JPH07307164A - Lithium ion secondary battery - Google Patents

Abstract

PURPOSE:To provide a lithium ion secondary battery with improved overdischarge resistance and high energy density. CONSTITUTION:A negative electrode contains heat treated meso-phase pitch spherules or meso-phase pitch green coke powder, covered with an oxygen- containing compound, and a positive electrode comprises a lithium composite oxide and a lithium manganese oxide, capable of doping/undoping lithium ions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement in over-discharge resistance of lithium ion secondary batteries.

[0002]

2. Description of the Related Art In recent years, a lithium ion secondary battery is a storage battery having a high energy density which can be reduced in size and weight, and includes an integrated video camera, a CD player, an MD player, a personal computer, a portable information data terminal, It has attracted attention as a power source for cordless portable electronic devices such as mobile phones. Generally, in the negative electrode of a lithium ion secondary battery,
Carbonaceous material powder such as petroleum pitch coke or coal pitch coke is used, and LiCoO ₂ or LiNiO ₂ is used for the positive electrode. Copper foil is used as a current collector for the negative electrode. However, even if cutoff is performed at a predetermined battery voltage, unless the leakage current of 1 μA or less is set in the electronic switch, it is over-discharged by leaving it. When the battery voltage drops to around 0 V due to over-discharge, the potential of the negative electrode rises, copper used as the negative electrode current collector is eluted, and the battery characteristics are significantly deteriorated. The conventional lithium-ion secondary battery has a drawback that it is necessary to add a control circuit to prevent such an over-discharged state and cannot be handled easily.

[0003]

SUMMARY OF THE INVENTION The present invention provides a high energy density lithium ion secondary battery for the purpose of improving the overdischarge resistance while maintaining a high discharge capacity.

[0004]

In order to achieve the above-mentioned object, the present invention comprises a heat-treated product of mesophase pitch globule or mesophase pitch green coke powder in which the negative electrode is mainly coated with an oxygen-containing compound, and the positive electrode is A lithium ion secondary battery mainly composed of a lithium metal composite oxide capable of being doped and dedoped with lithium ions and a lithium manganese oxide.

The negative electrode active material of the present invention includes a carbonaceous material obtained by coating the surface of mesophase pitch spherules or ground mesophase pitch green coke powder with an oxygen-containing compound and then heat-treating. For example, centrifugal separation of mesophase pitch spheres generated when tar pitch obtained by naphtha decomposition, crude oil decomposition, coal thermal decomposition, asphalt decomposition, etc. is heated from 300 ° C to 500 ° C, or small spheres are further fused. Separated lumps, crushed green coke powder into phenol resin,
Coated with furan resin, furfuryl alcohol and other oxygen-containing compounds, 800 ° C to 1300 ° C, preferably 900 ° C
To 1200 ° C. to a heat treatment in an inert gas atmosphere.

As a coating method, either a dry method or a wet method can be used. For example, a dry method by a high-speed air impact method using a high-pretizer or a raw material powder in a solution in which an oxygen-containing compound is dissolved in a medium is used. Examples thereof include a wet method of dispersing and evaporating a medium to dry, but the method is not particularly limited to these methods. The heat-treated product (A) of the present invention has a true density ρ (g / cm ³ ) of 1.7 ≦ ρ ≦ 2.
1, the crystallite thickness Lc is 0.8 nm to 10 nm, and the carbon interlayer distance d ₀₀₂ is preferably 0.35 nm to 0.37 nm. When the true density ρ, the crystallite thickness Lc, and the carbon interlayer distance d ₀₀₂ , which are the parameters expressing the physical properties of the heat-treated product of the present invention, deviate from the above ranges, a secondary battery that withstands over-discharge while maintaining a high discharge capacity is obtained. It is hard to become.

[0007] If necessary for the carbonaceous material, coke capable of doping / de-doping with lithium ions, graphite having low crystallinity or amorphous carbon, and graphite may be added and used. If necessary, graphite, carbon black or the like can be added and used as a conductive auxiliary agent between the active material particles. The lithium metal composite oxide that can be doped / undoped with lithium ions, which is the positive electrode active material of the present invention, uses lithium as a reference electrode (0
The potential defined as V) may be 3 V or more. For example, lithium cobalt oxide: Li _x CoO ₂ (0 <x ≦ 1) Li _x Co _y M _z O ₂ (where M is Al, In, S
n, Zr, Mn, and Ti represent at least one metal, and x, y, and z are 0 <x ≦ 1.1, 0.
It represents a number of 85 ≦ y ≦ 0.99 and 0.002 ≦ z ≦ 0.15. ) Li _x Co _y Ni _z O ₂ (0 <x ≦ 1, y + z = 1) Lithium nickel oxide: Li _x NiO ₂ (0 <x ≦ 1), Li _x Ni _y M _z O ₂
(However, M represents at least one metal selected from Mn, Fe, Ti, and Co, and x, y, and z are 0 <x ≦ 1 and 0.85 ≦ Y ≦ 0.95, respectively. , 0.05 ≦ z ≦
Represents a number of 0.15. ) And the like.

Lithium which is another positive electrode active material of the present invention
Lithium manganese that can be doped and dedoped with um ions
In the oxide, lithium is the potential from the reference electrode, that is, the metal
The potential based on the redox potential of thium as 0V is low.
If both have a potential of 2.1V to 2.9V,
Good. For example, Li_xMnO₂(0 <x ≦ 1), Li _x
Mn₂O_Four(0.5 ≦ x <2), Li_xMnO₃(0 <
x ≦ 1), LiCo_xMn_2-xO_Four(0 <x ≦ 0.2)
Etc.

The amount of positive electrode combined with the negative electrode of the present invention is
270 to 370 parts by weight of a lithium metal composite oxide (B) capable of doping and dedoping lithium ions, and 10 to 70 parts by weight of lithium manganese oxide (C) based on 100 parts by weight of the negative electrode (A). Used. When the lithium metal composite oxide (B) capable of doping / dedoping lithium ions is less than 270 parts by weight, the amount of lithium ions released from the positive electrode and used for charging is small,
If the battery does not have a high capacity and the amount is more than 370 parts by weight, the negative electrode side is overcharged, metal Li is deposited, and the battery characteristics are deteriorated. Further, when the lithium manganese oxide (C) is less than 10 parts by weight, the over-discharge resistance function is not exhibited, and when it is more than 70 parts by weight, the rapid chargeability is impaired and the battery capacity cannot be increased.

Electrolyte of non-aqueous electrolyte used in the present invention
For example, LiClO_Four, LiAsF₆, Li
_xPF₆, LiBF_Four, CH₃SO₃Li, (CF₃S
O ₂)₂Any one or two of lithium salts such as NLi
A mixture of two or more species can be used. Also, the electrolytic solution
Examples of the solvent include propylene carbonate and ethyl ether.
Ren carbonate, dimethyl carbonate, diethyl carbonate
Carbonate, 1,2-dimethoxyethane, 1,2-dieto
Xyethane, γ-butyrolactone, tetrahydrofuran
, 2-methyltetrahydrofuran, 1,3-dioxo
Orchid, sulfolane, methylsulfolane, acetonitril
, Propionitrile, methyl formate, ethyl formate, acetic acid
Methyl, ethyl acetate, butyl acetate, hexyl acetate, pro
Methyl pionate, ethyl propionate, propionate
Chill, hexyl propionate, trimethyl phosphate, phosphorus
Triethyl phosphate, triethylhexyl phosphate, triphosphate
Any one of octyl, trilauryl phosphate, etc., or
A mixture of two or more can be used.

As the separator used in the present invention, a single type of microporous film of polyolefin such as polyethylene or polypropylene or a single type or a laminated type of two or more types thereof can be used. Also, polyolefin,
Nonwoven fabrics such as polyester, polyamide, polyimide, and cellulose can also be used alone or in combination with the above porous membrane.

In constructing the lithium ion secondary battery of the present invention, active material particles are bonded to a current collector using a binder, and positive or negative is applied via a separator or via a Li ion conductive solid electrolyte. Of the electrode. Current collector,
The binder, separator, and solid electrolyte are not particularly limited, but the positive electrode current collector may be aluminum, nickel, stainless steel metal foil, mesh, porous metal, or the like having a thickness of 1 μm to 50 μm. Used.

In producing each of the positive electrode and the negative electrode, the active material particles and the binder are mixed and compression-molded as it is. After the binder and the active material particles are dispersed in an organic solvent, the coating is dried and further pressed if necessary. After dispersing the binder and active material particles in water, coating,
Dry and press if necessary. Any of these methods may be used. Alternatively, the active material particles may be mixed in advance and then the above-described methods 1 to 3 may be used.

In the lithium-ion secondary battery of the present invention, a metal such as stainless steel, nickel-plated steel, or aluminum is used as the exterior material, and the shape can be any shape such as a cylindrical shape, a sheet shape, and a square shape. . The lithium ion secondary battery of the present invention has a potential for a lithium electrode,
The heat-treated product of the surface of mesophase pitch particles capable of occluding lithium ions up to 2.8 V and having a high discharge electricity quantity was coated / treated with an oxygen-containing organic material was used as the negative electrode, and the positive electrode was used as the positive electrode during battery discharge. When lithium ions return, under normal use conditions of electronic equipment, that is, at a high rate discharge of 1 to 3 mA / cm ² in terms of current density of electrodes,
Cobalt- or nickel-based lithium composite oxide mainly works, and at low rate discharge due to weak leakage current flowing in the electronic circuit after switching off, the current density is 1 to 100 μA / cm ² , and lithium manganese oxide is also doped. Then, the potential of the positive electrode decreases, and when the potential is equal to or lower than the elution potential of copper, the potential becomes the same as that of the rising negative electrode, and the battery voltage becomes 0V. Therefore, even if the battery voltage is over-discharged to 0 V, the copper of the current collector can be prevented from being eluted, and if it is charged, the original battery performance is exhibited. The battery has excellent over-discharge resistance.

[0015]

EXAMPLES Examples of the present invention will be described below. However,
The following examples are examples for embodying the technical idea of the present invention, and the lithium ion secondary battery of the present invention is not limited to the following shapes, structures and members.

[0016]

Example 1 For a negative electrode, a surface of a mesophase pitch small sphere is coated with a phenol resin, and then heat-treated, indefinite-shaped carbon particles are applied to a copper foil using a binder and pressed. The positive electrode is lithium cobalt tin oxide and Li
A lithium manganese oxide of _1.5 Mn ₂ O ₄ and Li _0.9 MnO ₃ , acetylene black, and a conductive additive of graphite fine powder are coated on an aluminum foil with a binder and compression pressed.

A polyethylene microporous membrane is used for the separator. The electrolyte is lithium tetrafluoroborate (LiB
1 mol of F ₄ ) is used in a volume ratio of propylene carbonate: ethylene carbonate: γ-butyrolactone of 25: 2.
The one dissolved in a mixed solvent of 5: 50% is used. After spirally winding the electrode and the separator using these, it is made flat, and the thickness is 8.6 mm, the width is 34 mm, and the length is 4 mm.
An 8 mm prism size lithium ion secondary battery is assembled. Charged up to 4.20V at 700mA, 3
Discharge to 2.7 V at 50 mA. First discharge capacity is 9
It is 38 mAh. The lithium cobalt tin composite oxide of the positive electrode was 310 parts by weight, the Li _1.5 Mn ₂ O ₄ was 42 parts by weight, and the Li _0.9 MnO was 100 parts by weight of the negative electrode active material.
₃ is 5 parts by weight.

[0018]

[Comparative Example 1] For the negative electrode, pitch coke powder was used as in Example 1.
It is coated on copper foil with one binder and pressed.
The positive electrode is a lithium cobalt tin composite oxide (LiCo_0.97
Sn _0.03O₂) And acetylene black, conductive graphite fine powder
Apply the auxiliary agent to the aluminum foil with a binder and press
I will

Charging to 4.20V at 700mA,
Discharge to 2.7V. The first discharge capacity is 780mA
h. FIG. 1 shows the voltage of the example battery, and FIG. 2 shows the voltage of the reference lithium potential at the time of initial discharge and overdischarge, respectively.

[0020]

According to the present invention, a negative electrode containing a heat-treated product of mesophase pitch spherules or mesophase pitch green coke powder coated with an oxygen-containing compound, and a lithium metal composite oxide capable of being doped or dedoped with lithium ions. By using the positive electrode made of lithium manganese oxide and lithium manganese oxide, a high energy density lithium ion secondary battery with improved over-discharge resistance can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing discharge characteristics of a lithium ion secondary battery of an example.

FIG. 2 is a graph showing discharge characteristics of a conventional lithium-ion secondary battery of a comparative example.

Claims (2)

[Claims] 1. A lithium metal composite oxide in which the negative electrode contains a heat-treated product (A) of mesophase pitch spherules or mesophase pitch green coke powder coated with an oxygen-containing compound, and the positive electrode is capable of being doped / dedoped with lithium ions. Thing (B) and lithium manganese oxide (C)
A lithium-ion secondary battery comprising: 2. The true density ρ (g / cm) of the heat-treated product (A) is
³ ) is 1.7 ≦ ρ ≦ 2.1, and the crystallite thickness Lc is 0.8n.
m to 10 nm, the carbon interlayer distance d ₀₀₂ is in the range of 0.35 nm to 0.37 nm, the lithium metal composite oxide (B) is composed of at least one of cobalt and nickel, and the lithium manganese oxide (C) is The potential from the lithium reference electrode is 2.1V to 2.9V, and (A) 10
2. The lithium ion secondary battery according to claim 1, wherein (B) 270 to 370 parts by weight and (C) 10 to 70 parts by weight with respect to 0 parts by weight.