JPH10188979A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery having excellent charge and discharge cycle characteristics by using lithium manganese oxide subjected to washing, as a positive electrode active material. SOLUTION: Lithium manganese oxide includes by-product due to the uneven reaction and non-reactant in usual, and it is possible that charge and discharge cycle characteristics is lowered by an impure material. Since the impure material is eliminated by washing lithium manganese oxide, lowering of charge and discharge cycle characteristics can be reduced. Lithium manganese oxide is washed by dipping it in a washing liquid, and agitating or oscillating the liquid, and thereafter, filtering so as to recover the lithium manganese oxide, and drying it in an ordinary way. As the washing liquid, ordinary water is used, but if desired, organic solvent of alcohol such as methanol and ethanol and ketone such as acetone, methyletylketone can be used singly as a mixture.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the improvement of a lithium secondary battery using a non-aqueous electrolyte, particularly to the improvement of a positive electrode active material, and it is intended to improve the cycle characteristics of the battery.

[0002]

BACKGROUND OF THE INVENTION Problems to be Solved by the Invention
Of manganese and lithium as positive electrode active materials for secondary batteries
LiMn is a composite oxide_TwoO_FourHas been proposed,
It has been done. This LiMn is used for the positive electrode. _TwoO_FourUsing
Lithium secondary batteries are characterized by high voltage and high energy density.
Despite the characteristics, the charge / discharge cycle life is short.
It has not been used as a practical battery. Therefore the book
The invention is based on LiMn._TwoO_FourEquipped with a positive electrode that uses
And a small change in capacity due to charge / discharge cycles
To provide a secondary battery.

[0003]

The lithium secondary battery according to the present invention is characterized in that the lithium manganese oxide as the positive electrode active material has been subjected to a washing treatment, preferably a water washing treatment.

[0004]

DETAILED DESCRIPTION OF THE INVENTION Lithium manganese oxide (LiM
In a lithium secondary battery using n ₂ O ₄ ) as a positive electrode active material, the charge / discharge reaction of the positive electrode is represented by the following equation.

[0005]

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[0006] Lithium manganese oxide is produced by thoroughly mixing respective powders of a lithium compound and a manganese compound to form a uniform mixture, heating the mixture, and causing the solids to react with each other. It is considered that the lithium manganese oxide obtained by this method contains by-products and unreacted products due to the heterogeneous reaction. According to the study of the present inventors, the lithium manganese oxide obtained by this method is oxygen-deficient, that is,

[0007]

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(Δ = oxygen deficiency), and it is highly probable that this and the above-described impurities contained therein cause deterioration of the charge / discharge cycle characteristics. When the lithium manganese oxide is subjected to a washing treatment according to the present invention, these impurities are removed, and thus it is considered that the deterioration of the charge / discharge cycle characteristics can be reduced.

[0009] To clean the lithium manganese oxide, the lithium manganese oxide is put into a cleaning solution, stirred or shaken, and then filtered to collect the lithium manganese oxide.
What is necessary is just to dry by a conventional method. As the washing liquid, ordinary water is used. If desired, organic solvents such as alcohols such as methanol and ethanol, and ketones such as acetone and methyl ethyl ketone can be used alone or in combination. However, when an organic solvent is used, it is preferably used as an aqueous solution.

[0010] The temperature and time of the cleaning treatment are optional,
In general, the higher the processing temperature, the more the desired effect can be obtained in a shorter processing time. The lithium secondary battery according to the present invention has the same configuration as a conventionally known lithium secondary battery, except that the lithium manganese oxide that has been subjected to the above-described cleaning treatment is used as the positive electrode active material. Typically, it has a structure in which a positive electrode and a negative electrode are opposed to each other with a porous separator interposed therebetween, and a non-aqueous electrolyte is added thereto. As the negative electrode active material, for example, lithium, a lithium alloy, or a material that inserts and releases lithium is used. Among them, a carbon material that inserts and releases lithium is preferable because of its high safety. Such carbon materials include graphite, coal-based coke, petroleum-based coke, coal-based pitch carbide, petroleum-based pitch carbide,
Needle coke, pitch coke, phenolic resin,
Carbon materials such as crystalline cellulose and carbon materials partially graphitized thereof, furnace black, acetylene black,
And pitch-based carbon fibers.

As the conductive agent for the positive electrode, fine particles of graphite, carbon black such as acetylene black, and fine particles of amorphous carbon such as needle coke are used. Examples of a binder (binder) for the negative electrode active material and the positive electrode active material include polyvinylidene fluoride, polytetrafluoroethylene, EPDM (ethylene-propylene-diene terpolymer), SBR (styrene-butadiene rubber), and NBR.
(Acrylonitrile-butadiene rubber), fluorine rubber and the like are used.

The positive electrode may be prepared by adding a conductive agent and a binder to the positive electrode active material to form a uniform mixture, and then subjecting the mixture to pressure molding, or applying a slurry obtained by adding an appropriate solvent thereto to a conductive support. Can be created. When using a carbon material that inserts and releases lithium as the negative electrode, as in the case of the positive electrode, a binder is added to the material to form it under pressure, or a slurry obtained by adding a binder and a solvent is made conductive. It can be prepared by coating on a support.

As the separator, a microporous polymer film is used. As the polymer film, for example, a film of nylon, cellulose acetate, nitrocellulose, polysulfone, polyacrylonitrile, polyvinylidene fluoride, and the like can be used.However, from the viewpoint of chemical and electrochemical stability, polypropylene, polyethylene, A polyolefin film such as polybutene is preferable, and a polyethylene film is more preferable in terms of the self-closing temperature, which is one of the purposes of the battery separator.

The polyethylene separator is preferably ultra-high molecular weight polyethylene from the viewpoint of maintaining high-temperature shape, 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 electrolyte, an electrolyte obtained by dissolving a lithium salt in an organic solvent is used. As the organic solvent, for example, carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, chlorinated hydrocarbons, amines, esters, amides, phosphate ester compounds and the like can be used. it can. 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-dioxolan, 4-methyl-1,3
-Dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, benzonitrile, butyronitrile, valeronitrile, 1,
Examples thereof include 2-dichloroethane, dimethylformamide, dimethylsulfoxide, trimethyl phosphate, and triethyl phosphate, and these can be used alone or as a mixed solvent of two or more kinds.

As the electrolyte, any of conventionally known electrolytes can be used.
For example, LiClO_Four, LiAsF₆, LiPF₆, Li
BF_Four, LiB (C₆H_Five)_Four, LiCl, LiBr,
CH _ThreeSO_ThreeLi, CF_ThreeSO_ThreeLi or the like is used.
Hereinafter, the present invention will be described more specifically by way of examples.
However, the present invention is not limited to these. In addition,
Used as a positive electrode active material in the following Examples and Comparative Examples
Lithium manganese oxide is cubic by powder X-ray diffraction.
It was confirmed to be a crystalline spinel phase. Also, its composition
Was determined as follows.

Measurement of Li content: The sample lithium manganese oxide was vacuum dried at 50 ° C. for 1 hour. About 0.0
5 g was precisely weighed, added to the hydrochloric acid acidic solution, and dissolved by heating, and then adjusted to 100 ml with a volumetric flask. Using a 20-fold dilution of this solution, use an atomic absorption device,
The Li concentration was measured by the calibration curve method, and the Li content of the sample was calculated.

Measurement of Mn content (EDTA method): A lithium manganese oxide sample was vacuum-dried at 50 ° C. for 1 hour. Approximately 0.05 g thereof was weighed and added to the hydrochloric acid acid solution,
After heating to dissolve, the volume was adjusted to 100 ml with a volumetric flask. 10 ml of this solution was collected with a whole pipette, placed in a conical beaker, and a small excess of 0.01 M-EDTA was added.
The solution was added. The total volume was made up to 75 ml with pure water, adjusted to pH = 10 by adding an appropriate amount of an 8N aqueous sodium hydroxide solution and an ammonia buffer, and then made up to 100 ml with pure water. A BT indicator was added to the solution, and titration was performed using a 0.01 M-magnesium ion standard solution. The point at which the color of the BT indicator changed from blue to red was regarded as the end point. The manganese content of the sample was calculated from the results of the titration.

Measurement of Mn ³⁺ and Mn ⁴⁺ contents (iodometry): After vacuum-drying a sample of lithium manganese oxide at 50 ° C. for 1 hour, about 0.02 g thereof was precisely weighed and placed in a conical beaker. I put it. A small excess of a saturated aqueous solution of potassium iodide and concentrated hydrochloric acid were sequentially added thereto to completely dissolve the solution, and then the total amount was adjusted to 100 ml with pure water. This sample solution was titrated with a 0.1 N aqueous sodium thiosulfate solution. Immediately before the end point, the starch solution was added, and the point at which the color of the solution changed from light purple to colorless was regarded as the end point.

Calculation of Mn valence: From the measurement of Mn content by EDTA and the measurement of Mn ³⁺ and Mn ⁴⁺ contents by iodometry, the Mn valence of lithium manganese oxide was calculated by the following calculation. Calculated. Mn ⁴⁺ , Mn ³⁺
And the Mn ²⁺ is present k, m and n moles increments in a sample of unit amount, the iodometry (reaction with ^{Mn 4+) kMn 4+ + 4kI -} → kMnI 2 + kI 2 (2) kI 2 + 2kS 2 O _{^{3 -2 → kS 4 O 6 -2}} + 2kI - (3) ( reaction with ^{Mn 3+) mMn 3+ + 3mI -} → mMnI 2 + (1/2) mI 2 (4) (1/2) mI 2 + mS 2 O ₃ ⁻² → (1/2) mS ₄ O ₆ ⁻² + mI ⁻ (5) (Reaction with Mn ²⁺ ) nMn ⁴⁺ + 2nI ⁻ → nMnI ₂ (6) Since Mn ²⁺ does not release I ₂ , Does not react with sodium sulfate. From the above, in iodometry, (2k +
m) A titer equivalent to moles is obtained. On the other hand, in the EDTA chelate titration, a titration value corresponding to (k + m + n) mole is obtained. Therefore,

[0021]

Mn valence = 4 × k / (k + m + n) + 3 × m / (k + m + n) + 2 × n / (k + m + n) = (4k + 3m + 2n) / (k + m + n) = 2+ (2k + m) / (k + m + n) = 2 + (result of iodometry) / (result of EDTA chelate titration)

Determination of oxygen content From the measurement results of the Li content and the Mn content, L
The composition ratio of i and Mn was calculated. The composition ratio was determined such that the sum of the two became 3. From the molar ratio of Li and Mn determined here and the Mn valence determined above, the oxygen content was calculated using the rules of electrical neutrality.

Example 1 Lithium manganese oxide (Li _1.028 Mn _1.972 O
_3.963 ) 1 g was placed in a stoppered Erlenmeyer flask, and 50 ml of pure water was added _thereto . After the flask was covered with aluminum foil, it was mounted on a 50 ° C. constant temperature shaker and shaken under shaking conditions of 60 times / min for 48 hours. After suction filtration using a Millipore filter, air drying was performed at 120 ° C. This was crushed in an agate mortar and then vacuum dried at 100 ° C. to completely remove water. Acetylene black and polytetrafluoroethylene were added to this lithium manganese oxide (weight mixing ratio: 75: 20: 5) and mixed well to form a positive electrode mixture.
0.1 g of this positive electrode mixture is 1000 kg / cm ² at a diameter of 1
A positive electrode was prepared by press molding to 6 mm.

The positive electrode and the negative electrode (a lithium metal plate having a diameter of 16 mm) were arranged with a porous polypropylene film interposed therebetween, and an electrolytic solution was added to form a battery. As the electrolytic solution, a solution prepared by dissolving lithium perchlorate in an equal volume mixture of ethylene carbonate and 1,2-dimethoxyethane so as to be 1 mol / liter was used. The charging / discharging current of this battery was 2 mA, and the voltage range was 4.35 V to 3.2 V.
A charge / discharge cycle test for charging / discharging at a constant current was performed.
Table 1 shows the results.

Comparative Example 1 A lithium secondary battery was manufactured in exactly the same manner as in Example 1, except that the same lithium manganese oxide that had been subjected to the cleaning treatment in Example 1 was used as the positive electrode active material without performing the cleaning treatment. And a charge / discharge cycle test was performed. Table 1 shows the results. Example 2 Lithium manganese oxide (Li _1.024 Mn _1.976 O
_4.015 ), cleaning treatment, preparation of a lithium secondary battery, and charge / discharge cycle test were performed in exactly the same manner as in Example 1. Table 1 shows the results.

Comparative Example 2 A lithium secondary battery was produced in exactly the same manner as in Example 1 except that the same lithium manganese oxide that had been subjected to the cleaning treatment in Example 2 was used as the positive electrode active material without performing the cleaning treatment. And a charge / discharge cycle test was performed. Table 1 shows the results.

[0027]

[Table 1]

[0028]

According to the present invention, lithium manganese oxide

[0029]

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By washing and using as a positive electrode active material, a lithium secondary battery having excellent charge / discharge cycle characteristics can be obtained.

Claims (2)

[Claims] 1. A lithium secondary battery using lithium manganese oxide as a cathode active material, wherein the lithium manganese oxide as the cathode active material has been subjected to a washing treatment. 2. The lithium secondary battery according to claim 1, wherein the washing is water washing.