LITHIUM ION BATTERIES

Abstract

A lithium-ion battery and method for cycling lithium-ion batteries. The method includes providing a lithium-ion battery comprising a cathode, an anode, a separator and an electrolyte. The anode contains pre-lithiated silicon having a degree of pre-lithiation α1 of from 5 to 50% and the anode material is only partially lithiated during full charging of the lithium-ion battery by the lithiation capacity of silicon being utilized to a degree of lithiation α2 of from 5 to 50% by the partial lithiation of the anode material during full charging of the lithium ion battery. The total degree of lithiation α of the silicon is from 10 to 75%, the total degree of lithiation α is the sum of the degree of pre-lithiation α1 and the degree of lithiation α2, where the figures in % are based on the maximum lithiation capacity of silicon.

Claims

1-11. (canceled)

12. A method for cycling lithium ion batteries, comprising: providing a lithium ion battery comprising a cathode, an anode, a separator and an electrolyte, wherein the anode contains prelithiated silicon having a degree of prelithiation α1 of from 5 to 50% and the anode material is only partially lithiated during full charging of the lithium ion battery by the lithiation capacity of silicon being utilized to a degree of lithiation α2 of from 5 to 50% by the partial lithiation of the anode material during full charging of the lithium ion battery, wherein the total degree of lithiation α of the silicon is from 10 to 75%, wherein the total degree of lithiation α is the sum of the degree of prelithiation α1 and wherein the degree of lithiation α2, where the figures in % are based on the maximum lithiation capacity of silicon.

13. The method of claim 12, wherein the total degree of lithiation α of the silicon is from 20 to 60%, based on the maximum lithiation capacity of silicon.

14. The method of claim 12, wherein the ratio of lithium atoms to silicon atoms corresponds to the formula Li.sub.0.90 Si to Li.sub.2.90Si in the partially lithiated anode material of the fully charged lithium ion battery.

15. The method of claim 12, wherein the capacity of silicon is utilized to an extent of from 850 to 2700 mAh per gram of silicon in the partially lithiated anode material of the fully charged lithium ion battery.

16. The method of claim 12, wherein the from 7 to 46% of the maximum lithiation capacity of silicon is occupied by prelithiation of silicon.

17. The method of claim 12, wherein the amount of lithium introduced into the silicon by prelithiation corresponds to the formula Li.sub.0.25Si to Li.sub.1.80Si.

18. The method of claim 12, wherein the amount of lithium introduced into the silicon by prelithiation corresponds to a lithiation capacity of from 250 to 1700 mAh per gram of silicon.

19. The method of claim 18, wherein the ratio of lithium atoms to silicon atoms in the anode material changes by from 0.4 to 1.3 during cycling of the lithium ion battery.

20. The method of claim 18, wherein the from 10 to 45% of the lithiation capacity of silicon is utilized for the cycling of the lithium ion battery.

21. The method of claim 18, wherein the from 50 to 90% of the total degree of lithiation α is utilized for cycling of the lithium ion battery.

Description

EXAMPLE 1

[0103] Production of Unaggregated, Splinter-Shaped Silicon Particles by Milling:

[0104] The silicon powder was produced according to the prior art by milling of coarse crushed Si from the production 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).

[0105] The resulting product consisted of individual, unaggregated, splinter-shaped particles (SEM) and had a particle size distribution d.sub.10=2.23 μm, d.sub.50=4.48 μm and d.sub.90=7.78 μm and also a width (d90−d10) of 5.5 μm (determined by means of static laser light scattering, measurement instrument Horiba LA 950, using the Mie model in a greatly diluted suspension in ethanol).

EXAMPLE 2

Anode Comprising the Silicon Particles from Example 1

[0106] 29.709 g of polyacrylic acid (Sigma-Aldrich, Mw 450 000 g/mol) dried to constant weight at 85° C. and 751.60 g of deionized water were agitated by means of a shaker (290 1/min) for 2.5 h until complete dissolution of the polyacrylic acid. Lithium hydroxide monohydrate (Sigma-Aldrich) was added a little at a time to the solution until the pH was 7.0 (measured using WTW pH 340i pH meter and SenTix RJD) electrode. The solution was subsequently mixed by means of a shaker for a further 4 hours.

[0107] 7.00 g of the silicon particles from Example 1 were then dispersed in 12.50 g of the neutralized polyacrylic acid solution (concentration 4% by weight) and 5.10 g of deionized water by means of a high-speed mixer at a circumferential velocity of 4.5 m/s for 5 minutes and of 12 m/s for 30 minutes while cooling at 20° C. After addition of 2.50 g of graphite (Imerys, KS6L C), the mixture was then stirred for a further 30 minutes at a circumferential velocity of 12 m/s. After degassing, the dispersion was applied to a copper foil having a thickness of 0.030 mm (Schlenk Metallfolien, SE-Cu58) by means of a film drawing frame having a gap height of 0.10 mm (Erichsen, model 360). The anode coating produced in this way was subsequently dried for 60 minutes at 80° C. and 1 bar atmospheric pressure.

[0108] The anode coating dried in this way had an average weight per unit area of 2.85 mg/cm.sup.2 and a layer thickness of 32 μm.

EXAMPLE 3

Prelithiation of the Anode from Example 2

[0109] The electrochemical prelithiation was carried out in a button cell (type CR2032, Hohsen Corp.) in a two-electrode arrangement. The electrode coating from Example 2 was used as working electrode or positive electrode (diameter=15 mm) and Li foil having a thickness of 0.5 mm was used as counterelectrode or negative electrode (diameter=15 mm). A glass fiber filter paper (Whatman, GD Type D) impregnated with 120 μl of electrolyte served as separator (diameter=16 mm). The electrolyte used consisted of a 1.0 molar solution of lithiumhexafluorophosphate in a 3:7 (v/v) mixture of fluoroethylene carbonate and ethyl methyl carbonate admixed with 2.0% by weight of vinylene carbonate. The cell was constructed in a glove box (<1 ppm H.sub.2O, O.sub.2), and the water content in the dry mass of all components used was below 20 ppm.

[0110] The prelithiation was carried out by lithiating the anode from Example 2 at 20° C. using a constant current of 33.6 mA/g or 0.10 mA/cm.sup.2 (corresponds to C/25) for 31.25 hours and a constant current of 33.6 mA/g or 0.10 mA/cm.sup.2 up to attainment of the voltage limit of 1.0 V and then prelithiated at a constant current of 33.6 mA/g or 0.10 mA/cm.sup.2 for 12.5 hours (corresponds to 420 mAh/g). The specific current selected was based on the weight of the anode coating.

[0111] The details for formation and also the degrees of lithiation α, α1 and α2 are summarized in Table 1.

EXAMPLE 4 (EX. 4)

Lithium Ion Battery Comprising the Anode from Example 3

[0112] The electrochemical tests were carried out on a button cell (type CR2032, Hohsen Corp.) in a two-electrode arrangement. The prelithiated electrode coating from Example 3 was used as counterelectrode or negative electrode (diameter=15 mm), and a coating based on lithium nickel manganese cobalt oxide 6:2:2 having a content of 94.0% and an average weight per unit area of 14.5 mg/cm.sup.2 (procured from Custom Cells) was used as working electrode or positive electrode (diameter=15 mm). A glass fiber filter paper (Whatman, GD Type D) impregnated with 120 μl of electrolyte served as separator (diameter=16 mm). The electrolyte used consisted of a 1.0 molar solution of lithium hexafluorophosphate in a 3:7 (v/v) mixture of fluoroethylene carbonate and ethyl methyl carbonate admixed with 2.0% by weight of vinylene carbonate. The cell was again constructed in a glove box (<1 ppm H.sub.2O, 02), and the water content in the dry mass of all components used was below 20 ppm.

[0113] Electrochemical testing was carried out at 20° C. Charging of the cell was carried out by the cc/cv method (constant current/constant voltage) at a constant current of 75 mA/g (corresponds to C/2) and after attainment of the voltage limit of 4.2 V at a constant voltage until the current went below 19 mA/g (corresponds to C/8). Discharging of the cell was carried out by the cc method (constant current) at a constant current of 75 mA/g (corresponds to C/2) in subsequent cycles up to attainment of the voltage limit of 3.0 V. The specific current selected was based on the weight of the coating of the positive electrode.

[0114] On the basis of the anode formulation from Examples 2 and 3, the lithium ion battery was operated in combination with the cathode from Example 4 by the cell balancing set with partial lithiation of the anode.

[0115] In the first cycle (C/2), a reversible capacity of 2.24 mAh/cm.sup.2 was achieved.

[0116] After 250 charging/discharging cycles, the cell still had 89% of its initial capacity from the first cycle.

[0117] The test results are summarized in Table 2.

COMPARATIVE EXAMPLE 5 (CEX. 5)

[0118] The procedure of Example 4 was repeated, except that the anode was not prelithiated.

[0119] On the basis of the cell balancing resulting from the anode formulation of Example 2 and the cell balancing of Example 4, the Si anode was operated with partial lithiation.

[0120] In the first cycle (C/2), a reversible capacity of only 2.05 mAh/cm.sup.2 was observed.

[0121] After 250 charging/discharging cycles, the cell had only 75% of its capacity from the first cycle.

[0122] The details for formation and the degrees of lithiation α, α1 and α2 are summarized in Table 1, and the test results may also be found in Table 2.

EXAMPLE 6 (EX. 6)

[0123] The procedure of Example 4 was repeated, except that the anode was prelithiated at 252 mAh/g.

[0124] In the first cycle (C/2), a reversible capacity of 2.22 mAh/cm.sup.2 was achieved.

[0125] After 250 charging/discharging cycles, the cell still had 83% of its initial capacity from the first cycle.

[0126] The details for formation and the degrees of lithiation α, α1 and α2 are summarized in Table 1, and the test results may also be found in Table 2.

TABLE-US-00001 TABLE 1 Details for formation and also for the degrees of lithiation α, α1 and α2 for (comparative) examples 4~6: Formation C/25 C/25 Degree of lithiation [mAh/cm.sup.2] [mAh/g] α α1 α2 Ex. 4 2.39 839 0.43 0.14 0.29 CEx. 5 2.20 772 0.26 0.00 0.26 Ex. 6 2.37 832 0.37 0.09 0.28

COMPARATIVE EXAMPLE 7 (CEX. 7)

[0127] The procedure of Example 4 (prelithiation at 420 mAh/g; α1=0.14) was repeated, except that the partial lithiation was carried out with a degree of lithiation α2=0.85.

[0128] The total degree of lithiation α was 0.99.

[0129] The initial capacity was 3.37 mAh/cm.sup.2.

[0130] However, the capacity had dropped to 80% of the initial capacity after only four cycles.

TABLE-US-00002 TABLE 2 Results of the electrochemical tests using the (comparative) examples 4~6: capacity retention initial volumetric after capacity capacity 250 cycles [mAh/cm.sup.2] [mAh/cm.sup.3] [%] Ex. 4 2.24 700 89 CEx. 5 2.05 632 75 Ex. 6 2.22 680 83

COMPARATIVE EXAMPLE 8 (CEX. 8)

[0131] The procedure of comparative example 7 (degree of lithiation of the partial lithiation: α2=0.85) was repeated, except that the anode was not prelithiated.

[0132] The total degree of lithiation α was 0.85.

[0133] The initial capacity was 2.80 mAh/cm.sup.2.

[0134] However, the capacity had dropped to 80% of the initial capacity after only four cycles.

[0135] Compared to the batteries of the comparative examples, the batteries of the examples according to the invention surprisingly display a more stable electrochemical cycling behavior and also a high initial capacity.

[0136] The comparative examples show that when a procedure which is not according to the invention is employed, increased stressing of the Si-containing anode active material occurs, for example as a consequence of electrochemical milling or increased volume breathing of silicon. This results in electric decontacting and an impaired cycle behavior of the anode active material.

[0137] To achieve the advantageous effects according to the invention, it has been found to be essential to select the range according to the invention for the total degree of lithiation α, as comparison of the examples and the comparative examples shows.