METHOD FOR THE PREPARATION OF A MATERIAL COMPRISING SILICON NANOWIRES AND TIN

Abstract

A method for the preparation of a composite material including at least silicon nanowires and tin, the method including the use of a tin halide, as a catalyst for preparing silicon nanowires.

Claims

1-14. (canceled)

15. A method for the preparation of a composite material comprising at least silicon nanowires and tin, comprising at least the following stages: (1) introducing into a chamber of a reactor at least: a tin halide SnX.sub.2, with X selected from: F, Cl, Br, I, and a growth support, (1) solid/solid mixing of the tin halide SnX.sub.2 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), (1), (2), (3), and (4) can be implemented in this order or in another order.

16. The method of claim 15, wherein it is implemented in a fixed-bed reactor.

17. The method of claim 15, wherein it is implemented in the tubular chamber of a tumbler reactor set in motion by a rotating and/or a mixing mechanism.

18. The method as claimed in claim 15, wherein the tin halide is SnCl.sub.2.

19. The method as claimed in claim 15, wherein the tin halide and the growth support are mixed together before their introduction into the reactor.

20. The method as claimed in claim 15, wherein the thermal treatment is performed at a temperature ranging from 200? C. to 900? C.

21. The method as claimed in claim 20, wherein the thermal treatment is performed at a temperature ranging from 300? C. to 650? C.

22. The method as claimed in claim 15, wherein the thermal treatment is applied from 1 minute to 5 hours.

23. The method as claimed in claim 22, wherein the thermal treatment is applied from 10 minutes to 2 hours.

24. The method as claimed in claim 15, wherein it comprises a post-treatment step in order to transform organics, resulting from the precursor compound of the silicon nanowires, into carbon materials.

25. The method as claimed in claim 15, wherein it comprises an additional step (6) of treating the composite material obtained at the end of step (5) with an acidic solution.

26. The method as claimed in claim 15, wherein the precursor compound of the silicon nanowires is a silane compound or a mixture of silane compounds.

27. The method as claimed in claim 26, wherein the precursor compound of the silicon nanowires is silane SiH4 or diphenylsilane Si(C.sub.6H.sub.5).sub.2H.sub.2.

28. The method as claimed in claim 15, wherein the growth support is a carbon-based material, a silicon-based material, an ITO based material, or a carbonaceous polymer.

29. A method of making an electrode including a current collector, said method comprising (i) implementing the method of claim 15 to prepare composite material comprising at least silicon nanowires and tin, as an electrode active material, and (ii) covering at least one surface of the current collector with a composition comprising said electrode active material.

30. A method of making an energy storage device including a cathode, an anode, and a separator disposed between the cathode and the anode, wherein said method comprises implementing the method of claim 29 to make at least one of the electrodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0223] FIG. 1 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires/KS4 graphite composite at low magnification (example 1)

[0224] FIG. 2 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires/KS4 graphite composite at high magnification (example 1)

[0225] FIG. 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

[0226] FIG. 4 is a graphic representing the derivatives plot of 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

[0227] FIG. 5 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires/KS4 composite after HCI washing step at low magnification (example 2)

[0228] FIG. 6 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires/KS4 graphite composite after HCI washing step at high magnification (example 2)

[0229] FIG. 7 is a graphic representing the potential profile of the cell prepared from Si nanowires/KS4 graphite composite after HCI washing step (example 2): X=capacity of the cell in mA.h, Y=potential of the cell in V

[0230] FIG. 8 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires/BNB90 graphite composite at low magnification (example 3)

[0231] FIG. 9 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires/BNB90 graphite composite at high magnification (example 3)

[0232] FIG. 10 is a graphic representing the potential profile of the cell prepared from Si nanowires/BNB90 graphite composite (example 3): X=capacity of the cell in mA.h, Y=potential of the cell in V

[0233] FIG. 11 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires/SLP50 graphite composite at low magnification (example 4)

[0234] FIG. 12 is a photograph obtained by scanning electron microscopy (SEM) of Si nanowires/SLP50 composite at high magnification (example 4)

[0235] FIG. 13 is a graphic representing the potential profile of the cell prepared from Si nanowires/SLP50 graphite composite (example 4): X=capacity of the cell in mA.h, Y=potential of the cell in V.

[0236] FIG. 14 is a photograph obtained by scanning electron microscopy (SEM) at low magnification of a Si nanowires/Si nanoparticles composite (example 5).

[0237] FIG. 15 is a photograph obtained by scanning electron microscopy (SEM) at high magnification of a Si nanowires/Si nanoparticles composite (example 5).

[0238] FIG. 16 shows the potential profile (X=capacity of the cell in mA.h, Y=potential of the cell in V) of the cell prepared from Si nanowires/Si nanoparticles composite (example 5).

[0239] FIG. 17 is a photograph obtained by scanning electron microscopy (SEM) at low magnification of a Si nanowires/tin/KS4 composite obtained by treatment with SiH.sub.4 (example 6).

[0240] FIG. 18 is a photograph obtained by scanning electron microscopy (SEM) at high magnification of a Si nanowires/tin/KS4 composite obtained by treatment with SiH.sub.4 (example 6).

Experimental Part:

[0241] In the following examples, and unless otherwise indicated, the contents and percentages are given in mass.

Material

[0242] Reactor (fixed bed): stainless steel reactor (intern volume=1 L, diameter=100 mm, height=125 mm). [0243] ball-milling apparatus: Model PM100, commercialized by the company Retsch [0244] silicon precursor: diphenylsilane Si(C.sub.6H.sub.5).sub.2H.sub.2, commercialized by the company Sigma-Aldrich (CAS Number: 775-12-2), [0245] catalyst: SnCl2 commercialized by the company Strem Chemicals.Inc [0246] graphite growth support: BNB90 graphite (SSA=21.18 m.sup.2/g), KS4 graphite, (SSA=24.48 m.sup.2/g) and SLP50 graphite (SSA=4.97 m.sup.2/g) are commercialized by the company Imerys [0247] Silicon NPs: having an average particle size?50 nm commercialized by the company GetNanoMaterials under commercial reference Si-100 [0248] conductive fillers: graphite powder commercialized by the company Imerys under the name C-NERGY? Actilion GHDR-15-4. [0249] carbon black, commercialized by the company Imerys under the commercial reference Timcal C-NERGY C65 (CAS Number: 1333-86-4). [0250] carboxymethylcellulose (CMC) commercialized by the company Alfa-Aesar (CAS Number: 9004-32-4). [0251] styrene-butadiene rubber (SBR) commercialized by the company MTI Corporation (CAS Number: 9003-55-8). [0252] electrolyte: lithium hexaflurorophosphate LiPF.sub.6 (1M) dissolved in a mixture of ethylene carbonate (EC) and diethylcarbonate (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 KS4 Graphite/Si NWs Composite Material (M1)

a) Mixing KS4 Graphite and SnCl.SUB.2 .as Pre-Catalyst Material

[0253] 3 g of graphite KS4 is combined to 0.5 g of SnCl2 and introduced in a steel bowl of the ball-milling apparatus. Then, 40 g of 3 mm steel balls are introduced in the bowl before being tightly closed. The powders are mixed for 10 minutes 30 seconds at 400 rpm.

[0254] The pre-catalyst material is recovered by extracting the balls with a sieve.

b) Growth of the Silicon Nanowires (Process 1)

[0255] The material obtained at the end of step a) is placed on a glass cup inside the fixed-bed reactor. 50 mL of diphenylsilane, Ph.sub.2SiH.sub.2, are then poured at the bottom of the reactor.

[0256] 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 N.sub.2 to decrease the oxygen level. 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., the heating is stopped and then the reactor is cooled down to room temperature. The reactor is finally opened to recover the composite material.

c) Post-Treatment of the Graphite/Tin/Silicon Composite Material (Process 2)

[0257] The carbonization of the organics resulting from Ph.sub.2SiH.sub.2 decomposition is performed by thermal treatment.

[0258] 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 H.sub.2 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 composite M1.

[0259] FIGS. 1 and 2 are SEMs micrographs of composite M1 with Si NWs 101 and 201 having a mean diameter of 80 nm.

d) Description of FIG. 2

[0260] FIG. 2 shows a mixture of Si NWs 201 and small dots 202 dispersed onto the surface of the KS4 graphite 203. These small dots 202 are particles of Tin that have been too small to react with Si during the growth of Si NWs. This observation is in adequacy with the study made by Dusanes et al. (11).

Example 2: Synthesis of washed KS4 Graphite/Tin/Silicon NWs Composite Material (M2)

[0261] Steps a), b) and c) are the same than Example 1

d) Washing of the KS4 Graphite/Silicon Composite Material (Process 3)

[0262] 10 g of the composite M1 are introduced in a beaker equipped with a magnet. Then 100 ml of HCl 5% vol are added to the beaker. The mixture is stirred for an hour at 600 rpm. After an hour, the mixture is filtered on a Buchner funnel equipped with a filter, then washed with distilled water until the pH is back to pH 6-7. Finally, the excess of water is removed by addition of ethanol. Then, the filter cake is dried overnight in a heat chamber at 60? C. in order to recover composite material M2.

[0263] FIGS. 5 and 6 show composite material M3 with Si NWs 501 and 601 having a mean diameter of 80 nm.

Example 3: Synthesis of BNB90 Graphite/Tin/Si NWs Composite Material (M3)

a) Mixing BNB90 Graphite and SnCl.SUB.2 .as Pre-Catalyst Material

[0264] 3 g of BNB90 graphite is combined with 0.5 g of SnCl.sub.2 and introduced in a steel bowl of the ball-milling apparatus. Then, 40 g of 3 mm steel balls are introduced in the bowl before being tightly closed. The graphite-SnCl.sub.2 material is mixed for 10 minutes 30 seconds at 400 rpm.

[0265] The pre-catalyst material is recovered by extracting the balls with a sieve.

b) Growth of the Silicon Nanowires (Process 1)

[0266] The pre-catalyst material obtained at the end of step a) is placed on a glass cup inside the fixed-bed reactor. 50 mL of diphenylsilane, Ph.sub.2SiH.sub.2, are then poured at the bottom of the reactor.

[0267] 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 N.sub.2 to decrease the oxygen level. 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., heating is stopped and then the reactor is cooled down to room temperature. The reactor is finally opened to recover the composite material.

c) Post-Treatment of the BNB90 Graphite/Tin/Silicon Composite Material (Process 2)

[0268] The carbonization of organics resulting from Ph.sub.2SiH.sub.2 decomposition is performed by thermal treatment.

[0269] 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 H.sub.2 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 composite M3.

[0270] FIGS. 8 and 9 shows composite material M3 with Si NWs 801 and 901 having a mean diameter of 145 nm on BNB90 graphite 802.

[0271] Table 1 illustrates the ICP analysis of Si nanowires/KS4 composite from Example 1 (no post-treatment) and from Example 3 (after HCl washing step):

TABLE-US-00001 TABLE 1 Composite Si (% wt) C (% wt) Sn (% wt) Cl (% wt) O (% wt) M1 20 71 4.4 0.02 5.6 M3 20.8 70 2.28 0.007 5.7

Example 4: Synthesis of SLP50 Graphite/Tin/Si NWs Composite Material (M4)

a) Mixing SLP50 Graphite and SnCl.SUB.2 .as Pre-Catalyst Material

[0272] 3 g of SLP50 graphite is combined to 0.5 g of SnCl.sub.2 and introduced in a steel bowl of the ball-milling apparatus. Then, 40 g of 3 mm steel balls are introduced into the bowl before being tightly closed. The SLP50-SnCl.sub.2 material is mixed for 10 minutes 30 seconds at 400 rpm.

[0273] The pre-catalyst material is recovered by extracting the balls with a sieve.

b) Growth of the Silicon Nanowires (Process 1)

[0274] The pre-catalyst material obtained at the end of step a) is placed on a glass cup inside the fixed-bed reactor. 50 mL of diphenylsilane, Ph.sub.2SiH.sub.2, are then poured at the bottom of the reactor.

[0275] 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 N.sub.2 to decrease the oxygen level. 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., heating is stopped and then the reactor is cooled down to room temperature. The reactor is finally opened to recover the composite material.

c) Post-Treatment of the SLP50 Graphite/Tin/Silicon Composite Material (Process 2)

[0276] The carbonization of organics coming from Ph.sub.2SiH.sub.2 decomposition is performed by thermal treatment.

[0277] 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 H.sub.2 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 composite M4.

[0278] FIGS. 11 and 12 show composite material M4 with Si NWs 1101 and 1201 having a mean diameter of 158 nm on SLP50 graphite 1102 and 1202.

Example 5: Synthesis of Silicon NPs/Tin/Silicon NWs Composite Material (M5)

a) Mixing Si Nanoparticles and SnCl.SUB.2 .as Pre-Catalyst Material

[0279] 1 g of Silicon NPs is combined with 190 mg of SnCl.sub.2 and introduced in a zirconia bowl of the ball-milling apparatus. Then, 10 mm zirconia balls are introduced in the bowl before being tightly closed. The Si NPs-SnCl.sub.2 material is mixed for 10 minutes 30 seconds at 400 rpm.

[0280] The pre-catalyst material is recovered by extracting the balls with a sieve.

b) Growth of the Silicon Nanowires (Process 1)

[0281] The pre-catalyst material obtained at the end of step a) is placed on a glass cup inside the fixed-bed reactor. 50 mL of diphenylsilane, Ph.sub.2SiH.sub.2, are then poured at the bottom of the reactor.

[0282] 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 N.sub.2 to decrease the oxygen level. 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., heating is stopped and then the reactor is cooled down to room temperature. The reactor is finally opened to recover the composite material.

c) Post-Treatment of the Si Nanoparticles/Tin/Silicon Composite Material (Process 2)

[0283] The carbonization of organics resulting from Ph.sub.2SiH.sub.2 decomposition is performed by thermal treatment.

[0284] 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 H.sub.2 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 composite M5.

[0285] FIGS. 14 and 15 show composite material M5 with Si NWs 1401 and 1501 having a mean diameter of 165 nm.

Example 6: Synthesis of KS4 Graphite/Tin/Si NWS Composite Material (M6)

a) Mixing KS4 Graphite and SnCl.SUB.2 .as Pre-Catalyst Material

[0286] 35 g of SLP50 graphite is combined to 2.73 g of SnCl.sub.2 and introduced in a steel bowl of the ball-milling apparatus. Then, 50 steel balls of 10 mm are introduced into the bowl before being tightly closed. The KS4-SnCl.sub.2 material is mixed for 20 minutes 30 seconds at 300 rpm.

[0287] The pre-catalyst material is recovered by extracting the balls with a sieve.

b) Growth of the Silicon Nanowires (Process 1)

[0288] The pre-catalyst material obtained at the end of step a) is placed homogeneously in a quartz tube inside the fixed-bed reactor.

[0289] After connecting the gas line to the reactor and closing the heating chamber, the quartz tube is flushed with N.sub.2 at 5 slm for several minutes to decrease the oxygen level. Subsequently, the reactor is heated up by means of the heating device placed in contact with the exterior surface of the quartz tube. The heating and gas injection cycles are as follows: a heating ramp of 1 h from 20? C. to 650? C. under 5 slm Ar/H.sub.2 2.5% gas flow rate, a plateau of 4,3 hours at 650? C. under 5 slm N.sub.2/SiH.sub.4 0.9% (L/min), heating is stopped and then the reactor is cooled down to room temperature under 5 slm N.sub.2 gas flow rate. The reactor is finally opened to recover the composite material.

[0290] FIGS. 17 and 18 shows SEM micrograph of the Si nanowires/tin/KS4 composite M6 obtained in this example.

c) Description of FIGS. 17 and 18

[0291] FIGS. 17 and 18 show a mixture of Si NWs 1701 and 1801 and small dots 1803 dispersed onto the surface of the KS4 graphite 1702 and 1802. These small dots 1803 are particles of Tin that have been too small to react with Si during the growth of Si NWs. This observation is in adequacy with the study made by Dusanes et al. (11).

Example 7: Preparation of the Electrodes for Lithium Batteries

[0292] The electrochemical characterization of materials M1, M2, M3, M4 and M5 is performed by preparing coin-cells wherein the anode comprises one of the prepared materials as active material.

a) Mixing with Conductive Fillers

[0293] The composite material according to the invention M1, M2, M3, M4 or M5, is mixed with graphite powder using YSZ 3 mm diameter grinding balls, in an IKA? Ultra-Turrax disperser.

[0294] The composite material and the graphite are introduced into the disperser according to a weight ratio equal to 38:62.

[0295] Mixing is performed for 10 minutes at rotational speed 7.

[0296] The mixed material is finally recovered for further processing or characterization.

b) Preparation of a Coin-Cell

[0297] The synthesized material was mixed with graphite powder (Actilion GHDR-15-4) at a ratio of ca. 38:62. A reference graphite electrode of Actilion material 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 ?m. 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.sup.2 and weighted, and were finally dried overnight in vacuum at 110? C.

[0298] 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 1 M LiPF.sub.6 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. Seven formation cycles were performed prior to regular cycling at 1 C-rate. The formation cycles are made of 2 cycles at C/10 and 5 cycles at C/5 using galvanostatic and potentiostatic discharging (lithiation), and galvanostatic charging (delithiation).

c) Determination of the Electrochemical Performances

[0299] 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.

1Potential Profile

[0300] The potential profile of the cells C1, C2, C3, C4 and C5 has been determined during the cycling at C/10 by measuring the potential of the cell as a function of its capacity.

[0301] FIGS. 3, 7, 10, 13 and 16 represent the potential profiles obtained respectively from cells C1, C2, C3, C4 and C5 recorded during the second cycle at C/10 (third formation cycle).

[0302] On FIGS. 3, 7, 10 and 13, the potential profiles of the cells obtained from composite Materials M1, M2, M3, M4 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 highlighted by the inflexion/pseudo-plateau near 0.45 V during charge (delithiation), as is also clearly visible in the derivative plot (FIG. 13) of the capacity comparing a profile of a graphite electrode to that of a Si-graphite composite.

[0303] On FIG. 16, similarly to the other materials, the composite material M5 shows the cumulation of the electrochemical activity of the growth support, Silicon nanoparticules, and the silicon NWs material which evidences that the composite is electrically and electrochemically active as Li-ion anode material. The electrochemical activity of silicon with lithium ions is particularly highlighted by the inflexion/pseudo-plateau near 0.45 V during charge (delithiation).

2Initial Reversible Capacity

[0304] The initial reversible capacity of the cells, measured at C/10 during the first cycle, is given in the Table 2.

TABLE-US-00002 TABLE 2 Cell C1 C2 C3 C4 C5 Initial capacity (mA .Math. h/g) 822 872 864 713 1869

[0305] Cell C1, prepared from composite Material M1, and cell C2, prepared from composite Material M2, have similar initial reversible capacities. Therefore, composite Material M1 and composite Material M2 have the same silicon active content (ca. 20%). However, the initial capacity is increased for Material M2 compared to Material M1 due to the acidic washing that has been performed. Indeed, the purification of the material from the presence of potentially inhibiting compounds such as unreacted SnCl.sub.2, SnO.sub.x or SiO.sub.2, improves the initial capacity.

[0306] Moreover, a comparison of cells C1, C3 and C4 reveals an improvement of the initial capacity when the silicon active content increases (respectively 822 mA.h/g for ca. 15% vs. 864 mA.h/g for ca. 16% vs. 713 mA.h/g for ca. 11%), in composite Material M1 vs. composite Material M3 and composite Material M4.

[0307] 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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