METHOD FOR MANUFACTURING NEGATIVE ELECTRODE MATERIAL FOR LITHIUM-ION SECONDARY BATTERY, AND METHOD FOR MANUFACTURING LITHIUM-ION SECONDARY BATTERY

Abstract

A method of manufacturing a negative electrode for a lithium-ion secondary battery, the method comprising: (a) a process of obtaining a mixture that comprises a graphitizable binder and at least one selected from the group consisting of a graphitizable aggregate and graphite; (b) a process of obtaining a molded product by molding the mixture, in which the graphitizable binder is in a softened state; (c) a process of obtaining a graphitized product by graphitizing the molded product; and (d) a process of obtaining a pulverized product by pulverizing the graphitized product.

Claims

1. A method of manufacturing a negative electrode for a lithium-ion secondary battery, the method comprising: (a) a process of obtaining a mixture that comprises a graphitizable binder and at least one selected from the group consisting of a graphitizable aggregate and graphite; (b) a process of obtaining a molded product by molding the mixture, in which the graphitizable binder is in a softened state; (c) a process of obtaining a graphitized product by graphitizing the molded product; and (d) a process of obtaining a pulverized product by pulverizing the graphitized product.

2. The method of manufacturing a negative electrode for a lithium-ion secondary battery according to claim 1, wherein the process of obtaining a molded product is performed in a state in which the mixture has a temperature of 80 C. or more.

3. The method of manufacturing a negative electrode for a lithium-ion secondary battery according to claim 1, wherein the process of obtaining a molded product is performed by extrusion molding.

4. The method of manufacturing a negative electrode for a lithium-ion secondary battery according to claim 1, wherein the mixture further comprises a fluidity-imparting agent.

5. The method of manufacturing a negative electrode for a lithium-ion secondary battery according to claim 4, wherein the fluidity-imparting agent comprises a fatty acid.

6. The method of manufacturing a negative electrode for a lithium-ion secondary battery according to claim 1, wherein the pulverized product comprises a particle in which plural flat graphite particles are aggregated or bonded such that principal surfaces of the graphite particles are not parallel to each other.

7. A method of manufacturing a lithium-ion secondary battery, the method comprising a process of manufacturing a negative electrode using a negative electrode material that is manufactured by the method of manufacturing a negative electrode for a lithium-ion secondary battery according to claim 1.

8. A method of manufacturing a lithium-ion secondary battery, the method comprising: a process of manufacturing a negative electrode material for a lithium-ion secondary battery by the method of manufacturing a negative electrode for a lithium-ion secondary battery according to claim 1; and a process of manufacturing a negative electrode using the negative electrode material for a lithium-ion secondary battery.

Description

EXAMPLES

[0102] In the following, the embodiments as described above are explained more specifically based on the Examples. However, the embodiments are not limited to the Examples.

[0103] (1) Preparation of Negative Electrode Material

[0104] The materials shown in Table 1 were mixed at the amounts shown in Table 1 (parts by mass), thereby obtaining a mixture. Then, a molded product was obtained by molding the mixture at a temperature shown in Table 1 by extrusion molding. Subsequently, the molded product was subjected to a thermal treatment at 800 C. to 850 C. for 8 hours in a nitrogen atmosphere, and subjected to graphitization at 2600 C. to 2900 C. for 30 hours. The graphitized product was pulverized, thereby obtaining graphite powders (negative electrode materials) of Example 1 to 15.

[0105] The negative electrode materials of Example 16 and Example 17 were obtained by forming a molded product by molding and vibration molding, with the temperature of the mixture shown in Table 1, and performing graphitization and pulverization under the conditions as described above.

[0106] The negative electrode materials of Reference Examples 1 to 3 were obtained by cooling the mixture and pulverizing the same to have a particle size of 25 m, obtaining a molded product by packing the pulverized product into a rectangular container, and performing graphitization and pulverization under the conditions as described above.

[0107] The details of the materials shown in Table 1 are as follows.

[0108] Aggregate 1: mosaic coke having average particle size of 20 m

[0109] Aggregate 2: mosaic coke having average particle size of 100 m

[0110] Aggregate 3: semi-needle coke having average particle size of 20 m

[0111] Graphite 1: spherical natural graphite having average particle size of 23 m

[0112] Graphite 2: spherical natural graphite having average particle size of 14 m

[0113] Graphitization catalyst: silicon carbide (SiC)

[0114] Binder: tar pitch

[0115] Fluidity-imparting agent: stearic acid

[0116] The density of the molded product (g/cm.sup.3), the density of the graphitized product (g/cm.sup.3), and the average particle diameter (m), specific surface area (m.sup.2/g) and saturated tap density (g/cm.sup.3) of the negative electrode material are shown in Table 1, respectively.

[0117] The pulverized product obtained in each of the Examples and the Reference Examples included secondary particles in which graphite particles (graphitized product of the aggregate), were aggregated or bonded such that principal surfaces thereof were not parallel to each other. When spherical natural graphite was used as the material, the pulverized product also included composite particles in which the secondary particles and the spherical natural graphite were bonded to each other.

[0118] (2) Preparation of Negative Electrode and Evaluation of Orientation

[0119] A composition was prepared by mixing 98 parts by mass of the negative electrode material, 1 part by mass of styrene-butadiene rubber (BM-400B, Zeon Corporation) and 1 part by mass of carboxymethyl cellulose (CMC1380, Daicel Corporation), and adding water to adjust the viscosity. The composition was applied onto a current collector (copper foil of 10 m in thickness) in an amount of 10 mg/cm to form a composition layer. The composition layer was dried at 105 C. for 1 hour in the atmosphere, and pressurized so as to have a density of 1.70 g/cm.sup.3, thereby obtaining a negative electrode. The orientation of the negative electrode was evaluated by the method as mentioned above.

[0120] (3) Preparation of Cell for Evaluation

[0121] The negative electrode was cut into a round shape with 1.54 cm.sup.2, and a coin-shaped cell (2016-type) for evaluation was prepared using the negative electrode, metallic lithium as a positive electrode, a mixture of ethylene carbonate/ethylmethyl carbonate (3/7 in volume) including 1.0M LiPF.sub.6 and vinylene carbonate (0.5% by mass) as an electrolyte, a polyethylene microporous film of 25 m in thickness as a separator, and a copper foil of 230 m in thickness as a spacer.

[0122] (4) Evaluation of Battery Properties

[0123] The initial discharge capacity (Ah/kg) and the initial charge/discharge efficiency (%) were measured. Specifically, the cell for evaluation was placed in a thermostat chamber at 25 C., and subjected to constant-current charging at 0.434 mA until 0V. Then, the cell was further charged at a current voltage of 0V until the current attenuates to 0.043 mA, and the initial charge capacity was measured. After the charging and pausing for 30 minutes, the cell was discharged at 0.434 mA until 1.5V, and the initial discharge capacity was measured. The capacity was converted to a value per mass of the negative electrode material. The initial charge/discharge efficiency (%) was calculated by the following formula. The results are shown in Table 1.

Initial charge/discharge efficiency (%)=initial discharge capacity/initial charge capacity100

TABLE-US-00001 TABLE 1 Examples 1 2 3 4 5 6 7 8 9 10 Aggregate 1 (part by mass) 49 39 35 32 50 26 40 33 37 34 Aggregate 2 (part by mass) Aggregate 3 (part by mass) Graphite 1 (part by mass) 22 17 18 14 27 18 18 15 18 Graphite 2 (part by mass) Graphitization catalyst 15 18 25 21 18 16 17 15 17 (part by mass) Binder (part by mass) 27 27 27 27 27 27 24 30 33 27 Fluidity-imparting agent 2 2 2 2 2 2 2 2 0 4 (part by mass) Temperature at molding ( C.) 100 100 100 100 100 100 100 100 100 100 Density of molded product 1.65 1.75 1.75 1.82 1.76 1.74 1.66 1.75 1.74 1.74 (g/cm.sup.3) Density of graphitized 1.42 1.31 1.32 1.23 1.33 1.30 1.31 1.34 1.41 1.32 product (g/cm.sup.3) Average particle diameter 23.3 23.1 23.1 24.0 23.0 23.3 23.0 23.3 23.8 23.2 (m) Specific surface area (m.sup.2/g) 4.3 3.8 3.6 3.4 4.0 3.5 4.2 3.8 4.2 3.5 Saturated tap density (g/cm.sup.3) 0.86 0.88 0.87 0.88 0.82 0.88 0.86 0.87 0.88 0.86 Orientation 235 316 348 378 304 380 318 383 379 363 Initial discharge capacity 357 362 362 364 359 365 365 360 359 362 (Ah/kg) Initial charge/discharge 93.1 93.9 94.2 94.6 93.3 94.4 93.8 94.1 93.5 94.3 efficiency (%) Examples Reference Examples 11 12 13 14 15 16 17 1 2 3 Aggregate 1 (part by mass) 37 35 35 39 40 43 Aggregate 2 (part by mass) 42 43 Aggregate 3 (part by mass) 35 43 Graphite 1 (part by mass) 18 18 18 18 17 18 20 20 20 Graphite 2 (part by mass) 17 Graphitization catalyst 16 18 13 18 18 18 19 18 18 18 (part by mass) Binder (part by mass) 24 27 31 27 27 24 21 19 19 19 Fluidity-imparting agent 0 2 2 2 2 2 2 (part by mass) Temperature at molding 100 100 100 150 200 100 100 ( C.) Density of molded product 1.76 1.74 1.76 1.74 1.73 1.69 1.60 1.58 1.53 1.59 (g/cm.sup.3) Density of graphitized 1.42 1.30 1.31 1.31 1.31 1.32 1.27 1.32 1.26 1.34 product (g/cm.sup.3) Average particle diameter 23.4 23.6 23.5 23.2 23.7 23.0 23.0 23.2 23.4 23.0 (m) Specific surface area (m.sup.2/g) 4.6 3.1 4.1 3.5 3.6 3.4 3.3 3.5 3.1 4.5 Saturated tap density (g/cm.sup.3) 0.89 0.88 0.83 0.88 0.89 0.91 0.91 0.87 0.90 0.89 Orientation 451 470 372 352 363 363 370 318 453 445 Initial discharge capacity 355 366 362 361 362 364 365 364 362 356 (Ah/kg) Initial charge/discharge 93.3 94.9 93.2 94.0 94.1 94.6 94.4 94.3 94.7 93.2 efficiency (%)

[0124] As shown in Table 1, the cells of the Examples, in which the negative electrode materials are prepared by the method according to the disclosure, exhibit favorable initial charge/discharge efficiency that are comparable to the initial charge/discharge efficiency of the cells of the Reference Examples, in which the negative electrode materials are prepared by the conventional method (i.e., pulverized prior to molding).