JPH07169457A - Secondary battery - Google Patents

Abstract

PURPOSE:To improve the performance or a nonaqueous electrolytic secondary battery. CONSTITUTION:In a nonaqueous electrolytic secondary battery using a lithium manganese composite oxide (for example, LiMn2O4) with spinel structure as a positive active material, a carbonate of at least one element selected from alkali-earth metals is mixed with the positive active material, and a positive electrode 2 is formed. Capacity deterioration attendant on charge/discharge cycles is remarkably improved. A lithium ion secondary battery using graphite material as a negative active material and lithium manganese composite oxide with spinel structure as a positive active material has low material cost, long life, and high energy density superior to existing secondary batteries.

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, attention has been paid to non-aqueous electrolyte secondary batteries, and attempts have been made to put them into practical use. In particular, the so-called lithium secondary battery, which uses lithium metal for the negative electrode, seemed to have the greatest potential, but the metallic lithium negative electrode became powdered due to repeated charging and discharging, and its performance was significantly deteriorated. Because it deposits on dendrites and causes an internal short circuit,
There is a problem with the practical cycle life, and it is still difficult to put it into practical use. Therefore, recently, a non-aqueous electrolyte secondary battery having a carbon electrode as a negative electrode, which utilizes the movement of lithium ions into and out of carbon, is under development. This battery is
It was introduced to the world for the first time in 1990 under the name of lithium-ion secondary battery (Progress I magazine).
n Batteries & SolarCells,
Vol. 9, 1990, p209), and nowadays it is recognized in the battery industry and academic societies as to be called the next-generation secondary battery "lithium ion secondary battery", and its practical application is being spurred. Typically, a lithium-containing composite oxide (LiCoO ₂ , LiNiO ₂ , LiMn ₂₎ is used as a positive electrode material.
O ₄ etc.) and a carbonaceous material such as coke or graphite is used for the negative electrode. In fact, using LiCoO ₂ for the positive electrode and a special carbon material (pseudo-graphite material having a certain disordered layer structure) for the negative electrode, a lithium ion secondary battery having an energy density of about 200 Wh / l can be obtained. It is already in practical use. The energy density of the existing nickel-cadmium battery is 100 to 150 Wh / l, and the energy density of the lithium ion secondary battery is much higher than that of the existing battery. However, a major drawback of the lithium ion secondary battery is that the raw material cost is considerably high. When considering an inexpensive lithium-ion secondary battery, the price reduction of cobalt cannot be expected in the future due to resource reasons. Therefore, in terms of inexpensive materials, lithium manganese composite oxide (Li
Mn ₂ O ₄ ) is extremely attractive. If a lithium-ion secondary battery using a lithium-manganese composite oxide as a positive electrode active material instead of LiCoO ₂ is realized, a lithium-ion secondary battery will replace the nickel-cadmium secondary battery, which is worried about the environmental problems of cadmium. There is a high possibility that it will become the mainstream of secondary batteries. However, a lithium ion secondary battery using a lithium manganese composite oxide as a positive electrode material has a significantly large capacity deterioration particularly in a charge / discharge cycle at a high temperature (35 ° C. or higher). The carbon negative electrode is extremely charged because lithium ions are doped into carbon in the electrode during charging, and lithium ions are undoped from the carbon during discharging, and the carbon itself does not undergo a large change in crystal structure during charge / discharge, which is extremely high. Stable charge / discharge characteristics are shown, and characteristic deterioration due to charge / discharge is small.
It is possible to repeat charging and discharging zero or more times. However, the cycle characteristics of the lithium ion secondary battery using the lithium manganese composite oxide having the spinel structure are dominated by the deterioration of the characteristics of the positive electrode due to the cycle, and cannot be said to be at a sufficiently satisfactory level.

[0003]

DISCLOSURE OF THE INVENTION The present invention is directed to a lithium manganese composite oxide (LiMn ₂ O ₄ , Li) having a spinel structure.
The present invention relates to improvement of cycle characteristics of a non-aqueous electrolyte secondary battery in which M _x Mn _2-x O ₄ etc., where M is an element other than Mn) as a main positive electrode active material.

[0004]

[Means for Solving the Problems] A means for solving the problems is to add and mix a carbonate of one or more elements selected from alkaline earth metals to a lithium manganese composite oxide having a spinel structure as a positive electrode active material in a positive electrode. It will be done.

[0005]

When a lithium-containing manganese composite oxide having a spinel structure is used as the positive electrode active material, the positive electrode active material is dedoped with lithium ions in the charged state, and approaches the MnO ₂ (λ-MnO ₂ ) while maintaining the spinel structure. . λ-M
nO ₂ is MnO ₂ in other crystal structures (e.g., gamma-Mn
It is more unstable than O ₂ and β-MnO ₂ ). Therefore, as charging and discharging are repeated many times, the crystal structure of the positive electrode active material gradually changes and loses the charging and discharging function, and the capacity deteriorates with the cycle. Then, as a result of intensive research for stabilizing the positive electrode active material (λ-MnO ₂ ) in a charged state, the present inventor added a carbonate of one or more elements selected from alkaline earth metals to the positive electrode active material. It has been found that the mixing improves the stability of the positive electrode material, resulting in a non-aqueous electrolyte secondary battery with extremely small capacity deterioration with charge and discharge cycles.

[0006]

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

Example 1 The present invention will be described with reference to FIG. 1 for a specific cylindrical battery. FIG. 1 shows the overall structure of the battery of this embodiment. A battery element which is a power generation element for carrying out the present invention was prepared as follows. First, mesocarbon microbeads (d ₀₀₂ = heat treated at 2800 ° C.)
To 90 parts by weight of 3.37 Å), 10 parts by weight of polyvinylidene fluoride (PVDF) is added as a binder, and N as a solvent is added.
-Methyl-2-pyrrolidone was wet mixed to form a slurry (paste form). Then, this slurry was uniformly applied to both surfaces of a 0.01 mm-thick copper foil serving as a current collector, dried, and then pressure-molded with a roller press to form a strip-shaped negative electrode (1). Then, the positive electrode was prepared as follows. Commercially available manganese dioxide (MnO ₂ ) and lithium carbonate (Li ₂ C
O ₃ ) was mixed so that the atomic ratio of Li and Mn was 1: 2, and this was baked in air at 800 ° C. for 20 hours to prepare LiMn ₂ O ₄ . 87 of this LiMn ₂ O ₄
2 parts by weight of BaCO ₃ were mixed well with 8 parts by weight of graphite, 3 parts by weight of polyvinylidene fluoride as a binder and N-methyl-2-pyrrolidone as a solvent were added and wet mixed to form a slurry (paste). State). Subsequently, this slurry is used as a positive electrode current collector and has a thickness of 0.02 mm.
The aluminum foil was evenly applied on both sides, dried, and then pressure-molded with a roller press to form a strip-shaped positive electrode (2). The negative electrode (1) and the positive electrode (2) thus prepared are wound in a roll shape with a porous polypropylene separator (3) sandwiched therebetween to prepare a battery element as 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. Weld the negative electrode lead (6) taken out from the battery element to the bottom of the battery can and insert 1 into the battery can.
A mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) in which mol / liter of LiPF ₆ is dissolved is injected as an electrolytic solution. 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 body (10) serving as a positive electrode external terminal is overlapped with a doughnut-type PTC switch (11) sandwiched therebetween, and the edge of the battery can is crimped, and the battery structure shown in FIG. A battery (A) having a height of 65 mm was completed.

Comparative Example The positive electrode to be used was prepared by a conventional method, and the other examples were all prepared in Example 1.
A battery (X) according to the conventional method was prepared in the same manner as in. The positive electrode according to the conventional method is prepared as follows. 89 parts by weight of powdered LiMn ₂ O ₄ prepared in Example 1, 8 parts by weight of graphite, and polyvinylidene fluoride 3 as a binder
Part by weight is wet mixed with N-methyl-2-pyrrolidone which is a solvent to form a slurry (paste form). Next, the slurry was uniformly applied to both sides of a 0.02 mm-thick aluminum foil which was a positive electrode current collector, dried and pressure-molded with a roller press machine to prepare a strip-shaped positive electrode (2c). After that,
The positive electrode (2c) and the same negative electrode (1) prepared in Example 1 were wound into a roll with a porous polypropylene separator (3) sandwiched therebetween, and the average outer diameter was 15.
A battery element having a size of 7 mm was prepared, and thereafter, a battery (X) was prepared in the same manner as in Example 1.

Example 2 M was added to 87 parts by weight of LiMn ₂ O ₄ prepared in Example 1.
2 parts by weight of gCO ₃ was mixed well, 8 parts by weight of graphite, 3 parts by weight of polyvinylidene fluoride as a binder, and N-methyl-2-pyrrolidone as a solvent were further added and wet mixed to form a slurry (paste). State). Subsequently, this slurry is used as a positive electrode current collector and has a thickness of 0.02 mm.
The aluminum foil was evenly coated on both sides, dried, and then pressure-molded with a roller press to form a strip-shaped positive electrode (2b). After that, this positive electrode (2b) and the same negative electrode (1) as that prepared in Example 1 were wound into a roll with a porous polypropylene separator (3) interposed therebetween, and a battery having an average outer diameter of 15.7 mm. A device was prepared and a battery (B) was prepared in the same manner as in Example 1 after that.

Test Results In the batteries thus prepared in Examples 1 and 2 and Comparative Example, after the aging period of 12 hours was passed at room temperature for the purpose of stabilizing the inside of the battery, the charging upper limit voltage was 4.2 V. The battery was charged at room temperature for 8 hours, and all batteries were discharged at a constant current of 800 mA to a final voltage of 3.0 V at the same temperature to obtain the initial discharge capacity of each battery. Thereafter, each battery was subjected to a charge / discharge cycle test in a high temperature bath at 40 ° C. The charging current is 400 mA, the charging upper limit voltage is set to 4.2 V, and charging is performed for 4 hours. The discharging is performed at a constant current discharge of 800 mA up to the final voltage of 3.0 V, charging and discharging are repeated, and 40 cycles and 100 The discharge capacity at 800 mA discharge of each battery at the time of the cycle was determined. The results are summarized in Table 1. As shown in Table 1, the batteries (A) and (B) according to the present invention show a small decrease in capacity even after repeated charging and discharging, and the batteries (X) of the conventional method according to the comparative example at each of 40 and 100 cycles. The capacity difference with As seen in the battery (X) prepared by the conventional method, a spinel structure lithium manganese composite oxide (LiMn) is used as a positive electrode active material.
_In a lithium ion secondary battery using ₂ O ₄ ), capacity deterioration is extremely large in a charge / discharge cycle at high temperature,
At the time of 100 cycles, the capacity will be about half of the initial capacity. However, as shown in Table 1, BaCO ₃ and MgCO ₃
In the batteries (A) and (B) according to the present invention in which is added and mixed in the positive electrode, the degree of deterioration is extremely small, and 90% or more of the initial capacity is maintained even after 100 cycles.
A discharge capacity of 50-860 mAh is obtained. This has an energy density of about 230 Wh / l, which is higher than the initial energy density of the lithium ion secondary battery using cobalt which is currently commercialized and is commercially available. Regarding the change in internal resistance, at the end of 100 cycles, the battery according to the conventional method shows a change of (X) several tens of milliohms, whereas the change in the internal resistance of the battery according to the present invention shows both (A) and (B). It was confirmed to be very low in a few milliohms. As described above, according to the present invention, it is possible to significantly improve the capacity deterioration due to the cycle, which is the biggest drawback of the lithium ion secondary battery using the spinel structure lithium manganese composite oxide as the positive electrode active material. In the above-mentioned examples, L which is the most basic spinel structure lithium manganese oxide as a positive electrode active material.
Although described using iMn ₂ O ₄ , other lithium-manganese composite oxides having a spinel structure (for example, LiM _x Mn _2-x O ₄ in which a part of manganese is replaced with another element M).
Even when is used as the positive electrode active material, the improvement effect is of course exhibited. Further, in the above-mentioned examples, a positive electrode was prepared by adding and mixing BaCO ₃ and MgCO ₃ to the positive electrode active material, and EC and DEC in which LiPF ₆ was dissolved in the electrolytic solution.
The case where the mixed solution of is used was explained, but in addition to this, it is basically possible to add and mix a carbonate of one or more elements selected from the above alkali-metals to the positive electrode active material in combination with an appropriate electrolytic solution. Has the same effect.

[0011]

As described above, according to the present invention, a lithium manganese composite oxide having a spinel structure (for example, Li
By mixing with Mn ₂ O ₄ ) and adding a carbonate of one or more elements selected from alkaline earth metals to create a positive electrode, the capacity deterioration due to the charge / discharge cycle of the lithium ion secondary battery is significantly reduced. Can be improved. Lithium-manganese composite oxide can significantly reduce the material cost of the lithium-ion secondary battery, and thus can sufficiently replace the existing secondary battery.
A high-capacity, long-life and inexpensive lithium-ion secondary battery can be provided, and its industrial value is great.

[Brief description of drawings]

FIG. 1 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 positive electrode lead, and 10 is a closing lid.

Claims (1)

[Claims] 1. A battery having a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte, wherein the positive electrode uses a lithium manganese composite oxide having a spinel crystal structure as a positive electrode active material. A non-aqueous electrolyte secondary battery characterized in that a carbonate of one or more elements selected from alkaline earth metals is added to and mixed with the positive electrode active material in the positive electrode.