Process for the preparation of silicon-containing composite particles

Abstract

A process comprises providing a plurality of porous particles in a reactor having an outer wall defining an internal volume, stirring the particles with a stirring element located in the internal volu

Description

FIELD OF THE INVENTION This invention relates to processes for the production of silicon-containing composite 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 LiisSi^. 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 conductive particulate material, e.g. a porous carbon material (see WO 2020 / 095067 and WO 2020 / 128495). The silicon in these materials is finely divided with individual silicon 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 been synthesized by chemical vapour infiltration (CVI) in different reactor systems (static, rotary and fluidised bed reactors). More recently, close-clearance stirred bed reactors have been developed, as described in WO 2023 / 117047, to overcome problems present with each type of reactor system that lead to inhomogeneities in the final product. A problem with close-clearance stirred reactors of the sort disclosed in WO 2023 / 117047 is that they tend to produce compact 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 silicon-containing composite particles using stirred bed reactors that overcome the above problems. SUMMARY OF INVENTION In accordance with a first aspect of the invention, there is provided a process for preparing composite 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 nr3, 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 m3s'1 kg'1; 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. 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 is a region of the reactor in which there is a relatively high concentration of porous particles and a relatively low concentration of silicon precursor 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 silicon precursor gas. The aim of a stirred bed reactor, i.e. good aeration of the particle bed, may be considered in terms of the transition between these phases within the reactor, and to achieve good aeration it is desirable to minimise 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 high loading amount will therefore mean that a region of the bed may be aerated by a stirring process, but there may be large parts of the bed that cannot sufficiently aerated and remain in a highly concentrated phase. The present inventors have found 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'3 of 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 stirring surfaces affects the rate at which the stirrer may displace particles as it moves through the bed in order to move particles from the concentrated phase towards the dilute phase. As the surface area of the stirring surfaces decreases for a fixed amount of the particles, it becomes necessary to move the stirring surfaces through 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'2 of stirrer surface area. Finally, with a suitable loading amount of the particles, dependant on both the reactor volume and the stirrer surface area, it is necessary to ensure that 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 of the plurality of porous particles contained in the internal volume, being between 0.001 and 0.1 m3s’1 kg’1 during stirring. In other words, each kilogram of particles will require a certain volume to be swept per second by the stirring surfaces in order to appropriately aerate the particle bed. 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 urge the 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 are therefore not to be considered as part of the stirring surfaces, and as such their area 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 wall and perpendicular to its velocity vector will urge particles either away from the outer 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 along at 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%. 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 the internal volume, being no more than 400 kg nr3, preferably no more than 300 kg m'3, more preferably no more than 250 kg m'3 Similarly, preferably the mass 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, being no more than no more than 750 kg m'2, preferably no more than 500 kg m'2, more preferably no more than 400 kg nr2, most preferably no more than 300 kg nr2. Particularly preferable combinations include a mass of no more than 400 kg nr3 relative to reactor volume and no more than 1000 kg m'2 relative to stirrer area, or a mass of no more than 400 kg m'3 relative to reactor volume and no more than 750 kg m'2 relative to stirrer area, or a mass of no more than 400 kg m'3 relative to reactor volume and no more than 300 kg nr2 relative to stirrer area, or and a mass of no more than 300 kg m’3 relative to reactor volume and no more than 750 kg m'2 relative to stirrer area, or and a mass of no more than 300 kg m'3 relative to reactor volume and no more than 500 kg m'2 relative to stirrer area. While the upper loading limit for the porous particles has been discussed above in 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 the above upper limit will further improve aeration, but with diminishing returns and very 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 than 15 kg m’3, preferably no less than 50 kg m’3, more preferably no less than 100 kg m’3 Similarly, it is preferable that the mass 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, 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, more preferably 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 internal volume during stirring is between 15 kg m’3 and 500 kg m’3 of the volume of the internal volume, preferably between 50 kg m’3 and 500 kg m’3, more preferably between 50 kg m’3 and 400 kg m’3, more preferably between 100 kg m’3 and 400 kg m’3, more preferably between 100 kg m’3 and 300 kg m’3 most preferably between 100 kg m’3 and 200 kg m’3 Preferably, the mass of the plurality of porous particles contained in the internal volume during stirring is between 1 kg m’2 and 750 kg m’2 of the surface area of the one or more stirring surfaces, preferably between 3 kg m’2 and 500 kg m’2, more preferably between 10kgm’2 and 400 kg m’2, more preferably between 30 kg m’2 and 300 kg m’2, more preferably between 40 kg nr2 and 200 kg m’2. Likewise, to further improve the balance between displacement of the particle bed to cause aeration and the tendency to induce tangential movement and compaction of the particles at higher speeds, preferably the 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 of between 0.002 and 0.08 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’1 kg’1, most preferably between 0.005 and 0.02 m3s’1 kg’1. The geometry of the stirring surfaces may be varied within the above parameters to affect its arrangement relative to the particle bed. In some embodiments, the stirring 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 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 particles having a D50 particle diameter of more than 200 pm, 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 particles having a D5o particle diameter in the range 30 pm to 200 pm, 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 particles having a D50 particle diameter less than 30 pm. 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 clearance between the stirring element and an inner surface of the outer wall is no more than 10% 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 D5o particle diameter of more than 200 pm, wherein the clearance between the stirring element and an inner surface of the outer wall is less than 50 mm for particles having a D50 particle diameter in the range 30 pm to 200 pm, or wherein the clearance between the stirring element and an inner surface of the outer wall is less than 10 mm for particles having a D5o particle diameter less than 30 pm. Alternatively, the plurality of particles may have a D5o 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 D50 particle 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 surface of the outer wall and so limits build-up of particles on the outer wall. The invention is particularly useful for particles having a D5o particle diameter of less than 200 pm, preferably less than 100 pm, more preferably less than 50 pm. Particles of these sizes are considered particularly cohesive and so have a high tendency to agglomerate. 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. Preferably, the plurality of porous particles each comprise a porous carbon framework comprising micropores and / or mesopores. Preferably, the total pore volume of micropores and mesopores as measured by gas adsorption is in the range from 0.4 to 2.2 cm3 / g. Preferably, the PD5o pore diameter as measured by gas adsorption is no more than 20 nm. Preferably, the volumetric ratio of micropores to mesopores is from 90:10 to 55:45, or from 90:10 to 60:40, or from 85:15 to 65:35. 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 the volume moved through during stirring. 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 favourable movement 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 between the 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 extend substantially 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 preferably between 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°, more preferably within 10°, most preferably within 5°. Ina 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 a substantially vertical central axis. The stirring surfaces of a vertical reactor may typically 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, orsome combination of flat and curved. In a vertical reactor, 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, the direction of the velocity vector will be substantially perpendicular to the direction of 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. 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 silicon precursor gas is introduced into the internal volume of the reactor through a plurality of inlets into the internal volume, wherein the plurality of inlets are arranged to produce a silicon precursor gas flow rate into the 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 silicon precursor gas is introduced into the internal volume of the reactor through a plurality of inlets through the outer wall, wherein the plurality of inlets through the outer wall are arranged to produce a silicon precursor gas flow rate 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 are evenly exposed to the silicone precursor gas when the gas is introduced through the outer wall. The present arrangement allows the gas flow rate to be controlled to suitably match the characteristics of the stirred particle bed. Again, this variation in the silicon precursor gas flow rate 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 silicon precursor gas may be introduced into the internal volume of the reactor through a plurality of inlets located on the stirring element, wherein preferably the plurality of inlets are arranged to produce a silicon precursor gas flow rate into the internal 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. at approximately 100 kPa, but is also suited to use at high pressures, and so preferably the pressure inside the internal volume during step (c) is at least 100 kPa, more preferably at least 200 kPa, more preferably at least 500 kPa, most preferably at least 1000 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, or 370 to 420 °C, or 370 to 400 °C. The present process may be performed in modes in which the plurality of porous particles are introduced and removed in batches, but may also be performed in modes in which a feedstock of porous particles is provided and the porous particles 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 silicon precursor gas is introduced into the internal volume substantially continuously or semi-continuously during step (c), as well as modes with phased or staggered introduction of the silicon precursor gas. Preferably, the process also includes during step (c), withdrawing an effluent gas from the internal volume, preferably substantially continuously or semi-continuously. Preferably, 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 (S12H6), trisilane (SisHs), methylsilane, dimethylsilane and chlorosilanes. In a second aspect of the invention, there is provided a composition comprising or consisting silicon-containing composite particles obtainable by a process according to the first aspect of the invention. In a third aspect, the invention provides an electrode comprising silicon-containing composite 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 battery comprising an electrode according to the third aspect of the invention. 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 system of Figures 1 and 2; Figure 4 is a cross-section through the reactor and stirring element of the system of 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 a cylindrical closed 30 L (0.03 m3) internal volume within which the process according 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 3 is configured to rotate in use. Extending radially out from the central axis member 5 are a plurality of horizontal support rods that connect to and support two helical stirring surfaces 4, which extend outward from the horizontal support rods. Each helical stirring surface 4 is a ribbon shaped stirring surface that wraps around the central axis member 5 to form a double helix shape. The shape and configuration of the stirring surfaces 4 will be described further below. The stirring element of Figure 2 is for inserting inside the reactor 1 such that the central 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 supported by a gas-tight bearing, not shown, so that the motor 9 can drive the drive system 10 to cause the stirring element 3 to rotate inside the reactor by transmitting the rotational 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 stirring surfaces 4 extend away from the central axis member 5 towards the outer wall 2 of 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 outer radius, i.e. to the outer edge, proximate the outer wall 2, of 11.9 cm, so as to define a close clearance between the helical stirring surfaces 4 and the outer wall. The helical stirring surfaces may also have a width, i.e. along the radius direction, of approximately 5.5 cm. Each helical stirring surface 4 performs approximately one full turn over its height and extends substantially from the floor 6 of the reactor along 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 also possible to provide the helical stirring surface defining an angle along the radial direction so that rotation of the stirring element within the internal volume will also tend 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 silicone 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 described above and particularly shows the way a silicone precursor gas may be injected into the 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 gas may be injected into the internal volume through gas inlets 12 arranged on the cylindrical 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 silicone 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 silicone 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 silicone 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 an even flow rate per unit area of the floor, in this embodiment, the inlets are arranged such 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 silicone precursor gas may be injected into the reactor. In this embodiment, the silicone 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 ribbons to 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 silicone 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 silicone 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 perpendicular to the central axis 50. As shown in this Figure, a plurality of stirring surfaces 4 project outwards from the inside surface of the outer wall 2, into the internal volume of the reactor. Each stirring surface is inclined by an angle 0 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 of porous particles, each comprising a porous carbon framework comprising micropores and mesopores, which is a loading amount of 150 kg m'3 of the reactor internal volume and approximately 56 kg nr2 of 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 D5o particle diameter of less than 30 pm. The stirring element is rotated at an angular speed of 144 rpm, which results in a stirring rate per kilogram of particles of approximately 0.007 m3 s'1 kg'1. 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 at a flowrate of 2 grams of silicon in silane gas per minute per kilogram 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 wall of 31.3 cm. The reactor has a stirring element with two helical ribbons. Each ribbon defines an angle of 55° to a vertical direction. Each ribbon has a radius to its outer edge of 31.0 cm and a radius to its inner edge of 9 cm. Each ribbon makes one full turn over its height, and extends up approximately 76% of the total internal height of the reactor from the floor of the reactor. The reactor is loaded with 65 kg of porous particles comprising a porous carbon framework and having a D5o particle diameter of 30 pm. This loading amount is approximately 127 kg m'3 of the reactor internal volume 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 rate per kilogram of particles of approximately 0.008 m3 s'1 kg'1. 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 under pressure to raise the internal volume to a pressure of 1000 kPa. Effluent gas is continuously withdrawn while maintaining this pressure. Example Process 3 A third example process demonstrates a stirring element with a relatively low surface area compared to the particle loading amount. This 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 outer edge of 11.9 cm and a radius to its inner edge of 7 cm. The ribbon makes one full turn 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 carbon framework and having a D5o particle diameter of 250 pm. This loading amount is approximately 200 kg m'3 of the reactor internal volume and approximately 849 kg m’2 of 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 of approximately 0.021 m3 s'1 kg'1. The reactor is heated to a temperature of approximately 370 °C. During stirring, silane (SiH4) gas is continuously injected into the internal volume through the outer wall and the floor at a flowrate of 2 grams of 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 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 defined an angle of 65° to a vertical direction. Each ribbon had a radius to its outer edge of 11.9 cm and a radius to its inner edge of 4 cm. Each ribbon made approximately two full 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 2 kg of porous particles comprising a porous carbon framework and having a D5o particle diameter of 20 pm. This loading amount is approximately 67 kg nr3 of the reactor internal volume and approximately 5 kg nr2 of 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 of approximately 0.021 m3 s'1 kg'1. The reactor is heated to a temperature of in the range 330°C to 390°C suitable for gas injection. 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 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 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 an internal volume of 5000 L (5 m3) and an internal radius of 67.5 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 one full turns over its height, and extends up approximately 60% 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 carbon framework and having a D5o particle diameter of more than 100 pm. This loading amount is approximately 200 kg nr3 of the reactor internal volume and approximately 507 kg nr2 of 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 of approximately 0.004 m3 s’1 kg’1. 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, 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. 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 axis and outer wall of 12.1 cm. The reactor has a stirring element with two helical ribbons. Each helical ribbon defines an angle of 45° to a vertical direction. Each ribbon has a radius to its outer edge of 11.9 cm and a radius to its inner edge of 7.0 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 carbon framework and having a D50 particle diameter of less than 30 pm. This loading amount is approximately 167 kg m’3 of the reactor internal volume and approximately 100 kg m’2 of the surface area of the stirring surfaces. The stirring element is rotated at a speed of 150 rpm which results in a stirring rate per kilogram of particles of approximately 0.007 m3 s’1 kg’1. 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 Process 7 Another example of a particularly preferred process uses a vertical reactor having an internal volume of 5000 L (5 m3) and having a radius between the central axis and outer wall 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 outer edge of 66.5 cm and a radius to its inner edge of 25.5 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 carbon framework and having a D5o particle diameter of less than 30 pm. This loading amount is approximately 130 kg nr3 of the reactor internal volume and approximately 163 kg nr2 of 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 of approximately 0.005 m3 s’1 kg’1. 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 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 5 reactor. The reactor is loaded with 50 grams of porous particles comprising a porous carbon framework and having a D5o particle diameter of 30 pm. This loading amount is approximately 83 kg m'3 of the reactor internal volume and approximately 15 kg nr2 of the surface area of the stirring surfaces. The stirring 10 element is rotated at a speed of 250 rpm, which results in a stirring rate per kilogram of particles of approximately 0.02 m3 s'1 kg'1. 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 under pressure to raise the internal volume to a pressure of 1000 kPa. Effluent gas is continuously withdrawn while 15 maintaining this pressure.

Claims

1. A process for preparing composite 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; orthe 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 m3s'1 kg'1; 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 compositeparticles comprising a porous particle framework and elemental silicon within the pores of the porous particle framework.

2. A process according to claim 1, 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 400 kg m’3, preferably no more than 300 kg m’3, more preferably no more than 250 kg m’33. A process according to claim 1 or claim 2, 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 less than 15 kg m’3, preferably no less than 50 kg m’3, more preferably no less than 100 kg m’3.

4. A process according to any of the preceding claims, wherein the mass 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, being no more than no more than 750 kg m’2, preferably no more than 500 kg m’2, more preferably no more than 400 kg m’2, most preferably no more than 300 kg m’2.

5. A process according to any of the preceding claims, wherein the mass 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, being no less than 1 kg m’2, preferably no less than 3 kg nr2, more preferably no less than 5 kg m’2, most preferably no less than 10 kg m’2.

6. A process according to any of the preceding claims, wherein the 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 of between 0.002 and 0.08 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’1 kg’1, most preferably between 0.005 and 0.02 m3s’1 kg’1.

7. A process according to any of the preceding claims, wherein the clearance between the stirring element and an inner surface of the outer wall isless 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%.

8. A process according to any of the preceding claims, wherein 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 10% of an internal dimension of the reactor measured along the direction of the clearance for particles having a D50 particle diameter of more than 200 pm, 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 particles having a D5o particle diameter in the range 30 pm to 200 pm, 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 particles having a D50 particle diameter less than 30 pm.

9. A process according to any of the preceding claims, wherein the clearance between the stirring element and an inner surface of the outer wall is no more than 15 cm, preferably no more than 5 cm, more 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.

10. A process according to any of the preceding claims, wherein 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 pm, wherein the clearance between the stirring element and an inner surface of the outer wall is less than 50 mm for particles having a D50 particle diameter in the range 30 pm to 200 pm, or wherein the clearance between the stirring element and an inner surface of the outer wall is less than 10 mm for particles having a D50 particle diameter less than 30 pm.

11. A process according to any of the preceding claims, wherein the plurality of particles have a D5o 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 D50 particle 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.

12. A process according to any of the preceding claims, wherein the plurality of particles have a D50 particle diameter of less than 200 pm, preferably less than 100 pm, more preferably less than 50 pm.

13. A process according to any of the preceding claims, wherein 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%.

14. A process according to any of the preceding claims, wherein the one or more stirring surfaces are provided 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.

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

16. A process according to claim 15, wherein the internal volume has substantially continuous rotational symmetry about the central axis.

17. A process according to claim 15 or claim 16, wherein the stirring element defines an open centre around the central axis.

18. A process according to claim 17, wherein a radius of the open centre of the stirring element is between 10% and 90% of a distance between the central axis and the outer wall, preferably between 15% and 80%, further preferablybetween 20% and 70%, more preferably between 25% and 60%, most preferably between 30% and 50%.

19. A process according to any of the preceding claims, wherein each stirring surface 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 particles against the gravity vector direction.

20. A process according to claim 19, wherein 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°.

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

22. A process according to any of the preceding claims, wherein the silicon precursor gas is introduced into the internal volume of the reactor through a plurality of inlets into the internal volume, wherein the plurality of inlets are arranged to produce a silicon precursor gas flow rate into the internal volume per unit area in a plane perpendicular to a central axis of the internal volume that increases from the central axis towards the outer wall.

23. A process according to claim 19 or any preceding claim when dependent on claim 19, wherein the silicon precursor gas is introduced into the internal volume of the reactor through a plurality of inlets through the outer wall, wherein the plurality of inlets through the outer wall are arranged to produce a silicon precursor gas flow rate 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.

24. A process according to any of the preceding claims, wherein the silicon precursor gas is introduced into the internal volume of the reactor through a plurality of inlets located on the stirring element, wherein preferably the plurality of inlets are arranged to produce a silicon precursor gas flow rate into the internal volume that decreases along the stirring element, most preferably that decreases along a direction against the gravity vector direction.

25. A process according to any of the preceding claims, wherein the pressure inside the internal volume during step (c) is at least 100 kPa, preferably at least 200 kPa, more preferably at least 500 kPa, most preferably at least 1000 kPa.

26. A process according to any of the preceding claims, wherein the temperature inside the internal volume during step (c) is in the range from 350 to 500 °C, preferably from 350 to 450 °C, or 360 to 430 °C, or 370 to 420 °C, or 370 to 400 °C.

27. A process according to any of the preceding claims, wherein the silicon precursor gas is introduced 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.

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

29. An electrode comprising silicon-containing composite particles obtainable by a process according to any one of claims 1 to 27.

30. A rechargeable metal-ion battery comprising an electrode according to claim 29.

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