JPH10199536A - Electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform molding work easily in a process of press and the like and improve productivity of a battery and improve a battery life without causing any cracking and peeling off in an electrode mix layer. SOLUTION: In a lithium ion secondary battery formed of a positive electrode which has active material for storing and releasing the lithium ion and a negative electrode which has active material for storing and releasing he lithium ion, a polymer of chlorotrifluoroethylene and vinylidene fluoride in which content of chlorotrifluoroethylene is 8-20wt.% is used as binding agent used for the positive electrode and/or the negative electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte lithium ion secondary battery, and more particularly to a binder used for bonding an active material or the like to a current collector.

2. Description of the Related Art In recent years, miniaturization and weight reduction of electronic devices have been remarkable, and with this, high performance has been achieved for batteries as power sources.
Miniaturization and weight reduction are required. Among them, lithium ion secondary batteries are regarded as promising as high performance batteries with high energy density. In general, a lithium ion secondary battery is prepared by mixing an active material and a conductive auxiliary, forming a slurry using the mixture and a solvent in which a binder is dissolved in advance, and then forming a slurry on a metal foil as a current collector. The battery is applied, dried, pressed, and subjected to slitting, winding, and other steps to form a battery.

Examples of the binder used in this case include conventionally known Teflon, polyethylene, nitrile rubber, polybutadiene, butyl rubber, polystyrene, styrene / butadiene rubber, polysulfide rubber, nitrocellulose, cyanoethylcellulose, various latexes and the like. Examples thereof include polymers such as acrylonitrile, vinyl fluoride, vinylidene fluoride, propylene fluoride, and chloroprene fluoride, and mixtures thereof. Among them, Japanese Unexamined Patent Publication No. 4-95363 discloses that a copolymer of chlorotrifluoroethylene and vinylidene fluoride is mainly used as a binder having rubber elasticity and heat resistance temperature of 200 ° C. or more. There has been disclosed a technique for improving the charge-discharge cycle characteristics by improving the peeling off from the current collector or the like by using a fluorine rubber.

[0003]

When a vinylidene fluoride resin having almost no rubber elasticity among vinylidene fluoride polymers is used as a binder, the resin is rigid, so that an active material, a conductive agent and a binder are used. The electrode mixture layer portion composed of the above becomes hard, and cracks or peeling occurs in the electrode mixture layer portion in a processing step such as pressing, slitting, or winding, which causes a problem that productivity is reduced. In addition, if the electrode mixture layer portion peels off and falls off inside the battery can, various problems such as a decrease in battery capacity and a decrease in current collecting performance occur.

According to the present invention, a flexible electrode mixture layer is formed by using a specific vinylidene fluoride copolymer as a binder, so that the molding process can be easily performed in a process such as pressing, and the electrode mixture can be easily formed. It is an object of the present invention to improve the productivity of the battery and the battery life without causing any cracks or peeling in the layer.

SUMMARY OF THE INVENTION The present invention provides a lithium ion secondary battery comprising a positive electrode having an active material capable of storing and releasing lithium ions and a negative electrode having an active material capable of storing and releasing lithium ions. In the positive electrode and / or
Alternatively, the active material binder of the negative electrode is a copolymer of chlorotrifluoroethylene and vinylidene fluoride, and the content of chlorotrifluoroethylene is 8 to 20% by weight. The trifluoroethylene content is from 10 to 15% by weight.

The copolymer of the present invention has almost no rubber elasticity and has a property that the melting temperature is less than 200 ° C. Since the use of this copolymer makes the electrode mixture layer flexible, This has the effect of avoiding cracks, peeling and falling off of the electrode mixture layer in working steps such as pressing, slitting and winding and in a battery can.

If the chlorotrifluoroethylene content is less than 8% by weight, the rigidity characteristic of vinylidene fluoride resin is increased, so that the electrode mixture layer becomes hard and cracks and peeling occur during bending. On the other hand, when the content exceeds 20% by weight, the binder is significantly dissolved in the electrolytic solution, so that the active material and the conductive agent are easily peeled off and dropped off from the electrode after assembling the battery. In the practice of the present invention, the positive electrode active material, the negative electrode active material, the conductive auxiliary, the electrolytic solution, and the like are not particularly limited, and conventionally known materials can be used.

Any positive electrode active material may be used as long as it can store and release lithium ions. For example, lithium composite oxide _{Li x M Y N z O 2} (M represents at least one transition metal, N represents represents at least one non-transition metal,
X, Y, and Z are respectively 0.05 <X <1.10, 0.85 <
It has a number of Y ≦ 1.00, 0 ≦ Z <0.10, M represents one of cobalt, nickel, manganese and other transition metals or a mixture thereof, and N represents at least one of Al, In and Sn. Represents ). Any negative electrode active material may be used as long as it can store and release lithium ions. Examples thereof include carbon materials such as pyrolytic carbons, cokes (pitch coke, needle coke, petroleum coke, etc.), graphites, glassy carbons, organic polymer compound fired bodies, carbon fibers, activated carbon, and the like.

[0008] The conductive auxiliary agent is activated carbon, various coke,
Examples include carbon materials such as acetylene black, natural and artificial graphites, and the like. Organic solvents as the electrolyte include, for example, ethers, ketones, lactones, nitriles, amines, amides, sulfur compounds, halogenated hydrocarbons, esters, carbonates, nitro compounds, phosphate esters Compounds, sulfolane compounds and the like can be used, and among them, ethers, ketones, esters, lactones, halogenated hydrocarbons,
Carbonates and sulfolane compounds are preferred. Examples of these include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, anisole, monoglyme, 4-methyl-2-pentanone, ethyl acetate, methyl acetate, ethyl propionate, 1,2 dichloroethane, γ-butyrolactone, dimethoxyethane, Methyl formate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, vinylene carbonate, dimethylformamide, dimethyl sulfoxide,
Examples thereof include dimethylthioformamide, sulfolane, 3-methyl-sulfolane, trimethyl phosphate, triethyl phosphate, and a mixed solvent thereof. Examples of the electrolyte used for the electrolyte include LiClO ₄ , LiBF ₄ and LiA.
sF ₆ , LiPF _{6 and the} like, and mixtures thereof.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to examples and comparative examples.

【Example】

Example 1 A positive electrode was manufactured by first adding 10 parts of LiCoO ₂ as a positive electrode active material.
0 g, 5 g of graphite as a conductive agent, and 3 g of a copolymer of chlorotrifluoroethylene and vinylidene fluoride containing 15% by weight of chlorotrifluoroethylene as a binder were dispersed in N-methyl-2-pyrrolidone. Slurried. Next, this positive electrode mixture slurry was
After hand-coating with an applicator so as to be uniform on one side of an aluminum sheet of μm, it is dried in an oven at 120 ° C., and the dried electrode is cut into a size of 10 cm in width and 15 cm in length, using a roll press. 40kg / cm ² G
5 times. Example 2 10% by weight of chlorotrifluoroethylene as a binder
An electrode was produced in the same manner as in Example 1, except that the contained copolymer of chlorotrifluoroethylene and vinylidene fluoride was used.

Example 3 A negative electrode was prepared by first using 100% of carbon as a negative electrode active material.
g, 3 g of a copolymer of chlorotrifluoroethylene and vinylidene fluoride containing 15% by weight of chlorotrifluoroethylene as a binder was dispersed in N-methyl-2-pyrrolidone to form a slurry. Next, this negative electrode mixture slurry is applied by hand with an applicator so as to be uniform on one surface of a copper foil sheet having a thickness of 10 μm, and then dried in an oven at 120 ° C., and the dried electrode is 10 cm wide and 10 cm long. 1
Cut to a size of 5 cm and use a roll press machine
Pressing was performed 5 times at a pressure of kg / cm ² G.

Comparative Example 1 An electrode was produced in the same manner as in Example 1 except that a copolymer of chlorotrifluoroethylene containing 6% by weight of chlorotrifluoroethylene and vinylidene fluoride was used as a binder. . Comparative Example 2 22% by weight of chlorotrifluoroethylene as a binder
An electrode was produced in the same manner as in Example 1, except that the contained copolymer of chlorotrifluoroethylene and vinylidene fluoride was used.

With respect to the pressed electrodes obtained in the above Examples and Comparative Examples, the adhesion of the electrode mixture layer to the current collector (R evaluation) and the immersion in an ethylene carbonate solution (60 ° C.) (10 days) Was performed to evaluate the solvent resistance. In the R evaluation, the metal foil as an electrode current collector was pressed against a metal plate having a certain thickness and R-processed on one side so that the metal foil as the electrode current collector was on the inside, that is, the electrode mixture layer was on the outside, and the R-processed portion of the metal plate was pressed. At 180
The presence or absence of cracks or peeling was evaluated for the state of the electrode mixture layer at the bent and bent portions.

[0013]

[Table 1]

[0014]

According to the present invention, a lithium ion electrode having a flexible electrode mixture layer can prevent cracking and peeling in working steps such as pressing, slitting and winding, thereby improving battery productivity and battery life. An electrode for a secondary battery can be provided.

Claims (1)

[Claims] 1. A lithium ion secondary battery comprising a positive electrode having an active material capable of storing and releasing lithium ions and a negative electrode having an active material capable of storing and releasing lithium ions, wherein a positive electrode and / or a negative electrode are provided. An electrode for a lithium ion secondary battery, wherein the binder used is a copolymer of chlorotrifluoroethylene and vinylidene fluoride, and the content of chlorotrifluoroethylene is 8 to 20% by weight.