JPH03261075A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To obtain a battery of excellent safety and cycle characteristic by varying the thickness of a spirally wound positive electrode sheet in such a manner that the sheet is thinner at its upper and lower portions than at its center portion. CONSTITUTION:A positive electrode sheet having LiV3O8 fixed on a current collector of aluminium foil is fabricated. The positive electrode sheet is 40mm in width and 230mum in thickness at its center portion and 210mum in thickness at a portion 7mm-long from the upper side and 210mum in thickness at a portion 7mm-long from the lower side thereof. A negative electrode of lithium is wound around the positive electrode sheet using a separator comprising a porous film made from polypropylene. The positive and negative electrodes are then inserted in a can and an electrolyte which is a mixed solution of propylene carbonate having 1mol/l of LiPF6 dissolved therein is added thereto and the can is sealed so that a lithium secondary battery is fabricated. The battery is subjected to cycle tests at 3 to 2V with charging at a constant current of 70mA and discharge at a constant current of 150mA; then the battery yields 723mAh at the first test and about 560mAh after tests are repeated one hundred and fifty times. The safety and cycle characteristic of the battery are thus enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a battery with excellent safety and cycleability.

(Conventional technology) In recent years, non-aqueous electrolyte batteries that use alkali metals such as lithium as active materials have high voltage and large capacity, so they have been used for everything from memory backup for computers and VTRs to drive sources for cameras and other devices. Demand is rapidly increasing in a wide range of fields.

Furthermore, secondary batteries that can be charged and discharged, especially cylindrical and other spiral structure types, have the advantage of being able to draw large currents, so they are expected to be used as the future main power source for various electronic devices such as portable devices. .

Such main power batteries are required to fully and stably exhibit their battery performance under various usage conditions, and to have high safety without the risk of battery explosion.

However, non-aqueous electrolyte batteries, especially secondary batteries, may suddenly experience a decrease in capacity or may explode due to repeated use.

Such problems tend to occur particularly in secondary batteries having a spiral structure.

(Problems to be Solved by the Invention) The present invention was made in view of the above circumstances, and aims to provide a battery with excellent safety and cycle characteristics by controlling the thickness distribution of the positive electrode sheet.

(Means and effects for solving the problem) The present inventors,
As a result of intensive studies to achieve the above objective, we found that in the case of spiral structure secondary batteries, the thickness of the spirally wound positive electrode sheet is made thinner at the top and bottom than at the center. The inventors have discovered that it is possible to provide a battery with excellent safety and cycle characteristics, and have accomplished the present invention.

The present invention will be explained in more detail below.

First, a positive electrode sheet is created. Such a positive electrode sheet may be made of only the positive electrode active material, but in some cases it may be a composite material mixed with a binder such as Teflon or polyethylene, or a conductive agent such as carbon or metal powder, and in some cases it may be a composite material containing wire mesh, expanded metal, etc. A current collector such as a metal thin film may be used in combination with them.

In the case of a spiral structure type battery, a rectangular shape is usually adopted in consideration of winding. What is important here is to make the thickness of the upper and lower parts of the rectangular positive electrode sheet thinner than the thickness of the central part. The upper, lower, and central portions mentioned here indicate the positional relationship of the positive electrode sheet in the direction perpendicular to the winding direction, that is, they directly correspond to the upper and lower portions of the cylindrical battery.

Regarding the difference in thickness, the thickness of the upper and lower parts should be 5% or more, preferably 10% or more, than the thickness of the central part.
More preferably, it is suitable to reduce the thickness by 15% or more.

The difference in thickness may be created by creating a continuous slope from the center to the upper and lower parts, or by making only the upper and lower width parts thinner than the central part discontinuously. In any case, it is necessary that there be a difference from the center.

As mentioned above, these thickness differences are related to the positive electrode sheet, so it goes without saying that there is a thickness difference in the molded positive electrode active material itself, and in some cases, there may be a thickness difference in the molded positive electrode active material. Even without this, it is also possible to provide a thickness difference in the current collector and, as a result, a thickness difference in the positive electrode sheet.

The reason why safety and cycle characteristics are improved when the thickness of the upper and lower parts of the positive electrode sheet is made thinner than the thickness of the central part is not clear, but it is thought to be as follows.

Now, when we closely observed the spiral structure type secondary battery, we found that it tends to behave as follows. That is, as charging and discharging are repeated, the positive electrode sheet tends to expand, and the change is particularly severe in the vertical direction. This is probably due to the fact that in the case of the normal spiral structure type, the entire rolled electrode material is stuffed into an outer can to create a battery, so even if the positive electrode sheet tries to expand, the expansion is restricted by the outer can. This is thought to be because the positive electrode sheet does not have a degree of freedom in the thickness direction and cannot expand, and the expansion concentrates in the vertical direction, which has a relatively high degree of freedom.

This phenomenon tends to get worse the more charging and discharging is repeated, and in fact, in severe cases, cracks may occur in the winding direction at the top and bottom of the positive electrode sheet, and the space is relatively small. This is thought to be due to the positive electrode expanding to a certain upper and lower part escaping (5).

The problem is not just this, but the expanded positive electrode also seems to stretch the separator at the same time. For example, in the cracked areas of the positive electrode sheet mentioned above, the separator was stretched, and cracks were also observed in some areas.

Cracks in the separator have a high possibility of becoming a trigger for internal short-circuits, and therefore reduce the discharge capacity, worsen cycle characteristics, and in some cases, pose a risk of rupture or ignition, which is not desirable. be.

However, in the configuration of the present invention as described above, that is, when the thickness of the upper and lower parts of the positive electrode sheet is made thinner than the thickness of the central part, when the positive electrode expands, the separator It appears that the occurrence of stretching or cracking can be drastically reduced. Therefore, it is thought that this will lead to improvements in cycle characteristics and safety.

It should be noted that such an expansion phenomenon is not so much of a problem in primary batteries that are discharged only - times, but seems to become noticeable in secondary batteries that are repeatedly charged and discharged.

The positive electrode sheet, negative electrode sheet, and separator prepared as described above are prepared, rolled up, inserted into an outer can, filled with electrolyte, and sealed to create a battery.

The separator may be any separator as long as it prevents short circuit between the positive electrode and the negative electrode and allows ions to pass through. Examples thereof include a microporous film, a nonwoven fabric, and the like.

Further, the thickness may be any value, but it is usually preferably about 5 to 200 microns, and in some cases, two or more separators, and in some cases, different types of separators may be used in a stacked manner. There are no particular restrictions on the material, but polypropylene and polyethylene are usually preferred.

The positive electrode is not particularly limited, and examples include Va 0B , LiVa Oa , and LiC.

02, LiNt02, VaOa, Crys%
Mn0a %CubSCus'V20+a, Ti0a
, LLMn, 04. Metal oxides such as MnO□ compounds containing l-i, Ti5z, FeS, CuCo54
, M o S s and other metal oxides, selenides, graphite, polyacetylene, polypyridine, polybenzene, polyparaphenylene, polytriphenylamine, polypyridine, polythiophene, polyfuran, polypyrrole and other polymers. Among them, L
i Va Oa, L i Co.

2, LiNiOz, and Mn02% polyaniline have good compatibility and are particularly preferable as positive electrode active materials.

The negative electrode is not particularly limited, but lithium metal, lithium alloy, or carbon that occludes lithium is preferably used. Specifically, lithium alloys include lithium and aluminum, indium, tin, lead, bismuth, cadmium, zinc, etc. Examples include alloys with. Among these negative electrodes, lithium-aluminum alloy and carbon are particularly preferably used.

As electrolytes, L i P Fa , L i S
bFs, LiAsFg, LiC! 20< , Li
5O3CFa, Li I, LiBr, LiCl2, L
iBF4, LiAIcj2+% LiHF2,
One or more types of lithium salts such as Li5CN may be used, but the present invention is not necessarily limited to these lithium salts. Among these lithium salts, L i C
f204 , LiBF< , L, t PFa , L, 1
SO3CFa is particularly preferred. These lithium salts are usually appropriately dissolved and used as an electrolyte. In this case, the concentration of lithium salt is 0.01 to 10 mol/
It is desirable to set it to I2.

In this case, a relatively polar solvent is preferably used as the organic solvent, and specifically, propylene carbonate, ethylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, triethyl phosphate, triethyl Examples include one or a mixture of two or more of phosphite, dimethyl sulfate, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, dioxane, jetoxyethane, polyethylene glycol, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, etc. It is not limited. Among these, particularly suitable for (9) are propylene carbonate and a mixed solvent formed by mixing propylene carbonate with one or more solvents selected from dimethoxyethane, tetrahydrofuran, ethylene carbonate, and γ-butyrolactone.

(Example) Next, the present invention will be specifically explained with reference to Examples and Comparative Examples, but the present invention is not limited to the Examples below.

A positive electrode sheet was prepared by fixing LiVaOa on an aluminum foil current collector. The width of this positive electrode sheet is 40m.
m, with a thickness of 230 μm at the center and 7 mm from the top.
mm and the thickness between 7 mm from the bottom was controlled to be 210 μm.

This was wrapped with a lithium negative electrode and a polypropylene microporous film separator.

Next, this was inserted into a can, an electrolytic solution which was a mixed solvent with propylene carbonate in which 1mo°C/β LiPF6 had been dissolved was added, and the can was sealed to produce a AA size (10) lithium secondary battery.

Charging at 70mA constant current, discharging at 150mA constant current, 3~
A cycle test between 2V and 2V was performed.

The result was 723mAh the first time, and about 56mAh after 150 times.
It was 0mAh.

(Comparative Example) A battery was prepared in the same manner as in the example except that the thickness of the positive electrode sheet was changed to 230 LLm, and the same cycle test as in the example was conducted.

At 730 mAh for the first time, after 130 cycles, the capacity suddenly decreased drastically, and when it was disassembled and observed, it was confirmed that there were cracks in the winding direction at the upper and lower parts of the separator.

(Effects) As detailed above, the present invention improves the shortcomings that tend to occur in conventional secondary batteries by varying the thickness of the positive electrode depending on the central part and the upper and lower parts, and improves safety and cycle life. A secondary battery with excellent characteristics has been provided, and its effects are remarkable.

(11)

Claims (1)

[Claims] (1) In a spiral structure battery, a non-aqueous electrolyte secondary battery characterized in that the thickness of the spirally wound positive electrode sheet is thinner at the upper and lower parts than at the center.