Silicon electrode binder

Abstract

The present application relates to a binder. The present application can provide a binder which can be applied to production of silicon series negative electrodes to cope well with shrinkage and expansion by repeated charge and discharge, and has excellent binding force between active materials and adhesive force to a current collector, and an active material composition, an electrode and a secondary battery, comprising the same.

Claims

1. A silicon electrode binder comprising: a copolymer comprising: a first monomer unit, wherein a homopolymer of said first monomer unit has a glass transition temperature of 80° C. or higher; a second monomer unit, wherein a homopolymer of said second monomer unit has a glass transition temperature of 70° C. or lower; and a third monomer unit, wherein said third monomer unit has an aqueous solubility at room temperature of less than 5%, wherein the first monomer unit is a carboxyl group-containing monomer unit, wherein the second monomer unit is a hydroxyl group-containing monomer unit, a polyalkylene oxide unit-containing monomer unit, a phosphite group-containing monomer unit or a ureido group-containing monomer unit, wherein the third monomer unit is (meth)acrylonitrile unit, a styrene-based monomer unit or alkyl (meth)acrylate unit, wherein a ratio of the first monomer unit is in a range of 10 weight % to 40 weight % based on a total weight of the copolymer, and wherein the second monomer unit is comprised in an amount of 100 to 500 parts by weight relative to 100 parts by weight of the first monomer unit.

2. The silicon electrode binder according to claim 1, wherein an aqueous solubility of the copolymer is 5% or more.

3. The silicon electrode binder according to claim 1, wherein the copolymer has a weight average molecular weight in a range of 100,000 to 5,000,000.

4. The silicon electrode binder according to claim 1, wherein the carboxyl group-containing monomer is (meth)acrylic acid, 2-(meth)acryloyloxyacetic acid, 3-(meth)acryloyloxypropylic acid, 4-(meth)acryloyloxybutyric acid, acrylic acid dimer, itaconic acid, maleic acid or maleic anhydride.

5. The silicon electrode binder according to claim 1, wherein the second monomer unit is the hydroxyl group-containing monomer unit, the polyalkylene oxide unit-containing monomer unit or the ureido group-containing monomer unit.

6. The silicon electrode binder according to claim 1, wherein the second monomer unit is contained in a ratio of 150 to 500 parts by weight relative to 100 parts by weight of the first monomer unit.

7. The silicon electrode binder according to claim 1, wherein the third monomer unit is comprised in an amount of 0.1 to 30 parts by weight relative to 100 parts by weight of the first and second monomer units.

8. A silicon electrode binder composition comprising the silicon electrode binder of claim 1.

9. An active material composition comprising the silicon electrode binder of claim 1 and an electrode active material.

10. The active material composition according to claim 9, wherein the electrode active material is a silicon active material.

11. An electrode comprising a current collector, and the silicon electrode binder of claim 1 and an electrode active material, formed on one side of the current collector.

12. The electrode according to claim 11, wherein the electrode active material is a silicon active material.

13. A secondary battery comprising the electrode of claim 11 as a negative electrode.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The FIGURE is an exemplary schematic diagram of an electrode of the present application.

MODE FOR INVENTION

(2) Hereinafter, the device and method will be described in detail by way of Examples and Comparative Examples, but the scope of the device and method is not limited by the following examples.

(3) 1. Evaluation of molecular weight

(4) The weight average molecular weight (Mw) and the molecular weight distribution (PDI) were measured using GPC (gel permeation chromatograph) under the following conditions, and the measurement results were converted using standard polystyrene of Agilent system for production of calibration curves.

(5) <Measurement Conditions>

(6) Measuring instrument: Agilent GPC (Agilent 1200 series, U.S.)

(7) Column: PLGel-M, PLGel-L serial connection

(8) Column temperature: 35° C.

(9) Eluent: THF (tetrahydrofuran)

(10) Flow rate: 1.0 mL/min

(11) Concentration: ˜1 mg/mL (100 μL injection)

(12) 2. Polymer conversion rate and NMR evaluation

(13) The conversion rate was calculated according to Equation 1 below by applying integral values of a signal by a monomer and a signal by a polymer in the spectrum obtained by NMR analysis.

(14) The NMR analysis was performed at room temperature using an Agilent 500 MHz instrument, where an analyte (a polymerized reactant and the like) was diluted in a measuring solvent (CDCl.sub.3 and D.sub.2O) to a concentration of about 10 mg/ml or so and used, and a chemical shift was expressed in ppm.
Conversion rate (%)=100×polymer signal integral value/(polymer signal integral value+monomer signal integral value)  [Equation 1]

(15) 3. Solubility measurement method

(16) The solubility was evaluated by taking 1 g of the solubility measurement object (polymer or monomer), adding it to 5 g of water, stirring it at room temperature (25° C.) for 30 minutes, and then removing undissolved residual solute. The amount of the solute dissolved in the solvent was measured by measuring the removed residual solute, and the solubility was evaluated by converting the measured amount into the value for 100 g of the solvent. Here, the removal of the residual solute was performed by filtering the solution out by a sieve with a pore size of about 0.45 μm or so.

(17) The aqueous solubility was calculated as a percentage of the amount of the solute dissolved in the solvent based on the weight (aqueous solubility=100×B(B+A), where B is the weight (unit: g) of the solute and A is the weight (unit: g) of the solvent).

(18) In addition, the polymer solubility described in Table 1 below was measured in the above manner for the polymer itself prepared in each of Examples or Comparative Examples, and the aqueous solubility of the homopolymer of the monomer was measured for the homopolymer itself prepared by the method disclosed in the following example except for applying only the monomer to the method disclosed in the following example.

(19) 4. Adhesive force measurement

(20) A specimen was prepared by stamping the prepared negative electrode to a width of about 1.5 cm and a height of about 12 cm. Subsequently, a double-sided tape is attached on glass of a glass slide, a back surface of a 3M adhesive tape is attached on the double-sided tape, and the slurry surface of the stamped negative electrode is attached on the adhesive tape to obtain a measurement sample. Thereafter, one end of the negative electrode attached on the glass is peeled off about 0.5 cm and fixed to the lower clamp of a texture analyzer, and the other part of the drooping negative electrode is fixed with the upper clamp, and then pulled with a force of about 2 gf to measure the force at the time when the negative electrode slurry drops.

(21) 5. Maximum particle size measurement

(22) The maximum particle size was measured using a plate on which micropores with different sizes ranging from 1 μm to 100 μm are formed. 1 g of the prepared slurry is taken and placed on the end of the portion having a large pore. The slurry was scratched from the 100 μm portion toward the portion pitted with a small pore using a plate rod and the maximum particle size was determined by reading the pore size at the point where the slurry was no longer scratched.

(23) 6. Slurry viscosity

(24) The viscosity of the slurry was measured at room temperature using a Brookfield viscometer DV-I Prime as a measuring instrument.

Preparation Example 1. Preparation of Polymer (A1)

(25) 5 g of 2-hydroxyethyl acrylate (HEA, aqueous solubility: about 15%), 1.43 g of acrylic acid (AA, aqueous solubility: 99% or more), 0.71 g of acrylonitrile (AN, aqueous solubility: less than 1%) and 65 g of distilled water were placed in a 100 mL round bottom flask, and the inlet was sealed. The reaction was initiated by bubbling with nitrogen for 30 minutes to remove oxygen, placing the reaction flask in an oil bath heated to 65° C., and then introducing 7 mg of an initiator (VA-65, Wako Chem) and 4 mg of CTA (2-mercaptoethanol) thereto. The reaction was allowed to proceed for about 20 hours or so and then terminated to prepare a random polymer. The conversion rate calculated for the sum of the monomers HEA, AA and AN applied in the above reaction was about 99% or so.

(26) The ratio of AA units, HEA units and AN units in the polymer was about 2:7:1 (AA:HEA:AN) or so and the weight average molecular weight (Mw) was about 260,000 or so.

Preparation Example 2. Preparation of Polymer (A2)

(27) A random polymer was prepared in the same manner as in Preparation Example 1, except that methyl acrylate (MA, aqueous solubility: less than 1%) was used instead of acrylonitrile (AN) as the hydrophobic monomer. The conversion rate calculated for the sum of the monomers applied in the reaction was about 99% or so.

(28) The ratio of AA units, HEA units and MA units in the polymer was about 2:7:1 (AA:HEA:MA) or so and the weight average molecular weight (Mw) was about 240,000 or so.

Preparation Example 3. Preparation of Polymer (A3)

(29) A random copolymer was prepared in the same manner as in Example 1, except that the introduction amounts of the monomers were adjusted (AA: 1.8 g, HEA: 5 g, and AN: 0.36 g) so that the ratio of AA units, HEA units and AN units in the polymer was about 25:70:5 (AA:HEA:AN) or so, and the weight average molecular weight (Mw) of the prepared copolymer was about 280,000 or so.

Preparation Example 4. Preparation of Polymer (A4)

(30) A random copolymer was prepared in the same manner as in Example 1, except that the introduction amounts of the monomers were adjusted (AA: 2.3 g, HEA: 5 g, and AN: 0.38 g) so that the ratio of AA units, HEA units and AN units in the polymer was about 30:65:5 (AA:HEA:AN) or so, and the weight average molecular weight (Mw) of the prepared copolymer was about 220,000 or so.

Preparation Example 5. Preparation of Polymer (B1)

(31) 5 g of 2-hydroxyethyl acrylate (HEA, aqueous solubility: about 15%), 2.14 g of acrylic acid (AA, aqueous solubility: 99% or more) and 65 g of distilled water were placed in a 100 mL round bottom flask, and the inlet was sealed. The reaction was initiated by bubbling with nitrogen for 30 minutes to remove oxygen, placing the reaction flask in an oil bath heated to 65° C., and then introducing 7 mg of an initiator (VA-65, Wako Chem) and 4 mg of CTA (2-mercaptoethanol) thereto. The reaction was allowed to proceed for about 20 hours or so and then terminated to prepare a random polymer. The conversion rate calculated for the sum of the monomers applied in the above reaction was about 99% or so.

(32) The ratio of AA units and HEA units in the polymer was about 3:7 (AA:HEA) or so and the weight average molecular weight (Mw) was about 300,000 or so.

Preparation Example 6. Preparation of Binder (B2)

(33) The random copolymer prepared in Preparation Example 5 and polyacrylonitrile were mixed in a weight ratio of 9:1 (random copolymer:polyacrylonitrile) to prepare a binder.

EXAMPLE 1

(34) A negative electrode slurry composition was prepared by mixing the polymer (A1) (binder) prepared in Preparation Example 1, an active material mixture and a conductive material (Super C) in a weight ratio of 4:95:1 (binder:active material mixture:conductive material) and then adding water as a solvent thereto. Here, the active material mixture was a known silicon series mixture, where a mixture was used, in which a carbon active material and a silicon-based active material was mixed in a weight ratio (carbon active material:silicon-based active material) of about 90:10. Thereafter, the slurry was coated on a copper foil current collector having a thickness of about 20 μm to be a thickness of about 100 μm or so after drying and vacuum-dried at about 100° C. for about 10 hours to prepare a negative electrode having a loading amount of about 1.5 mAh/cm.sup.2 or so.

EXAMPLE 2

(35) A negative electrode was prepared in the same manner as in Example 1, except that the polymer (A2) prepared in Preparation Example 2 was used.

EXAMPLE 3

(36) A negative electrode was prepared in the same manner as in Example 1, except that the polymer (A3) prepared in Preparation Example 3 was used.

EXAMPLE 4

(37) A negative electrode was prepared in the same manner as in Example 1, except that the polymer (A4) prepared in Preparation Example 4 was used.

COMPARATIVE EXAMPLE 1

(38) A negative electrode was prepared in the same manner as in Example 1, except that the polymer (B1) prepared in Preparation Example 5 was used.

COMPARATIVE EXAMPLE 2

(39) A negative electrode was prepared in the same manner as in Example 1, except that the binder (B2) prepared in Preparation Example 6 was used.

(40) The physical properties measured for the above were summarized and described in Table 1 below.

(41) TABLE-US-00001 TABLE 1 Example Comparative Example 1 2 3 4 1 2 Viscosity (cp) 25000 30000 20000 23000 50000 70000 Maximum particle 60 72 58 58 100 more than 100 size of slurry (μm) Adhesive force (gf/cm) 98 75 85 95 47   20

(42) From Table 1, it can be seen that in the case of Examples, the binder or the active material has a small aggregation upon preparing the slurry, which is expected to be due to the characteristics of the polymer. For example, it is determined that the hydrophobic monomer units contained in the polymer interact with the other parts having high aqueous solubility to enhance the dispersibility. In addition, in the case of Examples, the excellent adhesion properties were also shown.

(43) Through this, it can be confirmed that if the binder of the present application is applied, there is an effect of increasing the dispersibility of the active material while maintaining the adhesive force in the silicon-based negative electrode material, whereby excellent cycle characteristics and the like can be secured.