comprising a porous carbon framework comprisingmicropores and mesopores, which is a loading amount of 150 kg m-3of the reactor internal volume and approximately 56 kg m-2of the surface area of the stirring surfaces 4. Due to the very close clearance of the stirring surfaces 4 to the outer wall 2, the porous particles are provided having a D50 particle diameter of less than 30 µm. In particular, the particles may be provided with a D1 particle diameter of0.9 µm, a D10 particle diameter of 3.2 µm, a D50 particle diameter of 16.7 µm, aD90 particle diameter of 41.1 µm, and a D98 particle diameter of 56.9 µm. Theparticles had a BET surface area of 1626 m2 g-1, a PD50 of 1.0 nm, a micropore tomesopore volume ratio of 69.6:30.4, and a pore volume of micropores andmesopores of 0.7 cm3 g-1. These particles were in Geldart Group C, had aHausner ratio of 1.73 and a skew of 0.87 in the volume-based particle diameterdistribution. The stirring element is rotated at an angular speed of 144 rpm, whichresults in a stirring rate per kilogram of particles of approximately 0.007 m3 s-1 kg-1.The stirring rate is calculated using the following equation: where ^^is the outer radius of each ribbon, ^^is the inner radius of each ribbon,^ is the height of each ribbon, ^ is the stirring speed in revolutions per minute,and ^ is the loading mass of the porous particles.In this embodiment, the stirring surfaces pass through approximately 45% of the reactor internal volume during stirring. This value is calculated using the equation: where ^^ is the proportion of the reactor internal volume stirred and ^^ is the totalinternal volume of the reactor.The reactor is heated to a temperature of approximately 380 °C. During stirring,silane (SiH4) gas is continuously injected into the internal volume through the outerwall and the floor at a flowrate of 2 grams of silicon in silane gas per minute perkilogram of porous particles, while effluent gas is continuously withdrawn. Example Process 2 A second example process uses a larger vertical reactor having an internal volume of 513 L (0.513 m3) and having a radius between the central axis and outer wallof 31.3 cm. The reactor has a stirring element with two helical ribbons. Eachribbon defines an angle of 55° to a vertical direction. Each ribbon has a radius toits outer edge of 31.0 cm and a radius to its inner edge of 9 cm. Each ribbonmakes one full turn over its height, and extends up approximately 76% of the totalinternal height of the reactor from the floor of the reactor.The reactor is loaded with 65 kg of porous particles comprising a porous carbonframework and having a D50 particle diameter of 30 µm. This loading amount isapproximately 127 kg m-3of the reactor internal volume and approximately85 kg m-2 of the surface area of the stirring surfaces. The stirring element isrotated at a speed of 90 rpm, which results in a stirring rate per kilogram ofparticles of approximately 0.008 m3 s-1 kg-. The stirring surfaces pass throughapproximately 70% of the reactor internal volume during stirring. The reactor isheated to a temperature of approximately 380 °C. During stirring, silane (SiH4)gas is injected into the internal volume through the outer wall under pressure toraise the internal volume to a pressure of 1000 kPa. Effluent gas is continuouslywithdrawn while maintaining this pressure. Example Process 3A third example process demonstrates a stirring element with a relatively lowsurface area compared to the particle loading amount. This example uses avertical reactor having an internal volume of 30 L (0.03 m3) and having a radiusbetween the central axis and outer wall of 12.1 cm. The reactor has a stirringelement with one helical ribbon. The helical ribbon defines an angle of 25° to avertical direction. The ribbon has a radius to its outer edge of 11.9 cm and a radiusto its inner edge of 7 cm. The ribbon makes approximately 0.2 turns over its height,such that it extends up approximately 50% of the total internal height of the reactor from the floor of the reactor. The reactor is loaded with 6 kg of porous particles comprising a porous carbonframework and having a D50 particle diameter of 250 µm. This loading amount isapproximately 200 kg m-3 of the reactor internal volume and approximately 849 kgm-2of the surface area of the stirring surfaces. The stirring element is rotated at a speed of 160 rpm, which results in a stirring rate per kilogram of particles ofapproximately 0.004 m3 s-1 kg-1. The reactor is heated to a temperature ofapproximately 370 °C. During stirring, silane (SiH4) gas is continuously injectedinto the internal volume through the outer wall and the floor at a flowrate of 2 gramsof silicon in silane gas per minute per kilogram of porous particles, while effluent gas is continuously withdrawn. Example Process 4 A fourth example process demonstrates a stirring element with a relatively high surface area compared to the particle loading amount. This example used a vertical reactor having an internal volume of 30 L (0.03 m3) and having a radiusbetween the central axis and outer wall of 12.1 cm. The reactor had a stirringelement with four helical ribbons. Each helical ribbon defined an angle of 65° toa vertical direction. Each ribbon had a radius to its outer edge of 11.9 cm and aradius to its inner edge of 4 cm. Each ribbon made approximately of 1.5 turns overits height, and extends up approximately 80% of the total internal height of thereactor from the floor of the reactor.The reactor is loaded with 2 kg of porous particles comprising a porous carbonframework and having a D50 particle diameter of 20 µm. This loading amount isapproximately 67 kg m-3 of the reactor internal volume and approximately 5 kg m-2of the surface area of the stirring surfaces. The stirring element is rotated at a speed of 120 rpm, which results in a stirring rate per kilogram of particles ofapproximately 0.021 m3 s-1 kg-1. The reactor is heated to a temperature of in therange 330°C to 390°C suitable for gas injection. This example operates in a full-batch mode. During stirring, silane (SiH4) gas is injected into the internal volume until a pressure of 1000 kPa is reached, upon which the supply of gas is paused.The reactor is then heated to a temperature in the range 370°C to 420°C suitablefor the reaction. After a predetermined amount of time, effluent gas is withdrawn from the reactor bringing the reactor to atmospheric pressure and then silane gas added again to bring the pressure up to 1000 kPa, with the temperature being controlled within the above ranges for each phase of gas injection and reaction. This process is repeated until the process is complete. Example process 5 A fifth example process demonstrates a very large vertical reactor, having aninternal volume of 5000 L (5 m3) and an internal radius of 67.5 cm. The reactorhas a stirring element with two helical ribbons. Each helical ribbon defined an angle of 50° to a vertical direction. Each ribbon has a radius to its outer edge of65.5 cm and a radius to its inner edge of 25.5 cm. Each ribbon makesapproximately two thirds of a turn over its height, and extends up approximately60% of the total internal height of the reactor from the floor of the reactor. The reactor is loaded with 1000 kg of porous particles comprising a porous carbonframework and having a D50 particle diameter of more than 100 µm. In particular,the particles may be provided with a D1 particle diameter of 5.6 µm, a D10 particlediameter of 40.5 µm, a D50 particle diameter of 104.0 µm, a D90 particle diameterof 185.0 µm, and a D98 particle diameter of 237.0 µm. The particles had a BETsurface area of 1540 m2 g-1, a PD50 of 5.0 nm, a micropore to mesopore volumeratio of 5.7:94.3, and a pore volume of micropores and mesopores of 1.76 cm3 g-1.These particles were in Geldart Group A, had a Hausner ratio of 1.02 and a skew of 0.44 in the volume-based particle diameter distribution. This loading amount isapproximately 200 kg m-3 of the reactor internal volume and approximately507 kg m-2 of the surface area of the stirring surfaces. The stirring element isrotated at a speed of 60 rpm, which results in a stirring rate per kilogram ofparticles of approximately 0.003 m3 s-1 kg-1. The reactor is heated to atemperature of approximately 380 °C. During stirring, silane (SiH4) gas iscontinuously injected into the internal volume through the outer wall, the floor, andthe stirring element at a flowrate of 2 grams of silicon in silane gas per minute perkilogram of porous particles, while effluent gas is continuously withdrawn. Example Process 6 An example of a particularly preferred process uses a vertical reactor having an internal volume of 30 L (0.03 m3) and having a radius between the central axisand outer wall of 12.1 cm. The reactor has a stirring element with two helicalribbons. Each helical ribbon defines an angle of 45° to a vertical direction. Eachribbon has a radius to its outer edge of 11.9 cm and a radius to its inner edge of7.0 cm. Each ribbon makes approximately 0.6 turns over its height and extends up approximately 70% of the total internal height of the reactor from the floor of the reactor. The reactor is loaded with 5 kg of porous particles comprising a porous carbonframework and having a D50 particle diameter of less than 30 µm. In particular, theparticles may be provided with a D1 particle diameter of 0.9 µm, a D10 particlediameter of 3.2 µm, a D50 particle diameter of 16.7 µm, a D90 particle diameter of41.1 µm, and a D98 particle diameter of 56.9 µm. The particles had a BET surfacearea of 1626 m2 g-1, a PD50 of 1.0 nm, a micropore to mesopore volume ratio of69.6: 30.4, and a pore volume of micropores and mesopores of 0.7 cm3 g-1. Theseparticles were in Geldart Group C, had a Hausner ratio of 1.73 and a skew of 0.87in the volume-based particle diameter distribution. This loading amount isapproximately 167 kg m-3 of the reactor internal volume and approximately100 kg m-2 of the surface area of the stirring surfaces. The stirring element isrotated at a speed of 150 rpm which results in a stirring rate per kilogram ofparticles of approximately 0.007 m3 s-1 kg-1. The reactor is heated to a temperatureof approximately 380 °C. During stirring, silane (SiH4) gas is continuously injectedinto the internal volume through the outer wall and the floor. The gas is injected through the floor with a flow rate per unit area of the floor that increases from the central axis towards the outer wall, and injected through the outer wall with a flow rate per unit area of the outer wall that decreases vertically along the outer wall. Effluent gas is continuously withdrawn during the process. Example Process 7 Another example of a particularly preferred process uses a vertical reactor havingan internal volume of 5000 L (5 m3) and having a radius between the central axisand outer wall of 67.0 cm. The reactor has a stirring element with three helicalribbons. Each helical ribbon defined an angle of 50° to a vertical direction. Eachribbon has a radius to its outer edge of 66.5 cm and a radius to its inner edge of25.5 cm. Each ribbon makes approximately 0.8 turns over its height and extends up approximately 80% of the total internal height of the reactor from the floor of the reactor. The reactor is loaded with 650 kg of porous particles comprising a porous carbonframework and having a D50 particle diameter of less than 30 µm. In particular, theparticles may be provided with a D1 particle diameter of 0.9 µm, a D10 particlediameter of 3.2 µm, a D50 particle diameter of 16.7 µm, a D90 particle diameter of41.1 µm, and a D98 particle diameter of 56.9 µm. The particles had a BET surfacearea of 1626 m2 g-1, a PD50 of 1.0 nm, a micropore to mesopore volume ratio of69.6: 30.4, and a pore volume of micropores and mesopores of 0.7 cm3 g-1. Theseparticles were in Geldart Group C, had a Hausner ratio of 1.73 and a skew of 0.87in the volume-based particle diameter distribution. This loading amount isapproximately 130 kg m-3 of the reactor internal volume and approximately 163 kgm-2of the surface area of the stirring surfaces. The stirring element is rotated at a speed of 60 rpm, which results in a stirring rate per kilogram of particles ofapproximately 0.005 m3 s-1 kg-1. The reactor is heated to a temperature ofapproximately 380 °C. During stirring, silane (SiH4) gas is continuously injectedinto the internal volume through the outer wall and the floor. The gas is injected through the floor with a flow rate per unit area of the floor that increases from the central axis towards the outer wall, and injected through the outer wall with a flow rate per unit area of the outer wall that decreases vertically along the outer wall. Effluent gas is continuously withdrawn during the process. Example Process 8 An eighth example process uses a small lab-sized reactor having an internal volume of 0.6 L (0.0006 m3) and having a radius between the central axis and outer wall of 4.10 cm. The reactor has a stirring element with three helical ribbons. Each ribbon defines an angle of 45° to a vertical direction. Each ribbon has a radius to its outer edge of 3.95 cm and a radius to its inner edge of 2.95 cm. Each ribbon makes almost half of a full turn over its height, such that it extends up approximately 74% of the total internal height of the reactor from the floor of the reactor. The reactor is loaded with 50 grams of porous particles comprising a porous carbon framework and having a D50 particle diameter of 30 µm. This loadingamount is approximately 83 kg m-3 of the reactor internal volume andapproximately 15 kg m-2of the surface area of the stirring surfaces. The stirring element is rotated at a speed of 250 rpm, which results in a stirring rate perkilogram of particles of approximately 0.02 m3 s-1 kg-1. The stirring surfaces passthrough approximately 40% of the reactor internal volume during stirring. Thereactor is heated to a temperature of approximately 380 °C. During stirring, silane (SiH4) gas is injected into the internal volume through the outer wall under pressure to raise the internal volume to a pressure of 1000 kPa. Effluent gas is continuously withdrawn while maintaining this pressure. Example Process 9A ninth example process uses a vertical reactor, having an internal volume of5000 L (5 m3) and an internal radius of 67.5 cm. The reactor with a stirring elementwith two helical ribbons. Each helical ribbon defined an angle of 50° to a vertical direction. Each ribbon has a radius to its outer edge of 65.5 cm and a radius toits inner edge of 25.5 cm. Each ribbon makes approximately one full turn over itsheight, and extends up approximately 96% of the total internal height of the reactor from the floor of the reactor. The reactor is loaded with 1000 kg of relatively large porous particles. In particular,the particles may be provided with a D1 particle diameter of 5.6 µm, a D10 particlediameter of 40.5 µm, a D50 particle diameter of 104.0 µm, a D90 particle diameterof 185.0 µm, and a D98 particle diameter of 237.0 µm. The particles had a BETsurface area of 1540 m2 g-1, a PD50 of 5.0 nm, a micropore to mesopore volumeratio of 5.7:94.3, and a pore volume of micropores and mesopores of 1.76 cm3 g-1.These particles were in Geldart Group A, had a Hausner ratio of 1.02 and a skew of 0.44 in the volume-based particle diameter distribution. This loading amount isapproximately 200 kg m-3 of the reactor internal volume and approximately277 kg m-2 of the surface area of the stirring surfaces. The stirring element isrotated at a speed of 60 rpm, which results in a stirring rate per kilogram ofparticles of approximately 0.004 m3 s-1 kg-1. The reactor is heated to atemperature of approximately 380 °C. During stirring, silane (SiH4) gas iscontinuously injected into the internal volume through the outer wall, the floor, andthe stirring element at a flowrate of 2 grams of silicon in silane gas per minute perkilogram of porous particles, while effluent gas is continuously withdrawn. Once reaction time was complete, the reactor atmosphere was switched to pure nitrogen whilst maintaining fluidisation, this purge lasted 30 minutes. After this the furnace was ramped to ambient temperature over several hours. On reaching ambient temperature, the furnace atmosphere was switched to air gradually over a period of hours.The composite particles were then added to the feed tray of a MC DecJet® 30 milland the grinding took place in an inert atmosphere. The ring pressure was set at 650 kPa and the Venturi pressure was set at 700 kPa. The particles were micronized and then collected in a suitable container. This process reduced theD50 particle diameter of the resulting composite particles to approximately 3 µm.Example Process 10The tenth example is another example of a large reactor, like the second example,but operated at a more moderate pressure to that described above in the secondexample. The reactor and stirring element are the same as described above in the second example.The particles used in this example have a D1 particle diameter of 0.8 µm, a D10particle diameter of 1.8 µm, a D50 particle diameter of 4.8 µm, a D90 particlediameter of 8.5 µm, and a D98 particle diameter of 10.7 µm. The particles had aBET surface area of 2584 m2 g-1, a PD50 of 1.9 nm, a micropore to mesoporevolume ratio of 51.8:48.2, and a pore volume of micropores and mesopores of1.25 cm3 g-1. These particles were in Geldart Group C, had a Hausner ratio of1.89 and a skew of 0.41 in the volume-based particle diameter distribution.Like the second example process, the reactor is loaded with 65 kg of porous particles. This loading amount is approximately 127 kg m-3of the reactor internalvolume and approximately 85 kg m-2 of the surface area of the stirring surfaces. The stirring element is rotated at a speed of 90 rpm, which results in a stirring rateper kilogram of particles of approximately 0.008 m3 s-1 kg-1. The stirring surfacespass through approximately 68% of the reactor internal volume during stirring.The reactor is heated to a temperature of approximately 380 °C. During stirring,silane (SiH4) gas is injected into the internal volume through the outer wall underpressure to raise the internal volume to a pressure of 600 kPa. Effluent gas iscontinuously withdrawn while maintaining this pressure. Example Process 11The eleventh example uses a vertical reactor having an internal volume of 30 L(0.03 m3) and having a radius between the central axis and outer wall of 12.1 cm.The reactor has a stirring element with one helical ribbon. The helical ribbon defines an angle of 25° to a vertical direction. The ribbon has a radius to its outeredge of 11.9 cm and a radius to its inner edge of 7 cm. The ribbon makes 0.2 turnsover its height, such that it extends up below approximately 50% of the totalinternal height of the reactor from the floor of the reactor..The particles used in this example have a D1 particle diameter of 5.6 µm, a D10particle diameter of 40.5 µm, a D50 particle diameter of 104.0 µm, a D90 particlediameter of 185.0 µm, and a D98 particle diameter of 237.0 µm. The particles hada BET surface area of 1540 m2 g-1, a PD50 of 5.0 nm, a micropore to mesoporevolume ratio of 5.7:94.3, and a pore volume of micropores and mesopores of 1.76cm3 g-1. These particles were in Geldart Group A, had a Hausner ratio of 1.02 anda skew of 0.44 in the volume-based particle diameter distribution.The reactor is loaded with 6 kg of porous particles, such that the loading amountis approximately 200 kg m-3 of the reactor internal volume and approximately852 kg m-2 of the surface area of the stirring surfaces. The stirring element isrotated at a speed of 90 rpm, which results in a stirring rate per kilogram ofparticles of approximately 0.002 m3 s-1 kg-1. The stirring surfaces pass throughapproximately 33% of the reactor internal volume during stirring. The other detailsof the reactor, stirring element, loading quantity of the particles, and stirring speed were kept unchanged. Example Process 12 The twelfth example process is another example of a stirring element with a relatively high surface area compared to the particle loading amount, like the fourth example, but the particles used are different and the process was operated at a higher pressure. This example used a vertical reactor having an internal volume of 30 L (0.03 m3) and having a radius between the central axis and outer wall of 12.1 cm. The reactor had a stirring element with four helical ribbons. Each helical ribbon definedan angle of 65° to a vertical direction. Each ribbon had a radius to its outer edgeof 11.9 cm and a radius to its inner edge of 4 cm. Each ribbon made approximately1.5 full turns over its height, and extends up approximately 80% of the total internalheight of the reactor from the floor of the reactor.The reactor is loaded particles comprising a porous carbon framework having aD1 particle diameter of 1.0 µm, a D10 particle diameter of 3.0 µm, a D50 particlediameter of 8.2 µm, a D90 particle diameter of 14.4 µm, and a D98 particle diameterof 18.4 µm. The particles had a BET surface area of 2222 m2 g-1, a PD50 of 0.9nm, a micropore to mesopore volume ratio of 70.3:29.7, and a pore volume ofmicropores and mesopores of 0.97 cm3 g-1. These particles were in Geldart GroupC, had a Hausner ratio of 1.74 and a skew of 0.43 in the volume-based particlediameter distribution.The reactor was loaded with 2 kg of these porous particles. This loading amountis approximately 67 kg m-3 of the reactor internal volume and approximately 5 kgm-2 of the surface area of the stirring surfaces. The reactor is heated to atemperature of in the range 330°C to 390°C suitable for gas injection. Stirring isperformed at a speed of 60 rpm, which results in a stirring rate per kilogram ofparticles of approximately 0.011 m3 s-1 kg-1. During stirring, silane (SiH4) gas isinjected into the internal volume until a higher pressure of 1900 kPa is reached, upon which the supply of gas is paused. The reactor is then heated to a temperature in the range 370°C to 420°C suitable for the reaction. After a predetermined amount of time, effluent gas is withdrawn from the reactor bringing the reactor to atmospheric pressure and then silane gas added again to bring the pressure up to 1900 kPa, with the temperature being controlled within the above ranges for each phase of gas injection and reaction. This process is repeated until the process is complete. Example Process 13 The thirteenth example process corresponds largely to the eighth example process, but differs in particle composition, and uses a pressure much closer to ambient pressure. In particular, the reactor stirring element, loading quantity of the particles, and stirring speed are the same as in the eighth example. This example differs in thatthe particles provided have a D1 particle diameter of 0.8 µm, a D10 particlediameter of 1.8 µm, a D50 particle diameter of 4.8 µm, a D90 particle diameter of8.5 µm, and a D98 particle diameter of 10.7 µm. The particles had a BET surfacearea of 2584 m2 g-1, a PD50 of 1.9 nm, a micropore to mesopore volume ratio of51.8:48.2, and a pore volume of micropores and mesopores of 1.25 cm3 g-1.These particles were in Geldart Group C, had a Hausner ratio of 1.89 and a skew of 0.41 in the volume-based particle diameter distribution. After loading 50 grams of porous particles, the stirring element is rotated at a speed of 250 rpm, which results in a stirring rate per kilogram of particles ofapproximately 0.02 m3 s-1 kg-, and the reactor is heated to a temperature ofapproximately 380 °C. The stirring surfaces pass through approximately 40% ofthe reactor internal volume during stirring. During stirring, silane (SiH4) gas isinjected into the internal volume through the outer wall under pressure to raise theinternal volume to a relatively low pressure of 190 kPa. Effluent gas iscontinuously withdrawn while maintaining this pressure. Example 14 Another example of a particularly preferred process uses a vertical reactor that isthe same as in the seventh example. In particular, this has an internal volume of5000 L (5 m3) and having a radius between the central axis and outer wall of 67.0cm. The stirring element has three helical ribbons. Each helical ribbon definedan angle of 50° to a vertical direction. Each ribbon has a radius to its outer edgeof 66.5 cm and a radius to its inner edge of 25.5 cm. Each ribbon makesapproximately 0.8 turns over its height and extends up approximately 80% of the total internal height of the reactor from the floor of the reactor.The reactor is again loaded with 650 kg of porous particles comprising a porouscarbon framework and having a D50 particle diameter of less than 30 µm. In particular, the particles may be provided with a D1 particle diameter of 0.9 µm, aD10 particle diameter of 3.2 µm, a D50 particle diameter of 16.7 µm, a D90 particlediameter of 41.1 µm, and a D98 particle diameter of 56.9 µm. The particles had a BET surface area of 1626 m2 g-1, a PD50 of 1.0 nm, a micropore to mesopore volume ratio of 69.6: 30.4, and a pore volume of micropores and mesopores of 0.7 cm3g-1. These particles were in Geldart Group C, had a Hausner ratio of 1.73 and a skew of 0.87 in the volume-based particle diameter distribution. This loading amount is approximately 130 kg m-3of the reactor internal volume andapproximately 163 kg m-2 of the surface area of the stirring surfaces.Due to the high cohesivity of the particles, in this example the stirring element isrotated at a higher speed of 120 rpm, which results in a stirring rate per kilogramof particles of approximately 0.010 m3s-1kg-1. The stirring surfaces pass throughapproximately 66% of the reactor internal volume during stirring. This improvedthe aeration of the particle bed compared to the lower stirring speeds of the seventh example. The reactor is heated to a temperature of approximately 380 °C. During stirring, silane (SiH4) gas is continuously injected into the internal volume through the outer wall and the floor. The gas is injected through the floor with a flow rate per unit area of the floor that increases from the central axis towards the outer wall, and injected through the outer wall with a flow rate per unit area of the outer wall that decreases vertically along the outer wall. Effluent gas is continuously withdrawn during the process. Example 15A fifteenth example process demonstrates a very large vertical reactor configuredto handle particles in Geldart Group A in such a way as to achieve good aerationof the particle bed whilst also achieving a high resulting quantity of compositeparticles. The reactor has an internal volume of 5000 L (5 m3) and an internalradius of 67.0 cm. The reactor has a stirring element with two helical ribbons.Each helical ribbon defined an angle of 50° to a vertical direction. Each ribbon has a radius to its outer edge of 65.5 cm and a radius to its inner edge of 25.5 cm.Each ribbon makes approximately 0.7 turns over its height, and extends upapproximately 60% of the total internal height of the reactor from the floor of the reactor. The reactor is loaded with 1500 kg of porous particles comprising a porous carbonframework having a D1 particle diameter of 5.6 µm, a D10 particle diameter of 40.5µm, a D50 particle diameter of 104.0 µm, a D90 particle diameter of 185.0 µm, anda D98 particle diameter of 237.0 µm. The particles had a BET surface area of 1540m2 g-1, a PD50 of 5.0 nm, a micropore to mesopore volume ratio of 5.7:94.3, and apore volume of micropores and mesopores of 1.76 cm3 g-1. These particles werein Geldart Group A, had a Hausner ratio of 1.02 and a skew of 0.44 in the volume-based particle diameter distribution. This loading amount is approximately300 kg m-3 of the reactor internal volume and approximately 760 kg m-2 of thesurface area of the stirring surfaces. The stirring element is rotated at a speed of50 rpm, which results in a stirring rate per kilogram of particles of approximately0.001 m3 s-1 kg-1. The stirring surfaces pass through approximately 53% of thereactor internal volume during stirring. The reactor is heated to a temperature ofapproximately 395 °C. During stirring, silane (SiH4) gas is continuously injectedinto the internal volume through the outer wall, the floor, and the stirring element at a flowrate of 2 grams of silicon in silane gas per minute per kilogram of porous particles, while effluent gas is continuously withdrawn.Compared to the fifth example, this process used a larger porous particle load andlower stirring speed. Since these particles are of Geldart Group A, and relativelyeasy to aerate, the particle bed still achieved good aeration even with the higherloading and lower stirring rate. Example 16A sixteenth example process demonstrates a very large vertical reactor configuredto handle particles in Geldart Group C exhibiting especially high cohesivity. Thereactor has an internal volume of 5000 L (5 m3) and an internal radius of 67.0 cm.The reactor has a stirring element with three helical ribbons. Each helical ribbon defined an angle of 50° to a vertical direction. Each ribbon has a radius to its outeredge of 66.5 cm and a radius to its inner edge of 10.5 cm. Each ribbon makesapproximately 0.8 turns over its height and extends up approximately 80% of the total internal height of the reactor from the floor of the reactor.The reactor is loaded with 100 kg of porous particles comprising a porous carbonframework and having a D1 particle diameter of 0.8 µm, a D10 particle diameter of1.8 µm, a D50 particle diameter of 4.8 µm, a D90 particle diameter of 8.5 µm, and a D98 particle diameter of 10.7 µm. The particles had a BET surface area of 2584 m2g-1, a PD50 of 1.9 nm, a micropore to mesopore volume ratio of 51.8:48.2, and a pore volume of micropores and mesopores of 1.25 cm3g-1. These particles werein Geldart Group C, had a Hausner ratio of 1.89 and a skew of 0.41 in the volume-based particle diameter distribution. This loading amount is approximately20 kg m-3 of the reactor internal volume and approximately 20 kg m-2 of the surfacearea of the stirring surfaces. The stirring element is rotated at a speed of 85 rpm,which results in a stirring rate per kilogram of particles of approximately0.054 m3 s-1 kg-1. The stirring surfaces pass through approximately 76% of thereactor internal volume during stirring. The reactor is heated to a temperature ofapproximately 365 °C. During stirring, silane (SiH4) gas is continuously injectedinto the internal volume through the outer wall and the floor. The gas is injected through the floor with a flow rate of 2 grams of silicon in silane gas per minute per kilogram of porous particles. Effluent gas is continuously withdrawn during the process.Compared to the seventh example, this example used particles with smallerdiameter. Although both particles are Geldart Group C, the smaller particles used in this example exhibit even greater cohesivity, resisting aeration to a greaterdegree. Therefore, this example uses a lower loading amount of particles, a wider stirring element and a faster stirring speed. This led to good aeration of the particle bed even despite the high cohesivity of the particles.

Claims

CLAIMS1. A process for treating particles, the process comprising the steps of:(a) providing a plurality of porous particles in a reactor, the reactor having an outer wall defining an internal volume for containing the plurality of porous particles; (b) stirring the plurality of porous particles with a stirring element located in the internal volume, the stirring element having one or more stirring surfaces, each stirring surface being an area of the stirring element defining an oblique angle to its velocity vector during stirring and arranged such that movement of the stirring surface through the plurality of porous particles urges the plurality of porous particles along the outer wall at an angle to the velocity vector and / or away from the outer wall, wherein either: the outer wall is movable for causing the stirring of the plurality of particles and the one or more stirring surfaces of the stirring element protrude from an inner surface of the movable outer wall; or the stirring element defines a clearance between the stirring element and an inner surface of the outer wall of less than 10% of an internal dimension of the reactor measured along the direction of the clearance; wherein the mass of the plurality of porous particles contained in the internal volume during stirring is dependent on the volume of the internal volume,being no more than 500 kg m-3, and is dependent on the surface area of the oneor more stirring surfaces, being no more than 1000 kg m-2, and wherein the oneor more stirring surfaces pass through a volume per second during stirring thatdepends on the mass of the plurality of porous particles contained in the internalvolume, being between 0.001 and 0.1 m3 s-1 kg-1; and(c) during stirring the plurality of porous particles, contacting the pluralityof porous particles with a gas at conditions effective to cause a reaction between the plurality of porous particles and the gas.

2. A process according to claim 1, wherein the process is a process ofpreparing composite particles, and wherein step (c) comprises during stirring theplurality of porous particles, contacting the plurality of porous particles with asilicon precursor gas at conditions effective to cause deposition of silicon in thepores of the porous particles to provide composite particles comprising a porousparticle framework and elemental silicon within the pores of the porous particle framework.

3. A process according to claim 1 or claim 2, wherein the mass of theplurality of porous particles contained in the internal volume during stirring isdependent on the volume of the internal volume, being no more than 400 kg m-3,preferably no more than 300 kg m-3, more preferably no more than 250 kg m-3.

4. A process according to any of the preceding claims, wherein the mass ofthe plurality of porous particles contained in the internal volume during stirring isdependent on the volume of the internal volume, being no less than 15 kg m-3,preferably no less than 50 kg m-3, more preferably no less than 100 kg m-3.

5. A process according to any of the preceding claims, wherein the mass ofthe plurality of porous particles contained in the internal volume during stirring is dependent on the surface area of the one or more stirring surfaces, being no morethan no more than 750 kg m-2, preferably no more than 500 kg m-2, morepreferably no more than 400 kg m-2, most preferably no more than 300 kg m-2.

6. A process according to any of the preceding claims, wherein the mass ofthe plurality of porous particles contained in the internal volume during stirring is dependent on the surface area of the one or more stirring surfaces, being no lessthan 1 kg m-2, preferably no less than 3 kg m-2, more preferably no less than5 kg m-2, most preferably no less than 10 kg m-2.

7. A process according to any of the preceding claims, wherein the one ormore stirring surfaces pass through a volume per second dependent on the massof the plurality of porous particles contained in the internal volume of between0.002 and 0.08 m3 s-1 kg-1 during stirring, preferably between 0.003 and0.06 m3 s-1 kg-1, more preferably between 0.004 and 0.04 m3 s-1 kg-1, mostpreferably between 0.005 and 0.02 m3 s-1 kg-1.

8. A process according to any of the preceding claims, wherein theclearance between the stirring element and an inner surface of the outer wall is less than 5% of an internal dimension of the reactor measured along the direction of the clearance, preferably less than 2%, most preferably less than 1%.

9. A process according to any of the preceding claims, wherein theclearance between the stirring element and an inner surface of the outer wall isdependent on the sizes of the plurality of porous particles provided in the reactor, wherein the clearance between the stirring element and an inner surface of the outer wall is less than 10% of an internal dimension of the reactor measured along the direction of the clearance for porous particles having a D50particle diameterof more than 200 µm, wherein the clearance between the stirring element and aninner surface of the outer wall is less than 3% of an internal dimension of the reactor measured along the direction of the clearance for porous particles havinga D50 particle diameter in the range 30 µm to 200 µm, or wherein the clearancebetween the stirring element and an inner surface of the outer wall is less than 2% of an internal dimension of the reactor measured along the direction of theclearance for porous particles having a D50 particle diameter less than 30 µm.

10. A process according to any of the preceding claims, wherein theclearance between the stirring element and an inner surface of the outer wall isno more than 15 cm, preferably no more than 5 cm, more preferably no more than2 cm, more preferably no more than 1 cm, more preferably no more than 5 mm, most preferably no more than 2 mm.

11. A process according to any of the preceding claims, wherein theclearance between the stirring element and an inner surface of the outer wall isdependent on the sizes of the plurality of porous particles provided in the reactor,wherein the clearance between the stirring element and an inner surface of the outer wall is less than 150 mm for porous particles having a D50particle diameter of more than 200 µm, wherein the clearance between the stirring element and an inner surface of the outer wall is less than 50 mm for porous particles having a D50particle diameter in the range 30 µm to 200 µm, or wherein the clearance betweenthe stirring element and an inner surface of the outer wall is less than 10 mm forporous particles having a D50 particle diameter less than 30 µm.

12. A process according to any of the preceding claims, wherein the pluralityof porous particles have a D50particle diameter of no less than 0.01% of the closest clearance between the stirring element and an inner surface of the outer wall, more preferably the D50 particle diameter is no less than 0.05%, morepreferably no less than 0.1%, more preferably no less than 0.5%, more preferablyno less than 1%, most preferably no less than 2% of the clearance between the stirring element and an inner surface of the outer wall.

13. A process according to any of the preceding claims, wherein the pluralityof porous particles have a D50particle diameter of less than 200 µm, preferably less than 100 µm, more preferably less than 50 µm.

14. A process according to any of the preceding claims, wherein the pluralityof porous particles have a D50 particle diameter in the range 0.5 µm to 200 µm,preferably in the range 1 µm to 30 µm, more preferably in the range 2 µm to 10µm.

15. A process according to any of the preceding claims, wherein the pluralityof porous particles have a D1 particle diameter of at least 0.1 µm, preferably atleast 0.5 µm, more preferably at least 1 µm, more preferably at least 1.5 µm.

16. A process according to any of the preceding claims, wherein the pluralityof porous particles have a D10 particle diameter of at least 0.2 µm, preferably atleast 0.5 µm, more preferably at least 1 µm, more preferably at least 1.5 µm, morepreferably at least 2 µm, more preferably at least 3 µm.

17. A process according to any of the preceding claims, wherein the pluralityof porous particles have a D90 particle diameter of no more than 250 µm,preferably no more than 150 µm, more preferably no more than 100 µm, morepreferably no more than 50 µm, more preferably no more than 30 µm.

18. A process according to any of the preceding claims, wherein the pluralityof porous particles have a D98 particle diameter of no more than 300 µm,preferably no more than 200 µm, more preferably no more than 100 µm, more preferably no more than 50 µm, more preferably no more than 30 µm.

19. A process according to any of the preceding claims, wherein the pluralityof porous particles have a span, defined as^^^^^^^^^^, of no more than 5, preferably no more than 4, more preferably no more than 3, more preferably no more than 2, more preferably no more than 1.5.

20. A process according to any of the preceding claims, wherein the pluralityof porous particles have a positive skew in the volume-based particle diameterdistribution.

21. A process according to any of the preceding claims, wherein the pluralityof porous particles have a D50 diameter that is less than the volume-based mean particle diameter.

22. A process according to any of the preceding claims, wherein the skew ofthe particle diameter distribution of the plurality of porous particles (as measuredby a Malvern MastersizerTM 3000 analyzer) is no more than 4, preferably no morethan 3, more preferably no more than 2, more preferably no more than 1.5.

23. A process according to any of the preceding claims, wherein the skew ofthe particle diameter distribution of the plurality of porous particles (as measuredby a Malvern MastersizerTM 3000 analyzer) is at least 0.2, preferably at least 0.3,more preferably at least 0.4.

24. A process according to any of the preceding claims, wherein the pluralityof porous particles are of Geldart Group A, and preferably the one or more stirringsurfaces may pass through a volume per second during stirring that is between0.001 and 0.005 m3 s-1 kg-1.

25. A process according to any of the preceding claims, wherein the pluralityof porous particles are of Geldart Group C, and preferably the one or more stirringsurfaces may pass through a volume per second during stirring that is between0.005 and 0.1 m3 s-1 kg-1.

26. A process according to any of the preceding claims, wherein the stirringelement moves through between 5% and 90% of the internal volume during stirring, preferably between 10% and 80%, more preferably between 20% and 70%, most preferably between 30% and 60%.

27. A process according to any of the preceding claims, wherein the one ormore stirring surfaces are provided along between 50% and 100% of a length ofthe outer wall, preferably along between least 60% and 90% of the length of theouter wall, more preferably between 60% and 80% of the length of the outer wall.

28. A process according to any of the preceding claims, wherein stirring theplurality of porous particles comprises rotating a stirring element located in the internal volume about a central axis of the internal volume.

29. A process according to claim 28, wherein the internal volume hassubstantially continuous rotational symmetry about the central axis.

30. A process according to claim 28 or claim 29, wherein the stirring elementdefines an open centre around the central axis.

31. A process according to claim 30, wherein a radius of the open centre ofthe stirring element is between 10% and 90% of a distance between the central axis and the outer wall, preferably between 15% and 80%, further preferably between 20% and 70%, more preferably between 25% and 60%, most preferably between 30% and 50%.

32. A process according to any of the preceding claims, wherein each stirringsurface is an area of the stirring element further defining an oblique angle to a gravity vector during stirring and arranged such that movement of the stirring surface through the plurality of porous particles lifts the plurality of porous particlesagainst the gravity vector direction.

33. A process according to claim 32, wherein each stirring surface is an areaof the stirring element further defining an angle of between 20° and 80° to the gravity vector during stirring, preferably between 30° and 70°, more preferablybetween 40° and 60°, and / or defining an angle to the gravity vector during stirring of more than 45°.

34. A process according to any of the preceding claims, wherein the stirringelement comprises a helical stirring surface, preferably at least two separate helical stirring surfaces, more preferably at least three separate helical stirring surfaces.

35. A process according to any of the preceding claims, wherein the gas isintroduced into the internal volume of the reactor through a plurality of inlets intothe internal volume, wherein the plurality of inlets are arranged to produce a gasflow rate into the internal volume per unit area in a plane perpendicular to a centralaxis of the internal volume that increases from the central axis towards the outerwall.

36. A process according to claim 32 or any preceding claim when dependenton claim 32, wherein the gas is introduced into the internal volume of the reactor through a plurality of inlets through the outer wall, wherein the plurality of inletsthrough the outer wall are arranged to produce a gas flow rate into the internalvolume per unit area of the outer wall that decreases along the outer wall along adirection against the gravity vector direction.

37. A process according to any of the preceding claims, wherein the gas isintroduced into the internal volume of the reactor through a plurality of inlets located on the stirring element, wherein preferably the plurality of inlets arearranged to produce a gas flow rate into the internal volume that decreases alongthe stirring element, most preferably that decreases along a direction against the gravity vector direction.

38. A process according to any of the preceding claims, wherein the pressureinside the internal volume during step (c) is at least 100 kPa, preferably at least200 kPa, more preferably at least 300 kPa, more preferably at least 500 kPa, morepreferably at least 650 kPa, more preferably at least 750 kPa, more preferably atleast 1000 kPa, more preferably at least 1500 kPa, more preferably at least 2000 kPa, more preferably at least 2500 kPa, more preferably at least 3000 kPa.

39. A process according to any of the preceding claims, wherein the pressureinside the internal volume during step (c) is no more than 6000 kPa, morepreferably no more than 5000 kPa, more preferably no more than 4000 kPa, morepreferably no more than 3000 kPa, more preferably no more than 2000 kPa, morepreferably no more than 1600 kPa, more preferably no more than 1500 kPa, morepreferably no more than 1200 kPa, more preferably no more than 1000 kPa, morepreferably no more than 900 kPa, more preferably no more than 800 kPa, morepreferably no more than 700 kPa, more preferably no more than 650 kPa, morepreferably no more than 600 kPa, more preferably no more than 300 kPa.

40. A process according to any of the preceding claims, wherein thetemperature inside the internal volume during step (c) is in the range from 350 to500 °C, preferably from 350 to 450 °C, or 360 to 430 °C, or 370 to 420 °C, or 370to 400 °C.

41. A process according to any of the preceding claims, wherein the gas isintroduced into the internal volume substantially continuously during step (c), and / or during step (c), withdrawing an effluent gas from the internal volume,preferably substantially continuously.

42. A process according to any of the preceding claims, further comprising:(d), after step (c), comminuting the plurality of particles so as to reducethe D50 particle diameter.

43. A process according to claim 42, wherein the porous particles provided instep (a) have a D50particle diameter of at least 20 µm, preferably at least 30 µm, more preferably at least 40 µm, more preferably at least 50 µm, more preferably at least 100 µm, and wherein step (d) comprises comminuting the plurality ofparticles so as to reduce the D50 particle diameter to less than 20 µm, preferablyless than 10 µm, more preferably less than 5 µm.

44. A composition comprising or consisting of silicon-containing compositeparticles obtainable by a process according to any one of the preceding claims.

45. An electrode comprising silicon-containing composite particles obtainableby a process according to any one of claims 1 to 41.

46. A rechargeable metal-ion battery comprising an electrode according toclaim 45.

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