Method for size-reduction of silicon and use of the size-reduced silicon in a lithium-ion battery

Abstract

The invention relates to a method for size-reducing silicon, wherein a mixture containing a suspension containing silicon to be size-reduced and silicon grinding media is set in motion in the grinding space of a grinding media mill. The size-reduced silicon is used as the active material in the anode of a lithium-ion battery.

Claims

1. A process for comminuting silicon, wherein: (a) a mixture containing a suspension comprising a silicon to be comminuted and milling media comprising silicon is set into motion in a milling space of a closed milling medium mill, (b) the milling media have a volume-weighted size distribution in a range of 50 m<d50<5 mm and a relative width of the volume-weighted size distribution of (d90d10)/d50<1, and (c) silicon particles produced by the process have a volume-weighted particle size distribution in a range of 50 nm<d50<1000 nm, wherein the suspension further comprises an organic liquid comprising at least one alcohol and the mixture comprises less than 5% by weight of water.

2. The process as claimed in claim 1, wherein a median d50 of diameters of the milling media is from 10 to 1000 times greater than a d90 value of a particle size distribution of the silicon to be comminuted.

3. The process as claimed in claim 1, wherein the silicon to be comminuted is a member selected from the group consisting of elemental silicon, doped silicon, metallurgical silicon, silicon oxide, binary silicon-metal alloy, ternary silicon-metal alloy and multinary silicon-metal alloy.

4. The process as claimed in claim 1, wherein the silicon to be comminuted has a volume-weighted particle size distribution having a d90<300 m.

5. The process as claimed in claim 1, wherein the milling media are silicon particles comprising round edges with approximately equal measurements in three dimensions.

6. The process as claimed in claim 1, wherein the organic liquid has a viscosity of less than 100 mPas at 20 C.

7. The process as claimed in claim 1, wherein the silicon comminuted by the process is used as an active material in an anode of a lithium ion battery.

8. An electrode material for a lithium ion battery, which contains silicon particles which have a volume-weighted particle size distribution with 50 nm<d50<1000 nm and metallic impurities of less than 1% by weight and is produced by the process as claimed in claim 1.

9. A lithium ion battery comprising a cathode, an anode, a membrane arranged as a separator between the cathode and the anode, and an electrolyte containing lithium ions, wherein the anode contains the electrode material as claimed in claim 8.

10. The process as claimed in claim 2, wherein the silicon to be comminuted is a member selected from the group consisting of elemental silicon, doped silicon, metallurgical silicon, silicon oxide, binary silicon-metal alloy, ternary silicon-metal alloy and multinary silicon-metal alloy.

11. The process as claimed in claim 10, wherein the silicon to be comminuted has a volume-weighted particle size distribution having a d90<300 m.

12. The process as claimed in claim 11, wherein the milling media are silicon particles comprising round edges with approximately equal measurements in three dimensions.

13. The process as claimed in claim 12, wherein the organic liquid has a viscosity of less than 100 mPas at 20 C.

14. The process as claimed in claim 13, wherein the silicon comminuted by the process is used as an active material in an anode of a lithium ion battery.

15. An electrode material for a lithium ion battery, which contains silicon particles which have a volume-weighted particle size distribution with 50 nm<d50<1000 nm and metallic impurities of less than 1% by weight and is produced by the process as claimed in claim 13.

16. A lithium ion battery comprising a cathode, an anode, a membrane arranged as a separator between the cathode and the anode, and an electrolyte containing lithium ions, wherein the anode contains the electrode material as claimed in claim 15.

17. The process as claimed in claim 1, wherein the silicon particles produced by the process have metallic impurities of less than 1% by weight.

18. The process as claimed in claim 1, wherein the suspension further comprises an organic liquid and the mixture comprises less than 1% by weight of water.

19. The process as claimed in claim 1, wherein the relative width of the volume-weighted size distribution of the milling media is (d90d10)/d50<0.5.

20. The process as claimed in claim 1, wherein the volume-weighted size distribution of the milling media is in a range of 50 m<d50<2 mm.

21. The process as claimed in claim 6, wherein the volume-weighted particle size distribution of the silicon particles produced by the process is in a range of 50 nm<d50<500 nm.

Description

EXAMPLES

Example 1

(1) A fraction of from 0.85 mm to 1.25 mm was sieved out from approximately spherical polysilicon granules (manufactured by Wacker Chemie AG) containing less than 10 g/g of metallic impurities.

(2) 30 ml of this fraction were introduced into a zirconium oxide-lined milling cup having a capacity of 50 ml.

(3) A mixture of 16 g of ethanol (purity 99.9%) and 10 g of a silicon powder having less than 10 g/g of metallic impurities was produced.

(4) The silicon powder was produced as per the prior art by premilling of a coarse crushed material from Wacker Produktion of Solar silicon in a fluidized-bed jet mill (Netzsch-Condux CGS16 using 90 m.sup.3/h of nitrogen at 7 bar as milling gas). The product obtained had a particle size distribution of d.sub.10=5 m, d.sub.50=11 m and d.sub.90=18 m, which was measured by static laser light scattering using a Horiba LA 950.

(5) This mixture was stirred for 20 minutes until all of the solid was finely dispersed in the suspension and the mixture was subsequently added to the granules in the milling cup.

(6) The milling cup was tightly closed under nitrogen as protective gas.

(7) It was inserted into a Retsch planetary ball mill PM 100 and then set into motion at a speed of rotation of 400 rpm for 240 minutes. Milling was interrupted for 10 minutes every 30 minutes in order to allow the milling cup to cool.

(8) After the milling procedure, the milling cup was emptied into a sieve having a mesh opening of 0.5 mm in order to separate the suspension containing the milled Si particles from the polysilicon granules.

(9) Ethanol was added thereto so that the solids concentration in the suspension was subsequently about 20% by weight.

(10) Measurement of the particle distribution by static laser light scattering using a Horiba LA 950 gave d.sub.10=0.24 m, d.sub.50=0.59 m and d.sub.90=1.32 m in a greatly diluted suspension in ethanol.

(11) About 5 ml of the suspension were dried at 120 C. and 20 mbar in a vacuum drying oven for 16 hours.

(12) The scanning electron micrographs of the milled Si powder showed that the sample consists of individual, unaggregated, splinter-shaped particles.

(13) Scanning electron micrographs of the polysilicon granules showed no visible changes on the milling media.

(14) ICP emission spectroscopy was carried out on the dried powder from the suspension in a Perkin Elmer Optima 7300 DV.

(15) No metallic contamination above the detection limit of 10 g/g was found.

Comparative Example 1a

(16) The experiment of example 1 was repeated with the polysilicon granules being replaced by zirconium oxide milling beads partially stabilized with yttrium oxide (Alpine Power Beads YSZ 0.8-1 mm).

(17) The measurement of the particle distribution by static laser light scattering using a Horiba LA 950 gave d.sub.10=0.10 m, d.sub.50=0.18 m and d.sub.90=0.33 m in a greatly diluted suspension in ethanol.

(18) About 5 ml of the suspension were dried at 120 C. and 20 mbar in a vacuum drying oven for 16 hours.

(19) The scanning electron micrographs of the milled Si powder showed that the sample consists of individual, unaggregated, splinter-shaped particles.

(20) Scanning electron micrographs of the zirconium oxide milled beads showed no visible changes on the milling media.

(21) ICP emission spectroscopy in a Perkin Elmer Optima 7300 DV indicated a proportion of 10 mg/g of zirconium in the sample.

Comparative Example 1b

(22) The experiment of example 1 was repeated with the polysilicon granules being replaced by steel balls (Netzsch SteelBeads Micro 1.0 mm).

(23) The measurement of the particle distribution by static laser light scattering using a Horiba LA 950 gave d.sub.10=0.17 m, d.sub.50=0.31 m and d.sub.90=1.03 m in a greatly diluted suspension in ethanol.

(24) About 5 ml of the suspension were dried at 120 C. and 20 mbar in a vacuum drying oven for 16 hours.

(25) The scanning electron micrographs of the milled Si powder showed that the sample consists of individual, unaggregated, splinter-shaped particles.

(26) ICP emission spectroscopy in a Perkin Elmer Optima 7300 DV indicated a proportion of 340 mg/g of iron in the sample.

Example 2

(27) A fraction of from 0.3 mm to 0.5 mm was sieved out from approximately spherical polysilicon granules (manufactured by Wacker Chemie AG) which contains less than 10 g/g of metallic impurities.

(28) The milling space of a laboratory stirred ball mill Netzsch LabStar LS1 using the milling system ZETA ceramic was filled with 490 ml of this fraction and closed.

(29) A mixture of 1600 g of ethanol (purity 99.9%) and 400 g of a silicon powder having less than 10 g/g of metallic impurities was produced.

(30) The silicon powder had a particle size distribution of d.sub.10=8 m, d.sub.50=15 m and d.sub.90=25 m after premilling in a fluidized-bed jet mill.

(31) The mixture was stirred for 20 minutes until all of the solid was finely dispersed in the suspension.

(32) The suspension composed of silicon powder and ethanol was then introduced into the cooled reservoir of the mill and circulated through the mill by pumping at a throughput of 40 kg/h.

(33) The particles in the suspension were milled at a speed of rotation of the mill of 4000 rpm for 300 minutes.

(34) The subsequent measurement of the particle distribution by static laser light scattering using a Horiba LA 950 gave a size distribution with d.sub.10=90 nm, d.sub.50=174 nm and d.sub.9=320 nm in a greatly diluted suspension in ethanol.

(35) About 5 ml of suspension were dried at 120 C. and 20 mbar in a vacuum drying oven for 16 hours.

(36) The scanning electron micrographs of the milled Si powder showed that the sample consists of individual, unaggregated, splinter-shaped particles.

(37) Scanning electron micrographs of the polysilicon granules showed no visible changes on the milling media.

(38) However, the mass of the granules decreased by about 14%.

(39) ICP emission spectroscopy was carried out on the dried powder from the suspension in a Perkin Elmer Optima 7300 DV.

(40) Only slight contamination amounting to 70 g/g of iron was found.

(41) Compared to example 1, this method offers the advantage that relatively large amounts of Si nanoparticles of from a few kg through to the industrial scale can be produced.

Comparative Example 2a

(42) The milling space of a laboratory stirred ball mill Netzsch LabStar LS1 using the milling system ZETA ceramic was filled with 490 ml of zirconium oxide milling beads which were partially stabilized with yttrium oxide and had an average diameter of 0.3 mm (Alpine Power Beads YSZ) and closed.

(43) A mixture of 1600 g of ethanol (purity 99.9%) and 400 g of a silicon powder having less than 10 g/g of metallic impurities was produced.

(44) The silicon powder had a particle size distribution of d.sub.10=7 m, d.sub.50=13 m and d.sub.90=24 m after premilling in a fluidized-bed jet mill.

(45) The mixture was stirred for 20 minutes until all of the solid was finely dispersed in the suspension.

(46) The suspension composed of silicon powder and ethanol was then introduced into the cooled reservoir of the mill and circulated through the mill by pumping at a throughput of 40 kg/h.

(47) The particles in the suspension were milled at a speed of rotation of the mill of 2500 rpm for 250 minutes.

(48) The subsequent measurement of the particle distribution by static laser light scattering using a Horiba LA 950 gave a size distribution with d.sub.10=82 nm, d.sub.50=166 nm and d.sub.90=325 nm in a greatly diluted suspension in ethanol.

(49) About 5 ml of suspension were dried at 120 C. and 20 mbar in a vacuum drying oven for 16 hours.

(50) The scanning electron micrographs of the milled Si powder showed that the sample consists of individual, unaggregated, splinter-shaped particles.

(51) Scanning electron micrographs of the zirconium oxide milling beads showed no visible changes after milling.

(52) A decrease in mass of the milling media was likewise not observed within the limits of measurement error and can therefore be estimated as being less than 2%.

(53) ICP emission spectroscopy on the dried powder from the suspension was carried out in a Perkin Elmer Optima 7300 DV.

(54) 6 mg/g of zirconium and 65 g/g of iron were detected in the sample.

Example 3

(55) A fraction of from 2 mm to 3.15 mm was sieved out from approximately spherical polysilicon granules (manufactured by Wacker Chemie AG) which contains less than 10 g/g of metallic impurities.

(56) The milling space of a stirred mill Netzsch LMZ 2 with milling system ZETA in PU configuration was filled with 1000 ml of this fraction and closed.

(57) A suspension composed of 25 kg of ethanol (purity 99.9%) and 7.5 kg of a silicon powder having less than 10 g/g of metallic impurities was produced.

(58) The silicon powder had a particle size distribution of d.sub.10=18 m, d.sub.50=96 m and d.sub.90=226 m after premilling in a fluidized-bed jet mill.

(59) The ethanol was introduced into the cooled reservoir of the mill and the silicon powder was slowly added while stirring.

(60) The suspension was circulated through the mill by pumping at a throughput of 200 kg/h.

(61) The particles in the suspension were milled at a speed of rotation of the mill of 1900 rpm for 140 minutes.

(62) The subsequent measurement of the particle distribution by static laser light scattering using a Horiba LA 950 gave a size distribution with d.sub.10=3 m, d.sub.50=7.3 m and d.sub.90=13 m in a greatly diluted suspension in ethanol.

(63) About 5 ml of suspension were dried at 120 C. and 20 mbar in a vacuum drying oven for 16 hours.

(64) The scanning electron micrographs of the milled Si powder showed that the sample consists of individual, unaggregated, splinter-shaped particles.

(65) Scanning electron micrographs of the polysilicon granules showed no visible changes on the milling media.

(66) However, the mass of the granules decreased by about 16%.

(67) ICP emission spectroscopy was carried out on the dried powder from the suspension in a Perkin Elmer Optima 7300 DV.

(68) Only slight contamination amounting to about 100 g/g of iron was found.

Example 4

(69) A fraction of from 2 mm to 3.15 mm was sieved out from crushed metallurgical silicon having less than 20 mg/g of metallic impurities (manufactured by Wacker Chemie AG).

(70) The milling space of a stirred mill Netzsch LMZ 2 with milling system ZETA in PU configuration was filled with 1000 ml of this fraction and closed.

(71) A suspension composed of 25 kg of ethanol (purity 99.9%) and 10 kg of a silicon powder having less than 10 g/g of metallic impurities was produced.

(72) The silicon powder had a particle size distribution of d.sub.10=18 m, d.sub.50=96 m and d.sub.90=226 m after premilling in a fluidized-bed jet mill.

(73) The ethanol was introduced into the cooled reservoir of the mill and the silicon powder was slowly added while stirring.

(74) The suspension was circulated through the mill by pumping at a throughput of 200 kg/h.

(75) The particles in the suspension were milled at a speed of rotation of the mill of 1950 rpm for 480 minutes.

(76) The subsequent measurement of the particle distribution by static laser light scattering using a Horiba LA 950 gave a size distribution with d.sub.10=2.8 m, d.sub.50=7.4 m and d.sub.90=14 nm in a greatly diluted suspension in ethanol.

(77) About 5 ml of suspension were dried at 120 C. and 20 mbar in a vacuum drying oven for 16 hours.

(78) The scanning electron micrographs of the milled Si powder showed that the sample consists of individual, unaggregated, splinter-shaped particles.

(79) The crushed silicon material was polished and rounded during milling. Its mass decreased by about 30%.

(80) ICP emission spectroscopy was carried out on the dried powder from the suspension in a Perkin Elmer Optima 7300 DV.

(81) Compared to example 3, a somewhat greater contamination of about 260 g/g of iron, 120 g/g of aluminum and 50 g/g of calcium was found.

Example 5

Production and Characterization of an Electrode for Li Ion Batteries

(82) 4.28 g of an 18.7% strength by weight Si suspension in ethanol as per example 2 and 0.48 g of conductive carbon black (Timcal, Super P Li) were dispersed in 24.32 g of a 1.3% strength by weight solution of sodium carboxymethylcellulose (Daicel, Grade 1380) in water by means of a dissolver at a circumferential velocity of 4.5 m/s for 15 minutes while cooling at 20 C.

(83) After addition of 2.41 g of graphite (Timcal, SFG6), the dispersion was then stirred at a circumferential velocity of 17 m/s for 45 minutes.

(84) After degassing, the dispersion was applied by means of a film drawing frame having a 0.10 nm gap height (Erichsen, model 360) to a copper foil (Schlenk Metallfolien, SE-Cu58) having a thickness of 0.030 mm.

(85) The electrode coating produced in this way was subsequently dried at 80 C. for 60 minutes.

(86) The average weight per unit area of the dry electrode coating was 0.90 mg/cm.sup.2.

(87) The electrochemical studies were carried out on a half cell with a three electrode arrangement (zero-current potential measurement).

(88) The electrode coating was used as working electrode, and a lithium foil (Rockwood Lithium, thickness 0.5 mm) was used as reference electrode and counterelectrode.

(89) A 6-layer nonwoven stack (Freudenberg Vliesstoffe, FS2226E) impregnated with 100 l of electrolyte served as separator.

(90) The electrolyte used consisted of a 1 molar solution of lithium hexafluorophosphate in a 3:7 (v/v) mixture of ethylene carbonate and diethyl carbonate which had been admixed with 2% by weight of vinylene carbonate.

(91) The construction of the cell was carried out in a glove box (<1 ppm H.sub.2O, O.sub.2), and the water content of all components used was below 20 ppm, based on the dry mass.

(92) Electrochemical testing was carried out at 20 C.

(93) The potential limits used were 40 mV and 1.0 V vs. Li/Li.sup.+.

(94) Charging or lithiation of the electrode was carried out at constant current by the cc/cv method (constant current/constant voltage) and, after the voltage limit had been reached, at constant voltage until the current went below 50 mA/g.

(95) Discharging or delithiation of the electrode was carried out at constant current by the cc method (constant current) until the voltage limit had been reached.

(96) At a specific current of 100 mA/g, based on the weight of the electrode coating, a reversible initial capacity of about 600 mAh/g was achieved in this way.

(97) After 100 charging/discharging cycles, the battery still had about 95% of its original capacity.