Method for producing carbon-coated silicon particles

Abstract

A method or process for producing non-aggregated carbon-coated silicon particles and lithium-ion batteries utilizing the same. The process includes providing or producing a dry mixture by mixing silicon particles and polyacrylonitrile present in solid form. Thermally decomposing the polyacrylonitrile present in solid form in the dry mixture to form gaseous carbon precursors. Forming gaseous carbon precursors that are carbonized in the presence of the silicon particles by CVD processes (chemical vapor deposition, chemical gas phase deposition). Where the non-aggregated carbon-coated silicon particles have an average particle diameters d.sub.50 of from 1 to 15 μm and containing ≤10% by weight of carbon and ≥90% by weight of silicon, each based on the total weight of the carbon-coated silicon particles.

Claims

1-10. (canceled)

11. A process for producing non-aggregated carbon-coated silicon particles, comprising: providing or producing a dry mixture by mixing silicon particles and polyacrylonitrile present in solid form; thermally decomposing the polyacrylonitrile present in solid form in the dry mixture to form gaseous carbon precursors; and forming gaseous carbon precursors that are carbonized in the presence of the silicon particles by CVD processes (chemical vapor deposition, chemical gas phase deposition); wherein the non-aggregated carbon-coated silicon particles have an average particle diameters d.sub.50 of from 1 to 15 μm and containing ≤10% by weight of carbon and ≥90% by weight of silicon, each based on the total weight of the carbon-coated silicon particles.

12. The process of claim 11, wherein the silicon particles and the polyacrylonitrile are present alongside one another as separate particles or granules in the dry mixtures.

13. The process of claim 11, wherein the dry mixture contains 2% to 50% by weight of polyacrylonitrile, based on the total weight of the dry mixture.

14. The process of claim 11, wherein the thermal decomposition of polyacrylonitrile is conducted at temperatures of ≥350° C.

15. The process of claim 11, wherein the proportion of polyacrylonitrile that is melted during the thermal decomposition and carbonization is ≤20% by weight, based on the total weight of the polyacrylonitrile used overall (determination method: thermogravimetric analysis).

16. The process of claim 11, wherein no polyacrylonitrile is melted during the thermal decomposition and carbonization steps.

17. The process of claim 11, wherein the carbon-coated silicon particles exhibit a degree of aggregation of ≤40% (determination by sieve analysis).

18. The process of claim 11, wherein the difference formed from the volume-weighted particle size distribution d.sub.50 of the carbon-coated silicon particles and the volume-weighted particle size distribution d.sub.50 of the silicon particles used as starting material for the production of the carbon-coated silicon particles is ≤5 μm.

19. The process of claim 11, wherein the thermal decomposition of polyacrylonitrile decomposition products from the group comprising acrylonitrile, acetonitrile, vinylacetonitrile and HCN are formed.

20. The process of claim 11, wherein the dry mixture does not contain any conductive additives selected from the group comprising graphite, conductive carbon black, graphene, graphene oxide, graphene nanoplatelets, carbon nanotubes, carbon fibers and copper.

21. A process for producing lithium-ion batteries, comprising: providing a lithium-ion battery comprising a cathode, an anode, a separator and/or an electrolyte; providing or producing a dry mixture by mixing silicon particles and polyacrylonitrile present in solid form; thermally decomposing the polyacrylonitrile present in solid form in the dry mixture to form gaseous carbon precursors; forming gaseous carbon precursors that are carbonized in the presence of the silicon particles by CVD processes (chemical vapor deposition, chemical gas phase deposition); wherein the non-aggregated carbon-coated silicon particles have an average particle diameters d.sub.50 of from 1 to 15 μm and containing ≤10% by weight of carbon and ≥90% by weight of silicon, each based on the total weight of the carbon-coated silicon particles; and wherein the carbon-coated silicon particles obtained are an anode active material for the Lithium-ion battery.

22. The process of claim 21, wherein the cathode, the anode, the separator and/or the electrolyte of the lithium-ion battery and/or another reservoir located in a battery housing contains one or more inorganic salts selected from the group comprising alkali metal, alkaline earth metal and ammonium salts of nitrate, nitrite, azide, phosphate, carbonate, borates and fluoride.

Description

EXAMPLE 1 (EX. 1)

[0125] Production of Silicon Particles by Means of Grinding:

[0126] Coarse Si chips from the production of polysilicon were ground using a fluidized bed jet mill (Netzsch-Condux CGS16 with 90 m.sup.3/h of nitrogen at 7 bar as grinding gas). The silicon particles thus obtained were in the form of individual, non-aggregated, chip-like particles, as shown in the SEM image (7500× magnification) in FIG. 1. Elemental composition: Si≥98% by weight; C 0.01% by weight; H<0.01% by weight; N<0.01% by weight; O 0.47% by weight.

[0127] Particle size distribution: monomodal; D.sub.10: 2.19 μm, D.sub.50: 4.16 μm, D.sub.90: 6.78 μm; (D.sub.90−D.sub.10)/D.sub.50=1.10; (D.sub.90−D.sub.10)=4.6 μm.

[0128] Specific surface area (BET): 2.662 m.sup.2/g.

[0129] Si tightness: 0%.

[0130] Powder conductivity: 2.15 μS/cm.

EXAMPLE 2 (EX. 2)

[0131] C-coating of silicon particles by means of gas phase coating from polyacrylonitrile (PAN):

[0132] 80.00 g of the silicon particles (Si) from example 1 and 20.00 g of polyacrylonitrile (PAN) were mechanically mixed at 80 rpm for 3 h using a ball mill roller bed (Siemens/Groschopp). 99.00 g of the Si/PAN mixture thus obtained were placed in a quartz glass boat (QCS GmbH) and carbonized, taking the following parameters into account:

[0133] nitrogen/H.sub.2 as inert gas, N.sub.2/H.sub.2 flow rate 200 ml/min, and with the following temperature treatment:

[0134] heating rate 10° C./min until the temperature reaches 1000° C., holding time 3 h. After cooling, 87.00 g of a black powder were obtained (carbonization yield 88%), which was freed from oversize by means of wet sieving. 79.00 g of C-coated Si particles having a particle size of D.sub.99<20 μm were obtained.

[0135] FIG. 2 shows an SEM image (7500× magnification) and FIG. 3 a TEM image (40 000× magnification) of the C-coated Si particles obtained.

[0136] Elemental composition: Si≥98% by weight; C 0.7% by weight; H 0.01% by weight; N 0.32% by weight; O 0.7% by weight.

[0137] Particle size distribution: monomodal; D.sub.10: 2.71 μm, D.sub.50: 4.57 μm, D.sub.99: 7.30 μm; (D.sub.90−D.sub.10)/D.sub.50=1.00.

[0138] Degree of aggregation: 9%.

[0139] Specific surface area (BET): 2.51 m.sup.2/g.

[0140] Si tightness: ˜100% (impermeable).

[0141] Powder conductivity: 70820.64 μS/cm.

COMPARATIVE EXAMPLE 3 (CEX. 3)

[0142] C-coating of silicon particles by means of melt coating from polyacrylonitrile (PAN):

[0143] As example 2, with the difference that the Si/PAN mixture was subjected to the following temperature treatment in the three-zone tubular furnace:

[0144] First: heating rate 10° C./min until a temperature of 300° C. is reached, holding time 90 min, N.sub.2/H.sub.2 flow rate 200 ml/min,

[0145] Thereafter: heating rate 10° C./min until a temperature of 1000° C. is reached, holding time 3 h, N.sub.2/H.sub.2 flow rate 200 ml/min.

[0146] After cooling, 92.12 g of a black powder were obtained (carbonization yield 94%), which was freed from oversize by means of wet sieving. 87.51 g of C-coated Si particles having a particle size of D.sub.99<20 μm were obtained.

[0147] Elemental composition: Si≥98% by weight; C 0.5% by weight; H<0.01% by weight; N 0.1% by weight; O 0.61% by weight.

[0148] Particle size distribution: monomodal; D.sub.10: 2.35 μm, D.sub.50: 4.51 μm, D.sub.90: 8.01 μm; (D.sub.90−D.sub.10)/D.sub.50=1.26.

[0149] Degree of aggregation: 5%.

[0150] Specific surface area (BET): 2.46 m.sup.2/g.

[0151] Si tightness: ˜100% (impermeable).

[0152] Powder conductivity: 50678.78 μS/cm.

COMPARATIVE EXAMPLE 4 (CEX. 4)

[0153] C-coating of silicon particles by means of liquid coating from polyacrylonitrile (PAN):

[0154] 20.00 g of polyacrylonitrile (PAN) were dissolved in 1332 ml of dimethylformamide (DMF) at room temperature. 80.00 g of the silicon powder (Si) from example 1 (D.sub.50=4.16 μm) were dispersed in the PAN solution by means of ultrasound (Hielscher UIS250V, amplitude 80%, cycle: 0.9; duration: 30 min). The resulting dispersion was sprayed and dried using a B-290 laboratory spray dryer (BÜCHI GmbH) with B-295 inert loop and B-296 dehumidifier (BÜCHI GmbH) (nozzle tip 0.7 mm; nozzle cap 1.4 mm; nozzle temperature 130° C., N.sub.2 gas flow 30; aspirator 100%; pump 20%). 58.00 g of a brown powder were obtained (58% yield).

[0155] 57.50 g of the Si/PAN powder thus obtained was placed in a three-zone tubular furnace and subjected to a temperature treatment as described in comparative example 3.

[0156] After cooling, 47.15 g of a black powder were obtained (carbonization yield 82%), which was freed from oversize by means of wet sieving. 39.61 g of C-coated Si particles having a particle size of D.sub.99<20 μm were obtained.

[0157] Elemental composition: Si≥98% by weight; C 0.4% by weight; N 0.17% by weight; O 0.73% by weight.

[0158] Particle size distribution: monomodal; D.sub.10: 3.69 μm, D.sub.50: 6.98 μm, D.sub.90: 11.12 μm; (D.sub.90−D.sub.10)/D.sub.50=1.06.

[0159] Degree of aggregation: 16%.

[0160] Specific surface area (BET): 2.13 m.sup.2/g.

[0161] Si tightness: ˜100%.

[0162] Powder conductivity: 56714.85 μS/cm.

COMPARATIVE EXAMPLE 5 (CEX. 5)

[0163] C-coating of silicon particles by means of gas phase coating from polystyrene (PS):

[0164] As example 2, with the difference that polystyrene (PS) was used instead of polyacrylonitrile.

[0165] After cooling, 80.00 g of a black powder were obtained (carbonization yield 80%), which was freed from oversize by means of wet sieving. 75.00 g of C-coated Si particles having a particle size of D99<20 μm were obtained.

[0166] Elemental composition: Si≥98% by weight; C 0.22% by weight; H<0.01% by weight; N<0.01% by weight; O 0.39% by weight.

[0167] Particle size distribution: monomodal; D.sub.10: 2.73 μm, D.sub.50: 5.02 μm, D.sub.90: 8.29 μm; (D.sub.90−D.sub.10)/D.sub.50=1.11.

[0168] Degree of aggregation: 6%.

[0169] Specific surface area (BET): 1.547 m.sup.2/g.

[0170] Si tightness: ˜86%.

[0171] Powder conductivity: 4084.782 μS/cm.

COMPARATIVE EXAMPLE 6 (CEX. 6)

[0172] C-coating of silicon particles by means of gas phase coating from ethene:

[0173] 20.00 g of the silicon particles from example 1 (D.sub.50=4.16 μm) were transferred at room temperature into the glass tube of the CVD reactor (HTR 11/150) from Carbolite GmbH. The introduction of the sample was followed by a purge procedure with the process gases (10 min argon 3 slm; 3 min ethene and H.sub.2 each 1 slm, 5 min argon 3 slm). With a heating rate of 20 K/min, the reaction zone was heated to 900° C. Even during the purging and heating, the tube was rotated (315° with an oscillation frequency of 8/min) and the powder was mixed. On attainment of the target temperature, there followed a hold time of 10 min. The CVD coating was conducted for a reaction time of 30 min with a total gas flow rate of 3.6 slm with the following gas composition:

[0174] 2 mol of ethene, 0.3 slm, 8.33% by volume; argon 2.4 slm, 66.67% by volume; H.sub.2 0.9 slm, 26% by volume.

[0175] After cooling, 15.00 g of a black powder were obtained (yield 75%), which was freed from oversize by means of wet sieving. 14.50 g of C-coated Si particles having a particle size of D.sub.99<20 μm were obtained.

[0176] FIG. 9 shows an SEM image (7500× magnification) and FIG. 10 a TEM image (20 000× magnification) of the C-coated Si particles obtained.

[0177] Elemental composition: Si≥94% by weight; C 2.54% by weight; H<0.01% by weight; N<0.01% by weight; O 0.10% by weight.

[0178] Particle size distribution: monomodal; D.sub.10: 2.79 μm, D.sub.50: 5.26 μm, D.sub.90: 8.77 μm; (D.sub.90−D.sub.10)/D.sub.50=1.44.

[0179] Degree of aggregation: 3%.

[0180] Specific surface area (BET): 2.1 m.sup.2/g.

[0181] Si tightness: ˜100% (impermeable).

[0182] Powder conductivity: 818267.37 μS/cm.

EXAMPLE 7 (EX. 7)

[0183] Anode comprising the C-coated silicon particles from example 2 and electrochemical testing in a lithium-ion battery:

[0184] 29.71 g of polyacrylic acid (dried at 85° C. to constant weight; Sigma-Aldrich, M.sub.w˜450 000 g/mol) and 756.60 g of deionized water were agitated by means of a shaker (290 1/min) for 2.5 h until dissolution of the polyacrylic acid was complete. Lithium hydroxide monohydrate (Sigma-Aldrich) was added in portions to the solution until the pH was 7.0 (measured by WTW pH 340i pH meter and SenTix RJD probe). The solution was then mixed by means of a shaker for a further 4 h.

[0185] 7.00 g of the carbon-coated silicon particles from example 2 were then dispersed in 12.50 g of the neutralized polyacrylic acid solution and 5.10 g of deionized water by means of a dissolver at a circumferential speed of 4.5 m/s for 5 min and of 12 m/s for 30 min while cooling at 20° C. After adding 2.50 g of graphite (Imerys, KS6L C), the mixture was stirred at a circumferential speed of 12 m/s for a further 30 min. After degassing, the dispersion was applied by means of a film applicator with gap height 0.20 mm (Erichsen, model 360) to a copper foil having a thickness of 0.03 mm (Schlenk Metallfolien, SE-Cu58). The anode coating thus produced was then dried at 50° C. and 1 bar air pressure for 60 min.

[0186] The average basis weight of the dry anode coating was 3.01 mg/cm.sup.2 and the coating density 1.0 g/cm.sup.3.

[0187] The electrochemical studies were conducted on a button cell (CR2032 type, Hohsen Corp.) in a 2-electrode arrangement.

[0188] The electrode coating from example 7 was used as counterelectrode or negative electrode (Dm=15 mm); a coating based on lithium-nickel-manganese-cobalt oxide 6:2:2 with a content of 94.0% and average basis weight of 15.9 mg/cm.sup.2 (sourced from SEI Corp.) was used as working electrode or positive electrode (Dm=15 mm). A glass fiber filter paper (Whatman, GD Type NE) soaked with 60 μl of electrolyte served as separator (Dm=16 mm). The electrolyte used consisted of a 1.0 molar solution of lithium hexafluorophosphate in a 1:4 (v/v) mixture of fluoroethylene carbonate and diethyl carbonate. The cell was constructed in a glovebox (<1 ppm H.sub.2O, O.sub.2); the water content in the dry matter of all components used was below 20 ppm.

[0189] The electrochemical testing was conducted at 20° C. The cells were charged by the cc/cv method (constant current/constant voltage) with a constant current of 5 mA/g (corresponding to C/25) in the first cycle and of 60 mA/g (corresponding to C/2) in the subsequent cycles and, on attainment of the voltage limit of 4.2 V, at constant voltage until the current went below 1.2 mA/g (corresponding to C/100) or 15 mA/g (corresponding to C/8). The cell was discharged by the cc method (constant current) with a constant current of 5 mA/g (corresponding to C/25) in the first cycle and of 60 mA/g (corresponding to C/2) in the subsequent cycles until attainment of the voltage limit of 3.0 V. The specific current chosen was based on the weight of the coating of the positive electrode.

[0190] On the basis of the formulation, the lithium-ion battery was operated by cell balancing with partial lithiation of the anode.

[0191] The results of the electrochemical testing are summarized in table 1.

EXAMPLE 8 (EX. 8)

[0192] Anode comprising the C-coated silicon particles from example 2 with lithium nitrate impregnation of the electrode and electrochemical testing in a lithium-ion battery:

[0193] An anode as described in example 7 was produced using the carbon-coated silicon particles from example 2. The anode was additionally modified with LiNO.sub.3 by the following procedure.

[0194] The anode from example 7 with a diameter of 15 mm was wetted with 30 μl of an ethanolic LiNO.sub.3 solution (21.7 mg/ml.sub.ethanol). The impregnated anodes were then dried for 2 h at 80° C. in a drying cabinet and the weight was determined. The amount of LiNO.sub.3 applied to the anode was calculated from the weight difference and given in mg of LiNO.sub.3 per mg of coating weight (mg/mg.sub.coating): 0.08 mg/g.sub.coating (0.24 mg/cm.sup.2.sub.anode). The impregnated anode was installed in a lithium-ion battery as described in example 7 and subjected to testing by the same procedure.

[0195] The results of the electrochemical testing are summarized in table 1.

COMPARATIVE EXAMPLE 9 (CEX. 9)

[0196] Anode comprising the C-coated silicon particles from comparative example 3 and electrochemical testing in a lithium-ion battery:

[0197] A lithium-ion battery was produced and tested, as described above with example 7, with the difference that the carbon-coated silicon particles from comparative example 3 were used.

[0198] The results of the electrochemical testing are summarized in table 1.

COMPARATIVE EXAMPLE 10 (CEX. 10)

[0199] Anode comprising the C-coated silicon particles from comparative example 4 and electrochemical testing in a lithium-ion battery:

[0200] A lithium-ion battery was produced and tested, as described above with example 7, with the difference that the carbon-coated silicon particles from comparative example 4 were used.

[0201] The results of the electrochemical testing are summarized in table 1.

COMPARATIVE EXAMPLE 11 (CEX. 11)

[0202] Anode comprising the C-coated silicon particles from comparative example 5 and electrochemical testing in a lithium-ion battery:

[0203] A lithium-ion battery was produced and tested, as described above with example 7, with the difference that the carbon-coated silicon particles from comparative example 5 were used.

[0204] The results of the electrochemical testing are summarized in table 1.

COMPARATIVE EXAMPLE 12 (CEX. 12)

[0205] Anode comprising the C-coated silicon particles from comparative example 6 and electrochemical testing in a lithium-ion battery:

[0206] A lithium-ion battery was produced and tested, as described above with example 7, with the difference that the carbon-coated silicon particles from comparative example 6 were used.

[0207] The results of the electrochemical testing are summarized in table 1.

TABLE-US-00001 TABLE 1 Test results of (comparative) examples 7 to 12: Discharge Si/C capacity Number parti- after of cycles cles C Nitrate cycle with ≥80% from C pre- coating impreg- 1 [mAh/ capacity C(Ex.) (C)Ex. cursor process nation cm.sup.2] retention 7 2 PAN CVD No 2.10 328 8 2 PAN CVD Yes 2.00 492 9 3 PAN Melt No 2.03 290 10 4 PAN liquid No 1.97 271 11 5 PS CVD No 2.06 179 12 6 Ethene CVD No 1.95 167

[0208] The lithium-ion battery from example 7 according to the invention surprisingly exhibited more stable electrochemical behavior compared to the lithium-ion batteries from comparative examples 9, 10, 11 and 12 with a comparably high discharge capacity after cycle 1.

[0209] The lithium-ion battery with added lithium nitrate of example 8 according to the invention surprisingly exhibited even more stable electrochemical behavior.