Process for the preparation of a material comprising at least silicon particles and silicon nanowires

Abstract

A process for the preparation of a material comprising at least silicon particles and silicon nanowires, said process comprising: (1) introducing, into a chamber of a reactor, at least: silicon particles, and a catalyst, (2) introducing, into the chamber, a precursor composition comprising at least a silane compound or a mixture of silane compounds as precursor compound of the silicon nanowires, (3) decreasing the content of molecular oxygen in the chamber, (4) applying a heat treatment to the chamber at a temperature ranging from 270° C. to 600° C., and (5) recovering the material comprising at least silicon particles and silicon nanowires. A material based on silicon particles and on silicon nanowires and its use for manufacturing electrodes, notably anodes, which can be used in an energy storage device.

Claims

1. A process for the preparation of a material comprising at least silicon particles and silicon nanowires, said process comprising at least the following stages: (1) introducing, into a chamber of a reactor, of at least: silicon particles, and a catalyst, (2) introducing, into the chamber of the reactor, of a precursor composition of the silicon nanowires which comprises at least one precursor compound of the silicon nanowires selected from the group consisting of: a silane compound and a mixture of silane compounds, (3) decreasing the content of molecular oxygen in the chamber of the reactor, (4) applying a heat treatment to the chamber of the reactor at a temperature ranging from 270° C. to 600° C., and (5) recovering the material comprising at least silicon particles and silicon nanowires.

2. The process as claimed in claim 1, additionally comprising a stage (6) of washing the material comprising at least silicon particles and silicon nanowires obtained on conclusion of stage (5).

3. The process as claimed in claim 1, wherein the heat treatment is performed at a temperature ranging from 350° C. to 550° C.

4. The process as claimed in claim 1, wherein the silicon particles of stage (1) are covered with a layer of silicon oxide.

5. The process as claimed in claim 1, wherein the catalyst is selected from the group consisting of: metals, bimetallic compounds, metal oxides and metal nitrides.

6. The process as claimed in claim 1, wherein the silane compound corresponds to formula (III): ##STR00002## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are independently selected from the group consisting of a hydrogen atom, a C.sub.1-C.sub.15 alkyl chain and an aryl group optionally substituted by a C.sub.1-C.sub.15 alkyl chain, at least one of the R.sub.1, R.sub.2, R.sub.3 and R.sub.4 groups being different from the hydrogen atom.

7. The process as claimed in claim 6, wherein the silane compound is chosen from mono-, di- and trialkylsilanes.

8. The process as claimed in claim 7, wherein the silane compound is selected from the group consisting of: monophenylsilane Si(C.sub.6H.sub.5)H.sub.3, diphenylsilane Si(C.sub.6H.sub.5).sub.2H.sub.2, triphenylsilane Si(C.sub.6H.sub.5).sub.3H and their mixtures.

9. The process as claimed in claim 1, wherein the weight ratio of precursor compound of the silicon nanowires with respect to the silicon particles ranges from 1:10 to 1000:10.

10. The process as claimed in claim 1, comprising, after stage (5) or after stage (6), applying at least one following cycle: (1′) introducing, into the chamber of the reactor, at least: the material comprising at least silicon particles and silicon nanowires product obtained in stage (5) or in stage (6), and a catalyst, (2′) introducing, into the reactor, a precursor composition of the silicon nanowires, (3′) decreasing the content of molecular oxygen in the chamber of the reactor, (4′) applying a heat treatment, and (5′) recovering the material comprising at least silicon particles and silicon nanowiresproduct.

11. The process as claimed in claim 10, comprising applying 1 to 10 cycles.

12. The process as claimed in claim 1, which comprises an additional stage of heat treatment of the material comprising at least silicon particles and silicon nanowires in the presence of a carbon source.

13. The process as claimed in claim 1, wherein the process produces a material wherein at least 10% of the silicon nanowires are not bonded via their ends to the surface of a silicon particle.

14. The process as claimed in claim 13, wherein the process produces a material wherein at least 25% of the silicon nanowires are not bonded via their ends to the surface of a silicon particle.

15. The process as claimed in claim 14, wherein the process produces a material wherein at least 50% of the silicon nanowires are not bonded via their ends to the surface of a silicon particle.

16. The process as claimed in claim 1, wherein the process produces a material wherein at least 50% of the silicon nanowires have a length greater than 5 μm.

17. The process as claimed in claim 1, wherein the process produces a material wherein the ratio of the mean length to the mean diameter of the silicon nanowires is from 250 to 10 000.

18. The process as claimed in claim 1, wherein the process produces a material wherein the ratio of the mean length of the silicon nanowires to the mean diameter of the silicon particles is greater than or equal to 2.

19. A process for the manufacture of an electrode comprising at least: a) the preparation of a material comprising at least silicon particles and silicon nanowires according to a process as claimed in claim 1, b) the preparation of an ink from the material obtained in step a), c) the deposition of the ink at least on a part of a face of a current collector, and d) the drying of the ink.

Description

FIGURES

(1) FIG. 1: photographs obtained by scanning electron microscopy (SEM) of silicon particles and of materials obtained by the implementation of the process according to the invention.

(2) FIG. 1a is a photograph obtained from a sample of silicon particles.

(3) FIG. 1b is a photograph obtained from a sample of the material A according to the invention.

(4) FIG. 1c is a photograph obtained from a sample of the material B according to the invention.

(5) FIG. 1d is a photograph obtained from a sample of the material C.sub.2 according to the invention.

(6) FIG. 2: enlargements of the photograph of FIG. 1d centered, on the one hand, on an isolated nanowire (FIG. 2a) and, on the other hand, on a group of nanowires (FIG. 2b).

(7) FIG. 3: enlargement of FIG. 1b.

(8) FIG. 4: diagrammatic representation of a reactor which can be used in the process of the invention.

(9) The invention is illustrated by the following examples, given without implied limitation.

EXPERIMENTAL PART

(10) In these examples, the parts and percentages are expressed by weight, unless otherwise indicated.

(11) Materials: silicon particles, the mean size of which is equal to 500 nm, commercially available from SkySpring Nanomaterials (CAS: 7440-21-3), diphenylsilane Si(C.sub.6H.sub.5).sub.2H.sub.2, commercially available from Sigma-Aldrich (CAS: 775-12-2), carbon black, commercially available from Imerys, under the commercial reference Timcal Super C65® (CAS: 1333-86-4), carboxymethyl cellulose (CMC), commercially available from Alfa-Aesar (CAS: 9004-32-4), lithium hexafluorophosphate LiPF.sub.6 electrolyte (1M) dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume) with addition of fluoroethylene carbonate (FEC, 10% by weight) and vinylene carbonate (VC, 2% by weight), commercially available from Solvionic.

(12) In the examples which follow, the materials are synthesized in two different reactors, the main characteristics of which are given below:

(13) Reactor 1: Reactor 1 is composed of a tube made of Pyrex glass (external diameter=16 mm, thickness of the glass=1 mm) exhibiting a constriction at approximately 5 cm from the bottom.

(14) Reactor 2: Reactor 2 is a reactor made of steel (internal volume=1 L, diameter=100 mm and height=125 mm).

(15) I—Preparation of the Materials

(16) a. Synthesis of the Gold Nanoparticles

(17) The gold nanoparticles are synthesized according to the protocol described in M. Brust et al., J. Chemical Society, Chemical Communications, 7(7), 801-802, 1994.

(18) The diameter of the gold nanoparticles obtained is from 1 to 4 nm. Their surface is covered with dodecanethiol molecules.

(19) A 50 mg/ml mother solution of gold nanoparticles is prepared by suspending the gold nanoparticles prepared above in toluene.

(20) b. Combining of the Silicon Particles and of the Catalyst

(21) The silicon particles are suspended in dry hexane (10 mL of hexane per 100 mg of silicon particles). The mother solution of gold nanoparticles prepared above is subsequently added to the suspension of silicon particles with stirring. The mixture obtained is stirred for 15 minutes and then the solvent is evaporated using a rotary evaporator. The dry solid obtained is transferred into the reactor.

(22) c. Growth of the Nanowires

(23) This stage c) corresponds to stages (1) to (5) of the process according to the invention.

(24) Depending on the reactor used, two different protocols are distinguished for carrying out the growth of the nanowires.

(25) c1—Growth in Reactor 1

(26) The dry solid obtained in stage b is transferred into a test tube made of Pyrex glass (diameter=11 mm and length=75 mm).

(27) Diphenylsilane Si(C.sub.6H.sub.5).sub.2H.sub.2 is deposited in the bottom of reactor 1, followed by the test tube containing the silicon particles impregnated with gold nanoparticles. Reactor 1 is subsequently placed on a vacuum line and sealed under vacuum with a blowtorch approximately 15 cm from the bottom.

(28) The reactor is subsequently placed in an oven at 450° C. for 1 h and then it is removed from the oven and left to cool at ambient temperature for 30 minutes. Finally, the reactor is opened using a glass knife in order to extract the crude product from it.

(29) c2—Growth in Reactor 2

(30) The dry solid obtained in stage b is transferred into a crystallizing dish made of Pyrex glass (diameter=80 mm, height=40 mm).

(31) Diphenylsilane Si(C.sub.6H.sub.5).sub.2H.sub.2 is deposited in the bottom of reactor 2. An empty beaker made of Pyrex glass (diameter=60 mm, height=60 mm) is then deposited in the reactor and, over the beaker, the crystallizing dish containing the silicon particles impregnated with gold nanoparticles. Reactor 2 is subsequently hermetically closed and connected to a pump in order to generate the vacuum inside the reactor.

(32) The reactor is subsequently heated by an electrical resistance in contact with the external wall of the reactor. The heating cycle is as follows: a heating gradient of 30 minutes between 20° C. and 450° C., a plateau of 60 minutes at 450° C., then the cutting of the heating and the cooling of the reactor to ambient temperature over 3 hours. Finally, the reactor is opened in order to recover the crude product.

(33) d. Washing of the Product Obtained

(34) This stage corresponds to stage (6) described above.

(35) The product obtained is subsequently washed in portions of 2 g according to the following protocol:

(36) The product is transferred into a 45 mL plastic centrifuge tube to which 15 mL of chloroform are added. The suspension in chloroform is then dispersed in an ultrasonic bath for 5 minutes before being centrifuged for 5 minutes at 8000 rpm (corresponding to 3500 g). After centrifuging, the solvent is removed and is replaced with a fresh portion of chloroform (15 mL).

(37) In total, this washing operation is repeated 3 times. During the fourth and final washing, the 15 mL of chloroform are replaced with a mixture of 5 mL of chloroform and 15 mL of ethanol.

(38) The washed product is subsequently dried in an oven at 80° C. in order to obtain a gray powder.

(39) e. Optional Repetition

(40) In some alternative forms, the product obtained at the end of stage d is reintroduced into the reactor in order to undergo one or more additional phases of growth of the nanowires corresponding to the repetition of stages b, c and d described above.

(41) The material obtained at the end of stage d of a cycle n then replaces the silicon particles and it is introduced into the reactor in stage b of the following cycle n+1. Thus, one and the same portion of silicon particles undergoes several growth phases.

(42) 5f. Materials Obtained

(43) The materials A, B, C1 and C2 are prepared according to the preceding protocol.

(44) The characteristics of the process employed and of the materials obtained are given in the following table 1:

(45) TABLE-US-00001 TABLE 1 Material A B C.sub.1 C.sub.2.sup.(4) Characteristics of the process employed Reactor used  1  1  2  2 Weight of SiP.sup.(1) 100 mg 100 mg 5 g — introduced into the reactor Volume of the mother 20 μl 20 μl 2.5 mL 2.5 mL solution of AuNP.sup.(2) Weight of 368 mg 368 mg 99.3 g 99.3 g diphenylsilane Number of  1  3  1  2 cycles.sup.(3) Characteristics of the product obtained Weight of product 112 mg 118 mg 7.9 g 6.3 g obtained Total weight 100 mg 100 mg 5 g 2.5 g of SiP.sup.(5) Total weight of 12 mg 18 mg 2.9 g 3.8 g nanowires.sup.(5) % by weight 90 85 63 40 of SiP.sup.(6) % by weight of 10 15 37 60 nanowires.sup.(6) .sup.(1)SiP: silicon particles .sup.(2)AuNP: gold nanoparticles .sup.(3)the number of times where stages b, c and d were carried out on one and the same portion of silicon particles .sup.(4)The material C.sub.2 is prepared from 3.8 g of the material C.sub.1. Thus, the material C.sub.2 is obtained after two growth cycles: the first cycle starting from silicon particles SiP, in order to form the material C.sub.1, and the second cycle starting from the material C.sub.1, in order to form the material C.sub.2. .sup.(5)Weights of silicon particles and/or of silicon nanowires present in the material obtained. .sup.(6)Percentages by weight of the silicon particles and/or of the silicon nanowires present in the material obtained, the percentages being expressed with respect to the total weight of the material.

(46) II—Characterization of the Material

(47) The microscopic topology of the materials obtained is observed using a Zeiss Ultra 55 scanning electron microscope (SEM).

(48) 1/ General Appearance of the Materials Obtained

(49) The images obtained by scanning electron microscopy (SEM) are appended.

(50) FIG. 1a is a photograph obtained by means of SEM from a sample of silicon particles.

(51) FIG. 1b is a photograph obtained by means of SEM from a sample of the material A.

(52) FIG. 1c is a photograph obtained by means of SEM from a sample of the material B.

(53) FIG. 1d is a photograph obtained by means of SEM from a sample of the material C.sub.2.

(54) In FIG. 1a, it is observed that the silicon particles do not exhibit any silicon nanowires at their surfaces. The sample is composed only of silicon particles.

(55) In FIGS. 1b, 1c and 1d, silicon particles around which are entangled a plurality of silicon nanowires, with which the particles are in contact, are observed.

(56) In FIG. 1c, a greater amount of silicon nanowires entangled around the silicon particles is also observed than on the particles observed in FIG. 1b. Likewise, in FIG. 1d, a greater amount of silicon nanowires entangled around the silicon particles is observed than on the particles observed in FIG. 1c.

(57) From these four figures, it is thus found that the process employed above made possible the growth of silicon nanowires at the surfaces of the silicon particles.

(58) Furthermore, it is found that the sequence of several growth phases on one and the same portion of silicon particles makes it possible to increase the number of silicon nanowires. It is found that the use of reactor 2 makes it possible to increase the length of the silicon nanowires.

(59) 2/ Length of the Nanowires

(60) The length of the silicon nanowires is determined from the images of the material C.sub.2.

(61) FIGS. 2a and 2b are enlargements of the photograph of FIG. 1d. FIG. 2a is centered on an isolated nanowire and FIG. 2b on a group of nanowires.

(62) In these two figures, the noncontinuous lines make it possible to identify the nanowires studied and the arrows make it possible to plot the ends of the visible part of these nanowires.

(63) The length of these noncontinuous lines is then regarded as the minimum observable length of the nanowires studied.

(64) The following results are thus obtained: from FIG. 2a, observable length of the silicon nanowire=6.8 μm. from FIG. 2b, observable length of the group of silicon nanowires=6.6 μm.

(65) The mean observable length of the nanowires in a material is determined from the measurement of the length of approximately twenty nanowires or groups of nanowires present in the material.

(66) 3/ Diameter of the Nanowires

(67) The mean diameter of the nanowires is estimated at equal to 13.3 nm, with a standard deviation of 3.1 nm, from the measurement of approximately one hundred nanowires.

(68) 4/ Particle/Nanowires Bond

(69) The arrangement at the microscopic scale of the material obtained and more particularly the arrangement between the silicon particles and the silicon nanowires synthesized is subsequently observed.

(70) FIG. 3 is an enlargement of the photograph 1b, centered on a silicon nanowire, the ends of which are located at the centers of the two circles.

(71) From this photograph, it is found that the ends of the nanowire are not bonded to any silicon particle. This is because the silicon nanowire is bonded to the surface of a silicon particle by its central part.

(72) III—Electrochemical Performance Qualities

(73) The electrochemical performance qualities of the materials obtained above are subsequently evaluated by preparing cells in which the anode comprises one of the materials prepared above.

(74) 1. Preparation of a Cell

(75) Each material obtained is mixed, using a mortar, with carbon black as conductive additive, carboxymethyl cellulose (CMC) as binder and distilled water as solvent, in order to obtain an ink. The weights of dry matter respect the ratio 50:25:25 (Si:C65:CMC). The distilled water added represents approximately 90% of the total weight of the ink; it makes it possible to adjust the viscosity of the ink.

(76) An electrode is subsequently prepared by tape casting ink over a copper sheet with a thickness of 25 μm. After drying in the oven at a temperature of 50° C., the copper sheet is cut out by means of a hollow punch in order to form a disk with a diameter of 15 mm. A half-cell is subsequently formed by the stacking of a lithium metal electrode sold by Alfa-Aesar (disk with a diameter of 15 mm and a thickness of 0.75 mm), of a Whatman® separator (glass fiber, thickness of 240 μm), of a Celgard® separator (membrane of 3 polypropylene/polyethylene/polypropylene layers, thickness of 25 μm) and of the electrode prepared from the material synthesized above. The separators are impregnated with a 1M lithium hexafluorophosphate LiPF.sub.6 electrolyte in EC:DEC (1/1 by volume) with addition of FEC (10% by weight) and of VC (2% by weight). The device is then sealed in a button cell press between two caps made of stainless steel constituting the positive and negative terminals, with a disk of stainless steel 15 mm in diameter and a spring for maintaining a uniform pressure over the area of the device.

(77) The cell P.sub.A, the cell P.sub.B and the cell P.sub.C2 were thus prepared, respectively from the materials A, B and C.sub.2 prepared above.

(78) A control cell P.sub.T was also prepared from silicon particles which had not undergone any modification. The cell P.sub.T was thus prepared according to the same protocol as that described above, except that the material according to the invention was replaced with silicon particles.

(79) The silicon particles used in the manufacture of the material of the invention were used in the manufacture of the control cell P.sub.T.

(80) 2. Measurement of the Electrochemical Properties

(81) The performance qualities of the cells P.sub.A, P.sub.B, P.sub.C2 and P.sub.T, prepared above, are determined by galvanostatic cycling, by means of a Biologic BCS-805 cycler equipped with 8 routes each having two electrodes.

(82) Each cell is lithiated a first time until a potential difference of 10 mV is achieved and with a current rate equal to C/20, that is to say with a current which makes it possible to lithiate, in 20 hours, an electrode having the theoretical capacity of silicon (3579 mA.Math.h/g.sub.Si). The first delithiation is carried out until a potential difference of 1 V is reached and with a current state also equal to C/20. The following galvanostatic cycles are carried out between 1 V and 10 mV at 1C, that is to say with a current which makes it possible to lithiate/delithiate, in 1 hour, an electrode having the theoretical capacity of silicon.

(83) The cyclability of the material used in each cell is subsequently evaluated from the change in the reversible capacity, that is to say the capacity during the delithiation of the silicon. The initial capacity corresponds to the reversible capacity at 1C when the latter stabilizes at its maximum value after 25 cycles for the cell P.sub.A, at the end of 2 to 3 cycles for the cells P.sub.B and P.sub.C2. As the capacity of the control cell P.sub.T does not stabilize and falls continuously, the capacity during the first cycle at 1C is reported. The differences in initial capacity are partly due to different weights of electrode per unit of surface area and partly to active materials of different natures (specific surface, compactness, and the like).

(84) The change in the reversible capacity is characterized by two limits relating to the value of the initial capacity. The number of cycles at the end of which the reversible capacity is less than 80% of the initial capacity and then the number of cycles at the end of which the reversible capacity is less than 50% of the initial capacity are reported.

(85) The results obtained for each of the cells are presented in the following table 2:

(86) TABLE-US-00002 TABLE 2 Cell P.sub.A P.sub.B P.sub.C2 P.sub.T Initial capacity 960 1470 1760 2440 (mA .Math. h/g.sub.Si) Electrode weight 0.92 0.53 0.45 1.22 (mg/cm.sup.2) 80% Limit 99 99 1071 5 (number of cycles) 50% Limit 394 848 >1800 12 (number of cycles)

(87) The capacity of the control cell P.sub.T decreases during the charge and discharge cycles. It has decreased by 20% after 5 cycles and by 50% after 12 cycles.

(88) The reversible capacity of the cells P.sub.A and P.sub.B also decreases during the cycles, but more slowly. After 99 charge/discharge cycles, the reversible capacity of the batteries P.sub.A and P.sub.B has decreased by only 20%. It subsequently decreases more strongly for the cell P.sub.A, by 50% after 394 cycles, than for the cell P.sub.B, by 50% after 848 cycles.

(89) As regards the cell P.sub.C2, its reversible capacity has decreased by only 20% after 1071 cycles. Even after 1800 cycles, it is still greater than 50% of the initial capacity.

(90) The process according to the invention thus makes it possible to obtain materials which can be used as active material in a lithium-ion battery and which exhibit an improved cyclability, with respect to the silicon particles of the prior art.