POLYMER COMPOSITION AS A BINDER SYSTEM FOR LITHIUM-ION BATTERIES

Abstract

The invention relates to an electrode coating for a lithium-ion battery, containing copolymer C or the salt thereof, copolymer C being synthesized by polymerizing a combined total of more than 70 wt % of vinyl-functional cyclic carbonate and one or more monomers from the group comprising acrylic acid and the derivatives thereof, the percentage by weight of vinyl-functional cyclic carbonate in relation to the weight of all monomers used being 5-90%.

Claims

1. An electrode coating for a lithium ion battery, said electrode coating comprising silicon as an active material and a copolymer C or a salt thereof, wherein the copolymer C or salt thereof can be prepared by polymerization of a combination of the following monomers: (a) vinyl-functional cyclic carbonate monomers, (b) one or more monomers selected from the group consisting of acrylic acid and derivatives thereof, and optionally (c) at least one further monomer, where monomers (a) and (b) together constitute more than 70% by weight of the combination of monomers, and monomer (a) constitutes 5-90% by weight of the combination of monomers.

2. The electrode coating as claimed in claim 1, wherein the acrylic acid in the copolymer C constitutes 30-85% by weight of the combination of monomers.

3. The electrode coating as claimed claim 1, wherein the copolymer C can be prepared exclusively from the monomers mentioned in claim 1.

4. The electrode coating as claimed in claim 1, wherein a number average molar mass of the copolymer C is Mn=500-30000.

5. The electrode coating as claimed in claim 1, which contains a sodium or lithium salt of the copolymer C.

6. (canceled)

7. The electrode coating as claimed claim 2, wherein the copolymer C can be prepared exclusively from the monomers mentioned in claim 1.

8. The electrode coating as claimed in claim 7, wherein a number average molar mass of the copolymer C is Mn=500-30000.

9. The electrode coating as claimed in claim 8, which contains a sodium or lithium salt of the copolymer C.

Description

COMPARATIVE EXAMPLE 1

polyVC

[0065] The preparation of polyVC was carried out as described by H. Zhao et al. Journal of Power Sources 263 (2014) 288-295.

[0066] 4.41 g of a 22% strength by weight silicon suspension in ethanol having a particle size of d50 =180 nm (unaggregated silicon particles, cf., for example, DE 102013211388) and 0.60 g of conductive carbon black (Timcal Super C65) were dispersed in 15.00 g of a 2.5% strength by weight solution of polyVC, in DMF by means of a speed mixer at a rotational speed of 2500 rpm for 5 minutes and subsequently by means of a dissolver at a circumferential velocity of 9 m/s for 15 minutes while cooling at 20 C. After addition of 2.83 g of graphite (Timcal SFG6) and 9.0 g of DMF, the mixture was then mixed by means of a speed mixer at a rotational speed of 2500 rpm for 5 minutes and by means of a dissolver at a circumferential velocity of 8 m/s for 15 minutes. After degassing in the speed mixer, the dispersion was applied by means of a film drawing frame having a gap height of 0.25 mm (Erichsen, model 360) to a copper foil (Schlenk Metallfolien, SE-Cu58) having a thickness of 0.030 mm. The electrode coating produced in this way was subsequently dried for 60 minutes at 80 C. and 1 bar of air pressure. The average weight per unit area of the dry electrode coating was 2.51 mg/cm.sup.2.

[0067] The electrochemical testing was carried out at 20 C. 40 mV and 1.0 V vs. Li/Li+ were used as potential limits. The charging or lithiation of the electrode was carried out by the cc/cv method (constant current/constant voltage) with a constant current and, after reaching the voltage limit, at a constant voltage until the current dropped below 50 mA/g. The discharging or delithiation of the electrode was carried out by the cc method (constant current) with a constant current until the voltage limit was reached. The specific current selected was based on the weight of the electrode coating.

[0068] The electrode coating from comparative example 1 had a reversible initial capacity of about 522 mAh/g and after 70 charging/discharging cycles had about 28% of its original capacity, and after 100 cycles 22% of its original capacity.

EXAMPLE 2

Poly(VC-co-AA)

Weights Used

[0069] 2.35 g of vinylene carbonate=27 mmol

[0070] 1.95 g of acrylic acid=27 mmol

[0071] 0.05 g=0.06 mmol of AIBN

Procedure

[0072] The two monomers were dissolved in one another without additional solvent and admixed with half of the AIBN (0.025 g) and degassed in a 50 ml 3-neck flask.

[0073] The mixture was subsequently polymerized at 70 C. After three hours, the remaining amount of AIBN (0.025 g) was added and polymerization was continued at 70 C. for 16 hours.

Work-Up

[0074] The polymer was dissolved in 30 ml of DMF at 90 C. and reprecipitated from 50 ml of diethyl ether. The polymer was dried at 70 C. under reduced pressure.

[0075] Mn=2500 g/mol (GPC)

Neutralization

[0076] 0.6 g of polymer was dissolved in 5.0 g of DMF.

[0077] 45 mg of NaOH were dissolved in 0.5 g of water.

Procedure

[0078] The aqueous NaOH was stirred dropwise into the polymer solution.

[0079] A precipitate is formed and this is redissolved by addition of water. Water is added until a solids content of 2% by weight is reached (=2.5% strength binder solution).

[0080] 4.41 g of a 22% strength by weight silicon suspension in ethanol having a particle size of d50=180 nm (unaggregated silicon particle) and 0.6 g of conductive carbon black (Timcal Super C65) were dispersed in 15.00 g of the abovementioned binder solution by means of a speed mixer at a rotational speed of 2500 rpm for 5 minutes and subsequently by means of a dissolver at a circumferential velocity of 9 m/s for 15 minutes while cooling at 20 C. After addition of 2.83 g of graphite (Timcal SFG6) and 7 g of water, the mixture was then mixed by means of a speed mixer at a rotational speed of 2500 rpm for 5 minutes and by means of a dissolver at a circumferential velocity of 8 m/s for 15 minutes. After degassing in the speed mixer, the dispersion was applied by means of a film drawing frame having a gap height of 0.25 mm (Erichsen, model 360) to a copper foil (Schlenk Metallfolien, SE-Cu58) having a thickness of 0.030 mm. The electrode coating produced in this way was subsequently dried for 60 minutes at 80 C. and 1 bar of air pressure. The average weight per unit area of the dry electrode coating was 2.51 mg/cm.sup.2.

[0081] The electrochemical testing was carried out at 20 C. 40 mV and 1.0 V vs. Li/Li+ were used as potential limits. The charging or lithiation of the electrode was carried out by the cc/cv method (constant current/constant voltage) with a constant current and, after reaching the voltage limit, at a constant voltage until the current dropped below 50 mA/g. The discharging or delithiation of the electrode was carried out by the cc method (constant current) with a constant current until the voltage limit was reached. The specific current selected was based on the weight of the electrode coating.

[0082] The electrode coating from example 2 had a reversible initial capacity of about 716 mAh/g and after 70 charging/discharging cycles still had about 99% of its original capacity. After 100 charging/discharging cycles, it had about 84% of its original capacity.

COMPARATIVE EXAMPLE 3

CMC Binder

[0083] 4.41 g of a 22% strength by weight silicon suspension in ethanol having a particle size of d50=180 nm (unaggregated silicon particles) and 0.6 g of conductive carbon black (Timcal Super C65) were dispersed in 15.00 g of a 2.5% strength by weight solution of CMC in water by means of a speed mixer at a rotational speed of 2500 rpm for 5 minutes and subsequently by means of a dissolver at a circumferential velocity of 9 m/s for 15 minutes while cooling at 20 C. After addition of 2.83 g of graphite (Timcal SFG6) and 4 g of water, the mixture was then mixed by means of a speed mixer at a rotational speed of 2500 rpm for 5 minutes and by means of a dissolver at a circumferential velocity of 8 m/s for 15 minutes. After degassing in the speed mixer, the dispersion was applied by means of a film drawing frame having a gap height of 0.25 mm (Erichsen, model 360) to a copper foil (Schlenk Metallfolien, SE-Cu58) having a thickness of 0.030 mm. The electrode coating produced in this way was subsequently dried for 60 minutes at 80 C. and 1 bar of air pressure. The average weight per unit area of the dry electrode coating was 2.20 mg/cm.sup.2.

[0084] The electrochemical testing was carried out at 20 C. 40 mV and 1.0 V vs. Li/Li+ were used as potential limits. The charging or lithiation of the electrode was carried out by the cc/cv method (constant current/constant voltage) with a constant current and, after reaching the voltage limit, at a constant voltage until the current dropped below 50 mA/g. The discharging or delithiation of the electrode was carried out by the cc method (constant current) with a constant current until the voltage limit was reached. The specific current selected was based on the weight of the electrode coating.

[0085] The electrode coating from comparative example 3 had a reversible initial capacity of about 694 mAh/g and after 70 charging/discharging cycles had about 78% of its original capacity, and after 100 cycles about 64% of its original capacity.

TABLE-US-00001 TABLE 1 Capacity and resistance trends over 70 and 100 cycles. Capacity Internal Reversible retention resistance initial after 70/ after 70/ capacity 100 cycles 100 cycles Material Binder [mAh/g] [%] [mOhm/cm.sup.2] Comp. Ex. 1* PolyVC 522 28/22 152.6/183.6 Ex. 2 Poly (VC- 744 99/84 33.5/31.7 co-AA) Comp. Ex. 3* CMC 694 78/64 72.5/119.0 *not according to the invention

[0086] The determination of the internal resistance was carried out by applying a current of 200 mA/g to the cell having a completely delithiated Si electrode, which cell was not being loaded with a current. From the difference of the cell voltages in the unloaded and loaded (measured after one second) state, the internal resistance was calculated according to Ohm's law RA=U/I (R=internal resistance, A=electrode area, U=voltage difference unloaded/loaded, I=current)

[0087] Compared to comparative example 1, example 2 showed a significantly higher initial capacity (=higher utilization of the theoretical capacity) together with a higher capacity retention. Only example 2 still had a high cycling stability with a capacity retention of >80% after 100 cycles.

[0088] Comparative example 3 using CMC as standard binder had a higher stability than polyVC. However, due to the significant increase in the internal resistance at high loadings, the cycling stability of the poly(VC-co-AA) binder according to the invention of example 2 could not be achieved.