JPH08162114A - Lithium secondary battery - Google Patents

Abstract

PURPOSE: To provide a lithium secondary battery capable of restraining increase in the contact resistance between a positive electrode active material and conductive agent caused by charge/discharge operation of a battery and having an improved cycle characteristic without decrease in capacity. CONSTITUTION: A positive electrode active material used for a positive electrode 1 of lithium secondary battery is composed of composite powder in which Lia MbO2 layer is formed on the surface of LixMnyO2 , powder, where M is one or two kind or more of elements selected from transition metal elements such as Cr, Fe, Co, Ni.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to a positive electrode active material thereof.

[0002]

_2. Description of the Related Art Conventionally, layered compounds such as lithium cobalt oxide and lithium nickel oxide, which can be represented by the general formula LiAO ₂ , have been used as a positive electrode active material for lithium secondary batteries, and LiMn ₂ O. A series of lithium manganese oxides including ₄ and LiMnO ₂ have been used as positive electrode materials.

[0003] However, lithium cobalt oxide and lithium nickel oxide have a problem in that cobalt, which is the main element, is expensive and has a resource problem, and that lithium nickel oxide has a material cost advantage due to the difficulty of the synthesis method. Since there is a problem such as the lack of a manganese oxide, there is a strong demand for the development of manganese oxide based on the demand for cost reduction.

However, common LiMnO ₂ and Li
A series of lithium manganese composite oxides containing Mn ₂ O ₄ are:
It is necessary to secure sufficient electron conduction due to problems such as high electron resistance and insufficient contact with the conductive agent that is responsible for electron conduction due to particle destruction due to expansion and contraction of particles during charging and discharging. Since the amount of conductive agent and the type of conductive agent are restricted in the configuration, the energy density is lowered.
There is also a problem that the cycle characteristics are not good due to distortion of the crystal lattice due to repeated charging and discharging, and a part where electron conduction between particles and diffusion of lithium ions are blocked due to particle destruction.

As a measure against these problems, Li _x M
By substituting manganese in n _y O ₂ with another element, attempts to reduce the electronic conductivity of the active material itself and particle distortion due to charge / discharge have been actively conducted, and the performance of the active material itself has been actually investigated. It is known that there is an effect on the improvement (JP-A-4-141954).

[0006]

However, for example, LiMn ₂ O ₄ having a spinel structure improves the cycle characteristics by substituting a part of manganese with another element, but on the other hand, the electron resistance of the particles is sufficiently reduced. Since this is not possible, a large amount of conductive agent is required in the positive electrode. Alternatively, since there is no elemental species that changes the valence involved in charging / discharging at a potential near 4 V in the spinel structure by substituting another element, lithium ions that do not participate in the charging / discharging reaction are generated correspondingly, and the capacity decreases as an adverse effect. There are still problems such as doing.

[0007]

The present invention as means for solving the above problems BRIEF SUMMARY OF THE INVENTION may, a positive electrode active material of a lithium secondary battery Li _x Mn _y O ₂ and Li _a M _b O ₂ (where M is Cr , Fe,
It is a composite powder with one or more elements selected from transition metal elements such as Co and Ni).

The composite powder is Li _x Mn _y O ₂ (where x and y are 0 <x ≦ 1.15 and 0.85 ≦ y ≦, respectively).
1.30) on the surface of the powder Li _a M _b O ₂ (where a and b are 0 <a ≦ 1.15 and 0.85 ≦ b ≦, respectively).
1.30) is formed.

In the positive electrode active material, _x of Li _x Mn _y O ₂ ,
The y value is in the range of 0 <x ≦ 1.15, 0.85 ≦ y ≦ 1.30, which is effective for a series of lithium manganese composite oxides, and other than Li _a M _b O _2. The element M is Cr, Fe, Co, N which participates in the charge / discharge reaction at a potential near 4V.
i is preferably a lithium composite oxide, especially LiCoO ₂ , L
iNiO ₂ is good.

In order to obtain the composite powder of the present invention, for example, LiMn ₂ O ₄ particles having a spinel structure are formed on the surface of LiMn ₂ O ₄ particles.
Taking a composite having a CoO ₂ layer as an example, a general method (for example, lithium carbonate Li ₂ CO ₃ and dimanganese trioxide M) is used.
n ₂ O ₃ was mixed at a molar ratio of 1 + x: 2 and calcined at about 800 ° C.) was used to synthesize Li _{1 + x} Mn ₂ O ₄ and the powder was crushed to contain x mol of cobalt. After dispersion in a cobalt nitrate Co (NO ₃ ) ₂ aqueous solution and thorough stirring, water is evaporated. By recalcining this powder at a relatively low temperature of about 300 to 400 ° C., a surface layer of LiCoO ₂ can be formed on the surface of the LiMn ₂ O ₄ particles without the diffusion of cobalt in the LiMn ₂ O ₄ crystal structure. .

[0011]

The lithium secondary battery using the positive electrode active material according to the present invention has a series of lithium manganese composite oxides or a positive electrode active material in which a part of manganese in the lithium manganese composite oxide is replaced with a different element. In comparison, the particle surface is coated with Li _a M _b O ₂ , the electron conductivity is good, and the particle destruction due to the expansion and contraction of the particle due to the charge and discharge of the battery is suppressed, and the surface layer is Forming L
Since iCoO ₂ or the like contributes to the reaction of the battery, the capacity of the battery does not decrease as compared with the case where it is replaced with another element, and thus the battery performance is remarkably improved.

[0012]

EXAMPLES The present invention will be described below by taking an example of LiMn ₂ O ₄ having a spinel structure and using cobalt as a composite element.

Example 1 Lithium carbonate Li ₂ CO ₃ and dimanganese trioxide Mn ₂ O ₃ were mixed in a molar ratio of Li: Mn = 1.0 + x: 2.0 and heat-treated at 800 ° C. to synthesize. Li
_{1 + x} Mn ₂ O ₄ was dispersed in an aqueous cobalt salt solution containing x mol of cobalt, stirred sufficiently, and water was evaporated.
It was obtained LiMn ₂ O ₄ -LiCoO ₂ composite powder by calcining again at a relatively low temperature of about 00-400 ° C..

The composite powder obtained by the above method was used as a positive electrode active material to prepare a coin-type lithium secondary battery shown in the sectional view of FIG.

First, for the positive electrode 1, the above positive electrode active material powder is used, and acetylene black is used as a conductive agent and polytetrafluoroethylene powder is used as an adhesive in 80:16:
The mixture was kneaded in a weight ratio of 4 and formed into a sheet having a thickness of 0.5 mm by a roller press, and then punched into a circle having a diameter of 16 mm, which was pressed to the positive electrode can 4 through the positive electrode current collector 6. I was there.

The negative electrode 2 was used by punching out a lithium foil having a thickness of 0.3 mm into a circular shape having a diameter of 15 mm and press-bonding it to the negative electrode can 5 via the negative electrode current collector 7.

On the other hand, a polypropylene microporous membrane is used for the separator 3, and the electrolyte is L in a mixed solvent of propylene carbonate and diethyl carbonate (volume ratio 1: 1).
A solution of iClO ₄ dissolved in 1 mol / l was used.

A coin type lithium battery having a diameter of 20 mm and a thickness of 1.6 mm was produced using the positive electrode 1, the negative electrode 2, the separator 3, the electrolytic solution and the insulating packing 8. This battery is designated as A1.

Example 2 The positive electrode active material synthesized in the above Example 1 was used, and the weight ratio of positive electrode active material: conductive agent: binder was 9 for the positive electrode.
Battery A2 of the present invention was produced in exactly the same manner as in Example 1 except that the ratio was set to 0: 6: 4.

Comparative Example 1 Lithium carbonate Li ₂ CO ₃ and dimanganese trioxide Mn ₂ O ₃ were weighed so as to have a molar ratio of Li: Mn = 1.0: 2.0,
By mixing and heat treating at about 800 ° C., LiMn
_{A 2} O ₄ positive electrode active material was obtained. Comparative Example Battery B1 was prepared in exactly the same manner as Example 1 except that this positive electrode active material was used.

Comparative Example 2 The LiMn ₂ O ₄ positive electrode active material synthesized in Comparative Example 1 was used, and the weight ratio of positive electrode active material: conductive agent: binder was 90: 6: 4 as the positive electrode. A comparative battery B2 was manufactured in exactly the same manner as in Example 1.

Comparative Example 3 Li: Mn: Co = lithium carbonate Li ₂ CO ₃ , dimanganese trioxide Mn ₂ O ₃ and cobalt hydroxide Co (OH) ₂
LiMn _{1.9 was} obtained by weighing and mixing so as to obtain a molar ratio of 1.0: 1.9: 0.1, and heat-treating at about 800 ° C.
A Co _0.1 O ₄ positive electrode active material was obtained. Comparative Example Battery B3 was produced in exactly the same manner as Example 1 except that this positive electrode active material was used.

COMPARATIVE EXAMPLE 4 Except that the LiMn _1.9 Co _0.1 O ₄ positive electrode active material synthesized in Comparative Example 3 was used and the weight ratio of positive electrode active material: conductive agent: binder was 90: 6: 4. Comparative Example Battery B4 was prepared in exactly the same manner as in Example 1.

Batteries A1, A2, B produced in this way
2 and 3 show the capacity-cycle characteristics for 1, B2, B3 and B4.

From FIG. 2, the battery A1 of the present invention is the comparative battery B.
It can be seen that the initial capacity is higher and the cycle characteristics are improved as compared with 1 and B3.

Further, it can be seen from FIG. 3 that the battery A2 of the present invention has good cycle characteristics as compared with the batteries B2 and B4 of the comparative example, with no decrease in capacity observed from the beginning of the cycle.

It is clear from FIG. 2 that the cycle characteristics are improved by substituting a part of manganese in LiMn ₂ O ₄ with cobalt, but the initial capacity is lowered by the substitution amount by cobalt. I understand. On the other hand, as in the present invention, the surface of the LiMn ₂ O ₄ particles is coated with LiCoO 2.
_The LiMn ₂ O ₄ —LiCoO ₂ composite oxide coated with _two layers can improve the cycle characteristics without lowering the initial capacity. It is speculated that this is because LiCoO ₂ coated on the surface of the LiMn ₂ O ₄ particles participates in the charge / discharge reaction of the battery.

From FIG. 3, the positive electrode active material of the present invention has good capacity-cycle characteristics at a level where the weight fraction of the conductive agent in the positive electrode is close to that of a practical battery, and LiMn ₂ O ₄
LiMn _2-x Co _x partially replaced with manganese and cobalt
It is clear when compared with the comparative batteries B2 and B4 of O ₄ .
It is considered that this is because the particle surface of LiMn ₂ O ₄ is coated with LiCoO ₂ , and the contact resistance with the conductive agent, which is the essence of LiMn ₂ O ₄ , is reduced. In addition, it is considered that electrons smoothly flow through LiCoO ₂ coating the particle surface, and it is speculated that the electrochemical reaction of the active material particles is promptly performed.

The present invention has been described above using LiMn ₂ O ₄ having a spinel structure as an example and using cobalt as a complex element. However, the present invention is described as Li _x Mn _y O ₂ (where x and y are 0 < x ≦ 1.15, 0.85 ≦ y ≦
It is effective for a series of lithium manganese composite oxides that can be represented by 1.30), and is also effective as a composite element for transition metal elements such as Cr, Fe, and Ni.

[0030]

INDUSTRIAL APPLICABILITY In the battery of the present invention, Li _a M _b O is formed on the surface of particles of lithium manganese composite oxide which is a positive electrode active material.
_{Since two} layers are formed, it is possible to suppress an increase in contact resistance between the positive electrode active material and the conductive agent due to charge and discharge of the battery, and it is possible to improve cycle characteristics without lowering the capacity, and as a whole. The energy density of the battery can be improved.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a coin type lithium secondary battery according to an example battery of the present invention.

FIG. 2 is a diagram showing capacity-cycle characteristics of batteries of Example A1 and Comparative Examples B1 and B3.

FIG. 3 is a diagram showing capacity-cycle characteristics of Example A2 and Comparative Examples B2 and B4 batteries.

[Explanation of symbols]

 1 Positive electrode 2 Negative electrode 3 Separator 4 Positive electrode can 5 Negative electrode can 6 Positive electrode current collector 7 Negative electrode current collector 8 Insulation packing

Claims (2)

[Claims] 1. A positive electrode active material Li _x Mn _y O ₂ and Li _a M _b
A lithium secondary battery comprising a composite powder with O ₂ (where M is one or more elements selected from transition metal elements such as Cr, Fe, Co and Ni). Wherein said composite powder is, Li _x Mn _y O ₂ (where x, y respectively 0 <x ≦ 1.15,0.85 ≦ y
≦ 1.30) on the surface of the powder Li _a M _b O ₂ (where a and b are 0 <a ≦ 1.15 and 0.85 ≦ b ≦, respectively).
1.30) is formed to form a layer.
The lithium secondary battery described.