JPH0773882A - Secondary battery - Google Patents

Abstract

PURPOSE:To manufacture lithium-containing manganese composite oxide excellent in a cycle characteristic by again mixing MnO2 of high purity obtained by acid treatment of LiMn2O4 with a lithium compound, followed by a heat treatment for synthesizing. CONSTITUTION:Manganese dioxide on the market is subjected to a heat treatment, and is mixed with a predetermined quantity of lithium nitrate, followed by a heat treatment, thus obtaining LiMn2O4. The resultant LiMn2O4 is oxidized with sulfuric acid or the like, and then, the precipitate is recovered, thereby obtaining MnO2 of high purity. The resultant MnO2 is mixed with a predetermined quantity of lithium nitrate, followed by a heat treatment, thus composing LiMn2O4. Use of the resultant lithium containing manganese composite oxide of spinel crystal structure as a positive electrode active material can improve a cycle characteristic of a non-aqueous electrolyte secondary battery.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improving the performance of a non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art As electronic devices are becoming smaller and lighter, there is a growing demand for high energy density secondary batteries as their power sources. In order to meet the demand, non-aqueous electrolyte secondary batteries have been attempted to be put into practical use because of their high potential as high energy density batteries. Above all, a non-aqueous electrolyte secondary battery using metallic lithium for the negative electrode and a lithium-containing manganese composite oxide for the positive electrode seemed to be quite promising. However, there is a problem in practical cycle life because the metallic lithium negative electrode becomes powdered by repeated charge and discharge and its performance is significantly deteriorated, and metallic lithium deposits on dendrites and causes an internal short circuit. Practical application is difficult. Therefore, recently, a non-aqueous electrolyte secondary battery having a negative electrode of a carbon electrode that utilizes the inflow / outflow of lithium ions from / to carbon instead of the lithium metal negative electrode is under development. This battery was named a lithium-ion secondary battery by the present inventors and was named in 1990 (Magazine Progr
ess in Batteries & Solar
Cells, Vol. 9, P.I. 209) for the first time, typically using LiCoO ₂ as the positive electrode material,
A carbonaceous material is used for the negative electrode. Now in the battery industry,
Next Generation Secondary Battery "Lithium Ion Secondary Battery" at Academic Society
It is said that it is attracting attention. In fact, a small amount of lithium ion secondary batteries having an energy density of about 200 Wh / l have already been put into practical use. The energy density of existing nickel-cadmium batteries is 100-150 Wh / l,
The energy density of lithium-ion secondary batteries is much higher than that of existing batteries. Another feature of lithium-ion secondary batteries is their long life. The carbon negative electrode is extremely stable because the carbon helium ion in the electrode is doped during charging and the lithium ion is undoped from the carbon during discharging, and the carbon itself does not undergo a large change in crystal structure during charging and discharging. The charging / discharging characteristics are shown, there is little deterioration of the characteristics due to charging / discharging, and specifically, the charging / discharging can be repeated 1000 times or more. However, the biggest drawback is that the raw material cost is much higher than that of existing batteries. Especially for the positive electrode material LiCoO
_In the above-mentioned lithium-ion battery in which No. ₂ is used and a carbonaceous material is used for the negative electrode, since expensive cobalt and a special carbon material are used, the raw material cost becomes extremely high. The existing nickel-cadmium battery has an energy density of 100-150.
It is 50 to 70% of the lithium ion battery in Wh / l, but the material cost is 20 to 30% or less. Therefore, if a positive electrode active material of a lithium ion battery is replaced with an inexpensive material (for example, LiMn ₂ O ₄ ), and an energy density of about 200 Wh / l can be achieved, a lithium ion secondary battery can be widely used instead of the existing nickel cadmium battery. Will be used. In addition to the lithium cobalt composite oxide (LiCoO ₂ ), lithium nickel composite oxide (LiNiO ₂ ) and lithium manganese composite oxide (LiMn ₂ O ₄ ) are the positive electrode materials that can be combined with the carbon negative electrode to form a lithium ion battery. Therefore, LiMn ₂ O ₄ is attractive in terms of an inexpensive material. But LiM
Only by replacing n ₂ O ₄ with LiCoO ₂ as a positive electrode material, an energy density of about 170 Wh / l can be achieved. Until now, the carbonaceous material suitable for the carbon negative electrode of the lithium-ion secondary battery has been a carbon material obtained by thermal decomposition of various organic compounds or carbonization by firing, and the thermal history temperature is adjusted to adjust the carbon material. It is said that the conditions are important, carbonization is not sufficient if the heat history temperature is too low, and it is said that the temperature is at least 800 ° C or higher. Further, the upper limit of the heat history temperature is more important, and crystal growth occurs at a temperature of 2400 ° C or higher. It was said that the battery characteristics would be significantly impaired if it proceeded too much. In other words, a carbon material with good performance is considered to be a pseudo-graphite material having a certain degree of disordered structure, and in a highly crystalline graphite material, the electrolytic solution decomposes on the graphite surface, and the intercalation reaction of lithium ions is difficult to proceed. Was reported. However, the most recent research result is that if an appropriate electrolytic solution is selected, it is even flatter to use a more graphitized carbon material that has been heat treated at 2400 ° C or higher, or graphite itself as the negative electrode carbon material. It has been found that the lithium ion secondary battery has a high discharge voltage (Japanese Patent Publication No. 4-115457). Therefore, if a graphite material is used as the negative electrode material, even if inexpensive LiMn ₂ O ₄ is used as the positive electrode material, the lithium ion secondary battery may exceed 200 Wh / l in terms of energy density. . However, the lithium-ion secondary battery using a carbon material for the negative electrode should have good cycle characteristics, but it should not be used as a positive electrode material for LiM.
It was found that the cycle characteristics of the lithium ion secondary battery using n ₂ O ₄ were not always good. In the most typical conventional synthesis method of spinel-type lithium-containing manganese composite oxide (LiMn ₂ O ₄ ), commercially available manganese dioxide is used as a manganese compound, and a lithium salt such as lithium carbonate or lithium nitrate is mixed with this. And 600 ~
Synthesize by firing at 800 ° C. Manganese dioxide is manufactured in large quantities for use in dry batteries, and as highly pure products, electrolytic manganese dioxide (EMD) and chemically synthesized manganese dioxide (CM).
D) is commercially available at a low price, so it is cheap LiM
In making n ₂ O ₄ , it can be said that it is a convenient material as a synthetic starting material. However, LiMn ₂ O prepared by the conventional method
_The lithium ion secondary battery using No. ₄ has a short cycle life, and the capacity of the battery deteriorates to almost half the initial capacity after 50 to 100 cycles of charging and discharging.
Although the cause of this deterioration is not clear, it is almost clear that the lithium ion secondary battery using LiCoO ₂ has excellent cycle characteristics, and the cause is related to LiMn ₂ O ₄ of the positive electrode material. In order to improve this cycle characteristic, LiMn ₂
Based on the hypothesis that the crystal of O ₄ may collapse,
For the purpose of improving the crystal stability, a part of Mn is replaced with various elements other than Mn (for example, Co, Cr, Ni, Ta, Zn).
, Etc., a lithium-containing manganese composite oxide has been proposed (Japanese Patent Publication No. 4-141954), but it has not reached the practical cycle life (300 to 500 cycles).

[0003]

SUMMARY OF THE INVENTION The present invention relates to improvement of cycle characteristics of a non-aqueous electrolyte secondary battery containing a lithium-containing manganese composite oxide as a main positive electrode active material, and particularly to lithium-containing manganese having good cycle characteristics. It is intended to provide a composite oxide.

[0004]

Means solving the problems SUMMARY OF THE INVENTION is the MnO ₂ obtained by LiMn ₂ O ₄ acid treatment synthesized in one conventional method, was synthesized by mixing heat-treating the lithium compound again A lithium-containing manganese composite oxide is used as the positive electrode active material.

[0005]

LiMn ₂ O ₄ has a cubic crystal structure having a spinel structure, and in a battery using this as a positive electrode active material,
Li ions are extracted from the crystal by charging, and Li enters the crystal again by discharging. However, when LiMn ₂ O ₄ after repeating the charge / discharge cycle was examined by x-ray diffraction, it was said that the crystallinity decreased with the charge / discharge cycle, and therefore the battery capacity decreased with the cycle, and the It is considered that the cycle life cannot be obtained. In the present invention, first, MnO ₂ is made from LiMn ₂ O ₄ . When LiMn ₂ O ₄ is treated with, for example, a sulfuric acid acidic solution, MnO ₂ is produced by the following reaction. 2LiMn ₂ O ₄ + 2H ₂ SO ₄ → Li ₂ SO ₄ +3
Impurities such as lower oxides of manganese contained in MnO ₂ + MnSO ₄ + 2H ₂ O LiMn ₂ O ₄ are dissolved in the solution by this acid treatment, so that high-purity M
nO ₂ is produced. Although the reason for the improvement of the cycle characteristics according to the present invention is not clear, a lithium-containing manganese composite oxide synthesized by mixing a lithium compound again with high-purity MnO ₂ obtained by treating LiMn ₂ O ₄ with an acid and performing heat treatment. (LiMn ₂ O ₄ ) is considered to have less impurities such as lower oxides of manganese, less crystal distortion, increased crystal stability, and significantly improved charge / discharge cycle characteristics. .

[0006]

The present invention will be described in more detail with reference to the following examples.

Synthesis of LiMn ₂ O ₄ by a conventional method Commercially available manganese dioxide (EMD (TAD-
I)) is heat-treated at 390 ° C. for 4 hours, and lithium nitrate (LiNO ₃ ) is added to the heat-treated product in an atomic ratio of Mn: Li = 1:
Mix at 0.51 and put in a porcelain container, put in an electric furnace 75
The temperature was raised to 0 ° C., the temperature was maintained at this temperature, and heat treatment was performed for 24 hours to synthesize a LiMn ₂ O ₄ sample (H).

[0008] Acid treatment of LiMn ₂ O ₄ the LiMn ₂ O ₄ sample (H) and stirred for 12 hours at 0.5 molar sulfuric acid solution, the precipitate filtered, washed with water dried M
A nO ₂ sample (G) was prepared.

Resynthesis of LiMn ₂ O ₄ MnO ₂ sample (G) prepared as described above was charged with lithium nitrate (LiN
O ₃ ) was mixed at an atomic ratio of Mn: Li = 1: 0.51 and placed in a porcelain container, put in an electric furnace and heated to 750 ° C.,
It is kept at this temperature and heat-treated for 24 hours to obtain LiMn.
_{A 2} O ₄ sample (F) was synthesized. As a result of the preliminary experiment, the mixing ratio of the lithium compound to manganese dioxide was L in atomic ratio.
When i / Mn <0.5, unreacted manganese oxide is produced, which is not preferable.

A specific battery of the present invention will be described with reference to FIG. A battery element which is a power generation element for carrying out the present invention was prepared as follows. First 2800
Mesocarbon microbeads (BE
T specific surface area = 0.8 m ² / g, d ₀₀₂ = 3.37 Å)
90 parts by weight of polyvinylidene fluoride (P
VDF) 10 parts by weight is added, and the solvent is N-methyl-2.
Wet mixed with pyrrolidone to form a slurry (paste). Then, this slurry is used as a current collector in a thickness of 0.01
mm copper foil was evenly applied on both sides, dried and pressure-molded with a roller press machine to prepare a strip-shaped negative electrode (1). 88 parts by weight of the LiMn ₂ O ₄ sample (F) prepared by resynthesis were mixed with 3 parts by weight of acetylene black as a conductive agent and 4 parts by weight of graphite together with 5 parts by weight of polyvinylidene fluoride as a binder, and the mixture was used as a solvent. Wet mix with N-methyl-2-pyrrolidone to form a paste. This paste was evenly applied to both sides of a 0.02 mm-thick aluminum foil serving as a positive electrode current collector, dried, and then pressure-molded with a roller press to form a strip-shaped positive electrode (2). Subsequently, the negative electrode (1) and the positive electrode (2) are wound into a roll with a porous polypropylene separator (3) sandwiched between them to form a battery element with a wound body having an average outer diameter of 15.7 mm. Next, the insulating plate (5) is placed on the bottom of the nickel-plated iron battery can (4) to house the battery element. Ethylene carbonate (EC) in which the negative electrode lead (6) taken out from the battery element was welded to the bottom of the battery can and 1 mol / liter of LiClO ₄ was dissolved as an electrolytic solution in the battery can.
And a mixed solution of diethyl carbonate (DEC) is injected. Then, the insulating plate (5) is also installed on the upper part of the battery element, the gasket (7) is fitted, and the explosion-proof valve (8) is installed inside the battery as shown in FIG. The positive electrode lead (9) taken out from the battery element is welded before injecting the electrolytic solution into this explosion-proof valve. On the explosion-proof valve, a closing lid (10) serving as a positive electrode external terminal is overlaid, the edge of the battery can is caulked, and a battery (A) having an outer diameter of 16.5 mm and a height of 65 mm is formed with the battery structure shown in FIG. Created.

[0011]

Comparative Example In order to confirm the effect of the present invention, a battery using LiMn ₂ O ₄ (H) synthesized by a conventional method as a positive electrode active material was prepared with the same structure as that of the example.

A battery (B) having the same battery structure as the battery of the embodiment shown in FIG. 2 was prepared in exactly the same manner as the embodiment except that the LiMn ₂ O ₄ sample (H) synthesized by the conventional method was used as the positive electrode active material. Created.

Test Results The batteries (A) and (B) thus prepared were each set at a charging voltage of 4.2 V after an aging period of 12 hours had elapsed for the purpose of stabilizing the inside of the battery. Also charged for 8 hours and discharged 800 mA for all batteries
Final voltage by constant current discharge of 3. A charging / discharging cycle test was conducted by going to OV. As a result, the discharge capacity at the time of 10 cycles was about 910 mAh for both the battery according to the example and the battery according to the comparative example, and the energy density was about 2
40 Wh / l. This value is more than 1.5 times that of the existing nickel-cadmium battery, and is 15% better than even the lithium-ion secondary battery using cobalt currently in practical use. However, as shown in FIG. 1, the capacity of the battery (B) according to the comparative example deteriorates considerably with the cycle of charging and discharging, and becomes less than half of the initial capacity after 50 cycles. On the other hand, in the battery (A) according to the embodiment of the present invention, 720 mA was obtained even after 200 cycles.
It is a lithium ion secondary battery that can be sufficiently put into practical use because it holds at least h and at least 190 wh / l in energy density. The batteries of (A) and (B) differ only in the positive electrode active material used, and the positive electrode active material (LiMn ₂ O ₄ ) is greatly related to the deterioration of capacity with the cycle. The starting materials of all LiMn ₂ O ₄ are the same MnO ₂ (Mitsui Kinzoku EMD / TAD
-I), and LiMn used as the positive electrode active material of the battery (A).
_{The 2} O ₄ sample (F) was used as the positive electrode active material of the battery (B), Li
It was obtained by synthesizing Mn ₂ O ₄ sample (H) again from MnO ₂ obtained by acid treatment, and LiM according to the example of the present invention.
It is clear that the method for synthesizing the n ₂ O ₄ sample is effective in synthesizing the positive electrode active material with less capacity deterioration with cycles. Although the reason for the improvement of the cycle characteristics according to the present invention is not clear, a lithium-containing manganese composite oxide synthesized by mixing a lithium compound again with high-purity MnO ₂ obtained by treating LiMn ₂ O ₄ with an acid and performing heat treatment. It is considered that the mixture of impurities such as lower oxides of manganese is reduced, the strain of the crystal is reduced, the stability of the crystal is increased, and the charge / discharge cycle characteristics are significantly improved. The present invention is not limited to the synthesis of a complex oxide composed of only lithium and manganese, that is, pure LiMn ₂ O ₄ , and for example, a spinel type crystal structure in which a part of Mn is replaced with an element other than Mn. the lithium-containing manganese composite oxide _{(LiM x Mn 2-x O} 4, where M is an element other than Mn) in order to synthesize, together with a lithium compound oxides of the elements to be substituted, LiMn ₂ O ₄ It is naturally applicable by mixing MnO ₂ obtained by acid treatment with MnO ₂ in a predetermined ratio and subjecting it to heat treatment, and LiM _x Mn _2-x O ₄ containing less impurities such as lower oxides of manganese is synthesized. It is possible to Although LiMn ₂ O ₄ was synthesized by using LiNO ₃ as a lithium compound in this example, the present invention is not limited to this, and various other lithium salts, lithium hydroxide, lithium oxide and the like can be used. Lithium compounds can be used. L before acid treatment
Although iMn ₂ O ₄ was synthesized by a conventional method using a commercially available EMD, the present invention is not limited to this, and LiMn having a spinel crystal structure synthesized using CMD or another manganese oxide as a starting material is used. _{In the case of 2} O ₄ , the present invention can be implemented by acid-treating this to obtain MnO ₂ . In the battery of this embodiment, 1 mol / liter of LiClO ₄ dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as an electrolytic solution, but other electrolyte salts (for example, LiPF ₆ and L
Similar effects can be obtained with an electrolytic solution composed of iBF ₄ or the like) or another solvent (for example, propylene carbonate or the like).

[0014]

Lithium-ion secondary battery using lithium-containing manganese composite oxide as a cathode active material according to the present invention is that the cycle life is short is the largest problem, LiMn ₂ O
_{If a} lithium compound is mixed with high-purity MnO ₂ obtained by treating ₄ with an acid and heat-treated to synthesize a lithium-containing manganese composite oxide as a positive electrode active material of a lithium-ion secondary battery, its cycle characteristics are improved. Is greatly improved. As a result, a lithium ion secondary battery with an energy density sufficiently higher than that of existing secondary batteries can be made at a low material cost, and a long-life, high-capacity secondary battery can be provided for a wide range of applications. The target value is great.

[Brief description of drawings]

FIG. 1 Cycle characteristics of batteries in Examples and Comparative Examples

FIG. 2 is a schematic cross-sectional view showing the structures of batteries in Examples and Comparative Examples.

[Explanation of symbols]

1 is a negative electrode, 2 is a positive electrode, 3 is a separator, 4 is a battery can, 5
Is an insulating plate, 6 is a negative electrode lead, 7 is a gasket, 8 is an explosion-proof valve, 9 is a negative electrode lead, and 10 is a closing lid.

Claims (2)

[Claims] 1. M obtained by acid treatment of LiMn ₂ O _4.
A method for producing a lithium-containing manganese composite oxide, which comprises again mixing a lithium compound with nO ₂ and performing heat treatment to synthesize. 2. A non-aqueous electrolyte secondary battery, wherein the lithium-containing manganese composite oxide having a spinel type crystal structure according to claim 1 is used as a positive electrode active material.