The invention is directed to a method for the growth of silicon nanowires using a zinc (II) halide, preferably zinc (II) chloride, as a metallic seed precursor. The method is cheap and robust, it proceeds at moderate temperature and allows control of silicon nanowires diameter. It is implemented in the presence of a growth support. Silicon nanowire-based composites, prepared by this method, can be used in various applications such as nano- and micro-electronics, spintronics, energy conversion and scavenging, sensors or anode material for lithium-ion batteries.

State of the art

Silicon, as a high earth abundant element with exceptional characteristics, is one of the centerpieces for many high-tech applications. Indeed, silicon is one of the leading components within solar cells technology as well as in microelectronics. Silicon has a low discharge potential and a very high theoretical charge capacity (>4000 mA.h.g ^-1 ), that make it very interesting for applications in Li-ion batteries. One of the other advantages of silicon is the possibility to modify its morphology via nano-structuration. Indeed, silicon is available as 0D (nanoparticles), ID (nanowires), 2D (nanosheets) morphologies. The nano-structuration of silicon is known to improve its capacity to withstand mechanical strains occurring during the lithiation/ de- lithiation process.

Among these morphologies, silicon nanowires (SiNWs) have attracted a lot of attention for their very high aspect ratio favoring efficient charge transport which is particularly beneficial for their application as anodes in Li-ion batteries.

In addition, their electrical conductivity can be easily improved by dopants which can extend their applications to supercapacitors (1, 2) and thermo-electrics (3).

Different techniques for SiNWs production are categorized mainly in two synthetic approaches: bottom-up (nanowire growth from elemental silicon) and top- down (etching of bulk silicon). Top-down approach is characterized by considerable waste of starting silicon and inevitable use of hazardous chemicals. Bottom-up technique, generally based on chemical vapor deposition (CVD), can produce high quality nanowires. This method is favored to produce composites of silicon nanowires and graphite/carbon. Industrial fabrication at acceptable price of such composites is an important challenge for battery market.

The synthesis of SiNWs did not evolve much since the 60s and the description of the bottom-up production of SiNWs by the vapor-liquid-solid (VLS) mechanism. Nowadays, the VLS is the most prominent and efficient method to synthesize SiNWs. More precisely, the VLS manufacturing process mainly focusses on the combination of a substrate, such as silicon wafer (2D), silicon or carbon nanoparticles (0D) and a growth seed usually in the form of a thin metallic film or nanoparticles.

Gold nanoparticles (Au NPs) are known as one the best seed for SiNWs growth. Indeed, the Au-Si binary phase diagram shows a first eutectic point at 363 °C. This low eutectic point allows the reaction to be carried out at relatively low temperature (compared to the melting point of gold around 1100 °C) and to be mostly driven by the temperature decomposition of the silane precursor.

The AuNPs “catalyzed” Si wire growth is usually performed by chemical vapor deposition (CVD) using a silicon precursor such as silane or diphenylsilane. However, this synthesis is exclusively performed at a laboratory scale for limited SiNWs quantities. Indeed, the “in-house” production of AuNPs is time consuming, expensive and could be difficult to scale up. This strategic material would be too expensive to allow economically viable mass production of SiNWs.

Other metals promote VLS mechanism and present a low eutectic point with silicon and no silicide phase in their binary phase diagram. For instance, Tin, Gallium, Cadmium, Indium, Strontium, Tellurium and Lead present this property. Among them, Zinc is earth abundant and presents a low eutectic point at 420 °C.

Zinc could be used in various forms as seed for the growth of Si NWs. For example, Uesawa et al. (4) reported Zinc (0) seeded SiNWs synthesis by CVD for use in device applications. SiNWs were grown from SiCL in presence of Zinc vapor (obtained by heating Zinc solid at 885 °C).

If this example proves that Zinc is an interesting candidate as seed for the growth of SiNWs, these materials remain expensive and do not allow a large scale production of SiNWs.

In the state of the art, ZnCL has been used as a precursor for Zinc-seeded Si NWs growth. For instance, Chung et al. (5) used a zinc chloride (II) ethanol solution to produce a zinc (0) deposit at low pressure (100 Torr) on a silicon substrate as seeds for the growth of SiNWs. This method permitted to produce thin SiNWs but the preparation of the zinc deposit is expensive and time consuming as it involves various steps and the use of a ZnCh solution.

CN105084365 describes a method for the preparation of Si nanomaterials which consists in mixing the silicon source, a reducing agent and a molten salt. The silicon source comes from silicon oxides, silicates, silicon-containing minerals, or biomass. The reducing agent is either potassium metal, calcium metal, sodium metal, magnesium metal, aluminum metal, or an alloy of the above metals. The molten salt could be aluminum chloride, zinc chloride, magnesium chloride, sodium chloride and lithium chloride. The method permits to produce silicon nanomaterials, but involves various steps which makes the process time consuming.

If these examples demonstrate that zinc is an interesting candidate as seed for the growth of Si NWs, these materials remain expensive and do not allow a large scale production of Si NWs.

A robust and economically viable technology for mass production of SiNWs is required to bring this unique and relevant material to several industrial applications.

The present invention describes a method for the growth of silicon nanowires using a zinc (II) halide, preferably zinc (II) chloride, as a metallic seed precursor. The method is simple, cost-efficient and robust. It uses in-situ transformation of a zinc (II) halide, preferably zinc (II) chloride, to zinc nanoparticles at moderate temperature. It also allows a satisfactory control of silicon nanowire diameter.

Summary of the invention

A first object of the invention consists in a method for the preparation of a composite material comprising at least silicon nanowires and zinc, said method comprising at least the following stages:

(1) introducing into a chamber of a reactor at least:

- a zinc halide ZnX2, with X selected from: F, Cl, Br, I, and

- a growth support,

(1 ’) solid/solid mixing of the zinc halide ZnX2 and the growth support,

(2) introducing, into the chamber of the reactor, at least one precursor compound of the silicon nanowires,

(3) decreasing the dioxygen content in the chamber of the reactor, (4) applying a thermal treatment at a temperature ranging from 200°C to 900°C, and

(5) recovering the product,

Wherein steps (1), (T), (2), (3), and (4) can be implemented in this order or in another order.

The invention also relates to a method of making an electrode including a current collector, said method comprising (i) implementing the method disclosed above to prepare a composite material comprising at least silicon nanowires and zinc, as an electrode active material, and (ii) covering at least one surface of the current collector with a composition comprising said electrode active material.

The invention also relates to a method of making an energy storage device, like a lithium secondary battery, including a cathode, an anode, and a separator disposed between the cathode and the anode, wherein at least one of the electrodes, preferably the anode, is obtained by the above disclosed method.

According to a first variant, the method for the preparation of a composite material is implemented in a fixed-bed reactor.

According to a second variant, the method for the preparation of a composite material is implemented in the chamber of a reactor comprising a rotating and/or a mixing mechanism.

According to an embodiment of this variant, the method is implemented in the tubular chamber of a tumbler reactor set in motion by a rotating and/or a mixing mechanism.

According to a favourite embodiment, the zinc halide is ZnCh.

According to a favourite embodiment, the zinc halide and the growth support are mixed together before their introduction into the reactor.

According to a favourite embodiment, the thermal treatment is performed at a temperature ranging from 400 °C to 650 °C.

According to a favourite embodiment, the thermal treatment is applied from 1 minute to 5 hours, preferably from 10 minutes to 2 hours, and more preferably from 30 minutes to 60 minutes.

According to a favourite embodiment, the method for the preparation of a composite material comprises a post-treatment step in order to transform organics, in particular organics resulting from the precursor compound of the silicon nanowires, into carbon materials.

According to a favourite embodiment, the method for the preparation of a composite material comprises an additional step (6) of treating the composite material obtained at the end of step (5) with an acidic solution.

According to a favourite embodiment, the precursor compound of the silicon nanowires is a silane compound or a mixture of silane compounds.

According to a favourite embodiment, the precursor compound of the silicon nanowires is silane (SiTU) or diphenylsilane Si(C6Hs)2H2.

According to a favourite embodiment, the growth support is a carbon-based material, a silicon-based material, an ITO based material, a carbonaceous polymer.

The method according to the invention is based on the use of a zinc halide, preferably ZnCb, as a catalyst for the preparation of SiNWs. It is advantageous in that it can be carried out in a one-pot reaction without pre-treatment of the catalyst.

The invention is implemented in the presence of a growth support.

The combination of the zinc halide, preferably ZnCb, and the growth support is simple and robust. Using a very stable product as ZnCb, or another zinc halide, allows an easy processing. Indeed, ZnCb and the other zinc halides only require solid/solid mixing with the growth support. Methods in which SiNWs growth is based, for example, on gold nanoparticles require a solid/liquid preparation, followed by an evaporation of solvents. The method according to the invention has the advantage of being implemented without any pre-treatment and without any solvent.

It is possible to control the nanowires’ diameters by the appropriate selection of the growth support physical properties. As illustrated in the examples, the diameters of nanowires prepared using ZnCb are directly impacted by the growth support characteristics.

Detailed description

The term "consists essentially of' followed by one or more characteristics, means that may be included in the process or the material of the invention, besides explicitly listed components or steps, components or steps that do not materially affect the properties and characteristics of the invention. The expression “comprised between X and Y” includes boundaries, unless explicitly stated otherwise. This expression means that the target range includes the X and Y values, and all values from X to Y.

A first object of the invention consists in a method for the production of a composite material comprising silicon nanowires through a chemical vapor deposition (CVD) based process. Said composite material is suitable for use as anode active material in lithium-ion batteries, while other uses are conceivable.

SiNWs composite materials obtained by this method can be used as produced, or can be submitted to post-production treatments.

The present invention relates to a process for the preparation of a silicon-based nanostructured material. It relates to a process for the preparation of a silicon-based composite material comprising at least nano-structured silicon material, zinc, and also a growth support material, and obtained from the chemical decomposition of a reactive silicon-containing gas species. The process is based on the chemical vapor deposition (CVD) principle.

The term "nanostructured material" is understood to mean, within the meaning of the invention, a material containing free particles, possibly in the form of aggregates or in the form of agglomerates, among which at least 5% by weight of said particles, with respect to the total weight of the material, have at least one of their external dimensions ranging from 1 nm to 100 nm, preferably at least 10%.

By “composite material”, we refer to a material made of at least two constituent materials with significantly different physical or chemical properties.

The external dimensions of the particles may be measured by any known method and notably by analysis of pictures obtained by scanning electron microscopy (SEM) of the composite material according to the invention.

The process for the preparation of the composite material

The invention relates to a process for the preparation of a composite material comprising at least zinc and SiNWs, the method comprising at least the following steps:

(1) introducing into a chamber of a reactor at least:

- a zinc halide Zn(X)2 with X selected from F, Cl, Br, I, preferably the zinc halide is zinc chloride ZnCh, and

- a growth support, (1 ’) solid/solid mixing of the zinc halide ZnX2 and the growth support,

(2) introducing, into the chamber of the reactor, at least one precursor compound of the silicon nanowires,

(3) decreasing the dioxygen content in the chamber of the reactor,

(4) applying a thermal treatment at a temperature ranging from 200°C to

900°C, and

(5) recovering the product.

The order of steps (1) to (4) can be the recited order or another order, depending essentially on: the characteristics of the reactor in which the method is implemented, the method for reducing dioxygen content and the state (liquid or gaseous) in which the precursor compound of the silicon nanowires is introduced into the reactor.

According to a first variant, the method is implemented in a fixed-bed reactor.

According to a second variant, the method is implemented in the tubular chamber of a tumbler reactor comprising a rotating and/or a mixing mechanism.

Preferably, the reactor is closed during the process.

By closed reactor is meant the introduction of gaseous species into the reactor is achieved at the beginning of the process and then the reactor is closed to gas flow during the thermal treatment step.

Process parameters

Process parameters which are reported here-under are common to all variants of the method (fixed-bed reactors, tumbler reactors with a rotating and/or a mixing mechanism).

The process according to the invention comprises the introduction of a growth support into the chamber of the reactor. The nature and characteristics of the growth support are detailed here-under.

The process according to the invention comprises a preliminary step of solid/solid mixing the growth support material with the ZnX2 here-after the catalyst.

According to a first variant, the ZnX2 catalyst (X=F, Cl, Br, I), preferably ZnCh, and the growth support are mixed together before their introduction into the reactor.

According to a second variant, the ZnX2 catalyst (X=F, Cl, Br, I), preferably ZnCb, and the growth support are mixed together in the chamber of the reactor after their introduction as separate raw materials into the reactor. Step (3) consisting in decreasing the dioxygen content in the chamber of the reactor can be performed by different methods:

Decreasing the dioxygen content in the chamber of the reactor can be implemented by placing the reactor under vacuum, preferably to a pressure inferior or equal to 10' ¹ bar (IO ^-2 MPa).

Alternately, decreasing the dioxygen content in the chamber of the reactor can be performed by washing the chamber of the reactor with an inert gas.

In the context of the invention, the expression “washing the chamber of the reactor with an inert gas” means that an inert gas flow is injected into the chamber of the reactor in order to replace the gas present in the reactor by the injected inert gas.

Preferably, the inert gas is chosen from dinitrogen N2, Argon Ar, and mixtures thereof.

Preferably, the chamber of the reactor is washed at least twice, more preferably at least 3 times with inert gas.

Preferably, at the end of step (3), the dioxygen content in the chamber of the reactor is inferior or equal to 1 % by volume, with respect to the total volume of the chamber of the reactor.

Preferably, the thermal treatment is performed at a temperature ranging from 200 to 900 °C, preferably from 300 °C to 700 °C, even more preferably from 400°C to 650°C.

Preferably, the thermal treatment is performed under low pressure, atmospheric pressure or pressure ranging from 0,1 to 30 MPa, preferably from 0,1 to 5 MPa, the pressure parameter being governed by the choice of the type of reactor.

During the process according to the invention, and because of the thermal treatment, the pressure in the reactor may increase. This internal pressure depends on the thermal treatment that is applied and is not necessarily controlled or monitored.

Preferably, the thermal treatment is applied from 1 minute to 5 hours, preferably from 10 minutes to 2 hours, and more preferably from 30 minutes to 60 minutes.

According to a variant embodiment, the process according to the invention comprises a post-treatment step, between steps (4) and (5), in order to transform organics into carbon materials. By “organics” it is meant organic chemical residues resulting from the decomposition of the silicon nanowire precursor, in particular silanes and/or diphenylsilane. When it is implemented, this step consists essentially of a thermal treatment. Advantageously, this step is performed under inert atmosphere, under a carrier gas atmosphere, like for example N2, Ar, a mixture of Ar/H2, at a temperature ranging from 500 °C to 700 °C, preferably from 550 °C to 650 °C, advantageously around 600 °C. Preferably, this thermal treatment (post-treatment step) is applied from 30 minutes to 5 hours, preferably from 1 hour to 3 hours.

According to a variant, the process according to the invention comprises an additional step (6) of washing the composite material obtained at the end of step (5).

The composite material obtained at the end of step (5) can be washed with an organic solvent, preferably chosen from: chloroform, ethanol, toluene, acetone, dichloromethane, petroleum ether and mixtures thereof.

Alternately, according to a favorite embodiment, the composite material obtained at the end of step (5) is washed with an acid solution.

According to this variant, preferably, after step (6), the process further comprises a supplementary step of drying the washed composite material.

Drying can be for example performed by placing the composite material into an oven, preferably at a temperature superior or equal to 40 °C, more preferably superior or equal to 60 °C.

Preferably, the drying step lasts from 15 minutes to 12 hours, more preferably from 2 hours to 10 hours, and even more preferably from 5 hours to 10 hours.

The precursor compound of the silicon nanowires

The process according to the invention comprises the introduction into the chamber of the reactor, of at least one precursor compound of the silicon nanowires. By “precursor compound of silicon nanowires”, we refer to a compound capable of forming silicon nanowires by implementing the method according to the invention, especially a compound capable of forming silicon nanowires under CVD process conditions.

This compound can be introduced into the chamber of the reactor as a liquid or as a gas. When the compound is introduced into the chamber of the reactor as a liquid, it is transformed to the gas state in the reactor chamber, by controlling the temperature and the pressure in the chamber of the reactor. When the precursor compound of silicon nanowires is in a gas state, it is designated « reactive silicon-containing gas species ». For example, if the precursor compound of SiNWs is a liquid, like for example diphenylsilane, when the reactor reaches appropriate temperature/pressure parameters, the liquid precursor evaporates to a gas species.

The precursor compound of silicon nanowires can be introduced into the reactor as a gas in mixture with a carrier gas.

If the precursor compound is in the form of a reactive silicon-containing gas species, it can be introduced into the chamber of the reactor in mixture with a carrier gas (forming a reactive silicon-containing gas mixture). For example, SiFU, a gas at ambient temperature/pressure, can be introduced directly into the chamber of the reactor alone or in mixture with a carrier gas. Alternately, a liquid precursor compound like diphenylsilane, Ph ₂ SiH ₂ , can be heated to be transformed to the vapour state in a preliminary stage of the process and then be introduced into the chamber of the reactor as a gas, alone or in mixture with a carrier gas.

Preferably, the precursor compound of silicon nanowires, or « reactive silicon- containing gas species », is a silane compound or a mixture of silane compounds.

For the purpose of the invention, the term “silane compound” refers to compounds of formula (I):

Ri-(SiR ₂ R ₃ )n-R4 (I) wherein:

- n is an integer ranging from 1 to 10, and

- Ri, R ₂ , R ₃ and R4 are independently chosen from hydrogen, C1-C15 alkyl groups, Ce-Ci ₂ aryl groups, C?-C ₂ o aralkyl groups and chloride.

According to this embodiment, preferably, the silicon-containing gas species is chosen from compounds of formula (I) wherein:

- n is an integer ranging from 1 to 5, and

- Ri, R ₂ , R ₃ and R4 are independently chosen from hydrogen, C1-C3 alkyl groups, phenyl, and chloride.

Even more preferably, n is an integer ranging from 1 to 3, and Ri, R ₂ , R ₃ and R4 are independently chosen from hydrogen, methyl, phenyl, and chloride.

According to this embodiment, preferably, the precursor compound of silicon nanowires is chosen from silane, disilane, trisilane, chlorosilane, dichlorosilane, trichlorosilane, dichlorodimethylsilane, phenylsilane, diphenylsilane or triphenylsilane or a mixture thereof. According to a preferred embodiment, the precursor compound of silicon nanowires is silane (SilH ) or diphenylsilane Si(C6Hs)2H2. The nature and physical state of the precursor compound of silicon nanowires is selected according to the type of reactor and the other parameters of the method.

The reactive silicon-containing gas mixture

The precursor compound of the silicon nanowires according to the invention can be introduced into the reactor as a gas, or as a liquid which is transformed to a gas in the reactor. The silicon nanowires are obtained from the chemical decomposition at high temperature of a reactive silicon-containing gas species, which may be in mixture with a carrier gas. This mixture is referred to hereinafter as « reactive silicon- containing gas mixture ».

By “carrier gas”, we refer to a gas that is chosen from a reducing gas, an inert gas, or a mixture thereof.

Preferably, the reducing gas is hydrogen (H2).

Preferably, the inert gas is chosen from argon (Ar), nitrogen (N2), helium (He), or a mixture thereof.

According to a preferred embodiment, the silicon-containing gas mixture is composed of at least 1 % by volume of silicon-containing gas species, preferably at least 10 % by volume, more preferably at least 50 % by volume, still more preferably 100 % by volume.

The proportions of silicon-containing gas species and carrier gas can be modulated at different levels at different steps of the process.

The catalyst

The process according to the invention comprises the introduction into the chamber of the reactor of a ZnX2 catalyst, with X a halide selected from the group consisting of: F, Cl, Br and I.

In the context of the invention “catalyst” designates a compound selected from compounds of the formula ZnX2, with X a halide selected from the group consisting of: F, Cl, Br and I.

Preferably, in the context of the invention, “catalyst” designates ZnCh.The function of the catalyst is to promote the growth of SiNWs. Preferably ZnX2, especially ZnCh, is under the form of particles. The process according to the invention comprises a step of solid/solid mixing of the ZnX2 catalyst and the growth support.

For the purposes of the invention, the term "solid/solid mixing" means that the growth support material and the catalyst undergo an association and/or combination and/or blending step corresponding to mixing of the catalyst with the growth support as raw materials in the solid state in order to obtain a material of essentially homogeneous composition. The solid/solid mixing is performed in the absence of any medium or solvent.

According to a favourite embodiment of the invention, the mixture of the ZnX2, preferably ZnCb, catalyst and the growth support is advantageously prepared before their introduction into the chamber of the reactor. The obtained solid mixture is then introduced into the chamber of the reactor and the steps (2) to (5) as disclosed above are implemented.

This step of the process according to the invention can be implemented with any industrial mixing apparatus known to the skilled professional such as ball-milling, attrition-milling, hammer milling, high energy milling, pin-milling, turbo-milling, fine cutting milling, impact milling, fluidized bed milling, conical screw milling, rotor milling, agitated bead milling, or jet milling. Preferably, this step of the process does not take more than 30 minutes.

According to another favourite embodiment of the invention, the solid/solid mixing of the ZnX2, preferably ZnCb, catalyst and the growth support is implemented in the chamber of the reactor after the introduction into the chamber of the reactor of the ZnX2 catalyst and the growth support as raw materials. According to this embodiment, the solid/solid mixing can, for example, be achieved through the mixing means and/or mechanism of the reactor. This can be the case, for example, when using a tumbler reactor with a rotating mechanism. Alternatively, the solid/solid mixing can be achieved by injecting an inert gas flow into the chamber of the reactor that can create a movement of particles by mechanical fluidization and thus their mixing.

According to the invention, the growth support material and the catalyst are associated before or after their introduction into the reactor.

For the purposes of the invention, the term "associated" means that the growth support material and the catalyst have undergone an association step corresponding to mixing of the catalyst with the growth support material in order to obtain a material of essentially homogeneous composition.

The combination of ZnX2, preferably ZnCb, and the growth support, according to the invention, is simple and robust. ZnCh, like the other zinc halides, as raw materials being a very stable product allows an easier processing compared to other catalysts. Indeed, ZnCh, like the other zinc halides, only requires solid/solid mixing with the growth support, whereas the growth medium based, for example, on gold nanoparticles requires a solid/ liquid preparation followed by an evaporation of solvents.

In case the growth support is a 2D support (ITO glass or Si wafers for example), advantageously, the solid/solid mixing results in a mixture of the catalyst and the growth support wherein the growth support is coated on part or all of its surface with the catalyst.

This coating can be implemented for example by dropping off the catalyst powder on the support.

Preferably, the catalyst and the growth support material are used according to a mass ratio catalyst/growth support ranging from 0.01 to 1, more preferably from 0.02 to 0.5, and still more preferably from 0.05 to 0.15.

The step of solid/solid mixing of the catalyst with the growth support material according to the invention allows the formation of a plurality of particles growth sites on the surface of the growth support material.

The growth support

The method according to the invention is implemented in presence of a growth support.

For example, the growth support can be a carbon-based material, a silicon-based material, an ITO based material, a carbonaceous polymer.

The growth support can be a 0D, ID, 2D or 3D material.

For example, 0D materials could be silicon nanoparticles or carbon black nanoparticles.

For example, ID materials could be carbonaceous polymer fibers or carbon nanotubes.

For example, 2D materials could be a silicon wafer, graphene, or an ITO glass. For example, 3D materials could be powders such as silicon microparticles, graphite (natural, artificial or expanded), or fine graphite, or a carbonaceous medium such as a polymer material.

The silicon-based support may be any material selected from the group consisting of silicon nanoparticles, silicon microparticles, silicon wafers.

Preferably, silicon nanoparticles have a mean particle size from 1 to 100 nm, more preferably from 30 to 50 nm.

Preferably, silicon microparticles have a mean particle size from 0,1 to 30 pm, advantageously from 1 to 15 pm.

Preferably, silicon wafers have a mean width size from 1 cm to 45 cm, advantageously from 1 to 10 cm. The ITO-based support may be any material selected from the group consisting of ITO glass have a mean width size from 1 cm to 100 cm, advantageously from 1 to 10 cm.

The carbon-based support may be any material selected from the group consisting of graphite, graphene, carbon, and more specifically natural graphite, artificial graphite, hard carbon, soft carbon, carbon nanotubes or amorphous carbon, carbon nanofibers, carbon black, expanded graphite, graphene or a mixture of two or more thereof.

The average particle size of the support may be measured by using a laser diffraction method.

In case the growth support is a carbon-based support, it can be under the form of particles, particulate agglomerates, non-agglomerated flakes, or agglomerated flakes.

According to this variant, aadvantageously, the carbon-based support has a Brunauer-Emmett-Teller (BET) surface ranging from 1 to 100 m ² /g, more preferably in the range of 1-70 m ² /g, even more preferably in the range of 3-50 m ² /g.

According to a favourite embodiment of this variant, the carbon-based material is selected from graphite, graphene, carbon, preferably graphite powder with a mean particle size from 0.01 to 50 pm.

According to another variant, the growth support is a carbonaceous polymer material. The use of a polymer as growth support has been disclosed in WO2021/018598.

When the growth support is a carbonaceous polymer material, preferably, the polymer material has a decomposition temperature, determined by thermal gravimetric analysis, superior or equal to 200 °C, preferably superior or equal to 300 °C, more preferably superior or equal to 400 °C, advantageously superior or equal to 500 °C.

Advantageously, according to this variant, the polymer material is chosen from fibrous polymer materials of synthetic or natural origin, preferably from fibrous polymer materials of synthetic origin.

More advantageously, according to this variant, the polymer material is chosen from polybenzothiazoles, polyamines, polyimides, polyurethanes, polybenzoxazoles, polyamides, polybenzimidazoles and mixtures thereof, preferably chosen from polyamides.

Even more advantageously, according to this variant, the polymer material is poly-paraphenylene terephtalamide, also known as Kevlar®.

In case the growth support is a polymer material, the method according to the invention comprises: i) preparing a composite material comprising a polymer material and SiNWs according to the process defined above, and ii) carbonizing the polymer material of the polymeric composite material. Detailed process parameters appropriate to prepare a silicon/polymer composite material and a silicon/carbon composite material have been disclosed in WO2021/018598.

It is possible to control the silicon nanowires’ diameter via the growth support physical properties. This possibility is illustrated in the experimental part for 3D growth supports:

As noted in the experimental part, the nanowires’ diameter prepared using ZnCh are directly impacted by the growth support material. Indeed, it has been observed that with an increase of the specific surface area (SSA > 10 m ² /g) of the growth support material, Si NWs present an average diameter around 650 nm. However, when the growth is made in presence of a growth support material with a lower specific surface area (SSA < 10 m ² /g), average diameters were found to be around 130 nm.

The morphology of the growth support material can also be used to control the nanowires’ diameters. For example, expanded graphite gives access to Si NWs with an average diameter around 200 nm (example 4). Doping material

According to one embodiment, the process according to the invention can comprise the introduction, into the reactor, of at least one doping material.

The term "doping material" is understood to mean, within the meaning of the invention, a material capable of modifying the conductivity properties of the silicon. A doping material within the meaning of the invention is, for example, a material rich in phosphorus, boron or also nitrogen atoms.

Preferentially, and according to this embodiment, the doping material is introduced into the chamber of the reactor by means of a precursor chosen from diphenylphosphine, triphenylborane and di- and triphenylamine. According to a first variant, this introduction is implemented before the growth of SiNWs has started.

According to another variant, the precursor of the doping material is introduced as a gas simultaneously with (and possibly as part of) the reactive silicon-containing gas mixture.

Preferably, the molar proportion of doping material, with respect to the precursor compound of the silicon nanowires, is from 10' ⁴ molar % to 10 molar %, preferably from 10' ² molar % to 1 molar %.

The reactor

The reactor is preferably closed during the thermal treatment step because ZnCh sublimates at low temperature (1,33 hPa at 428 °C). Silane decomposes at a similar or higher temperature which is not compatible with the use of an open reactor. For example, the decomposition temperature of SiFU at atmospheric pressure begins at 380 °C, but has a better yield at 500 °C.

According to a first variant, the method is implemented in a fixed-bed reactor.

According to a second variant, the method is implemented in the tubular chamber of a tumbler reactor comprising a rotating and/or a mixing mechanism.

First variant

According to a first variant, the method is implemented in a fixed-bed reactor.

• Characteristics of the reactor:

A reactor which can be used to implement the method according to the invention is disclosed for example in W02019020938. In this document, it is used in the “closed reactor” mode. • Parameters:

According to this first variant, the reactor is closed during the thermal treatment step. Decreasing the dioxygen content in the chamber of the reactor can be performed by placing the reactor under vacuum, preferably to a pressure inferior or equal to 10’ ¹ bar (IO ^-2 MPa).

Alternately, decreasing the dioxygen content in the chamber of the reactor can be performed by washing the chamber of the reactor with an inert gas.

In the context of the invention, the expression “washing the chamber of the reactor with an inert gas” means that an inert gas flow is injected into the chamber of the reactor in order to replace the gas present in the reactor by the injected inert gas before the reactor is closed.

Preferably, the inert gas is chosen from dinitrogen N2, Argon Ar, and mixtures thereof. Preferably, according to this variant, the chamber of the reactor is washed at least twice, more preferably at least 3 times with inert gas before the reactor is closed.

Preferably, at the end of step (3), the dioxygen content in the chamber of the reactor is inferior or equal to 1 % by volume, with respect to the total volume of the chamber of the reactor.

According to this variant, generally the precursor compound of the silicon nanowires is introduced into the reactor as a liquid.

According to this variant, the catalyst, the precursor compound of the silicon nanowires, and the growth support, can be introduced into the reactor in the form of a mixture.

According to this variant, preferably the reactor comprises at least two charging zones, a first zone which makes it possible to receive the precursor compound of the silicon nanowires and a second zone which makes it possible to receive the growth support and the catalyst.

According to a first alternative form, the first charging zone and the second charging zone are located at the same level in the chamber of the reactor.

According to a preferred alternative form, the second charging zone is raised with respect to the first charging zone.

• Steps of the process:

According to this variant, the process according to the invention advantageously comprises: (1’) solid/solid mixing of the zinc halide ZnX2 and the growth support in a mixing device,

(1) introducing into a chamber of a reactor the mixture resulting from step O’)

(2) introducing, into the chamber of the reactor, at least one precursor compound of the silicon nanowires,

(3) decreasing the dioxygen content in the chamber of the reactor,

(4) applying a thermal treatment at a temperature ranging from 200°C to 900°C, and

(5) recovering the product,

Wherein steps (T) and (1) are implemented in the recited order, but steps (2), (3), and (4) can be implemented in this order or in another order.

Second variant

According to a second variant, the method according to the invention is implemented in the chamber of a closed reactor comprising a rotating and/or a mixing mechanism.

• Characteristics of the reactor:

According to a first embodiment of this variant the method is implemented in the tubular chamber of a tumbler reactor comprising a rotating and/or a mixing mechanism. The tumbler reactor here-above mentioned is composed of at least a tubular chamber, heated by a furnace, in which the growth support and the zinc halide catalyst can be loaded either as separate materials or as a mixture. The reactor integrates a rotating mechanism and/or a mixing mechanism. The reactor can comprise two tubular chambers. The tubular chamber longitudinal axis is horizontal or can be tilted to make an angle with the horizontal axis up to 20°. The reactor further comprises a product feeding system and a product discharge system, allowing a semi- continuous production of silicon-zinc and growth support composite material. The tumbler reactor comprises a system of valves in order to allow the introduction of reactants in the closed chamber. The tumbler reactor comprises a reactor pressure control device, like for example a needle valve, or a pressure controller.

A typical mechanical tumbler reactor is a Lbdige’s type fluidized-bed reactor, where fluidization is generated by the rotation of a horizontal axis helix in the tubular chamber. Another typical mechanical tumbler reactor comprises a rotating tubular chamber where fluidization is generated by the rotation of the tubular chamber around its longitudinal axis.

According to a second embodiment of this variant the method is implemented in the chamber of a mixing reactor comprising a vertical mixing mechanism.

For example, the mixing chamber can be conical in shape.

Mixing of the reactants can be operated by the rotation of a vertical blade propeller or an end-less screw. The speed of the blades or the screw causes circular movements of the powders in the mixing chamber. The reactor further comprises a product feeding system and a product discharge system, allowing a semi-continuous production of silicon-zinc and growth support composite material. The reactor comprises a system of valves in order to allow the introduction of reactants in the closed chamber. The reactor comprises a reactor pressure control device, like for example a needle valve, or a pressure controller.

• Parameters:

This variant is of particular interest as fluidization mechanically generated by the reactor is beneficial to the contact between the growth support and the silicon comprising reactive gas species. This variant is also of particular interest since it permits the direct introduction of the catalyst and the growth support and their mixing in the chamber of the reactor.

According to this variant, preferably the precursor compound of the silicon nanowires is introduced into the reactor as a gas.

• Steps of the process:

According to this variant, the process according to the invention comprises, for example:

(1) introducing into the tubular chamber of the reactor at least the ZnX2 catalyst, preferably the ZnCb catalyst, and the growth support,

(1’) rotating the tubular chamber and/or starting the mixing mechanism,

(3) decreasing the dioxygen content in the chamber of the reactor by flowing inert gas,

(2A) introducing a reactive silicon-containing gas mixture into the tubular chamber,

(2B) closing the reactor, (4) applying a thermal treatment at a temperature ranging from 200 °C to 900 °C, in the tubular chamber under rotation and/or mixing,

(5) recovering the obtained product,

According to this variant, most steps have to be accomplished according to this order, however, the rotation and/or mixing at step (1’) can start before or after step (1).

It is being understood that, alternatively, according to this variant, the process can be implemented by first solid/solid mixing of the ZnX2 catalyst, preferably the ZnCb catalyst, and the growth support prior to their introduction into the tubular chamber, and then implementing the same steps (1) to (5) as above recited.

At the end of step (4), the reactor can be opened and another iteration of steps (2 A), (2B) and (4) can be implemented in order to continue SiNWs growth before the product is recovered.

According to this variant, the thermal treatment of step (4) is applied at low pressure (lower than atmospheric), or at atmospheric pressure or at a pressure superior to atmospheric.

Preferably, when the reactor is a reactor comprising a rotating and/or a mixing mechanism, the thermal treatment of step 4) is applied at a pressure superior to atmospheric.

Material composition:

The above disclosed method gives access to a composite material comprising, preferably consisting essentially of: the growth support, silicon nanowires and zinc particles. The material may comprise halides, especially chloride, as traces.

Advantageously, in the composite obtained, Si content is advantageously superior to 5 %, preferably superior to 20 %, by weight of silicon, with regards to the total weight of the material.

Si content can be calculated from the weight of the final material and the theoretical capacity of the components, combined with an electrochemical reverse dosage consisting in measuring the capacity of the final material. Alternately, Si content could be evaluated by ICP analysis (Inductively Coupled Plasma).

Zinc particles come from the decomposition of zinc (II) halide, especially zinc (II) chloride, during the reaction. The composite material preferably comprises zinc particles in amounts ranging from 1 % to 10 % by weight of zinc with regards to the total weight of the material, preferably ranging from 1% to 5 % by weight. Remaining zinc (II) halide, especially zinc chloride, can be partially removed by acidic treatment of the composite.

By remaining zinc (II) halide, especially zinc chloride, we refer to the fact that not all zinc (II) halide, reacts with silicon during the process. Traces of halide, especially chloride, can be found in the composite. Typical values are below 1 % by weight of halide, especially chloride, with regards to the total weight of the material, preferably below 0.1 %.

The silicon material, resulting from chemical vapor decomposition of the silicon-containing gas species, can be under the form of wires, worms, rods or filaments.

According to a preferred embodiment, the silicon material is in the form of nanowires.

The term "nanowire" is understood to mean, within the meaning of the invention, an elongated element, the shape of which is similar to that of a wire and the diameter of which is nanometric.

Preferably, silicon nanowires have a diameter ranging from 1 nm to 1000 nm, more preferentially ranging from 10 nm to 900 nm and more preferentially still ranging from 50 nm to 800 nm.

The size of the silicon material may be measured by several techniques well known by the skilled person such as for example by analysis of pictures obtained by scanning electron microscopy (SEM) from one or more samples of the carbon-silicon composite material.

Advantageously silicon, preferably silicon nanowires, represents from 1 % to 70 % by weight with regards to the total weight of the silicon-based composite material, preferably from 10 % to 70 % by weight.

The silicon-based composite material is preferably obtained in the form of a powder.

Uses of the silicon-zinc composite material

The silicon-zinc composite material according to the invention may be used as an anode active material and for the manufacture of a lithium-ion battery.

An electrode including a current collector is prepared by a preparation method classically used in the art. For example, the anode active material consisting in the silicon composite material of the present invention is mixed with a binder, a solvent, and a conductive agent. If necessary, a dispersant may be added. The mixture is stirred to prepare a slurry. Then, the current collector is coated with the slurry and pressed to prepare the anode.

Various types of binder polymers may be used as the binder in the present invention, such as a poly vinylidene fluori de-hexafluor opropylene copolymer (PVDF- co-HEP), polyvinylidene fluoride, polyacrylonitrile, and polymethylmethacrylate.

The electrode may be used to manufacture a lithium secondary battery including a separator and an electrolyte solution which are typically used in the art and disposed between the cathode and the anode.

Brief description of the drawings

Figure 1 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires / KS4 graphite composite at low magnification (example 1)

Figure 2 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires / KS4 graphite composite at high magnification (example 1)

Figure 3 is a graphic representing the potential profile of the cell prepared from Si nanowires / KS4 graphite composite (example 1): X = capacity of the cell in mA.h, Y = potential of the cell in V

Figure 4 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires / SFG15L graphite composite at low magnification (example 3)

Figure 5 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires / SFG15L graphite composite at high magnification (example 3)

Figure 6 is a graphic representing the potential profile of the cell prepared from Si nanowires / SFG15L graphite composite (example 3)

Figure 7 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires / BNB-90 graphite composite at low magnification (example 4)

Figure 8 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires / BNB-90 graphite composite at high magnification (example 4)

Figure 9 is a graphic representing the potential profile of the cell prepared from Si nanowires / BNB-90 graphite composite (example 4)

Experimental part:

In the following examples, and unless otherwise indicated, the contents and percentages are given in mass. Material

- Reactor (fixed bed): stainless steel reactor (intern volume = IL, diameter = 100 mm, height = 125 mm).

- ball-milling apparatus: Model PM100, commercialized by the company Retsch

- silicon precursor: diphenylsilane Si(C6Hs)2H2, commercialized by the company Sigma-Aldrich (CAS Number: 775-12-2),

- catalyst: ZnC12 commercialized by the company Thermo Scientific Alfa Aesar

- graphite growth support: BNB90 graphite (SSA = 21.18 m ² /g), KS4 graphite, (SSA = 24.48 m ² /g) and SFG15L graphite (SSA = 9,45 m ² /g) are commercialized by the company Imerys

- conductive fillers: graphite powder commercialized by the company Imerys under the name C-NERGY™ Actilion GHDR-15-4.

- carbon black, commercialized by the company Imerys under the commercial reference Timcal C-NERGY C65 (CAS Number: 1333-86-4).

- carboxymethylcellulose (CMC) commercialized by the company Alfa-Aesar (CAS Number: 9004-32-4).

- styrene-butadiene rubber (SBR) commercialized by the company MTI Corporation (CAS Number: 9003-55-8).

- electrolyte: lithium hexaflurorophosphate LiPFe (IM) dissolved in a mixture of ethylene carbonate (EC) and di ethyl carb onate (DEC) (1 : 1 in volume) comprising 10 % by weight of fluoroethylene carbonate (FEC) and 2 % by weight of vinylene carbonate (additive), commercialized by the company Solvionic.

Example 1: Synthesis of a batch of graphite/silicon composite material (Ml) a) Preparation o f the KS4 graphite / ZnC pre-catalyst material

3 g of graphite KS4 from Imerys is combined to 360 mg of ZnCh and introduced in a steel bowl of a Retsch PM100 ball-milling apparatus. Then, 40 g of 3 mm steel balls are introduced into the bowl before being tightly closed. The KS4/ZnC12 material is mixed for 10 minutes 30 seconds at 400 rpm.

The KS4 graphite / ZnCh pre-catalyst material is simply recovered by extracting the balls with a sieve. b) Growth of the silicon nanowires (Process 1)

This step corresponds to the steps (1) to (5) of the process for the preparation of a growth base/silicon composite material according to the invention. The growth base/pre-catalyst material obtained at the end of step a) is installed on a glass cup inside the fixed-bed reactor. 50 mL of diphenylsilane, Pt^SiEE, are then poured at the bottom of the reactor.

After sealing the reactor, gas lines and temperature heating elements are connected to the reactor. The reactor is then put under vacuum and purged several times with N2 to remove air/moisture contaminants. Subsequently, the reactor is heated up by means of an electric resistance placed in contact with the exterior surface of the reactor. The heating cycle is as follows: a heating ramp of 30 minutes from 20 °C to 430 °C, a plateau of 60 minutes at 430 °C, then heating is stopped, the reactor is left to cool down to room temperature and maintained at ambient temperature for 3 hours. The reactor is finally opened to recover the obtained material. c) Post-treatment of the growth base/silicon composite material (Process 2)

The carbonization of the organics coming from Ph2SiH2 decomposition is performed by thermal treatment.

The composite material obtained at the end of Process 1 is placed in a crucible which is then introduced in a horizontal quartz tube furnace. The inlet of the furnace is connected to argon Ar and dihydrogen H2 gas lines with controlled amounts in a ratio of 97.5:2.5 (v/v) that are continuously flowed over the material. Thermal treatments are performed with a heating ramp of 6 °C/min up to a temperature equal to 600 °C for a duration of 2 h, followed by natural cooling. The furnace is finally opened to recover the carbonaceous composite material.

The silicon content is calculated from the results of electrochemical measurement (combined with the theoretical capacity of Si and graphite). The active silicon content of the final material is ca. 15 wt% Si.

Figures 1 and 2 are SEMs micrographs of composite Ml with Si NWs 102 and 202 having a mean diameter of 648 nm on its KS4 graphite support 101 and 201.

Example 2: Preparation of the electrodes for lithium batteries

The electrochemical characterization of all prepared materials Ml, M2, and M3, respectively issued as Examples 1, 3, and 4, was performed by preparing coin-cells wherein the anode comprises as active material one of the materials Ml, M2 or M3. a) Mixing with conductive fillers

The starting composite material obtained at the end was mixed with graphite powder using YSZ 3 mm diameter grinding balls, in an IKA® Ultra-Turrax disperser. The composite material and the graphite were introduced into the disperser according to a weight ratio equal to 38:62.

Mixing was performed for 10 minutes at rotational speed 7.

The mixed material was finally recovered for further processing or characterization. b) Preparation o f a coin-cell

The synthesized material was mixed with a blend graphite powder (IMERYS Actilion GHDR-15-4 and SFG15L) at a weight ratio of ca. 38:62. A reference graphite electrode was made using pure graphite as the active material. For both systems, carbon black C-NERGY C65 was added as an electronic conductive additive, sodium carboxymethyl cellulose (Na-CMC) with styrene-butadiene rubber (SBR) were used as binders, and deionized water was employed as solvent. The weight ratios are 95: 1:4 for the active material :C65:binders. Water is added to reach a viscosity allowing electrode processing, yielding to a dry content of about 40 wt%. Wet mixing was performed for 30 minutes at speed 5. Each electrode ink was cast on a copper foil of 20 pm. After drying in air, the electrodes were further dried at 65 °C in an oven for 2 hours. The electrodes were then cut into discs of 14 mm diameter, calendered at ca. 1 t/cm ² and weighted, and were finally dried overnight in vacuum at 110 °C.

Half coin-cells (Kanematsu KGK Corp®, stainless steel 316L) were prepared inside an Ar glovebox using metallic Li as counter and reference electrodes, a layer of Whatman glass fiber and a layer of Celgard 2325 separator, and the electrode of interest. The electrolyte used to impregnate the electrode and separator materials was I M LiPFe dissolved in EC:DEC (1/1 v/v) with 10 wt% FEC (fluoroethylene carbonate) and 2 wt% VC (vinylene carbonate) additives. The cell was subsequently sealed with an automated press and taken out of the glovebox to be measured on a battery cycler.

Five formation cycles were performed prior to regular cycling at 1 C-rate. The formation cycles are made of 2 cycles at C/7 and 3 cycles at C/5 using galvanostatic and potentiostatic discharging (lithiation), and galvanostatic charging (delithiation). c) Determination of the electrochemical performances

The performances of the cells are determined by galvanostatic cycling using a Biologic BCS-805 cycling system equipped with 8 ways, each of the 8 ways comprising 2 different electrodes. 1- Potential profile

The potential profile of the cells Cl, C2, and C3 has been determined during the cycling at C/7 by measuring the potential of the cell as a function of its capacity.

Figures 3, 6 and 9 represent the potential profiles obtained from cells Cl, C2, and C3 recorded during the first cycle at C/7

On Figure 3, 6, and 9, we observe that the potential profiles of the cell obtained from composite Material Ml, M2, and M3 show the cumulation of the electrochemical activity of graphite and silicon materials, which evidences that the composite materials are electrically and electrochemically active. The electrochemical activity of silicon with lithium ions is well visible with the inflexion/pseudo-plateau near 0.45 V during charge (delithiation).

2- Initial reversible capacity

The initial reversible capacity of the cells, measured at C/7 during the first cycle, is given in the Table 1.

Table 1

A comparison of cell Cl, cell C2 and cell C3 reveals an improvement of the initial capacity when the silicon active content increases (respectively 696 mA.h/g for ca. 15 wt% Si vs. 638 mA.h/g for ca. 12 wt% Si vs. 854 mA.h/g for ca. 21 wt% Si), in composite Material Ml vs. composite Material M2 vs. composite Material M3. Overall, these results demonstrate that the specific surface area and the morphology of the medium help to tune the growth ratio of the Si NWs onto the medium surface and allow the desired control of the electrical and electrochemical performances of the carbonaceous composite materials.

Example 3: Synthesis of a batch of SFG15L graphite / Silicon NWs material (M2) a) Preparation o f the SFG15L graphite / pre-catalyst ZnCl^material

3 g of SFG15L from Imerys is combined to 360 mg of ZnCh and introduced in a steel bowl of a PM100 ball-milling apparatus. Then, 40 g of 3 mm steel balls are introduced in the bowl before being tightly closed. The SFGlSL-ZnCh material is mixed for 10 minutes 30 seconds at 400 rpm. The SFG15L graphite / pre-catalyst ZnCh is simply recovered by extracting the balls with a sieve. b) Growth of the silicon nanowires (Process 1)

The SFG15L graphite / pre-catalyst ZnCh material obtained at the end of step a) is installed on a glass cup inside the fixed-bed reactor. 50 mL of diphenylsilane, Pt^SiEE, are then poured at the bottom of the reactor.

After sealing the reactor, gas lines and temperature heating elements are connected to the reactor. The reactor is then put under vacuum and purged several times with N2 to remove air/moisture contaminants. Subsequently, the reactor is heated up by means of an electric resistance placed in contact with the exterior surface of the reactor. The heating cycle is as follows: a heating ramp of 30 minutes from 20 °C to 430 °C, a plateau of 60 minutes at 430 °C, then heating is stopped, the reactor is left to cool down to room temperature and maintained at ambient temperature for 3 hours. The reactor is finally opened to recover the obtained material. c) Post-treatment o f the SFG15L graphite /silicon composite material (Process 2X

The carbonization of the organics coming from PhiSiEE decomposition is performed by thermal treatment.

The composite material obtained at the end of Process 1 is placed in a crucible which is then introduced in a horizontal quartz tube furnace. The inlet of the furnace is connected to argon Ar and dihydrogen H2 gas lines with controlled amounts in a ratio of 97.5:2.5 (v/v) that are continuously flowed over the material. Thermal treatments are performed with a heating ramp of 6 °C/min up to a temperature equal to 600 °C for a duration of 2 h, followed by natural cooling. The furnace is finally opened to recover the carbonaceous material.

The active silicon content of the final material is ca. 12 wt% Si.

Figures 4 and 5 are SEMs micrographs of the composite material M2 with Si NWs 402 and 502 having a mean diameter of 131 nm on its SFG15L graphite support 401 and 501. Example 4: Synthesis of a batch of BNB-90 graphite / Silicon NWs material (M3) a) Preparation of the BNB-90 graphite / pre-catalyst ZnCl^material

3 g of BNB-90 from Imerys is combined to 360 mg of ZnCh and introduced in a steel bowl of a PM100 ball-milling apparatus. Then, 40 g of 3 mm steel balls are introduced in the bowl before being tightly closed. The BNB-90-ZnCh material is mixed for 10 minutes 30 seconds at 400 rpm.

The BNB-90 graphite / pre-catalyst ZnCh is simply recovered by extracting the balls with a sieve. b) Growth of the silicon nanowires (Process 1)

The BNB-90 graphite / pre-catalyst ZnCh material obtained at the end of step a) is installed on a glass cup inside the fixed-bed reactor. 50 mL of diphenylsilane, Pt^SiJB, are then poured at the bottom of the reactor.

After sealing the reactor, gas lines and temperature heating elements are connected to the reactor. The reactor is then put under vacuum and purged several times with N2 to remove air/moisture contaminants. Subsequently, the reactor is heated up by means of an electric resistance placed in contact with the exterior surface of the reactor. The heating cycle is as follows: a heating ramp of 30 minutes from 20 °C to 430 °C, a plateau of 60 minutes at 430 °C, then heating is stopped, the reactor is left to cool down to room temperature and maintained at ambient temperature for 3 hours. The reactor is finally opened to recover the obtained material. c) Post-treatment of the BNB-90 graphite /silicon composite material (Process 2)

The carbonization of the organics coming from Pt^SiJB decomposition is performed by thermal treatment.

The composite material obtained at the end of Process 1 is placed in a crucible which is then introduced in a horizontal quartz tube furnace. The inlet of the furnace is connected to argon Ar and dihydrogen H2 gas lines with controlled amounts in a ratio of 97.5:2.5 (v/v) that are continuously flowed over the material. Thermal treatments are performed with a heating ramp of 10 °C/min up to a temperature equal to 600 °C for a duration of 2 h, followed by natural cooling. The furnace is finally opened to recover the carbonaceous material.

The active silicon content of the final material is ca. 21 wt% Si. Figures 7 and 8 show SEMs micrographs of the composite material M3 with Si NWs 702 and 802 with a mean diameter of 198 nm on its BNB-90 graphite support 701 and 801.

Overall, these results demonstrate that the specific surface area and the morphology of the growth support permit to tune the growth ratio of Si NWs on the growth support and allow control of the electrical and electrochemical performances of the composite materials.

References

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(3) P. Yang et al., Nature, 2008, 451, 163-167

(4) Uesawa et al. Journal of Physical Chemistry C, 114, 2010, 4291-4296

(5) Chung et al. Applied Physics Letters 76, 15, 2000, 2068-2070