JPH08162115A - Lithium secondary battery - Google Patents

Abstract

PURPOSE: To improve cycle characteristics without decrease in capacity with a very small amount of substitution by performing uniform substitution in a lithium secondary battery using a positive electrode active material where part of manganese in lithium manganese composite oxide is substituted with another element. CONSTITUTION: A positive electrode active material used for a lithium secondary battery positive electrode 1 is composed of lithium manganese composite oxide composed by using a manganese compound where part of manganese is preliminarily substituted with another element M as the raw material. M is one or two kind or more of elements selected from nonmetal elements and semi-metal elements of IIIB, IVB and VB group in the periodic table.

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, a general formula LiAO ₂ has been used as a positive electrode active material for a lithium secondary battery from the viewpoint of high energy density.
A layered compound such as lithium cobalt oxide or lithium nickel oxide that can be represented by the following, or a series of lithium manganese composite oxides containing LiMn ₂ O ₄ and LiMnO ₂ is used as a positive electrode material.

In the above-mentioned positive electrode active material, a series of lithium manganese composite oxides causes crystal structure destruction due to the progress of cycle, or particle destruction due to expansion and contraction of particles causes insufficient contact with a conductive agent that is responsible for electron conduction. Due to this, there was a problem that the capacity decreased with the passage of cycles.

As a measure against these problems, by substituting a part of manganese in the lithium-manganese composite oxide with another element, it is possible to reduce the crystal structure strain due to charge and discharge of the active material itself and improve the electronic conductivity. A lot of attempts have been made aiming at it, and it is known that it is actually effective in improving the performance of the active material itself (Japanese Patent Laid-Open No. 14195/1992).
4 publication).

[0005]

As a method for synthesizing a compound in which a part of manganese in the lithium-manganese composite oxide is substituted with another element, generally, a lithium compound, a manganese compound, and a desired substitution element-containing compound are used. Firing the powder mixture.

However, according to this method, since the raw material powders are mixed and fired, the solid phase reaction between lithium and other elements must occur continuously or simultaneously in the manganese raw material. Therefore, the reaction is likely to be uniform and sufficient substitution cannot be obtained, or LiM synthesized at a relatively low temperature is used.
The problem with nO ₂ is that the solid phase reaction does not proceed sufficiently.

Further, as described above, LiMn ₂ O ₄ has improved cycle characteristics by substituting manganese with another element, but in the spinel structure, changes in valence involved in charge and discharge occur at a potential near 4V. Since there is no elemental species to perform, lithium ions that do not participate in the charging / discharging reaction are generated correspondingly, and there is a problem that the capacity decreases as an adverse effect.

The present invention has been made to solve the above problems, and an object thereof is to easily and uniformly prepare a compound in which a part of manganese of a lithium manganese composite oxide is replaced with another element. Intended for synthesis.

[0009]

In the lithium secondary battery of the present invention for solving the above problems, the positive electrode active material is previously composed of other elements (non-metallic elements and semi-metallic elements of Group IIIB, IVB and VB of the periodic table). , One or more elements selected from metal elements other than alkaline earth metals and Mn)
A lithium-manganese composite oxide obtained by firing a manganese compound substituted with and a lithium compound is used.

[0010]

In the lithium secondary battery using the lithium manganese composite oxide according to the present invention, since the positive electrode active material is synthesized by a reaction consisting of only a manganese compound in which manganese is replaced with another element in advance and a lithium source, Is performed uniformly, the effect of substituting other elements is sufficiently exhibited, and the battery performance is improved.

Further, since the other element is already substituted in the manganese compound, the solid-phase (diffusion) reaction with lithium is smoothly performed, and the other element having a larger ionic radius than the manganese ion is present. It becomes easier to replace, and as a result, the synthesizing method such as shortening the firing time and lowering the firing temperature is simplified, and it is difficult to substitute manganese with other elements because it is synthesized at a low temperature like LiMnO ₂ . Can also be synthesized.

Further, LiMn ₂ O ₄ has improved cycle characteristics by substituting manganese with another element, but in the spinel structure, there is no element species that changes the valence involved in charge and discharge at a potential near 4 V, and therefore, the Lithium ions that do not participate in the charge / discharge reaction are generated, and as a reverse effect, the capacity decreases. However, since the other element is uniformly substituted in the lithium manganese composite oxide of the present invention, it is possible to improve the cycle characteristics with a small amount of substitution, and the capacity decrease due to the substitution of other element is small, so that the conventional synthesis The active material performance is improved over that obtained by the method.

Of the above lithium manganese composite oxides,
One particularly preferred one is LiMn _2-a C synthesized using a series of manganese oxides represented by Mn _x Co _y O _z.
o _a O ₄ .

This LiMn _2-a Co _a O ₄ composite powder is
For example, lithium carbonate Li ₂ Co ₃ and MnO
₂ and Co (OH) ₂ were mixed and heat treated at a predetermined temperature.
n _x Co _y O _z is Li / (Mn + Co) = 1.0 / 2.0
Weigh and mix to obtain a molar ratio of 800
It can be obtained by heat treatment at about ° C.

Suitable x, y and z values of the manganese raw material are 0.8 ≦ x <1, 0 <y ≦ 0.2 and x + y, respectively.
= 1,1 ≦ z <2.2. If the x, y, and z values are outside the above range, the effect of substitution of other elements will not be exhibited, or the energy density will be reduced, which is not preferable.

As a more preferable lithium manganese composite oxide, LiMn _1-α C synthesized by using a series of manganese oxides represented by γ-Mn _1-α Co _α OOH.
o _α O ₂ may be mentioned.

This LiMn _1-α Co _α O ₂ composite powder is, for example, γ-composited and composited by a coprecipitation method at the stage of liquid phase synthesis of lithium hydroxide LiOH.H ₂ O and γ-MnOOH. Mn _1-α Co _α OOH to Li / M
Weigh so that the molar ratio of n + Co = 1.0 / 1.0,
It can be obtained by mixing and heat-treating at about 450 ° C. in a nitrogen stream.

The preferable α value of the manganese raw material is 0 <α
≦ 0.3. If the α value is outside the above range, the effect of substituting other elements will not be exhibited, or the energy density will decrease, which is not preferable.

[0019]

EXAMPLES The present invention will be described below based on examples in which cobalt is used as a substitution element.

Example 1 LiMn using Li ₂ CO ₃ and Mn ₂ O ₃ as main raw materials
_In the synthesis of _1.95 Co _0.05 O ₄ , Mn ₂ O _{3 was} previously prepared.
Mn _1.95 Co _0.05 O ₃
And Li ₂ CO ₃ are Li: (Mn + Co) = 1.0: 2.0
The raw materials are weighed and mixed so that the molar ratio becomes 80
The compound was obtained by baking at 0 ° C.

The compound 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, to which acetylene black as a conductive agent and polytetrafluoroethylene powder as a binder are mixed in a weight ratio of 80: 1.
After kneading at a weight ratio of 6: 4 and molding this into a sheet having a thickness of 0.5 mm by a roller press, the diameter is 14 mm.
The circular punched product was used by pressure bonding to the positive electrode can 4 via the positive electrode current collector 6.

As the negative electrode 2, a lithium foil having a thickness of 0.3 mm was punched into a circular shape having a diameter of 16 mm, and the negative electrode can was pressed through the negative electrode current collector 7 and used.

On the other hand, a polypropylene microporous film is used as the separator 3, and the electrolyte is Li in a mixed solvent of propylene carbonate and diethyl carbonate (volume ratio 1: 1).
The ClO ₄ was used after dissolving 1mol / l.

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.

[0026] Except for using Example 2 using Mn _1.90 Co _0.10 O ₃ in the synthesis of the positive electrode, synthetic LiMn _{1 .90} which obtained was then Co _0.10 O ₄ positive electrode active material, in the same manner as in Example 1 Inventive Battery A
2 was produced.

Example 3 LiOH.H ₂ O and γ-type manganese oxide hydroxide (γ-Mn)
(OOH) as the main raw material in the synthesis of LiMn _0.95 Co _0.05 O ₂ , γ-Mn _0.95 Co _0.05 OO in which manganese in γ-type oxidized manganese hydroxide was replaced with cobalt in advance.
H and LiOH.H ₂ O are Li: (Mn + Co) = 1.
Weigh and mix the raw materials so that the molar ratio is 0: 1.0,
The compound was obtained by firing at 450 ° C. in a nitrogen atmosphere.

The battery A of the present invention was manufactured in the same manner as in Example 1 except that the positive electrode active material powder obtained by the above method was used.
3 was produced.

Example 4 In exactly the same manner as in Example 1 except that γ-Mn _0.90 Co _0.10 OOH was used in the synthesis of the positive electrode and the LiMn _0.90 Co _0.10 O ₂ positive electrode active material powder obtained by the synthesis was used. The present invention battery A4 was produced.

Comparative Example 1 A battery B1 was prepared in the same manner as in Example 1 except that a LiMn ₂ O ₄ positive electrode active material synthesized by using Li ₂ Co ₃ and Mn ₂ O ₃ was used as a raw material. .

Comparative Example 2 Except for using LiMn _1.95 Co _0.05 O ₄ positive electrode active material synthesized by using Li ₂ CO ₃ , Mn ₂ O ₃ and Co (OH) ₂ as a raw material, the same as Example 1. Battery B2 was prepared in the same manner.

Comparative Example 3 Except for using LiMn _1.90 Co _0.10 O ₄ positive electrode active material synthesized by using Li ₂ CO ₃ , Mn ₂ O ₃ and Co (OH) ₂ as a raw material, the same as Example 1. Battery B3 was produced in the same manner.

Comparative Example 4 A battery B4 was prepared in the same manner as in Example 1 except that a LiMnO ₂ positive electrode active material synthesized by using LiOH.H ₂ O and γ-MnOOH was used as a raw material.

Comparative Example 5 As raw materials, LiOH.H ₂ O, γ-MnOOH, Co (O
A battery B5 was made in exactly the same manner as in Example 1 except that the LiMn _0.95 Co _0.05 O ₂ positive electrode active material synthesized using H) ₂ was used.

Comparative Example 6 LiOH.H ₂ O, γ-MnOOH, Co (O
A battery B6 was manufactured in exactly the same manner as in Example 1 except that the LiMn _0.90 Co _0.1 O ₂ positive electrode active material synthesized using H) ₂ was used.

Batteries A1, A2, A produced in this way
Tables 1 and 2 show the initial capacity (mAh) of 3, A4, B1, B2, B3, B4, B5 and B6 and the capacity (mAh) after 50 and 100 cycles.

[0037]

[Table 1]

[0038]

[Table 2]

As is clear from Table 1, Examples A1 and A2 using the lithium manganese composite oxide of the present invention have better cycle characteristics than Comparative Examples B1, B2 and B3. Further, in the battery of the example, compared with the battery of the comparative example, the cycle characteristics are good and the capacity is large even if the substitution amount of other element (cobalt) other than manganese is small.

LiMn ₂ O ₄ has improved cycle characteristics by substituting manganese with another element, but in the spinel structure, since there is no element species that changes the valence involved in charging / discharging in the vicinity of 4 V, charging / discharging correspondingly. Lithium ions that do not participate in the reaction are generated, and as a reverse effect, the capacity decreases. However, since the lithium-manganese composite oxide of this example is uniformly substituted with cobalt, it is possible to improve the cycle characteristics with a small amount of substitution, and the capacity reduction due to cobalt substitution is small, so the conventional synthesis method The performance of the active material is higher than that obtained in the above.

As is clear from Table 2, Examples A3 and A4 using the lithium manganese composite oxide of the present invention.
It is clear that, as compared with Comparative Examples B4, B5 and B6, the capacity is large and the cycle characteristics are good.

Further, the cycle characteristics of the batteries of the Examples were improved by replacing the manganese with cobalt, but the capacity of the batteries of the Comparative Example decreased with the substitution of cobalt for the manganese, and no improvement in the cycle characteristics was observed.

Since LiMnO ₂ is synthesized at a low temperature, cobalt does not sufficiently undergo solid phase reaction with the manganese raw material in the heat treatment of the powder mixture of the lithium raw material, the manganese raw material, and the cobalt raw material. Remains,
This is because the capacity is reduced accordingly.

The lithium secondary battery in which cobalt is used as an example of the substitution element of manganese as the positive electrode active material in the present invention has been described above. The substitution element M in the present invention has been described above.
Is not limited to cobalt, but the periodic table III
Non-metal elements and metalloid elements of B, IVB and VB groups,
One or more elements selected from alkaline earth metals and metal elements other than Mn are targeted, and are effective as well as cobalt.

[0045]

EFFECTS OF THE INVENTION In the battery of the present invention, the lithium manganese composite oxide, which is the positive electrode active material, is synthesized using the manganese raw material in which manganese has been previously substituted with another element, so that uniform substitution is performed. Therefore, it is effective in improving cycle characteristics with a small amount of substitution and without reducing the capacity.

[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.

[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

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[Procedure amendment]

[Submission date] December 29, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0006

[Correction method] Change

[Correction content]

However, according to this method, since the raw material powders are mixed and fired, the solid-state reaction between lithium and multi-elements must occur continuously or simultaneously in the manganese raw material. Therefore, the reaction tends to be non- uniform, and when sufficient substitution cannot be obtained, or when Li synthesized at a relatively low temperature is used.
MnO _{2 and the} like have a problem that the solid phase reaction does not proceed sufficiently.

Claims (3)

[Claims] 1. A part of manganese is used as another element M in advance.
(Where M is a non-metal element and a metalloid element of Group IIIB, IVB, and VB of the periodic table, an alkaline earth metal, Mn
Lithium secondary manganese oxide prepared by using as a raw material a manganese compound substituted with one or more elements selected from metal elements other than battery. 2. The manganese compound is Mn _x M _y O.
_z (x, y, z are 0.8 ≦ x <1, 0 <y ≦
0.2, x + y = 1, 1 ≦ z <2.2)), The lithium secondary battery according to claim 1. 3. The lithium secondary battery according to claim 1, wherein the manganese compound is γ-type manganese oxide hydroxide Mn _1-α M _α OOH (0 <α ≦ 0.3).