JPH11214003A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain capacity deterioration accompanying cycling, especially while keeping a high capacitance by improving electrochemical performance of a nonaqueous electrolytic solution battery that a spinel-based lithium manganese oxide is used as an electrode active material. SOLUTION: This lithium secondary battery uses lithium, including metallic oxide which can feeds/discharges lithium in an positive electrode. Lithium including metallic oxide is represented by a general formula (chemical formula 1): Li(Mn2-x /Lix )O4 (0<=X<=0.05), and nonuniform distortion η of a crystallite for a distortion X-ray diffraction pattern is within a range 0<=η<=1.25×10<-3> in formula (1); βcosθ=λ/D+2ηsinθ(where represents β integrated width (rad), θthe diffraction angles (degrees), λ is 1.54056 Å (CuK α), D the crystal size (Å), and η the non-uniform distortion of crystallite are represented respectively).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the improvement of a lithium secondary battery, particularly to the improvement of a positive electrode active material, and is intended to improve the charge / discharge capacity and cycle characteristics of the battery.

[0002]

2. Description of the Related Art As a positive electrode active material of a lithium secondary battery,
LiMn ₂ O ₄ which is a composite oxide of manganese and lithium
Has been proposed and research is being actively conducted. Although it has the characteristics of high voltage and high energy density, it has the problem that the charge / discharge cycle life is short, and has not been used as a practical battery. Until now, JP-A-7-
Or the _{Li 1 + X Mn 2-X} O 4 with an excess of lithium as disclosed such as 282,798, JP-A-3-10
_{8261, LiMn 2-X Co X} O 4 a part of manganese as disclosed in JP-A-3-219571, etc. Co, and replaced with other metals such as _{Cr, LiMn 2-X Cr X 0} 4
It has been proposed to improve the cycle characteristics by reforming the lithium manganese oxide.However, since these reforming methods cause a decrease in the charge / discharge capacity, the cycle characteristics are reduced without reducing the charge / discharge capacity. Improved lithium manganese oxides have been desired.

[0003]

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a spinel-based lithium manganese oxide having a small charge / discharge cycle and a small capacity deterioration while maintaining a high charge / discharge capacity. An object of the present invention is to provide a lithium ion secondary battery as a material.

[0004]

Means for solving the problem is to use a general formula as a main active material of the positive electrode.

[0005]

Embedded image Li (Mn _2-x Li _x ) O ₄

Lithium manganese having a non-uniform strain η in the X-ray diffraction pattern of a cubic spinel crystal represented by (0 ≦ X ≦ 0.05) in the range of 0 ≦ η ≦ 1.25 × 10 ^-3 The use of oxides. In a battery using a spinel-based lithium manganese oxide (LiMn ₂ O ₄ ) as a positive electrode active material, the charge / discharge reaction of the positive electrode in a 4 V (when Li counter electrode) region is represented by the following equation.

[0007]

[Number 2] _{LiMn 2 O 4 ⇔Li 1-n} Mn 2 O 4 + nLi + + ne - (2)

[0008] Conventionally, spinel lithium manganese oxide has a property that the charge / discharge function is easily lost when the above charge / discharge reaction is repeatedly performed. Therefore, an excessive amount of Li is charged for synthesis, or a part of the Mn site is changed to Co, C
By substituting with a metal such as r, the crystal structure can be made denser, and the deterioration of the charge / discharge function can be improved. But,
The conventional method focusing on the other cation substitution of the Mn site has a drawback that the capacity is reduced, so it cannot be said to be a fundamental improvement method. Thus, the present inventors have conducted intensive studies, and as a result, have found that there is a correlation between the battery characteristics and the non-uniform strain of crystallites, and that the battery characteristics decrease as the non-uniform strain increases. That is, a spinel-based lithium manganese oxide having a small capacity deterioration due to a charge / discharge cycle while maintaining a high capacity over a certain level has a Li / Mn molar ratio close to a constant ratio and a crystallite in an X-ray diffraction pattern. It was experimentally found that the non-uniform strain of was smaller than a specified value.

It is considered that such non-uniform distortion is generated for several reasons.
It is presumed that this has had a significant effect. One is Li /
When synthesizing a lithium manganese oxide having a Mn ratio close to a constant ratio, the closer the ratio is to 1/2, the more the Mn ratio increases.
The average valence decreases, and Mn is reduced as compared with the case of excess Li.
The proportion of ³⁺ Jahn-Teller ions increases. For this reason, uneven distortion is more likely to occur in the crystal structure.
Secondly, if the particle size of the Mn starting material varies, the reaction proceeds non-uniformly, thereby causing a composition variation and generating non-uniform distortion of crystallites. In this case, L
It is considered that non-uniform distortion is generated even if i is excessively large. It is considered that the cycle characteristics were poor for these reasons. Therefore, it is not easy to use a suitable starting material or to obtain a material within a range defined without applying a preferable synthesis method.

The correlation between the heterogeneous strain of cubic LiMn ₂ O ₄ and the cycle characteristics has already been described in the summary of the 64th Annual Meeting of the Institute of Electrical Engineers of Japan [3A1
0], p31 (1997), and Battery Engineering Committee Material 9-7, which point out that those with small non-uniform strain have good cycle characteristics. However, there is no mention of the amount of Li substitution at the Mn site or the range of non-uniform strain. In the present invention, the correlation between the X value and the non-uniform strain η of Li (Mn _2-x Li _x ) O ₄ and the capacity-cycle characteristic balance was examined.・
It was found that the cycle characteristic balance was excellent, and if it deviated from this, the performance satisfying the present invention was not exhibited.

From the above, the general formula Li (Mn _2-x Li _x )
By limiting the X value of O ₄ and limiting the non-uniform distortion of crystallites in the X-ray diffraction pattern, the cycle characteristics can be improved while maintaining a high capacity, and the present invention has been completed. That is, according to the present invention,
General formula

[0012]

Embedded image Li (Mn _2-x Li _x ) O ₄

(Where 0 ≦ X ≦ 0.05)
In addition, the non-uniform strain η of the crystallite in the X-ray diffraction pattern is given by the equation (1)

[0014]

(3) βcos θ = λ / D + 2η sin θ (1)

(Where β is the integral width (rad), θ is the diffraction angle (゜), λ is 1.54056 ° (CuKα), D is the crystallite size (Å), and η is the nonuniform strain of the crystallite. ), The use of a spinel-based lithium-containing metal oxide in the range of 0 ≦ η ≦ 1.25 × 10 ⁻³ results in a large initial discharge capacity and excellent cycle characteristics. With a discharge capacity of 115 mAh / g or more.

It should be noted that in the present invention, Li (
Mn _2-x Li _x ) O ₄ (provided that 0 ≦ X ≦ 0.05) where the Li excess amount in the cubic spinel crystal is 0 ≦
X ≦ 0.05, non-uniform strain η in X-ray diffraction pattern
^Satisfies the range of 0 ≦ η ≦ 1.25 × 10 ⁻³ , which indicates the range of the initial state before lithium is dedoped from the positive electrode active material in the battery system. . Outside of this range, it becomes difficult to obtain desired battery characteristics. Here, the more preferable upper limit of X is 0.30,
The most preferable upper limit is 0.20, the more preferable value of η is 1.00 × 10 ⁻³ or less, and the most preferable value is 0.75 × 1.
0 ^-3 or less. The amount of oxygen is formally described as 4, but its non-stoichiometric property is known and is not particularly limited, but the closer to the constant ratio, the more preferable.

Such Li (Mn _2-x Li _x ) O
₄ is produced by firing a mixture of a manganese compound and a lithium compound as starting materials. Mixing may be a usual method, a method of dry-mixing both raw materials, a method of wet mixing, a method of suspending a manganese compound in a lithium salt aqueous solution, a method of drying the suspension, a method of coprecipitation, or Any method can be used as long as it can be uniformly mixed, such as a method of pulverizing and mixing with a ball mill.

Lithium compound used for this starting material
As Li_TwoCO_Three, LiNO _Three, LiOH, Li
OH ・ H_TwoO, LiCl, CH_ThreeCOOLi, Li_TwoO
Licarboxylic acid Li and the like, among which LiOH.H_Two
Use of O, LiOH or Li dicarboxylate
preferable.

As the manganese compound, a manganese compound having no variation in particle size is preferable. Specifically, Mn
Manganese oxides such as ₂ O ₃ , MnCO ₃ , Mn (N
O ₃ ) ₂ , manganese salts such as manganese dicarboxylate and the like, among which Mn ₂ O ₃ and manganese dicarboxylate are preferably used. In this case, Mn ₂ O ₃
A material prepared by appropriately heat-treating a compound such as CO ₃ or MnO ₂ may be used. Since γ-MnO ₂ has a relatively large variation in the aggregated particle size,
It is not preferred to react the starting materials directly.

The lithium manganese oxide of the present invention is disclosed in, for example, Japanese Patent Application Nos. 08-120068, 8-125574,
It can be obtained relatively easily by using the methods disclosed in Japanese Patent Application Nos. 9-077235 and 9-089568.
As an example of a specific manufacturing method, γ-MnO ₂ is 800
Mn ₂ O ₃ and Li obtained by heating in air at 24 ° C. for 24 hours
A mixture obtained by mixing OH.H ₂ O at a molar ratio of Li and Mn of 1: 2 is calcined, then calcined at 800 ° C. for 24 hours in the air, and then 600 ° C., 500 ° C., 450 ° C. , 40
While maintaining the temperature at 0 ° C. and 350 ° C. for 3 hours, 1
° C / min. And a method in which a non-aqueous solution of a lithium salt and a manganese salt is co-precipitated by a dicarboxylate co-precipitation method, and a method of firing the powder is performed. As a cooling method of the sample at this time, rapid cooling is not preferable because oxygen deficiency tends to occur. The present invention is not limited by the manufacturing method described above.

The general formula Li (Mn _2-x L
i _x) O ₄ (provided that, 0 ≦ X ≦ 0. 05 ) has, as the negative electrode active material used in combination with a positive electrode made of the spinel type lithium manganese oxide which defines the scope of the non-uniform strain of crystallite, In general, any of the materials used for this type of non-aqueous electrolyte secondary battery can be used.

For example, lithium or a lithium alloy may be used, but a compound that can insert and release lithium with higher safety, particularly a carbon material, is preferable. This carbon material is not particularly limited, graphite, coal-based coke, petroleum-based coke, coal-based pitch carbide, petroleum-based pitch carbide, needle coke, pitch coke, phenolic resin, carbides such as crystalline cellulose and the like. Examples thereof include partially graphitized carbon materials, furnace black, acetylene black, and pitch-based carbon fibers.

As the negative electrode, a negative electrode active material and a binder (binder) slurried with a solvent, applied and dried, can be used. The positive electrode consists of a positive electrode active material and a binder
A slurry obtained by slurrying the binder and the conductive agent with a solvent is applied and dried. As the binder (binder) for the negative electrode and the positive electrode active material, for example, polyvinylidene fluoride, polytetrafluoroethylene, EP
DM (ethylene-propylene-diene terpolymer),
Examples include, but are not limited to, SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), and fluororubber.

Examples of the conductive agent for the positive electrode include, but are not limited to, graphite fine particles, carbon black such as acetylene black, and amorphous carbon fine particles such as needle coke. Also, when using a separator, usually, a microporous polymer film is used as a separator, nylon, cellulose acetate, nitrocellulose, polysulfone, polyacrylonitrile, polyvinylidene fluoride, polypropylene, polyethylene, polybutene, etc. What consists of a polyolefin polymer is used. The chemical and electrochemical stability of the separator is an important factor. In this respect, a polyolefin-based polymer is preferable, and it is preferable that the battery separator be made of polyethylene from the viewpoint of the self-closing temperature, which is one of the purposes of the battery separator.

In the case of a polyethylene separator, ultrahigh molecular weight polyethylene is preferable from the viewpoint of high-temperature shape retention, and the lower limit of the molecular weight is preferably 500,000, more preferably 1,000,000, and most preferably 1.5 million. On the other hand, the upper limit of the molecular weight is preferably 5,000,000, more preferably 4,000,000, and most preferably 3,000,000. If the molecular weight is too large, the fluidity is too low and the pores of the separator may not be closed when heated.

As the ionic conductor in the lithium secondary battery of the present invention, for example, known organic electrolytes, polymer solid electrolytes, gel electrolytes, inorganic solid electrolytes and the like can be used. Is preferred. The organic electrolyte is composed of an organic solvent and a solute.

The organic solvent is not particularly restricted but includes, for example, carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, chlorinated hydrocarbons, ethers, amines and esters. ,
Amides, phosphate compounds and the like can be used. When these representatives are listed, propylene carbonate, ethylene carbonate, vinylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 4-methyl-2-pentanone, 1,2-dimethoxyethane, 1,2 -Diethoxyethane, γ-butyrolactone, 1,3-dioxolane,
4-methyl-1,3-dioxolan, diethyl ether, sulfolane, methylsulfolane, acetonitrile,
Propionitrile, benzonitrile, butyronitrile,
A single solvent or a mixture of two or more solvents such as valeronitrile, 1,2-dichloroethane, dimethylformamide, dimethylsulfoxide, trimethyl phosphate, and triethyl phosphate can be used.

[0028] As the solute to be dissolved in the solvent, and any known can be used, LiCl _4, Li
AsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C
_{6 H 5) 4, LiCl,} LiBr, CH 3 SO 3 Li,
CF ₃ SO ₃ Li or the like is used. In the case of using a polymer solid electrolyte, a known polymer compound can be used, and it is particularly preferable to use a polymer compound having high ion conductivity with respect to lithium ions. For example, polyethylene oxide, polypropylene Oxide, polyethyleneimine and the like are preferably used. It is also possible to add the above solvent together with the above solute to the polymer compound and use it as a gel electrolyte.

When an inorganic solid electrolyte is used,
Use of known crystalline and amorphous solid electrolytes for inorganic substances
Can be. As a crystalline solid electrolyte, for example, Li
l, Li_ThreeN, Li_{1 + x}M_xTi_2-x(PO_Four)_Three(M
= Al, Sc, Y, La), Li_0.5-3xRE_{0.5 + x}Ti
O_Three(RE = La, Pr, Nd, Sm) and the like,
As an amorphous solid electrolyte, for example, 4.9 Lil-3
4.1Li_TwoO-61B _TwoO_Five, 33.3Li_TwoO-6
6.7 SiO_TwoOxide glass such as 0.45Lil-
0.37Li_TwoS-0.18P_TwoS_Five, 0.44Lil
−0.30Li_TwoS-0.26B_TwoS_Three, 0.30Li
1-0.42Li_TwoS-0.28SiS_TwoSulfide gas
Lath and the like. At least one of these
Can be used.

[0030]

EXAMPLES The method of the present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto. Table 1 shows examples and comparative examples of the present invention. Here, in evaluating the initial discharge capacity and the capacity retention ratio, metal Li was used as the negative electrode, and the initial discharge capacity was converted per 1 g of the positive electrode active material.

Example 1 1 mol / l of LiNO ₃ and 1 mol / l of Mn (NO
_{3) 2} · 6H ₂ 0 in ethanol solution was stirred mixed solution such that the molar ratio of Li / Mn is 1/2, was slowly added 1.5 equivalents of oxalic acid in ethanol solution therein . This coprecipitation solution was stirred for 2 hours, then adjusted to pH 6.87 with triethylamine, and further stirred for 3 hours, and allowed to stand for coprecipitation. Thereafter, the precipitate is filtered and dried to form a coprecipitated powder (Li ₂ (C ₂ O ₄ ) and Mn (C ₂ O ₄ ).
Was prepared. By burning the obtained coprecipitated powder, LiMn ₂ O ₄ was synthesized. The firing conditions were as follows: temperature: 400 ° C., time: 6 hr, temperature rise / fall rate;
5 ° C / min. And pyrolyze in air, then temperature; 7
50 ° C., time: 24 hr, heating rate: 5 ° C./min. Then, firing is performed in the air, and then 0.2 ° C./min. At 4
After cooling to 50 ° C, the temperature is maintained for 6 hours, and then to room temperature at 5 ° C /
min. And cooled.

Example 2 Li ₂ CO ₃ and MnCO ₃ were wet-mixed using a mortar at a ratio of Li / Mn = 1/2, and the mixture was air-mixed for 45 minutes.
The temperature was raised stepwise while maintaining the temperature at 0, 500, 550, and 600 ° C. for 12 hours, and then calcined at 750 ° C. for 48 hours in the air, and then 0.2 ° C./min. Cooled slowly at the speed.

Example 3 Mn ₂ O ₃ obtained by heating γ-MnO ₂ in air at 800 ° C. for 24 hours and LiOH · H ₂ O were used as starting materials.
It was blended so that the atomic ratio of lithium and manganese was 1: 2. Ethanol was added to this formulation and ground well in a mortar to form a uniform mixture. The obtained mixture was calcined in air at 500 ° C. for 24 hours. Then 800 in the atmosphere
After firing for 24 hours at 600 ° C, 500, 450, 4
While maintaining the temperature at 00 and 350 ° C. for 3 hours each,
° C / min. Then, the temperature was lowered at 350 ° C. and the furnace was cooled after 350 ° C.

[0034] 4 hours Comparative Example 1 γ-MnO ₂ 800 ℃, and Mn ₂ O ₃ and LiOH · H ₂ O was obtained by heating in air was used as a starting material, the atomic ratio of lithium and manganese 1 / 2 was added. Ethanol was added to this formulation and ground well in a mortar to form a uniform mixture. The obtained mixture was calcined in air at 500 ° C. for 24 hours. Then 800 in the atmosphere
After firing for 24 hours at 600 ° C, 500, 450, 4
While maintaining the temperature at 00 and 350 ° C. for 3 hours each,
° C / min. Then, the temperature was lowered at 350 ° C., and the furnace was cooled after 350 ° C.

Comparative Example 2 LiOH.H ₂ O and γ-MnO ₂ were mixed with Li / Mn = 1/2.
, And the mixture was calcined in the air at 400 ° C. for 6 hours. Next, at 750 ° C in air
After firing for 24 hours, the temperature was increased to 450 ° C. at 0.2 ° C./min.
Cooled slowly at the speed.

Comparative Example 3 LiOH.H ₂ O and γ-MnO ₂ were mixed with Li / Mn = 1/2.
, Using a mortar, wet-mixing the mixture at 450, 500, 550, and 600 ° C. in the air in a stepwise manner while maintaining the temperature for 12 hours, and then 750 ° C. for 48 hours in the air. After firing at 0.2 ° C / mi up to 450 ° C
n. Cooled slowly at the speed. By powder X-ray diffraction measurement, it was confirmed that all the samples synthesized in Examples and Comparative Examples were cubic single phases at room temperature. The powder X-ray diffraction patterns in Examples and Comparative Examples of the present invention were measured under the following conditions.

Measurement model: LINT 1500 manufactured by Rigaku Corporation Irradiated X-ray: Cu Kα ray Tube voltage: 50 kV Tube current: 200 mA Divergence, scattering slit: 1/4 mm Light receiving slit: 0.15 mm Scan type: Step Step size: 0.004時間 Time per step: 5 seconds Measurement range: 30 to 31, 35 to 37, 37 to 2θ
38.5, 63-65, 75-78, 80-82 degrees

The non-uniform strain η of the crystallite can be calculated from the results obtained under the above conditions by the following equations.

[0039]

Equation 4 βcos θ = λ / D + 2η sin θ (1)

(Where β is the integral width (rad), θ is the diffraction angle (゜), λ is 1.54056 ° (CuKα), D is the crystallite size (Å), and η is the nonuniform strain of the crystallite. Surface reflection (220) (311) (222) (44)
0), (533), (622), and (444) were substituted by the diffraction angle θ and the integration width β, and the slope (2η) when the horizontal axis was sin θ and the vertical axis was βcos θ was divided by 2. . X values in Examples and Comparative Examples in Table 1 were determined by the following methods.

(Analysis of total amount of Li) After vacuum drying at 50 ° C for 1 hour, a precisely weighed lithium manganese oxide was heated in an acid solution of hydrochloric acid to prepare a measurement solution. Using an atomic absorption spectrometer, the Li concentration was determined by the calibration curve method, and the Li content per constant weight was calculated.

(Total analysis of Mn: EDTA chelate titration) 50
After vacuum drying at 1 ° C. for 1 hour, the precisely weighed lithium manganese oxide was heated in a hydrochloric acid solution to prepare a measurement solution. The solution was fractionated into a conical beaker with a whole pipette, and a small excess of 0.01M-EDTA solution was added. Next, pure water was added to adjust the total volume to 75 ml, and the pH was adjusted to 10 by adding an appropriate amount of an 8N-NaOH aqueous solution and an ammonia buffer. The total volume was adjusted to 100 ml with pure water, and then the BT indicator was added dropwise. Using a 0.01 M-Mg ion standard solution as a titration solution, titration was performed with a microburette. The point at which the color of the BT indicator changed from blue to red was defined as the end point. From the titration result at this time, Mn per a constant weight
The content was calculated.

(Composition ratio of Li and Mn) The composition ratio of Li and Mn was calculated from the quantification results of Li and Mn obtained above. When the molar ratio of Li / Mn is 1/2 or more,
The composition of the product is crystallographically Li (Mn _2-x Li _x ) O ₄
Therefore, the composition ratio was determined so that the total sum would be 3.

The method for producing the battery and the charge / discharge conditions will be described below. The cathode active material synthesized as shown in Table 1 was mixed with acetylene black as a conductive agent and polytetrafluoroethylene resin as a binder at a weight ratio of 75: 20: 5 to form a cathode mixture. . In addition, 0.1 g of the positive electrode mixture
Was press-formed at a diameter of 16 mm at 1 ton / cm ² to obtain a positive electrode. A porous polypropylene film was placed as a separator 3 on the positive electrode 1 in FIG. A lithium plate having a diameter of 16 mm and a thickness of 0.4 mm serving as the negative electrode 4 was pressure-bonded to a sealing tube 6 provided with a polypropylene gasket 5. As a non-aqueous electrolyte, a solution of ethylene carbonate + 1,2-dimethoxyethane (50 vol%: 50 vol%) in which 1 mol / l of lithium perchlorate was dissolved was used, and the solution was added on the separator 3 and the negative electrode 4. Thereafter, the battery was sealed. The charge / discharge cycle characteristics of these batteries were compared. In the charge / discharge cycle test in this example, the charge / discharge current was 2 mA, and the voltage range was 4.35 V to 3.35 V.
This was performed by charging and discharging at a constant current between 2V. The results are shown in Table 1 and FIG.

[0045]

[Table 1] Non-uniform strain η of crystallite, discharge capacity after initial (1th) / 100th, and 100th discharge capacity retention ratio

In the examples according to the present invention, since the initial discharge capacity is large and the cycle characteristics are excellent,
It can be seen that the discharge capacity after 0th is 115 mAh / g or more. In this embodiment, lithium metal is used as the negative electrode material of the battery. However, similar results are obtained when a lithium alloy or a compound capable of inserting and releasing lithium is used.

[0047]

As described above, as the active material of the positive electrode, the general formula Li (Mn _2-x Li _x ) O ₄ (where 0 ≦ X ≦ 0.0
If a lithium manganese oxide having a non-uniform strain η in the range of 0 ≦ η ≦ 1.25 × 10 ⁻³ in the X-ray diffraction pattern of the cubic spinel crystal shown in 5) is used, a high charge / discharge capacity can be obtained. A battery with improved cycle characteristics is maintained. As a result, inexpensive lithium manganese oxide can be used as a cathode material, and a high-performance, safe and inexpensive lithium-ion secondary battery can be supplied to a wide range of uses, and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is a correlation diagram between η and battery performance (100th discharge capacity).

FIG. 2 is a longitudinal sectional view of a coin-type battery used in a test of a method of manufacturing an active material for a non-aqueous electrolyte secondary battery according to an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Case 3 Separator 4 Negative electrode 5 Gasket 6 Sealing can

Claims (1)

[Claims] In a lithium secondary battery using a lithium-containing metal oxide capable of inserting and releasing lithium into a positive electrode, the lithium-containing metal oxide has a general formula: Li (Mn _2-x Li _x ) O ₄ (where 0 ≦ X ≦ 0.05), and the X
The non-uniform strain η of the crystallite in the line diffraction pattern is expressed by the following equation (1): β cos θ = λ / D + 2η sin θ (1) (where β is the integral width (rad), θ is the diffraction angle (゜), λ Is 1.54056 ° (CuKα), D is the crystallite size (Å), and η is the non-uniform strain of the crystallite.)
.Ltoreq..ltoreq.≤1.25.times.10.sup.- ³ .