JP2002042787A - Lithium secondary battery and manufacturing method for it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery and a manufacturing method for it with excellent binding strength of a negative electrode active material layer and a collector, binding strength of negative electrode active materials, and binding strength to a separator and having excellent discharging characteristics and cycle characteristics. SOLUTION: A carbon material of a negative electrode active material and a binder powder particle are mixed while being applied with pressure and a shearing action, and a part or the whole of the front surface of the carbon material is coated with binder powder particles.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery and a method for manufacturing the same, and more particularly to a surface modification of a carbon material as an active material of a negative electrode.

[0002]

2. Description of the Related Art In recent years, the development of electronic devices has been remarkable, and various devices have been reduced in size and weight. Accordingly, there is a great demand for a battery as a power source to be smaller and lighter. Therefore, a lithium secondary battery using a carbon material capable of occluding and releasing lithium ions as a negative electrode active material and a lithium-containing composite oxide as a positive electrode active material as a small and lightweight battery capable of charging and discharging with high capacity has been developed. Has been put to practical use.

As a method of further reducing the thickness of a lithium secondary battery, a lithium polymer battery using a polymer as a separator material for holding an electrolyte is disclosed in US Pat. No. 5,296,963.
No. 18 discloses this.

[0004] Such a positive electrode plate and a negative electrode plate of a lithium secondary battery are prepared by mixing a binder (binder) with a powdery electrode forming material to which a conductive agent is added as required for an electrode active material. An electrode active material mixture paste obtained by kneading and dispersing in an appropriate organic solvent is applied on a current collector to form an electrode active material layer.

[0005] The binder must have durability against a non-aqueous electrolyte using ethylene carbonate, propylene carbonate or the like as a non-aqueous solvent. Conventionally, a binder such as a fluororesin such as polytetrafluoroethylene or a styrene-butadiene copolymer has been used. In recent years, a vinylidene fluoride polymer (PVD) having a lower specific resistance and a better thin film forming property has been used.
A lithium secondary battery using F) as a binder has been put to practical use. However, the binding property between these binders and the electrode active material is not sufficient, and particularly when a carbon material is used as the negative electrode active material, the binding property is poor. In addition to being easily peeled off from the anode, there was a problem that the negative electrode active material particles fell off from the negative electrode active material layer and were released because the binding strength between the negative electrode active materials was weak, thereby deteriorating the cycle life characteristics of the battery. . From a microscopic point of view, a certain portion contains only graphite and a certain portion contains only a binder, and is not necessarily in a uniformly mixed state.

In order to solve the above-mentioned problem, a silane-modified vinylidene fluoride-based polymer (JP-A-6-93025) has been proposed.
JP-A-6-17245, a vinylidene fluoride-based polymer containing a carboxyl group or a carbonate group.
No. 2) and a sulfonated vinylidene fluoride polymer (JP-A-10-298386).

[0007]

The vinylidene fluoride polymer obtained by introducing a polar group by copolymerization or the like has an improved binding property, but the electrode mixture layer is formed as described above. In addition, if necessary, a binder and a conductive agent are mixed with the electrode active material, and the mixture is kneaded and dispersed in an appropriate organic solvent to form an electrode active material mixture paste. However, the electrode active material was present without a uniform binder distribution.

The present invention is excellent in the bonding strength between the negative electrode active material layer and the current collector, the bonding strength between the negative electrode active materials, and the bonding strength with the separator even with a small amount of the binder, and has excellent discharge characteristics. A main object of the present invention is to provide a lithium secondary battery having excellent cycle life characteristics and a method for manufacturing the same.

[0009]

In order to achieve the above object, a lithium secondary battery of the present invention comprises a positive electrode using a lithium-containing composite oxide as an active material, and a carbon material capable of inserting and extracting lithium ions. In a lithium secondary battery comprising a negative electrode having an active material of, a separator, and a non-aqueous electrolytic solution impregnated therein, a part or all of the surface of the carbon material is coated with a binder. Things.

[0010] The method for producing a lithium secondary battery according to the present invention further comprises a positive electrode using a lithium-containing composite oxide as an active material;
In the method for producing a lithium secondary battery comprising a negative electrode comprising an active material comprising a carbon material capable of inserting and extracting lithium ions and a binder, a separator, and a non-aqueous electrolyte impregnated therein, the method comprises the steps of: And the binder powder particles are mixed while giving pressure and shearing action to coat a part or all of the surface of the carbon material with resin powder particles.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing the structure of a lithium polymer battery according to the present invention. The positive electrode plate 1 includes a positive electrode active material layer 4 and a positive electrode current collector 5 buried in the positive electrode active material layer. Similarly, the negative electrode plate 2 includes a negative electrode active material layer 6 and a negative electrode current collector 7 buried in the negative electrode active material layer. Separator 3 made of polymer that absorbs and retains non-aqueous electrolyte
Is located between the positive electrode plate and the negative electrode plate, and is laminated and integrated with the positive electrode plate and the negative electrode plate by thermal welding.

The positive electrode current collector 5 is made of aluminum foil, punched metal, and expanded metal, and the negative electrode current collector 7 is made of copper or nickel foil, punched metal, and expanded metal.

The positive electrode active material layer is formed by applying a paste obtained by mixing a positive electrode active material, a conductive agent, a binder, and a plasticizer with an organic solvent to the current collector and then drying and removing the organic solvent. I do. The negative electrode active material layer is formed by applying a paste obtained by kneading and dispersing a negative electrode active material, a binder, and a conductive agent and a plasticizer, if necessary, with an organic solvent to the current collector, and then drying and removing the organic solvent. To make.

As the positive electrode active material, a lithium-containing composite oxide capable of accepting lithium ions as a guest is used. For example, a composite metal oxide of lithium and at least one metal selected from cobalt, manganese, nickel, chromium, iron and vanadium, LiCoO ₂ ,
LiMnO ₂ , LiNiO ₂ , LiCo _x Ni _(1-x) O
₂ (0 <x <1), LiCrO ₂ , αLiFeO ₂ , Li
VO _{2 and the} like are preferred.

As the conductive agent, a carbon-based conductive agent such as acetylene black, graphite, or carbon fiber is preferable.

Examples of the carbon material as the negative electrode active material include graphite, activated carbon, and calcination and carbonized phenol resin or pitch, but are not particularly limited thereto.

Examples of the binder include polytetrafluoroethylene, styrene-butadiene copolymer, vinylidene fluoride polymer (PVDF), and copolymer P of vinylidene fluoride and hexafluoropropylene (VDF + HFP). It is preferable to use the same polymer as the separator that absorbs and retains the electrolytic solution, and the polymer is preferably a copolymer P (VDF + HFP) of vinylidene fluoride and hexafluoropropylene.

Organic solvents include organic organic solvents such as N-methyl-2-pyrrolidone, N, N-dimethylformamide, tetrahydrofuran, dimethylacetamide, dimethylsulfoxide, hexamethylsulfformamide, tetramethylurea, acetone, methylethylketone and the like. Any water may be used, and these may be used alone or as a mixture.

The separator is prepared by applying a paste obtained by mixing a polymer and a plasticizer with an organic solvent on, for example, a film made of a polyethylene terephthalate resin and then removing the organic solvent by drying. As the polymer, PVDF or P (VDF-HFP) is preferable.
HFP) is optimal.

The above-described battery in which the positive electrode, the negative electrode, and the separator are laminated and integrated is immersed in xylene to extract and remove the plasticizer, and the xylene is dried. This is inserted into a case and a non-aqueous electrolyte is injected to produce a lithium secondary battery.

Here, the non-aqueous electrolyte used in the lithium secondary battery of the present invention is a solution in which an electrolyte is dissolved in a non-aqueous solvent. Examples of the electrolyte include LiClO ₄ , L
Lithium salts such as iBF ₄ and LiPF ₆ are used. On the other hand, as the non-aqueous solvent, for example, one obtained by mixing non-aqueous solvents such as propylene carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and vinylene carbonate, and a mixture of a plurality of non-aqueous solvents are used.

The lithium secondary battery of the present invention is constructed by appropriately combining the above-described positive electrode, negative electrode, separator and non-aqueous electrolyte. The battery itself has a sheet shape,
It can be applied to any shape such as a cylinder, a square, a coin, and a button.

FIG. 2 is a schematic view of a mechanofusion apparatus, in which a carbon material and binder powder particles are put into a processing case 8, and a drive motor 10 rotates a stirring / shearing blade 9 so that a pressure and a pressure are reduced. This is a device that mixes while giving a shearing action.

[0024]

The present invention will be described in detail below with reference to examples and comparative examples.

Example 1 Graphite was used as the active material of the negative electrode, and P (VDF-HF) was used as a binder on the surface of the powder.
P) Powder is mixed with a mechanofusion device to which pressure and shearing action is applied, and P (VDF-H
FP) Powder particles were coated. The coating conditions were as follows.

For 100 parts by weight of graphite, P (VDF-H
FP) 10 parts by weight were mixed, and the high-speed stirring shear blade was rotated at 1000 rpm with a mechanofusion device as shown in FIG.
The mixture was treated for 10 minutes while rotating at m to prepare a negative electrode active material mixture. A 0.6 μm P (VDF-HFP) coating layer was formed on the surface of the obtained graphite.

The thus prepared negative electrode active material mixture 1
The organic solvent N-methyl-2-pyrrolidone 45 parts by weight and the plasticizer n-butyl phthalate (DBP) in 00 parts by weight
15 parts by weight were mixed to form a paste. Here, this paste was uniformly applied to both surfaces of a copper foil having a thickness of 15 μm by a doctor blade method and dried, and then rolled by a rolling mill to produce a negative electrode plate having a thickness of 230 μm. The negative electrode plate manufactured in this manner is referred to as a negative electrode plate a. The positive electrode plate was prepared by adding 100 parts by weight of LiCoO ₂ as a positive electrode active material to P (V
8 parts by weight of DF-HFP) and 5 parts by weight of acetylene black as a conductive agent were mixed with 50 parts by weight of N-methylpyrrolidone and 15 parts by weight of DBP as a plasticizer to form a paste. This paste was uniformly applied to both sides of a 20-μm-thick aluminum foil by a doctor blade method and dried, and then rolled by a rolling mill to produce a 250-μm-thick positive electrode plate.

Also, 10 parts by weight of P (VDF-HFP) were dissolved in 50 parts by weight of acetone as an organic solvent to prepare a mixed solution to which 10 parts by weight of a plasticizer DBP was added, and this was placed on a polyethylene terephthalate film by a doctor. It is applied by a blade method to produce a polymer electrolyte separator sheet. The negative electrode, the positive electrode, and the separator are laminated and passed through a heating roller to be integrated to produce a constituent battery. The above-mentioned laminated integrated battery is immersed in xylene, DBP is extracted and removed, and after vacuum drying, an aluminum foil is inserted into a laminated film bag used as an intermediate layer, an electrolytic solution is injected, and sealing is performed. , Battery capacity is 500mAh
Of battery A was obtained.

Example 2 A negative electrode plate b and a battery B were obtained in the same manner as in Example 1 except that the amount of P (VDF-HFP) was changed to 20 parts by weight to prepare a negative electrode active material mixture. Was. On the surface of the obtained graphite, 1.0 μm of P (VDF-HF
The coating layer of P) was formed.

Comparative Example 1 P (VDF-HFP) powder particles as a binder were not mixed with a graphite surface of a negative electrode active material by a mechanofusion apparatus. That is, a negative electrode plate c and a battery C were obtained in the same manner as in Example 1, except that a negative electrode active material mixture was used in which only graphite and P (VDF-HFP) powder were stirred.

Comparative Example 2 A negative electrode plate d and a battery D were obtained in the same manner as in Comparative Example 1, except that the amount of P (VDF-HFP) was changed to 20 parts by weight.

(Measurement of binding strength) Using the electrodes a, b, c and d obtained in Examples 1 and 2 and Comparative Examples 1 and 2, an adhesive tape was adhered to the surface of the negative electrode active material layer. Table 1 shows the results of measuring the binding strength between the current collector and the negative electrode active material layer by a 90-degree peel test using a tensile tester.

[0033]

[Table 1]

As is clear from Table 1, the electrodes a and b obtained in Examples 1 and 2 have a higher binding strength than the electrodes c and d obtained in Comparative Examples 1 and 2. I understand. As described above, the binding strength between the electrode c and the electrode d is low because graphite as the negative electrode active material and P (VDF-
HFP) is present individually, is non-uniform when viewed microscopically, and is considered to be due to poor binding strength between the negative electrode active material particles. On the other hand, the electrode a and the electrode b are mixed using a mechanofusion device while applying pressure and shearing action, so that the surface of the negative electrode active material is uniformly coated with the binder powder particles, so that the negative electrode active material is It is considered that the binding strength between the particles and the binding strength with the current collector were improved.

(Discharge Characteristics and Cycle Life Characteristics)
Using the batteries A, B, C, and D obtained in Examples 1 and 2 and Comparative Examples 1 and 2, discharge characteristics and cycle life characteristics were evaluated. The results are shown in Table 2 and FIG. . The discharge characteristics were 300 mA (0.6
C), a 4.2V constant current constant voltage charge is performed, and the current value becomes 5
When the current reached 0 mA (0.1 C), charging was terminated.
Thereafter, constant current discharge was performed to 3.0 V in an environment at a temperature of 20 ° C. At this time, the current value of the discharge current is set to 100 mA (0.
2C), 500 mA (1.0 C), 1000 mA (2.
0C). The cycle life characteristics were as follows. In an environment of a temperature of 20 ° C., a current value of 50 mA was obtained by performing 4.2 V constant current constant voltage charging at 300 mA (0.6 C).
(0.1C), the charging is terminated, and the temperature becomes 20
3.0V at 300mA (0.6C) constant current in an environment of ° C
The cycle of discharging until 400 cycles was repeated.

[0036]

[Table 2]

As shown in Table 2, the batteries A obtained in Examples 1 and 2
And the battery B, compared with the batteries C and D obtained in Comparative Examples 1 and 2, had a discharge characteristic, particularly 1000 mA (2.0 mA).
It can be seen that the discharge characteristics of C) are excellent. FIG.
As is clear from the graphs, the batteries A and B have better cycle life characteristics than the batteries C and D. The reason why the discharge characteristics of the batteries A and B are excellent is that the negative electrode active material surface is covered with the binder powder particles and uniformly distributed as described in the measurement of the binding strength. It is considered that the binding strength between the active material particles and the binding strength with the current collector have been improved, and the cycle life characteristics are excellent because the above-mentioned reasons and the binder and the polymer used for the separator are the same material. This is probably because even if the charge / discharge cycle is repeated, it does not peel off.

[0038]

As is apparent from the above, according to the lithium secondary battery and the method of manufacturing the same of the present invention, since the negative electrode active material is covered with the binder powder particles, the discharge characteristics and the cycle life are reduced. A high-performance lithium secondary battery having excellent characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view of the configuration of a battery.

FIG. 2 is a schematic diagram of a mechanofusion device.

FIG. 3 is a cycle life characteristic diagram.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode plate 2 Negative electrode plate 3 Separator 4 Positive electrode active material layer 5 Positive electrode current collector 6 Negative electrode active material layer 7 Negative current collector 8 Processing case 9 Stirring shearing blade 10 Drive motor

Continued on front page F-term (reference) 5H021 EE10 EE15 5H029 AJ02 AJ05 AK03 AL07 AM03 AM04 AM05 AM07 BJ04 BJ13 CJ03 CJ08 DJ08 DJ12 DJ16 EJ12 EJ14 5H050 AA02 AA07 BA17 CA07 CA08 CA09 CB08 DA03 DA10 FA11 FA18 EA18 FA18 GA10 GA22 GA27

Claims (4)

[Claims] 1. A positive electrode comprising a lithium-containing composite oxide as an active material, a negative electrode comprising an active material comprising a carbon material capable of inserting and extracting lithium ions and a binder, a separator, and a separator impregnated therein. A lithium secondary battery comprising a non-aqueous electrolyte, wherein part or all of the surface of the carbon material is coated with a binder. 2. A positive electrode comprising a lithium-containing composite oxide as an active material, a negative electrode comprising an active material comprising a carbon material capable of inserting and extracting lithium ions and a binder, a separator, and a separator impregnated therein. In the method for producing a lithium secondary battery comprising a non-aqueous electrolyte, the carbon material and the binder powder particles are mixed while applying a pressure and a shearing action to bind a part or all of the surface of the carbon material. A method for producing a lithium secondary battery, wherein the lithium secondary battery is coated with an agent powder particle. 3. The lithium secondary battery according to claim 1, wherein the separator and the binder are polymers of the same material. 4. The lithium secondary battery according to claim 3, wherein the polymer is a copolymer of vinylidene fluoride and hexafluoropropylene.