JP2000021408A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the charging and discharging cycle lifetime characteristics and stability of a nonaqueous electrolyte secondary battery, using the a manganese compound oxide for positive electrode. SOLUTION: This secondary battery has a positive electrode, including the lithium manganese compound oxide expressed with formula Li(1+x)Nm(2-x)MO4 (0<=x<=0.2, M is at least one or more kinds of metallic elements except for Mn, 0.01×(2-x)<=M<=0.2×(2-x)) in the mix, a negative electrode including the material capable of repeating the doping and de-doping of the lithium with charge and discharge in the mix, and the nonaqueous electrolyte, in which lithium ion can be moved. In this case, the mix of the positive electrode or the negative electrode includes the conductive material and the binder, and the binder includes thermoplastic resin having a carboxyl group or an amide group, and this resin is bridged by the cross-linking agent.

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 secondary battery, and more particularly to an improvement in a binder for an electrode mixture for the purpose of increasing its capacity and improving cycle life characteristics.

[0002]

2. Description of the Related Art In recent years, the performance of electronic devices has been improved due to advances in electronic technology, miniaturization and portability have progressed, and high energy density batteries have been desired as power sources. Conventional secondary batteries include a lead storage battery and a nickel cadmium battery, but they are still insufficient in obtaining a battery having a high energy density. Therefore, as an alternative to these batteries, non-aqueous electrolyte secondary batteries with a high energy density have been developed and are rapidly becoming popular. General non-aqueous electrolyte secondary batteries have a lithium composite metal oxide such as a lithium-cobalt composite oxide for the positive electrode and lithium for the negative electrode. Using a small amount of carbon material, these and a binder were dispersed in N-methyl-2-pyrrolidone (NMP) to form a slurry, and the mixture was coated on both sides of a metal foil as a current collector, and the solvent was dried. Thereafter, the positive and negative electrode plates are obtained by compression molding with a roller press. The binder is mainly polyvinylidene fluoride (PVD)
F) is frequently used. However, when polyvinylidene fluoride is used as the binder, the adhesion at the interface between the current collector and the mixture and the adhesion between the material particles in the mixture are inferior. In such a case, a part of the mixture is peeled or dropped off from the current collector, which causes a minute short circuit or a variation in battery capacity. In addition, since the carbon material of the negative electrode expands and contracts in particular due to repeated charge and discharge, the mixture is peeled or dropped from the current collector, and the adhesion between the material particles in the mixture is reduced, so that the current collection efficiency is reduced And the reaction with lithium is not uniform, which causes a problem that the battery capacity is gradually reduced. Further, as described in JP-A-6-172452, a vinylidene fluoride copolymer obtained by copolymerizing a monomer containing vinylidene fluoride as a main component and an unsaturated dibasic monoester is used. When the union is used as a binder, the adhesion strength at the interface between the current collector and the union is improved, but the vinylidene fluoride copolymer is decomposed due to an abnormal temperature rise under a high voltage, and the fluoropolymer is decomposed. Hydrogen was generated and reacted with the lithium intercalation compound (GIC) on the surface of the negative electrode plate or precipitated metallic lithium, causing abnormal heat generation, which could cause the battery to burst or explode.

Further, examples of the binder other than the fluororesin-based resins include synthetic rubbers such as acrylic rubber and styrene-butadiene rubber, and thermosetting resins such as epoxy resins.
These are used alone, dissolve or largely swell in the electrolytic solution, and cannot maintain the adhesion at the interface between the current collector and the mixture and the adhesion between the material particles in the mixture for a long period of time. Due to the high transition temperature, it takes time to remove water and N-methyl-2-pyrrolidone adsorbed or remaining in the mixture. When using a thermosetting resin,
In order to cure this, it is necessary to heat at a temperature of 180 ° C. or higher. If the curing is insufficient, the resistance to the electrolytic solution is significantly reduced as described above. Diene-based synthetic rubbers have resistance to electrolytes, but they are very difficult to disperse evenly in the mixture, so when cellulose and surfactants are added and dispersed, they dissolve in the electrolyte. Decrease the charging and discharging efficiency of the battery.

[0004]

The problem to be solved by the present invention is to improve the adhesion at the interface between the current collector and the mixture and the adhesion between the material particles in the mixture to achieve a fine short circuit. In addition to reducing battery capacity fluctuations and reducing the amount of binder added, the battery capacity is increased, the battery capacity is reduced due to charge / discharge cycles, and the battery ruptures even if the battery temperature rises abnormally. -To provide a highly safe non-aqueous electrolyte secondary battery that is not likely to explode.

[0005]

According to the present invention, there is provided a compound of the general formula Li
_{(1 + x)} Mn _(2-x) MO ₄ (0 ≦ x ≦ 0.2, M is at least one metal element other than Mn and 0.01 × (2-x) ≦ M ≦ 0.2 × (2-x) )
A positive electrode containing lithium manganese composite oxide in the mixture, a negative electrode containing a substance capable of repeatedly doping and undoping lithium by charge and discharge in the mixture, and a non-aqueous solution capable of moving lithium ions. A non-aqueous electrolyte secondary battery having an electrolyte solution, comprising a conductive material and a binder in the mixture of the positive electrode or the negative electrode, the binder comprises a thermoplastic resin having a carboxyl group or an amide group. The resin is cross-linked by a cross-linking agent, the content of the binder is 1 to 15% by weight in the mixture, and the cross-linking agent is a polyisocyanate compound, an epoxy resin, melamine. It is at least one of a resin and a metal chelate, and is contained in an amount of 0.1 to 10 parts by weight based on 100 parts by weight of the thermoplastic resin.

[0006] Since the binder has a carboxyl group or an amide group, it has excellent adhesion to a metal foil as a current collector, and the mixture is separated from the current collector even after repeated charge and discharge. -It is possible to improve battery capacity reduction due to charge / discharge cycles without falling off. In addition, by being cross-linked with a cross-linking agent, it has excellent resistance to electrolytes and heat, and even when used at high temperatures, adheres to the interface between the current collector and the mixture for a long period of time and interacts with the material particles in the mixture. The adhesion strength between them can be maintained. When the adhesion at the interface between the current collector and the mixture and the adhesion strength between the material particles in the mixture are improved, the amount of the binder in the mixture can be reduced, and as a result, the amount of the active material can be increased. As a result, the battery volume energy density can be increased. Furthermore, since fluorine is not contained in the molecules of the binder, the battery does not decompose even if the battery temperature rises abnormally, and there is no fear of the battery exploding or exploding, so that the safety is high. Furthermore, since a binder is present so as to cover a part of the surface of the lithium manganese composite oxide particles of the positive electrode, the amount of Mn eluted from the positive electrode can be reduced, and the electron conductivity of the positive electrode is secured, while the elution is performed. Since the deterioration of the negative electrode due to the Mn can also be suppressed, a decrease in battery capacity due to a charge / discharge cycle can be improved.

Examples of the thermoplastic resin as the binder include C4-C12 alkyl esters of acrylic acid and / or methacrylic acid such as isobutyl acrylate, octyl acrylate, nonyl acrylate, lauryl acrylate, butyl methacrylate and 2-ethylhexyl methacrylate, and methacrylic acid. Copolymers with unsaturated monomers having a carboxyl group or amide group functional group such as acid, itaconic acid, maleic acid, fumaric acid, polyacrylic acid such as acrylamide and methacrylamide, and polyamides, polyamideimides, and polyamidebis Examples thereof include polyesters such as maleimide, polybutylene terephthalate, and polyethylene terephthalate. These may be used alone or in combination.

Examples of the crosslinking agent include an epoxy resin, a melamine resin, and a metal chelate in addition to a polyisocyanate compound.
Tolylene diisocyanate, hexamethylene isocyanate, isophorone diisocyanate and derivatives thereof can be used.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Preparation of binder 90 parts by weight of n-butyl acrylate, 10 parts by weight of acrylic acid, 0.3 parts by weight of azobisisobutylnitrile and 100 parts by weight of ethyl acetate are mixed. 30% of this solution is thermometer,
Place in a flask equipped with a stirrer, burette and reflux device and heat to 80 ° C. with stirring. 1 at 80 ° C
After stirring for an hour, stirring is continued and the remaining solution is added slowly over 1 hour. After all the solution was added, stirring and reflux were continued for another 2 hours to produce an acrylic copolymer having a carboxyl functional group.

[0010] 2. Preparation of Positive Electrode A lithium-manganate having an average particle diameter of 10 μm, a conductive material of carbon powder having an average particle diameter of 3 μm, a binder of an acrylic copolymer having a carboxyl group or an amide group, and a cross-linking agent of a polyisocyanate compound were used. The mixture is added to and mixed with methyl-2-pyrrolidone to prepare a slurry mixture solution. 20μm thickness
Apply this mixture solution to both sides of the aluminum foil of
Curing (crosslinking of the binder), rolling, and then cutting to a width of 54 mm produced a strip-shaped positive electrode sheet. In this embodiment, Li
_Although lithium manganate having a composition of _1.107 Mn _1.893 O ₄ was used, any transition metal oxide capable of reversibly inserting and releasing lithium ions may be used. Li, V, Cr, Fe, C
at least one selected from o, Ni, Mo, W, Zn, B, Mg
A lithium manganese composite oxide in which a manganese site or a lithium site is substituted with at least one kind of metal may be used.

3. Preparation of Negative Electrode A negative carbon material of amorphous carbon having an average particle diameter of 20 μm, a binder of an acrylic copolymer having a carboxyl group or an amide group, and a crosslinking agent of a polyisocyanate compound were converted into N-methyl-2-pyrrolidone. The mixture is charged and mixed to prepare a slurry mixture solution. This mixture solution is applied to both sides of a 10 μm thick copper foil, dried, cured (crosslinking of binder), rolled, and then
It was cut to a width of 56 mm to produce a strip-shaped negative electrode sheet. The negative electrode carbon material may be pitch coke, petroleum coke, graphite, carbon fiber, activated carbon, or a mixture thereof.

4. Preparation of Electrolyte Solution 1 m of electrolyte LiPF ₆ was added to an organic solvent in which propylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 1.
The solution was dissolved at a concentration of ol / l to prepare an electrolytic solution. As the organic solvent, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl carbonate, γ-butyl lactone, tetrahydrofuran, diethyl ether, sulfolane, acetonitrile, etc. alone or among these Two or more mixed solvents can be used, and the electrolyte is LiClO ₄ , LiPF
6, LiPF ₄ , LiBF ₄ , LiCl, LiBr, CH ₃ SO ₃ Li, Li
AsF ₆ or the like can be used.

4. Preparation of the battery The positive electrode and the negative electrode prepared above were wound around a separator made of a microporous polyethylene film having a thickness of 25 μm and a width of 58 mm,
A spiral wound group is prepared. The wound group is inserted into a battery can, and a nickel tab terminal previously welded to the copper foil of the negative electrode current collector is welded to the bottom of the battery can. 5 ml of the electrolyte was injected into the battery container. Next, an aluminum tab terminal previously welded to the aluminum foil of the positive electrode current collector was welded to the lid,
The lid is placed on top of the battery can via an insulating gasket, this part is caulked and sealed, diameter 18 mm, height 65 m
m was manufactured.

[0014]

EXAMPLE FIG. 1 shows the adhesion strength of negative electrode plates having different addition amounts of a crosslinking agent of a polyisocyanate compound to a binder of an acrylic copolymer. As the amount of the cross-linking agent increases, the adhesion of the interface between the current collector and the mixture and the reduction in the adhesion strength between the material particles in the mixture, and the decrease in charge / discharge efficiency due to the excessive cross-linking agent remaining, Problems such as the gelling of the binder start at an early stage occur. In addition, when the amount of the crosslinking agent added is small, the crosslinking density of the binder is reduced, and as a result, the electrolyte resistance is poor.
Therefore, it is desirable to add 0.1 to 10 parts by weight, preferably 0.5 to 5 parts by weight, based on 100 parts by weight of the binder.

Next, FIG. 2 shows the adhesion strength of the negative electrode plates with different addition amounts of the binder of the acrylic copolymer to the negative electrode mixture. As the amount of binder added increases, the adhesion strength increases. The amount of the binder added is 1 to 15% by weight, preferably 3 to 10% by weight. Even if 15% by weight or more of the binder is added to the negative electrode mixture, the adhesion strength is not changed, but the volume energy density is rather lowered.

The prepared battery was charged at a constant current of 1400 mA and a limit voltage of 4.2 V, and then discharged at a discharge current of 280 mA until the discharge end voltage reached 2.5 V to measure the initial capacity. With this charge / discharge as one cycle, the same charge / discharge was repeated at an ambient temperature of 50 ° C. until the capacity reached 70% or less of the initial capacity. The results are shown in FIG. The non-aqueous electrolyte secondary battery using the binder of the present invention maintains excellent adhesion between the current collector and the mixture layer interface and between the mixture layers. No decrease is observed.

[0017]

According to the non-aqueous electrolyte secondary battery of the present invention,
Because the binder has a carboxyl group or an amide group, it has excellent adhesion to the metal foil that is the current collector, and the mixture does not peel off or fall off the current collector even after repeated charge and discharge. Battery capacity reduction due to charge / discharge cycles can be improved. In addition, by being cross-linked with a cross-linking agent, it has excellent resistance to electrolytes and heat, and even when used at high temperatures, adheres to the interface between the current collector and the mixture for a long period of time and interacts with the material particles in the mixture. The adhesion strength between them can be maintained. When the adhesion at the interface between the current collector and the mixture and the adhesion strength between the material particles in the mixture are improved, the amount of the binder in the mixture can be reduced, and as a result, the amount of the active material can be increased. As a result, the battery volume energy density can be increased. Furthermore, since fluorine is not contained in the molecules of the binder, it does not decompose even if the battery temperature rises abnormally,
High safety with no risk of the battery exploding. Furthermore, since a binder is present so as to cover a part of the surface of the lithium manganese composite oxide particles of the positive electrode, the amount of Mn eluted from the positive electrode can be reduced, and the electron conductivity of the positive electrode is secured, while the elution is performed. Since the deterioration of the negative electrode due to the Mn can also be suppressed, a decrease in battery capacity due to a charge / discharge cycle can be improved.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the ratio of a crosslinking agent to a binder used in this example and the adhesive strength of a negative electrode.

FIG. 2 is a diagram showing a relationship between a ratio of a binder in a negative electrode mixture used in the present example and a negative electrode adhesion strength.

FIG. 3 is a diagram showing a cycle life test result of the nonaqueous electrolyte secondary battery of the present example.

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Claims (3)

[Claims] (1) The general formula Li _{(1 + x)} Mn _(2-x) MO ₄ (0 ≦ x ≦ 0.2,
M is at least one metal element other than Mn and is 0.01 × (2
-x) ≦ M ≦ 0.2 × (2-x)) A positive electrode containing a lithium manganese composite oxide in the mixture and a substance capable of repeating doping and undoping of lithium by charge and discharge in the mixture A non-aqueous electrolyte secondary battery having a negative electrode and a non-aqueous electrolyte capable of moving lithium ions, wherein the mixture of the positive electrode or the negative electrode contains a conductive material and a binder, and A non-aqueous electrolyte secondary battery, wherein the adhesive comprises a thermoplastic resin having a carboxyl group or an amide group, and the resin is cross-linked by a cross-linking agent. 2. The content of the binder is 1 to 3 in the mixture.
The non-aqueous electrolyte secondary battery according to claim 1, wherein the content is 15% by weight. 3. The crosslinking agent is at least one of a polyisocyanate compound, an epoxy resin, a melamine resin, and a metal chelate, and is contained in an amount of 0.1 to 10 parts by weight based on 100 parts by weight of the thermoplastic resin. The non-aqueous electrolyte secondary battery according to claim 1.