JPH10172607A - Sheet-like lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent generation of dendrite in the vicinity of a negative electrode active substance layer by installing a positive electrode sheet in a folding of a folded negative electrode sheet so that each edge of an active substance layer of each side of the negative electrode sheet is not projected outside over the edge of the active substance layer of the negative electrode sheet. SOLUTION: A negative electrode sheet 1 and a separator 3 are folded in a stacking manner, and a positive sheet 2 is inserted between the separators 3. Every edge of a positive electrode active substance layer 22 of the positive electrode sheet 2 is not projected outside over the edge of a negative active substance layer of the negative electrode sheet 1, and a gap (d) between the edges is at least 0.5-5mm. The lithium ion to be emitted from each edge of the positive active substance layer 22 of the positive sheet 2 is affordably received by the negative electrode active substance layer, generation of dendrite can be substantially eliminated, the service life can be improved, and the battery capacity can be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium secondary battery, and more particularly to a sheet-shaped lithium secondary battery.

[0002]

2. Description of the Related Art A lithium secondary battery having a large discharge capacity has been spotlighted as a battery for electronic equipment such as a portable telephone and a personal computer. Conventionally, three-dimensional batteries such as columnar and box-type batteries have mainly been used as such lithium secondary batteries, but recently, interest in sheet-shaped lithium secondary batteries has increased due to space factors and light weight. ing. A sheet-shaped lithium secondary battery basically has a structure in which an electrolyte is interposed between positive and negative bipolar sheets and sealed with an appropriate exterior sheet without being wound.
As for the electrolyte, solid and liquid electrolytes have been proposed as in the case of the three-dimensional battery. The liquid electrolyte is used in a state where the liquid electrolyte is impregnated in the separator, as in the case of the three-dimensional battery. The advantage of a sheet-shaped lithium secondary battery is that unlike a three-dimensional battery, it is thin and has a low degree of heat trapped in the battery for good heat dissipation, so even if an overcurrent flows for some reason, Alternatively, even if a penetrating scratch is caused by a nail or the like, an explosion accident due to the burning of lithium inside the battery is unlikely to occur, which is extremely safe. On the other hand, there is a problem that a large capacity cannot be obtained because the electrode area is generally smaller than that of a three-dimensional battery.

For the purpose of increasing the capacity of a sheet-shaped lithium secondary battery, for example, Japanese Patent Application Laid-Open No. Hei 8-96789 proposes to use a separator made of a polysaccharide polymer having a large ability to retain an electrolytic solution. However, this proposal has a problem in practicality from the viewpoint of manufacturing cost because the separator is expensive.

The inventors of the present invention have proposed a method of increasing the battery capacity by interposing a positive / negative bipolar sheet via a separator, as proposed in three-dimensional batteries, such as JP-A-2-78152 and JP-A-2-301973. Many attempts to fold. A large number of folding increases the electrode area, which leads to an increase in the capacity of the battery. However, this attempt has revealed the following problem. That is, when folded many times, a portion where the positive electrode sheet enters the folded portion of the negative electrode sheet and a portion opposite thereto are alternately generated. There is no problem with the former part where the positive electrode sheet enters the folded part of the negative electrode sheet, but in the case of the latter part, the negative electrode active material layer at the tip of the negative electrode sheet that has entered the folded part of the positive electrode sheet has the positive electrode It will be surrounded by the positive electrode active material layer of the sheet. In this case, a large amount of lithium ions emitted from the surrounding positive electrode active material layer generated during charging of the battery cannot be received by the negative electrode active material layer having a very small area, and as a result, dendrite is generated at the tip portion. Easier to do.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a sheet-shaped lithium secondary battery having a folded electrode structure in which the problem of dendrite generation has been reduced or solved, and an increased battery capacity. It is in.

[0006]

The present invention has the following features. (1) A separator in which a positive electrode sheet having an active material layer on both sides is installed in the folded negative electrode sheet, and a non-aqueous liquid electrolyte is impregnated between each active material layer of the negative electrode sheet and the positive electrode sheet. A sheet-shaped lithium secondary battery characterized in that a sheet is interposed. (2) The number of the negative electrode sheet is one, and the sheet is folded in two at the center or in the vicinity thereof.
(1) The sheet-shaped lithium secondary battery according to (1). (3) The entire edge of the active material layer on both sides of the positive electrode sheet does not extend beyond the edge of the active material layer of the negative electrode sheet (1) or
(2) The sheet-shaped lithium secondary battery according to (2). (4) The sheet-shaped lithium secondary battery according to any one of the above (1) to (3), wherein the negative electrode active material is graphite and the positive electrode active material is a lithium-containing transition metal oxide.

[0007]

According to the present invention, a structure in which a positive electrode sheet having an active material layer on both sides is inserted and installed in the folded negative electrode sheet so that a small portion of the inserted end of the positive electrode sheet has a large area It will be surrounded by the active material layer. Therefore, a small amount of lithium ions emitted from a small portion of the insertion end of the positive electrode sheet is sufficiently received by the large-area negative electrode active material layer, and the problem of dendrite generation in that portion is substantially solved. . Furthermore, in a more preferred embodiment of the present invention, all edges of the active material layers on both sides of the positive electrode sheet do not extend beyond the edges of the active material layer of the negative electrode sheet. Since all edges exist outside the entire edges of the active material layers on both surfaces of the positive electrode sheet, lithium ions released from all edges of the active material layers on both surfaces of the positive electrode sheet are received by the negative electrode active material layer. The problem of dendrite generation near the edge of the negative electrode active material layer does not substantially occur.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a negative electrode active material, a positive electrode active material, positive and negative current collectors, a non-aqueous liquid electrolyte, and a separator are all used in the field of lithium secondary batteries or lithium ion secondary batteries. May be conventionally known. Above all, as the negative electrode active material and the positive electrode active material, a nonmetallic lithium-based material is generally preferable in order to further enhance the safety and handleability of the sheet-shaped lithium secondary battery.

Preferred examples of the negative electrode active material include:
Various natural graphite and artificial graphite, for example, fibrous graphite, flaky graphite, graphite such as spherical graphite, as a binder,
Examples include polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, and ethylene-propylene-diene-based polymers. The amount of the negative electrode active material used is about 80 to 96 parts by weight per 100 parts by weight of the total amount of the negative electrode active material and the binder.

Preferred examples of the positive electrode active material include:
A potential difference from the negative electrode of at least 1 V, for example, V
₂ O ₅ , MnO ₂ , LiMn ₂ O ₄ , LiCoO ₂ , L
iNi _0.5 Co _0.5 O ₂ , LiNiO ₂ , Li-Co-
P-based composite oxide _{(LiCo 0. 5 P 0.5 O 2} , LiCo
_{0.4 P 0.6 O 2, LiCo 0.6} P 0. 4 O 2, LiCo 0.
₃ Ni _0.3 P _0.4 O ₂ , LiCo _0.2 Ni _0.2 P _0.6 O
₂ ), TiS ₂ , MoS ₂ , MoO ₃ and the like. Among these, a Li-Co-P-based composite oxide that can particularly increase the electromotive force and charge / discharge voltage of the secondary battery is particularly preferable. Examples of the binder include polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, and an ethylene-propylene-diene-based polymer. As the conductive agent, various types of conductive graphite and conductive carbon black may be used. The amount of the positive electrode active material used is about 80 to 95 parts by weight per 100 parts by weight of the total amount of the positive electrode active material, the binder, and the conductive agent,
The amount of the binder used is 1 to 1 per 100 parts by weight of the positive electrode active material.
The conductive agent is used in an amount of about 3 to 15 parts by weight per 100 parts by weight of the positive electrode active material.

As the negative electrode current collector, copper, nickel, silver,
About 5-100 μm in thickness of conductive metal such as SUS,
Especially about 8 to 50 μm foil or perforated foil, thickness 20 to 30
Expanded metal having a thickness of about 0 μm, particularly about 25 to 100 μm is preferable. As the positive electrode current collector, aluminum, aluminum alloy, conductive metal such as titanium,
Foil or perforated foil having a thickness of about 10 to 100 μm, particularly about 15 to 50 μm, and a thickness of about 25 to 300 μm, particularly 30 to
An expanded metal of about 150 μm or the like is preferable.

The negative electrode sheet is prepared by applying a mixed composition of a negative electrode active material and a binder to one surface of a negative electrode current collector, drying the mixture sufficiently,
The thickness formed by rolling is about 20 to 500 μm, particularly 50
One having a negative electrode active material layer of about 200 μm is exemplified. On the other hand, for the positive electrode sheet, a mixed composition of the positive electrode active material and the binder is applied to both surfaces of the positive electrode current collector, and after being sufficiently dried,
Rolled thickness is 20 to 500 μm on both sides
Those having a positive electrode active material layer having a thickness of about 50 to 200 μm are exemplified.

The size and shape of the positive electrode sheet vary depending on the use of the present invention. For example, in the case of a portable telephone, the positive electrode active material layer on one side is a square or a rectangle having an area of about 30 to 100 cm ² . For the purpose of the present invention, a negative electrode sheet having an area at least twice or more than one side of the positive electrode sheet is used.

As the non-aqueous liquid electrolyte, an electrolyte in which salts are dissolved in an organic solvent can be used. The salts include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAs
Examples thereof include F ₆ , LiAlCl ₄ , and Li (CF ₃ SO ₂ ) ₂ N, and one or a mixture of two or more thereof is used. As the organic solvent, ethylene carbonate,
Propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl sulfoxide, sulfolane, γ-butyrolactone,
1,2-dimethoxyethane, N, N-dimethylformamide, tetrahydrofuran, 1,3-dioxolan, 2,
-Methyltetrahydrofuran, diethyl ether and the like, and one or a mixture of two or more thereof is used. The concentration of the salts in the electrolyte is 0.1.
About 1 to 3 mol / liter is appropriate.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in more detail with reference to the drawings. FIG. 1 is a top view of an embodiment of the present invention, and FIG. 2 is a cross-sectional view taken along line XX in FIG. 1 and 2
1, 1 is a negative electrode sheet, 11 is a negative electrode current collector, 12 is a negative electrode active material layer, 13 is a folded portion of the negative electrode sheet 1, 2 is a positive electrode sheet, 21 is a positive electrode current collector, 22 is a positive electrode active material layer,
23 is a front end portion of the positive electrode sheet, 3 is a separator interposed between the negative electrode sheet 1 and the positive electrode sheet 2, 4 is an outer sheet, 41 is a sealing portion of the outer sheet, 5 is a negative electrode terminal,
6 is a positive electrode terminal. The separator 3 is impregnated with a non-aqueous liquid electrolyte.

The exterior sheet 4 is preferably a sheet impermeable to gas and water, for example, a composite sheet having a metal foil such as copper or aluminum and a thermoplastic resin laminate layer such as polyester or polypropylene on both sides.
The contents can be heat-sealed and sealed using the thermoplastic resin laminate layer.

The positive electrode active material layer 22 at the front end 23 of the positive electrode sheet is surrounded by the negative electrode active material layer 12, as shown in the drawing. Does not extend beyond the edge of the layer. From the positional relationship between the positive and negative electrode active material layers, the problem of dendrite generation on the negative electrode active material layer 12 can be overcome. In the present invention, the negative electrode active material layer 12 and the positive electrode active material layer 2
The interval between them is about the same as the interval in a normal three-dimensional lithium secondary battery, for example, 10 to 100 μm. Also,
The difference d between the edges of the positive and negative electrode active material layers shown in FIG. 2 (the same applies to other portions) may be at least about 0.5 mm, and preferably about 0.5 to 5 mm.

The embodiment shown in FIGS. 1 and 2 can be manufactured by the following method example. A negative electrode sheet 1 having a length approximately twice as long as the positive electrode sheet 2 and a separator 3 are stacked,
The separator 3 is folded inside at a substantially central portion with the separator 3 inside, and then the positive electrode sheet 2 is inserted and installed between the separators 3 as shown in the figure. As the above-mentioned positive and negative electrode sheets, those obtained by welding terminals 5 and 6 in advance are used. Thereafter, the above-described assembly is installed between the two exterior sheets 4, and the three sides of the exterior sheet 4 are sealed by heat fusion. Then, the separator 3 is impregnated with a non-aqueous liquid electrolyte under reduced pressure, and the remaining one of the exterior sheets 4 is sealed by heat fusion while maintaining the reduced pressure state to obtain a sheet-shaped lithium secondary battery.

[0019] Length 30cm, width 4.1cm, thickness 14μ
A negative electrode sheet having a 70 μm-thick negative electrode active material layer made of a composition of 90 parts by weight of fibrous graphite and 10 parts by weight of polyvinylidene fluoride on the entire surface of one side of a copper foil having a length of at least A polypropylene separator having a width and a length of 14 cm, a width of 3.9 cm and a thickness of 2
LiCoO ₂ 90 over the entire surface of both sides of a 0 μm aluminum foil
A positive electrode sheet having a positive electrode active material layer having a thickness of 73 μm, which was composed of a composition of 3 parts by weight of conductive graphite, 7 parts by weight of conductive graphite, and 3 parts by weight of polyvinylidene fluoride, was prepared. Further, an envelope was prepared by heat-sealing two sides of a composite sheet having a polyester laminate layer on one surface of a 10-μm-thick aluminum foil and a polypropylene laminate layer on the other surface, thereby forming an envelope. A separator was placed on the negative electrode active material layer of the negative electrode sheet, and the separator was folded at substantially the center with the separator inside. Next, the positive electrode sheet was inserted between the folded separators. The thus obtained assembly was loaded into the above outer package, and a non-aqueous liquid having a composition in which 1 mol of LiPF ₆ was dissolved per liter of a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a ratio of 1: 1 (by volume). The electrolyte was impregnated, the pressure was reduced to 50 mmHg or less, and the other of the outer package was sealed by heat sealing while maintaining the reduced pressure to obtain a sheet-shaped lithium secondary battery of Example.

A sheet-like lithium secondary battery of a comparative example was obtained which was different from the above-mentioned example only in that the dimensions and the positional relationship of the negative electrode sheet and the positive electrode sheet were completely reversed.

In the above embodiment, the initial discharge capacity is 250 m
Ah, and the capacity retention after 100 cycles of charge and discharge was 96%. In contrast, the above comparative example
Although the initial discharge capacity was 250 mAh, an abnormality appeared on the charge / discharge curve after 15 cycles of charge / discharge. On the other hand, in the example, no generation of dendrite was observed even after 100 cycles of charging and discharging.

In the present invention, the number of times of folding of the negative electrode sheet may be three or more, and a positive electrode sheet having an active material layer on both surfaces may be provided in each fold. However, when the number of times of folding of the negative electrode sheet increases, the restoring force of the negative electrode sheet to the state before folding increases, which generally complicates the manufacture of the sheet battery. Accordingly, it is actually particularly preferable that the number of the negative electrode sheets used is one, and this is only folded in two at the center or in the vicinity thereof.

[0023]

The sheet-like lithium secondary battery of the present invention comprises:
Since the problem of dendrite generation is substantially solved and the life is extended and the battery capacity is increased, the advantages of the sheet-shaped lithium secondary battery, such as space factor, light weight,
Safety based on good heat dissipation is utilized to a high degree.

[Brief description of the drawings]

FIG. 1 is a top view of an embodiment of the present invention.

FIG. 2 is a sectional view taken along the line XX in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Negative electrode sheet 13 Folded part of negative electrode sheet 1 2 Positive electrode sheet 3 Separator 4 Exterior sheet 5 Negative electrode terminal 6 Positive electrode terminal

Claims (4)

[Claims] A positive electrode sheet having an active material layer on both sides is provided in the folded negative electrode sheet, and a non-aqueous liquid electrolyte is impregnated between each active material layer of the negative electrode sheet and the positive electrode sheet. A sheet-shaped lithium secondary battery characterized by having a separator interposed therebetween. 2. The sheet-type lithium secondary battery according to claim 1, wherein the number of the negative electrode sheets is one, and the negative electrode sheet is folded in two at a center portion or a vicinity portion thereof. 3. The sheet-shaped lithium secondary battery according to claim 1, wherein all edges of the active material layers on both surfaces of the positive electrode sheet do not extend beyond edges of the active material layer of the negative electrode sheet. 4. The sheet-shaped lithium secondary battery according to claim 1, wherein the negative electrode active material is graphite, and the positive electrode active material is a lithium-containing transition metal oxide.