JPH07296816A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To enhance a charge and discharge cycle characteristic by using a polymer, which is composed chiefly of vinylidene fluoride crosslinked by the irradiation of an electron beam, as a binder which is contained in an anode mix and/or a cathode mix. CONSTITUTION:An anode comprises an anode active material held within an anode collector by a binder, and a cathode comprises a cathode active material held within a cathode collector by the binder. A polymer composed chiefly of vinylidene fluoride is used as the binder, and electron ray crosslinks are formed in the binder by the irradiation of an electron beam. The polymer contains a 70mol% or more vinylidene fluoride monomer and has an average degree of polymerization of 100-5000. Therefore, the active materials are so strongly held on the collector surfaces that they hardly peel off during repeated charging and discharging, resulting in an enhanced charge and discharge cycle characteristic.

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 improvement of a binder contained in an electrode mixture.

[0002]

2. Description of the Related Art In recent years, with the spread of portable devices such as video cameras and radio cassettes, there is an increasing demand for rechargeable secondary batteries instead of disposable primary batteries.

Most of the secondary batteries currently in use are nickel-cadmium batteries using an alkaline electrolyte. However, this battery has a low voltage and it is difficult to improve the energy density. There is also a drawback that the self-discharge rate is high.

Therefore, a non-aqueous electrolyte secondary battery using a light metal such as lithium for the negative electrode has been studied. This non-aqueous electrolyte secondary battery has the advantages of high energy density, low self-discharge, and light weight. However, in a non-aqueous electrolyte secondary battery using this lithium or the like as a negative electrode, when charge and discharge are repeated, metallic lithium or the like crystallizes in a dendrite form from the negative electrode and contacts the positive electrode, resulting in an internal short circuit. There is a possibility that it is difficult to put into practical use.

Therefore, a non-aqueous electrolyte secondary battery has been proposed in which lithium or the like is alloyed with another metal and this alloy is used for the negative electrode. However, this battery has a problem that the alloy forming the negative electrode becomes fine particles when the charge and discharge are repeated, and it is also difficult to put it into practical use.

Therefore, a non-aqueous electrolyte secondary battery using a carbonaceous material such as coke as a negative electrode active material has been proposed. This non-aqueous electrolyte secondary battery utilizes doping / dedoping of lithium ions into the carbon layer in the negative electrode reaction, and does not use metallic lithium or lithium alloy as a negative electrode active material. No precipitation or alloy atomization occurs. Therefore, good cycle characteristics can be obtained. Then, as the positive electrode active material, for example, L
The battery capacity is improved by using the lithium-transition metal composite oxide represented by i _x MO ₂ (M represents one kind or more than one kind of transition metal, and 0.05>x> 1.10.). Thus, a non-aqueous electrolyte secondary battery with high energy density can be obtained.

[0007]

In the non-aqueous electrolyte secondary battery as described above, for example, when a negative electrode is formed by using a carbonaceous material as a negative electrode active material, the carbonaceous material is powdered to obtain powdery carbon. A high-quality material is dispersed in a solvent together with a binder to prepare a negative electrode mixture coating material, which is applied to the negative electrode current collector. As a result, the negative electrode is formed such that the negative electrode active material is held on the surface of the negative electrode current collector by the binder. Similarly, for example, when a positive electrode is formed by using a lithium transition metal composite oxide as a positive electrode active material, this is powdered and the powdery lithium transition metal composite oxide is dispersed in a solvent together with a binder to form a positive electrode mixture paint. It is prepared and applied to a positive electrode current collector. As a result, the positive electrode is formed such that the positive electrode active material is held on the surface of the positive electrode current collector by the binder.

Conventionally, polyvinylidene fluoride has been used as an electrode binder for holding the active material on the current collector because of its excellent organic solvent resistance.

The object of the present invention is to further improve the adhesion to the electrode current collector and the retention of the electrode active material, while maintaining the characteristics of polyvinylidene fluoride, and thereby the non-excellent cycle characteristics It is intended to provide a water electrolyte secondary battery.

[0010]

The non-aqueous electrolyte secondary battery of the present invention is intended to solve the above problems and achieve the above objects.
A negative electrode in which a negative electrode mixture composed of a negative electrode active material and a binder is held by a negative electrode current collector, and a positive electrode in which a positive electrode mixture composed of a positive electrode active material, a conductive agent and a binder is held by a positive electrode current collector, In a non-aqueous electrolyte secondary battery comprising a water electrolyte, the binder contained in the negative electrode mixture and / or the positive electrode mixture is a polymer containing vinylidene fluoride as a main component, and an electron beam. It is characterized in that it is electron beam crosslinked by irradiation.

In the non-aqueous electrolyte secondary battery, the negative electrode is formed by holding the negative electrode active material on the negative electrode current collector by the binder, and the positive electrode is held on the positive electrode current collector by the binder. Composed of.

In the present invention, in order to improve the cycle characteristics of the battery, a polymer containing vinylidene fluoride as a main component is used as a binder for holding each of the negative electrode active material and the positive electrode active material on a current collector. In addition, electron beam cross-linking is formed in this polymer by electron beam irradiation.

In a positive electrode or a negative electrode in which a polymer containing polyvinylidene fluoride as a main component is used as a binder and electron beam cross-linking is formed on the polymer, the active material is firmly held on the surface of the current collector, and when repeating charging and discharging. The active material hardly peels off, and exhibits excellent charge / discharge cycle characteristics.

The polymer containing vinylidene fluoride as a main component is polyvinylidene fluoride or a copolymer of vinylidene fluoride and monomer units of the second and third components.
However, in this case, in order to maintain the characteristics of polyvinylidene fluoride such as resistance to organic solvents, it is necessary to set the monomer composition so that the content of the vinylidene fluoride monmer unit is at least 70 mol% or more.

The average degree of polymerization of the polymer containing vinylidene fluoride as a main component is preferably 100 to 5,000. If this degree of polymerization is less than 100, the cycle characteristics will be significantly deteriorated, and if it exceeds 5,000, the thixotropy of the electrode mixture coating material will be extremely large, making it difficult to apply it to the electrode current collector.

For electron beam cross-linking to a polymer containing vinylidene fluoride as a main component, for example, an electrode mixture paint prepared by dissolving an active material, a polymer and various additives in an organic solvent is applied to a current collector and dried. Then, an electrode coating film is formed, and the formed electrode coating film is irradiated with an electron beam.

The dose of this electron beam is 5 to 100 Mra.
It is preferably d. If it is less than 5 Mrad, crosslinking does not proceed and cycle characteristics are not improved. Conversely, 100 Mrad
If it is larger than the above range, decomposition progresses and cycle characteristics deteriorate.

As the negative electrode active material and the positive electrode active material retained by the above-mentioned cross-linking polymer, any of those usually used in this type of non-aqueous electrolyte secondary battery can be used.

The negative electrode active material is doped with lithium /
A carbon material that can be dedoped is used. For example, in addition to conductive polymers such as polyacetylene and polypyrrole, coke, polymer carbon, carbon fiber, etc., pyrolytic carbons and coke because of their large energy density per unit volume. Class (petroleum coke, pitch coke, coal coke, etc.), carbon black (acetylene black, etc.), glassy carbon, organic polymer material fired body (organic polymer material in an inert gas stream at an appropriate temperature of 500 ° C. or higher) , Or those fired in vacuum), carbon fibers, etc. are preferred.

On the other hand, examples of the positive electrode active material include transition metal oxides such as manganese dioxide and vanadium pentoxide, transition metal chalcogenides such as iron sulfide and titanium sulfide, and composite compounds of these with lithium. Can be used. In particular, high voltage and high energy density are obtained,
Lithium-cobalt composite oxide and lithium-cobalt-nickel composite oxide are preferable because they have excellent cycle characteristics.

The organic solvent used in the electrolytic solution is not particularly limited, but propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyl lactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran. , 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,
A single solvent or a mixture of two or more kinds of 3-dioxolane, diglymes, triglymes, sulfolane, dimethyl carbonate, diethyl carbonate, dipropyl carbonate and the like can be used.

Any known electrolyte may be used as the electrolyte.
Can be used, LiClO_Four, LiAsF ₆, LiPF₆, L
iBF4, LiB (C6 H5) 4, LiCl, LiB
r, CH₃SO₃Li, CF₃SO₃Li or the like is used
It

[0023]

In the non-aqueous electrolyte secondary battery of the present invention, a polymer containing at least vinylidene fluoride as a main component is used as a binder for holding the negative electrode active material and the positive electrode active material on the electrode current collector, and Electron beam crosslinks are formed in the polymer by electron beam irradiation. In the polymer in which electron beam cross-linking is formed in this manner, the active material holding power is enhanced by the cross-linking structure while maintaining excellent properties of polyvinylidene fluoride as a binder for electrodes. Therefore, in the positive electrode and the negative electrode using such a binder, the active material is firmly held on the surface of the current collector, the capacity deterioration due to repeated charging / discharging is small, and good charge / discharge cycle characteristics are exhibited.

[0024]

EXAMPLES Preferred examples of the present invention will be described based on experimental results. Table 1 shows the monomer constitution of vinylidene fluoride (polymer A to polymer E) used in the following experimental examples 1 to 10. In Table 1, VdF
Indicates vinylidene fluoride, and HFP indicates hexafluoropropylene.

[0025]

[Table 1]

Experimental Example 1 First, the negative electrode 1 was manufactured as follows. Weigh each paint component according to the paint composition below and use a ball mill to make 10
Mixed for hours. Negative electrode mixture coating composition Carbon (specific surface area; 8 m ² / g) 60 parts by weight Polymer A 5 parts by weight N-methyl-2-pyrrolidone 35 parts by weight

This negative electrode mixture coating material was applied on both surfaces of a copper foil having a thickness of 10 μm to serve as a negative electrode current collector with a coating thickness of 100 μm to prepare a strip-shaped negative electrode.

Next, the positive electrode 2 was produced as follows.

Each paint component was weighed according to the following paint composition and mixed in a ball mill for 10 hours. Positive electrode mixture coating composition LiCoO 2 (specific surface area; 0.35 m ² / g) 60 parts by weight carbon (specific surface area; 220 m ² / g) 5 parts by weight Polymer A 5 parts by weight N-methyl-2-pyrrolidone 30 parts by weight

This positive electrode mixture coating material was applied on both sides of an aluminum foil having a thickness of 20 μm to be a positive electrode current collector with a coating thickness of 100 μm to produce a strip positive electrode. The average degree of polymerization of the binder mixed with the negative electrode mixture paint and the positive electrode mixture paint was 1,000.
Is a polymer A obtained by copolymerization so that

Next, these belt-shaped negative electrodes and belt-shaped positive electrodes were irradiated with an electron beam irradiator at an irradiation energy (accelerating voltage) of 200 k.
The electron beam irradiation was performed under the conditions of V and electron beam irradiation amount of 50 Mrad. Then, these strip-shaped positive electrodes and strip-shaped negative electrodes were laminated with a 25 μm-thick polypropylene film serving as the separator 3 and wound many times to produce a spirally wound electrode body having an outer diameter of 18 mm. The electron beam irradiator is manufactured by ESI (Headquarters / Boston) in the United States, and its specifications are shown in Table 2.

[0032]

[Table 2]

Then, the spiral electrode body was housed in a nickel-plated iron battery can 5, and insulating plates 4 were placed above and below the spiral electrode body. Then, the aluminum positive electrode lead 12 was led out from the positive electrode current collector and welded to the battery lid 7, and the nickel negative electrode lead 11 was led out from the negative electrode current collector and welded to the battery can 5.

An electrolytic solution prepared by dissolving LiPF ₆ at a concentration of 1 mol / 1 in a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 was placed in a battery can 5 containing this spiral type electrode body. Was injected. Then, the safety valve device 8 having a current cutoff mechanism and the battery lid 7 are fixed to the battery can 5 by caulking with an insulating sealing gasket 6 whose surface is coated with asphalt, and are fixed to a cylindrical non-shaped cylinder having a diameter of 18 mm and a height of 65 mm. A water electrolyte secondary battery was created.

Experimental Example 2 to Experimental Example 10 A battery was prepared in the same manner as in Experimental Example 1 except that the polymer shown in Table 3 was used instead of the polymer A.

The non-aqueous electrolyte secondary battery produced as described above was charged at room temperature under the conditions of maximum charging voltage 4.2V and charging current 1A for 2.5 hours to obtain a constant resistance of 6.2Ω. The change of discharge capacity was observed by repeating the charge and discharge cycle such as discharging at 5 ℃, and the discharge capacity was 5% of the initial capacity.
Number of cycles down to 0% (50% capacity number of cycles)
I checked. The results are shown in Table 3 together with the polymer species used.

[0037]

[Table 3]

In Table 3, in particular, the non-aqueous electrolyte secondary batteries of Experimental Examples 1 to 5 had a 50% capacity cycle number as compared with the battery of Experimental Example 10 which was not subjected to electron beam irradiation. Also showed a large value. From this, it is effective to use a polymer containing vinylidene fluoride as a main component as a binder of an electrode and to cross-link the polymer by electron beam irradiation by electron beam irradiation is effective in improving charge-discharge cycle characteristics of a battery. Recognize.

However, in the battery of Experimental Example 6 in which the average polymerization degree of polyvinylidene fluoride was 50, the 50% capacity cycle number was as small as 425, while the average polymerization degree was 60.
In the battery of Comparative Example 2 of polymer No. 00, the paint was so viscous that application of the paint was impossible. From this, it is understood that the average degree of polymerization of polyvinylidene fluoride is suitably 100 to 5000.

Also, using Polymer A, 50 Mrad
The battery of Experimental Example 1 irradiated with an electron beam showed a very large value of 980 with a 50% capacity cycle number. However, even if the same polymer A was used, the battery of Experimental Example 4 irradiated with 10 Mrad electron beam was 50%. The number of capacity cycles was 875, which was slightly small. On the other hand, the 50% capacity cycle number was 823, which was slightly small, in the battery of Experimental Example 5 which emitted 100 Mrad electron beam. Also, using the same polymer A, 5Mr
The battery of Experimental Example 8 which emitted ad electron beam had a smaller 50% capacity cycle number of 513, and the battery of Experimental Example 9 which emitted 150 Mrad electron beam had a smaller 50% capacity cycle number of 437. From this, it is understood that the electron beam irradiation amount is suitably 5 to 100 Mrad.

[0041]

According to the present invention, a negative electrode in which a negative electrode mixture composed of a negative electrode active material and a binder is held by a negative electrode current collector,
A non-aqueous electrolyte secondary battery comprising a positive electrode in which a positive electrode mixture composed of a positive electrode active material, a conductive agent and a binder is held by a positive electrode current collector, and a non-aqueous electrolyte secondary battery comprising: Or, the binder contained in the positive electrode mixture is a polymer containing vinylidene fluoride as a main component, and electron beam cross-linking is formed on the polymer, so that
It is possible to obtain a non-aqueous electrolyte secondary battery exhibiting excellent charge / discharge cycle characteristics, in which the active material hardly peels off from the current collector during repeated charge / discharge cycles.

[Brief description of drawings]

FIG. 1 is a schematic vertical cross-sectional view showing the configuration of a non-aqueous electrolyte secondary battery to which the present invention has been applied.

[Explanation of symbols]

 1 Negative electrode 2 Positive electrode 3 Separator 4 Insulating plate 5 Battery can 6 Insulation sealing gasket 7 Battery lid 8 Safety valve device

Claims (5)

[Claims] 1. A negative electrode formed by holding a negative electrode mixture composed of a negative electrode active material and a binder on a negative electrode current collector, and a positive electrode mixture formed of a positive electrode active material, a conductive agent and a binder held on a positive electrode current collector. In a non-aqueous electrolyte secondary battery comprising a positive electrode and a non-aqueous electrolyte solution, the binder contained in the negative electrode mixture and / or the positive electrode mixture is a polymer containing vinylidene fluoride as a main component. And a non-aqueous electrolyte secondary battery, which is cross-linked by electron beam irradiation. 2. The non-aqueous electrolyte solution according to claim 1, wherein the negative electrode active material is a carbonaceous material capable of being doped and dedoped with lithium, and the positive electrode active material is a lithium-transition metal composite oxide. Secondary battery. 3. The non-aqueous electrolyte solution according to claim 1, wherein the polymer containing vinylidene fluoride as a main component is a copolymer containing 70 mol% or more of vinylidene fluoride monomer. Secondary battery. 4. The non-aqueous electrolyte secondary battery according to claim 3, wherein the polymer containing vinylidene fluoride as a main component has an average degree of polymerization of 100 to 5,000. 5. An electron beam cross-link is formed on the vinylidene fluoride by irradiating the electrode coating film made of a polymer containing vinylidene fluoride as a main component and an active material with an electron beam, and the electron irradiating the electrode coating film. The non-aqueous electrolyte secondary battery according to claim 4, wherein the radiation dose is 5 to 100 Mrad.