JPH07201316A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To provide a nonaqueous electrolyte secondary battery in which electrode active materials scarcely peel out of current collectors even after charge/ discharge cycles are repeated and which has excellent charge/discharge cycles. CONSTITUTION:Regarding a nonaqueous electrolyte secondary battery provided with a negative electrode 1, a positive electrode 2, and a non-aqueous electrolytic solution, wherein the negative electrode is prepared by keeping a negative electrode mix consisting of a negative electrode active material and a binder in a negative electrode current collector and the positive electrode is prepared by keeping a positive electrode mix consisting of a positive electrode active material and a binder in a positive electrode current collector; polyfluorovinylidene having hydrophilic polar group is used as at least a part of the binder to be contained in the nagative electrode mix and the positive electrode mix.

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 conductive agent and a binder to mix the positive electrode. An agent paint is prepared and applied to the 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. However, this polyvinylidene fluoride has insufficient adhesion to the electrode current collector and holding power of the electrode active material, and in a conventional battery using this as a binder for an electrode, the active material becomes the current collector when repeatedly charged and discharged. There is a problem in that it peels off more, and the capacity drops to 50% after about 500 times of charging and discharging.

Therefore, the present invention has been proposed in view of such conventional circumstances, and realizes an electrode binder excellent in adhesion to an electrode current collector and retention of an electrode active material, which is excellent. It is an object of the present invention to provide a non-aqueous electrolyte secondary battery that exhibits excellent cycle characteristics.

[0010]

In order to achieve the above-mentioned object, the present inventors modified polyvinylidene fluoride by various modifications, and by modifying polyvinylidene fluoride with a hydrophilic polar group, an electrode assembly was obtained. We have come to the knowledge that the adhesion to electric materials and the retention to electrode active materials are improved.

The non-aqueous electrolyte secondary battery of the present invention has been completed on the basis of the above findings, and the negative electrode mixture composed of the negative electrode active material and the binder is held on the negative electrode current collector. A non-aqueous electrolyte secondary battery comprising a negative electrode, a positive electrode in which a positive electrode mixture composed of a positive electrode active material, a conductive agent and a binder is held on a positive electrode current collector, and a non-aqueous electrolyte secondary battery comprising: The binder contained in the agent and the positive electrode mixture is characterized by being polyvinylidene fluoride having a hydrophilic polar group.

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 by the positive electrode active material 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, polyfluoride having a hydrophilic polar group is used as a binder for holding the negative electrode active material and the positive electrode active material on the respective current collectors. Vinylidene will be used.

Polyvinylidene fluoride having a hydrophilic polar group is a polymer composed of a polyvinylidene fluoride monomer unit and a monomer unit having a hydrophilic polar group as shown in Chemical formula 1, and is excellent in organic solvent resistance and Excellent adhesion to the electrode current collector and retention of the electrode active material.

[0015]

[Chemical 1]

Therefore, in a battery in which the electrode active material is held on the current collector, the electrode active material does not peel off from the current collector even after repeated charging and discharging, and good charge / discharge cycle characteristics are obtained. To be

In the polyvinylidene fluoride represented by Chemical formula 1, the content of the monomer unit having a hydrophilic polar group is preferably 0.01 to 0.5 mol%. When the content of the monomer unit having a hydrophilic polar group is less than 0.01 mol%, further improvement in dispersibility cannot be obtained and cycle characteristics cannot be sufficiently enhanced. On the other hand, if the content of the monomer unit having a hydrophilic polar group exceeds 0.5 mol%, the thixotropy of the electrode material mixture coating material becomes very large, and it is difficult to apply it to the current collector.

As the hydrophilic polar group, --SO
₃ M, -OSO ₃ M, -COOM, OPO ₃ M (however,
In each hydrophilic polar group, M represents an alkali metal), as well as an amine-based polar group represented by Chemical formula 2 and Chemical formula 3 (however, in Chemical formula 2 and Chemical formula 3, R ₂ and R ₃ represent alkyl groups) Etc.

[0019]

[Chemical 2]

[0020]

[Chemical 3]

The degree of polymerization of the polyvinylidene fluoride having a hydrophilic polar group is preferably 300-5000. If this degree of polymerization is less than 300, 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, and it will be difficult to apply it to the electrode current collector.

The polyvinylidene fluoride contains not only a vinyl fluoride monomer unit and a monomer unit having a hydrophilic polar group, but also a third component and a fourth component for the purpose of improving solubility in a solvent. It doesn't matter. However, in this case, in order to maintain the characteristics of polyvinylidene fluoride such as resistance to organic solvents, it is necessary to design the monomer composition so that the content of the vinyl fluoride monomer unit is at least 70 mol% or more.

Polyvinylidene fluoride having such a hydrophilic polar group can be obtained, for example, by copolymerizing vinylidene fluoride with a vinyl monomer which is copolymerizable with the vinylidene fluoride and has a hydrophilic polar group. can get. Thus, as the vinyl monomer copolymerizable with vinylidene fluoride and having a hydrophilic polar group,
Chemical formula 4 to chemical formula 12 (wherein, in chemical formula 4 to chemical formula 12, R ₁ represents an alkyl group and Y represents a hydrophilic polar group, respectively).

[0024]

[Chemical 4]

[0025]

[Chemical 5]

[0026]

[Chemical 6]

[0027]

[Chemical 7]

[0028]

[Chemical 8]

[0029]

[Chemical 9]

[0030]

[Chemical 10]

[0031]

[Chemical 11]

[0032]

[Chemical 12]

As the negative electrode active material and the positive electrode active material dispersed in polyvinylidene fluoride having a hydrophilic polar group as described above, any of those usually used in non-aqueous electrolyte secondary batteries of this type is used. Can also be used.

As the negative electrode active material, a carbon material capable of being doped / dedoped with lithium or the like is used. For example, a conductive polymer such as polyacetylene or polypyrrole, or coke, polymer charcoal, carbon fiber, or the like, per unit volume Because of its high energy density, pyrolytic carbons, cokes (petroleum coke, pitch coke, coal coke, etc.), carbon black (acetylene black, etc.), glassy carbon, organic polymer material fired body (organic polymer material) Is fired at a suitable temperature of 500 ° C. or higher in an inert gas stream or in vacuum), carbon fiber, etc.

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 can be used as the electrolyte. LiClO ₄ , LiAsF ₆ , LiPF ₆ , L
iBF ₄ , LiB (C ₆ H ₅ ) ₄ , LiCl, LiB
r, CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, etc. are used.

[0038]

[Function] Polyvinylidene fluoride having a hydrophilic polar group is excellent in adhesion to the electrode current collector and retention of the electrode active material. Therefore, in a non-aqueous electrolyte secondary battery using this as a binder for electrodes, the electrode active material is firmly held by the current collector, and the electrode active material may peel off from the current collector even after repeated charging and discharging. Absent. Therefore,
Good charge / discharge cycle characteristics can be obtained.

[0039]

EXAMPLES Preferred examples of the present invention will be described based on experimental results.

Example 1 First, the negative electrode 1 was manufactured as follows.

A negative electrode mixture coating material was prepared by weighing out each coating material component according to the following coating composition and mixing it in a ball mill for 10 hours. Negative electrode mixture coating composition carbon (specific surface area; 10 m ² / g) 60 parts by weight binder 5 parts by weight N-methyl-2-pyrrolidone 35 parts by weight

Then, this negative electrode mixture coating material is applied to the negative electrode current collector 9
Then, a copper foil having a thickness of 10 μm was coated on both surfaces with a coating thickness of 100 μm to prepare a strip-shaped negative electrode.

Next, the positive electrode 2 was manufactured as follows.

According to the following paint composition, the paint components were weighed and mixed in a ball mill for 10 hours to prepare a positive electrode mixture paint.

Positive electrode mixture coating composition LiCoO 2 (specific surface area; 0.5 m ² / g) 60 parts by weight carbon (specific surface area; 200 m ² / g) 5 parts by weight binder 5 parts by weight N-methyl-2-pyrrolidone 30 parts by weight

Then, this positive electrode mixture coating material is applied to the positive electrode current collector 1
Coating thickness of 100 on both sides of 20 μm thick aluminum foil
It was applied in a thickness of μm to prepare a positive electrode. In addition, negative electrode mixture paint,
The binder mixed in the positive electrode mixture paint had a vinylidene fluoride monomer unit content of 99.9 mol% and a hydrophilic polar group-containing monomer unit CH ₂ ═CH—SO ₃ Na
Is a hydrophilic polar group-containing polyvinylidene fluoride having a content of 0.1 mol% and a degree of polymerization of 1500.

Next, these strip-shaped positive electrodes and strip-shaped negative electrodes were laminated with a 25 μm-thick polypropylene film serving as a separator interposed therebetween and wound many times to give an outer diameter of 18 mm.
The spiral electrode body of was produced.

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.

Battery can 5 containing this spiral electrode body
The volume ratio of ethylene carbonate and diethyl carbonate is 1: 1.
An electrolytic solution in which LiPF ₆ was dissolved at a concentration of 1 mol / l was injected into the mixed solvent mixed in. 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.

Examples 2 and 3 A non-aqueous electrolyte solution was prepared in the same manner as in Example 1 except that the hydrophilic polar group-containing polyvinylidene fluoride having the monomer composition shown in Table 1 was mixed as the binder in the negative electrode coating material and the positive electrode coating material. A secondary battery was produced.

Comparative Example 1 A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that polyvinylidene fluoride containing no hydrophilic polar group as a binder was mixed in the negative electrode coating material and the positive electrode coating material.

[0052]

[Table 1]

The non-aqueous electrolyte secondary battery produced as described above was charged at room temperature for 2.5 hours under the conditions of maximum charging voltage 4.2V and charging current 1A and a constant resistance of 6.2Ω. The change of discharge capacity is observed by repeating the charge / discharge cycle such as discharging at 5 ℃
Number of cycles down to 0% (50% capacity number of cycles)
I checked. The results are shown in Table 2.

[0054]

[Table 2]

As can be seen from Table 2, compared with the battery of Comparative Example 1 in which polyvinylidene fluoride containing no hydrophilic polar group as a binder was mixed with the negative electrode coating material and the positive electrode coating material, the hydrophilic polar group was added to the negative electrode coating material and the positive electrode coating material. The non-aqueous electrolyte secondary batteries of Examples 1 to 3 in which the contained polyvinylidene fluoride is mixed have a large 50% capacity cycle number and exhibit good charge / discharge cycle characteristics.

From the above, using polyvinylidene fluoride containing a hydrophilic polar group as the binder is
It was found to be effective in improving the cycle characteristics of the battery.

Experimental Examples 1 to 7 In this experimental example, the optimum monomer composition of polyvinylidene fluoride containing a hydrophilic polar group to be mixed with the negative electrode coating material and the positive electrode coating material was examined.

The content of the hydrophilic polar group-containing monomer unit CH ₂ ═CH—SO ₃ Na and the degree of polymerization of the hydrophilic polar group-containing polyvinylidene fluoride mixed with the negative electrode coating material and the positive electrode coating material were changed as shown in Table 3. A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except for the above.

[0059]

[Table 3]

Then, the 50% capacity cycle number of the produced non-aqueous electrolyte secondary battery was examined in the same manner as described above.
The results are shown in Table 4.

[0061]

[Table 4]

In Table 4, first, when comparing the batteries of Experimental Example 1 to Experimental Example 4 in which the content of the hydrophilic polar group-containing monomer unit of polyvinylidene fluoride was changed, the comparison of the hydrophilic polar group-containing monomer of polyvinylidene fluoride was made. Content is 0.0
In the batteries of Experimental Example 2 and Experimental Example 3 with 1 mol% and 0.5 mol%, the 50% capacity cycle number was as large as 870 or more. However, the battery of Experimental Example 1 in which the content of the hydrophilic polar group-containing monomer unit of polyvinylidene fluoride is 0.001 mol% has a small 50% capacity cycle number of 521,
On the other hand, in Experimental Example 4 in which the content of the hydrophilic polar group-containing monomer unit of polyvinylidene fluoride was 1.0 mol%, the thixotropy of the electrode mixture paint was high, and the current collector could not be applied. From this, the content of the hydrophilic polar group-containing monomer of polyvinylidene fluoride is 0.01 to 0.5.
It turns out that mol% is suitable.

Next, comparing Experimental Examples 5 to 8 in which the degree of polymerization of polyvinylidene fluoride was changed, the batteries of Experimental Example 6 and Experimental Example 7 in which the polymerization degrees of polyvinylidene fluoride were 300 and 5000 respectively were 50. % Capacity cycle number is 800
It is a large value. However, in the battery of Experimental Example 5 in which the polymerization degree of polyvinylidene fluoride was 150, the 50% capacity cycle number was as small as 307, while the polymerization degree was 8000.
In Experimental Example 8 using the above polyvinylidene fluoride, the thixotropy of the paint was high and it was impossible to apply it to the current collector. From this, the degree of polymerization of polyvinylidene fluoride is 300
It can be seen that ~ 5000 is suitable.

[0064]

As is apparent from the above description, in 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 positive electrode active material, a conductive agent and a binder. At least one of a negative electrode mixture and a binder to be contained in the positive electrode mixture in a non-aqueous electrolyte secondary battery comprising a positive electrode in which a positive electrode mixture containing the above is held on a positive electrode current collector and a non-aqueous electrolyte solution. Since polyvinylidene fluoride containing a hydrophilic polar group is used as the part, the electrode active material does not peel off from the current collector even after repeated charge and discharge, and a non-aqueous electrolyte solution that exhibits good charge and discharge cycle characteristics. A secondary battery can be obtained.

[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 (1)

[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, at least a part of the binder contained in the negative electrode mixture and the positive electrode mixture has a hydrophilic polar group. And a non-aqueous electrolyte secondary battery.