JPH113708A - Lithium ion secondary battery positive electrode and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery which imparts improved high energy density, while maintaining features of a conventional lithium ion secondary battery. SOLUTION: A lithium ion secondary battery constituted of a positive electrode containing a lithium salt, a negative electrode comprising a substance which stores lithium alloy or lithium, and a separator comprising micro-porous plastic film separating both electrodes. The lithium salt of 7,7,8,8- tetracyanoquinodimethane(TCNQ) is used as a positive electrode active material. A lithium ally such as Wood's alloy or a substance storing atomic lithium between layers such as graphite are used conventionally as the negative electrode. A shut-down separator, a film such as micro-porous polyethylene film, is used as the separator for the positive electrode and the negative electrode, in order to block the penetration of ions so as to prevent firing or bursting or the like in case of the rapid rise of a battery temperature, resulting from an external short-circuit or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium ion secondary battery, and more particularly to a lithium ion secondary battery having an improved high energy density.

[0002]

2. Description of the Related Art In recent years, with the development of electronics, electronic equipment, especially portable equipment, has been rapidly reduced in size.
Also, in order to suppress air pollution due to automobile exhaust gas, development of an electric vehicle using a secondary battery as a power source is strongly desired.

[0003] Therefore, a lithium ion secondary battery having features of high energy density and long life is considered to be most suitable for such a field.

In the structure of a lithium ion secondary battery, lithium cobalt oxide (LiCoO ₂ ) is generally most often used as a positive electrode. On the other hand, for the negative electrode, a lithium alloy such as a wood alloy or a substance that stores atomic lithium between layers (intercalate) such as graphite is used. As the electrolyte, since lithium directly reacts with water, an organic electrolyte obtained by dissolving a lithium salt in an organic solvent such as ethylene carbonate is used. In order to prevent direct contact between the positive electrode and the negative electrode, a film made of microporous polyethylene or the like is used as a separator.

The features of the lithium ion secondary battery include:
High output voltage (3-4V), high weight energy density, high volume energy density, high output energy density, low self-discharge rate, good cycle characteristics, no memory effect, high charging efficiency, and residual amount Display is easy.

[0006] However, the lithium ion secondary battery has a disadvantage that the resources of cobalt contained in LiCoO ₂ used as a positive electrode active material are scarce. Therefore, lithium manganate (LiMn ₂ O) is used as a positive electrode active material instead of this.
₄ ) and the like have been studied, but no performance exceeding LiCoO ₂ has been obtained so far.

In order to increase the energy density of a battery, it is important to use a material having a small specific gravity. For a negative electrode of a lithium ion secondary battery, a mixture of a carbon material and a binder is pasteurized with an organic solvent. This objective has been achieved to some extent by the use of a slab. However, as for the positive electrode material, polyacetylene or the like has been tried as a binder for an active material and as a conductive agent, but it cannot be said that sufficient performance has been obtained.

[0008]

Thus, it can be said that the problem of the lithium ion secondary battery is to search for a new positive electrode active material for increasing the energy density. An object of the present invention is to provide a lithium ion secondary battery that uses a positive electrode active material that is made of a material that is abundant in the natural world and that can increase energy density.

[0009]

As a first aspect of the present invention, 7,7,8,8-tetracyanoquinodimethane (TC
Provided is a positive electrode for a lithium ion secondary battery, comprising a lithium salt of NQ) as a positive electrode active material.

Further, as a second aspect, the present invention provides a lithium ion secondary battery having the positive electrode of the first aspect, a negative electrode made of a lithium alloy or a substance occluding lithium, and a separator for separating both electrodes.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventor has calculated the formula (1)

[0012]

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[0013] TCNQ and Li represented by stable ion radical salt Li ⁺ TCNQ ^- to form a (L. R.Melby
J. Am. Chem. Soc. , Vol. 84, No. 5, P. 3374 (196
2)). The lithium ion battery according to the present invention uses this Li ⁺ TCNQ ⁻ as a positive electrode active material. The battery reaction at the positive electrode and the negative electrode is represented by the reaction formula (2)

[0014]

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In the negative electrode, the reaction formula (3)

[0016]

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## EQU1 ## By charging, Li ⁺
TCNQ ^- is TCNQ become oxidized, lithium ion Li ⁺ and electrons e ^- to generate. At the negative electrode, Li ⁺ is reduced, and lithium atoms L
i ⁰ is saved. The discharge reaction is the reverse of the above, and Li ⁰ of the negative electrode is ionized and moves to the positive electrode, and Li ⁺ TC
NQ ^- is played.

[0018] The positive electrode, Li ⁺ TCNQ ^- by kneading a powder and a conductive agent molded into a pellet. Here, examples of the conductive agent include acetylene black and furnace black, and acetylene black is particularly preferable. Further, artificial graphite or the like may be added. Examples of the binder include polytetrafluoroethylene (PTFE) and the like.

As the negative electrode, a lithium alloy such as a wood alloy and a substance such as graphite, which stores atomic lithium between layers, are conventionally used.

As the electrolyte, an organic electrolyte obtained by dissolving a lithium salt of a supporting electrolyte in an organic solvent is used. The organic solvent should have a high dielectric constant, an appropriate solvation ability with lithium ions, and a low viscosity that does not hinder the movement of ions.
There are ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and the like, and preferably ethylene carbonate.

In addition, a separator for preventing contact between the positive electrode and the negative electrode is provided between the positive electrode and the negative electrode. As a separator, if the battery temperature rises suddenly due to an external short circuit, etc.
A shutdown separator is used to prevent the permeation of ions to prevent ignition or rupture, and a film such as a microporous polyethylene film is used.

Hereinafter, the present invention will be described in detail with reference to examples.

[0023]

FIG. 1 is a block diagram showing one embodiment of a lithium ion battery according to the present invention. Positive electrode 1 in this embodiment
Is, Li ⁺ TCNQ ^- powder, acetylene black and PTFE as a binder as a conductive agent 70: 2
The mixture was kneaded at a weight ratio of 5: 5 and formed into pellets. Reference numeral 2 denotes a negative electrode, which is formed by molding graphite into pellets. Reference numeral 3 denotes a microporous polyethylene film as a separator, and reference numeral 4 denotes a case made of stainless steel, which functions as an electrode. The electrolytic solution was a 1: 1 mixture of propylene carbonate solution of 1M lithium chlorate (LiClO ₄ ) and ethylene carbonate, and the mixture was mixed with the positive electrode 1.
The negative electrode 2 and the separator 3 were impregnated.

[0024]

The lithium ion secondary battery according to the present invention is lighter in weight than the conventional lithium ion secondary battery and can provide a high energy density by using TCNQ, which is an organic material, as the positive electrode active material. it can.

[Brief description of the drawings]

FIG. 1 is a structural diagram of one embodiment of a lithium ion secondary battery according to the present invention.

[Explanation of symbols]

 1 Positive electrode 2 Negative electrode 3 Separator 4 Case

Claims (2)

[Claims] 1. A positive electrode for a lithium ion secondary battery, comprising a lithium salt of 7,7,8,8-tetracyanoquinodimethane (TCNQ) as a positive electrode active material. 2. A lithium ion secondary battery comprising: the positive electrode according to claim 1; a negative electrode made of a lithium alloy or a substance occluding lithium; and a separator for separating the two electrodes.