JPH087883A - Nonaqueous electrolytic secondary battery - Google Patents

Abstract

PURPOSE:To increase charge/discharge capacity and enhance productivity by using a mixture of lithium-containing manganese oxides having spinel crystal structure and layer structure each having a specified chemical formula as a positive active material. CONSTITUTION:A nonaqueous electrolytic secondary battery is constituted by using a carbon material as a negative active material and and a mixture of LiMn2O4 having a spinel crystal structure and LiMnO2 having a layer structure as a positive active material. In the first charge in charge/discharge processes of the battery, lithium is released from the mixed oxides and lithium, equivalent to the amount of lithium released, dissolved in an electrolyte is taken in the carbon material. When the battery is discharged after completion of the first charge, lithium is released from the carbon material and dissolved in the electrolyte, and lithium, equivalent to the amount of lithium released, in the electrolyte is taken in the mixed oxides. High discharge capacity is obtained and performance is enhanced without decrease in productivity of the battery.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to improvement of a positive electrode active material.

[0002]

2. Description of the Related Art In recent years, due to advances in electronic technology, high performance, miniaturization, and portability of electronic devices have progressed, and secondary batteries used in these electronic devices are required to have high energy density. Has been done.

Therefore, recently, a carbon material capable of being doped or dedoped with lithium is used as a negative electrode active material, a metal oxide containing lithium is used as a positive electrode active material, and a non-aqueous solution of a lithium salt is used as an electrolytic solution. Non-aqueous electrolyte secondary batteries have been proposed. This non-aqueous electrolyte secondary battery has a high battery voltage, a high energy density, little self-discharge, and excellent cycle characteristics.

Here, when the material of the non-aqueous electrolyte secondary battery is specifically exemplified, the lithium-containing metal oxide serving as the positive electrode active material is as high as about 4 V with respect to the lithium potential (Li / Li ⁺ ). Examples thereof include LiMO ₂ having a layered structure (where M is Co and Ni), LiMn ₂ O ₄ having a spinel crystal structure, and the like because they show a potential.

On the other hand, as the carbon material used as the negative electrode active material, graphitizable carbon such as coke, which exhibits a potential close to lithium potential, non-graphitizable carbon such as a furfuryl resin fired body, or graphite is used.

The electrolytic solution contains Li solvent in a mixed solvent prepared by mixing a solvent having a relatively high dielectric constant such as propylene carbonate or ethylene carbonate and a solvent having a relatively low viscosity such as diethyl carbonate or dimethyl carbonate.
A non-aqueous solution in which a lithium salt such as PF ₆ or LiBF ₄ is dissolved is used.

By the way, in the above non-aqueous electrolyte secondary battery, the negative electrode made of a carbon material is usually incorporated in the battery together with the positive electrode and the electrolytic solution in a state where lithium is not contained.

Therefore, lithium is taken into the negative electrode only when the assembled battery is charged for the first time.

That is, the first charging process of this battery proceeds with a reaction in the direction of the arrow pointing to the right in the following chemical formula 1.

[0010]

Embedded image

First, in the battery before charging, the positive electrode active material is L
It is represented by i _x MO _y and contains x in an amount of Li. On the other hand, Li is not incorporated in the negative electrode active material C.

When this battery is charged, an amount dx of Li is eluted from the positive electrode active material into the electrolytic solution, and the positive electrode active material is Li.
_The composition is represented by _x-dx MO _y . At the same time, in the negative electrode active material C, Li of the amount a · dx corresponding to Li eluted from the positive electrode active material out of Li dissolved in the electrolytic solution is taken in and represented by C _{b / (adx)} Li. Become a shape. The first charge is completed by the elution of Li from the positive electrode active material and the incorporation of Li into the negative electrode active material.

On the other hand, the first charging is completed in this way.
In the discharging process of the battery, _{b / (adx)}Expressed in Li
Li desorbs from the negative electrode active material and dissolves in the electrolyte
Put out. At the same time, the positive electrode active material Li_x-dxMO_yTo
Out of the Li dissolved in the electrolyte, from the negative electrode active material
An amount of Li corresponding to the eluted Li is taken in.

At this time, if the same amount of Li as the Li taken in during the first charging process is desorbed from the negative electrode active material and taken into the positive electrode active material, 100% charge / discharge efficiency should be obtained.

However, in the carbon material as the negative electrode active material,
Even if Li is taken in, it may remain as it is and not be desorbed, or a film may be formed on the surface due to decomposition of the electrolytic solution or the like, and as a result, a part of the taken Li is lost. Due to the reason, in the first discharging process, a discharging capacity less than 100% of the first charging capacity can be obtained.

Also in the subsequent charge / discharge process, only the remaining Li partially lost is involved and only a charge / discharge capacity smaller than the theoretical value can be obtained. (However, in the subsequent charge / discharge, there is almost no further loss of Li, and the discharge capacity for each charge capacity is about 100%).

Therefore, a method of compensating for the loss of charge / discharge capacity by preliminarily containing an excessive amount of Li corresponding to Li that is irreversibly taken into the carbon material in the battery system has been studied.

That is, in the lithium-containing metal oxide, the electrode potential rises when lithium is desorbed. On the other hand, the electrode potential of the carbon material also rises when lithium is desorbed. Considering this also, in order to construct a battery having a large charge / discharge capacity, the lithium-containing oxide serving as the positive electrode active material is made to contain an excessive amount of Li corresponding to the irreversible incorporation of the carbon material. It is desirable to set it.

As such a technique, for example, LiI ₊ xMn ₂ having a spinel crystal structure is prepared by chemically inserting lithium into LiMn ₂ O ₄ having a spinel crystal structure by using LiI.
The method of setting O ₄ (0 ≦ x ≦ 1) is described in J. Electro
chem. Soc. , 138 , 2864, 1991.

[0020]

However, it is possible at the experimental level to chemically insert lithium into the synthesized LiMn ₂ O ₄ by further using LiI, but it is introduced into industrial mass production. Is too complicated to use and lacks practicality.

Therefore, the present invention has been proposed in view of such conventional circumstances, and it is possible to supplement the amount of Li corresponding to Li which is irreversibly incorporated into the negative electrode active material, Moreover, it is an object of the present invention to obtain a positive electrode active material that can be mass-produced and provide a non-aqueous electrolyte secondary battery having a large charge / discharge capacity and excellent productivity.

[0022]

In order to achieve the above object, the present invention provides a negative electrode having a negative electrode active material of a carbon material capable of doping and dedoping lithium, a positive electrode and a non-aqueous electrolyte. In a non-aqueous electrolyte secondary battery comprising:
The positive electrode active material constituting the positive electrode is a lithium-containing manganese oxide having a spinel crystal structure represented by the chemical formula LiMn ₂ O _4, and a lithium-containing manganese oxide having a layer structure represented by the chemical formula LiMnO _2. Is a mixed oxide of.

Further, the mixing amount of LiMnO _{2 in} the mixed oxide of LiMn ₂ O ₄ and LiMnO ₂ is LiMn ₂ O ₄
It is characterized in that the molar ratio is 0.2 to 40% with respect to the amount.

The non-aqueous electrolyte secondary battery comprises a negative electrode having a negative electrode active material of a carbon material capable of doping and dedoping lithium, a positive electrode and a non-aqueous electrolyte.

In the present invention, a lithium-containing manganese oxide having a spinel crystal structure represented by a chemical formula of LiMn ₂ O _{4 and} a chemical formula of LiMnO ₂ are used as a positive electrode active material of such a non-aqueous electrolyte secondary battery. A mixed oxide with a lithium-containing manganese oxide having a layer structure shown is used.

For LiMnO ₂ having a layered structure,
When excess Li is desorbed, the LiMnO ₂ may undergo a phase change to LiMn ₂ O ₄ having a spinel crystal structure.
t. Res. Bull. , 28 , 1249, 1993.
Have been reported in.

In the present invention, such characteristics of LiMnO ₂ are irreversibly incorporated into the negative electrode active material.
It is used to make up for.

That is, the charge and discharge process of a battery using a carbon material as a negative electrode active material and a mixed oxide of LiMn ₂ O ₄ having a spinel crystal structure and LiMnO ₂ having a layer structure as a positive electrode active material is as follows. In the first charging, Li is desorbed from the mixed oxide, and at the same time, Li dissolved in the electrolytic solution is taken into the carbon material in an amount corresponding to the desorbed Li.

When discharging is performed after the first charging is completed in this way, Li is desorbed from the carbon material and is eluted into the electrolytic solution. At the same time, the mixed oxide is dissolved in the electrolytic solution. L from the carbon material
An amount of Li corresponding to i is taken in.

At this time, in the carbon material, the incorporated L
A part of i is lost, and the mixed oxide takes in only a smaller amount of Li than the eluted Li.

Here, in the above-mentioned mixed oxide, even when a part of Li is lost in this way, when charging and discharging are further repeated, excess L from the mixed LiMnO ₂ is added.
i is desorbed, and the LiMnO ₂ undergoes a phase change to LiMn ₂ O ₄ having a spinel crystal structure. That is, this Li
L desorbed during the phase change of MnO ₂ to LiMn ₂ O ₄
When i is supplied into the battery system, the phase-changed LiMn ₂ O ₄ thereafter functions as a positive electrode active material like other LiMn ₂ O ₄ . Therefore, irreversibly incorporated Li
The charge and discharge proceed in a manner that is compensated for, and a large charge and discharge capacity can be obtained as compared with the case where LiMn ₂ O ₄ alone is used as the positive electrode active material.

Moreover, LiM mixed with the above mixed oxide
nO ₂ is a raw material that is chemically stable in the atmosphere and is about 250 to 1
It is obtained by firing in the temperature range of 000 ° C., and can be manufactured by a simple method without using a special operation. Therefore, L
It is more suitable for mass production than the method of chemically inserting Li into iMn ₂ O ₄ and is also advantageous for improving the productivity of the non-aqueous electrolyte secondary battery.

In the above mixed oxide, LiMn
For the purpose, it is desirable to mix O ₂ only in an amount capable of supplying the amount of Li irreversibly incorporated into the carbon material. Usually, the mixing ratio of LiMnO ₂ is LiMn ₂
A suitable molar ratio is 0.2 to 40% with respect to O ₄ .

As described above, the non-aqueous electrolyte secondary battery of the present invention is used.
In the pond, LiMn is used as the positive electrode active material. ₂O_FourAnd LiMnO
₂A lithium-containing metal oxide mixture consisting of
Normally used as a carbon material and non-aqueous electrolyte for the negative electrode
Any of the above can be used.

As the carbon material, pyrolytic carbons, cokes (pitch cokes, needle cokes, petroleum cokes, etc.), graphites, glassy carbons, organic polymer compound fired bodies (phenol resin, furan resin, etc.) are suitable. Carbonized by firing at various temperatures), carbon fiber, activated carbon,
Examples include graphite.

As the electrolytic solution, for example, a non-aqueous electrolytic solution obtained by using a lithium salt as an electrolyte and dissolving this in an organic solvent is used.

The organic solvent is not particularly limited, but propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, γ-butyl lactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, sulfolane, A single solvent such as acetonitrile, diethyl carbonate, dipropyl carbonate or a mixed solvent of two or more kinds can be used.

As the electrolyte, LiClO ₄ , LiAs
F 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 can be used.

[0039]

When LiMnO ₂ having a layer structure is desorbed from excess Li, LiMn ₂ O ₄ having a spinel crystal structure is formed.
Changes to.

Such LiMnO ₂ and LiMn ₂ O ₄
In a non-aqueous electrolyte secondary battery having a mixed oxide composed of a mixed oxide as a positive electrode active material, even if Li is irreversibly taken into the negative electrode active material during the first charging process, LiMnO 2 is repeatedly charged and discharged. Li is desorbed from ₂ to form LiMn ₂ O
_The phase changes to ₄ and then functions as a positive electrode active material like other LiMn ₂ O ₄ . Therefore, charging / discharging progresses in the form of being supplemented with Li that is irreversibly taken in, and L
A large charge / discharge capacity can be obtained as compared with the case where iMn ₂ O ₄ alone is used as the positive electrode active material.

[0041]

EXAMPLES Preferred examples of the present invention will be described below based on experimental results.

Example 1 First, a non-graphitizable carbon material such as a fired furfuryl resin material was prepared as a negative electrode active material, and the ratio (desorption amount / insertion amount) η of the lithium desorption amount to the lithium insertion amount was determined. It was

The lithium insertion amount and the lithium desorption amount of the above-mentioned non-graphitizable carbon material were evaluated by preparing an evaluation cell by using an electrode made of this non-graphitizable carbon material and lithium metal which is the opposite electrode. It was investigated by measuring the first charge capacity and discharge capacity of the cell. As a result, in the non-graphitizable carbon material used in this example, the lithium desorption / insertion ratio η was 0.75, and the lithium irreversible uptake / insertion ratio was determined to be 0.25. It was

Next, a positive electrode active material was prepared as follows. LiMn ₂ O ₄ having a spinel crystal structure was produced by mixing manganese dioxide and lithium carbonate at a predetermined ratio and firing the mixture at a temperature of 700 to 850 ° C. in an air atmosphere.

LiMnO ₂ having a layer structure was produced by mixing dimanganese trioxide and lithium oxide in a predetermined ratio and firing the mixture at a temperature of about 750 ° C. in an argon atmosphere.

Then, these LiMn ₂ O ₄ and LiMn
O ₂ was mixed at a ratio of 1: 0.25 (molar ratio) based on an irreversible uptake / insertion ratio of 0.25 of the non-graphitizable carbon material to obtain a lithium-containing metal oxide mixture.

The above non-graphitizable carbon material was mixed with 5% by weight of a binder and pressure-molded to prepare a pellet-shaped negative electrode, while a lithium-containing metal oxide was added with 5% by weight of a binder and 10% by weight. A conductive additive was mixed and pressure-molded to produce a pellet-shaped positive electrode.

Then, a coin type cell having a thickness of 20 mm and a radius of 25 mm was assembled using the prepared negative electrode, positive electrode, electrolytic solution in which LiPF ₆ was dissolved in a mixed solvent of propylene carbonate and diethyl carbonate, and a separator of polyolefin. .

Comparative Example 1 A coin type cell was assembled in the same manner as in Example 1 except that LiMn ₂ O ₄ alone was used as the positive electrode active material.

The non-aqueous electrolyte secondary battery prepared as described above was charged and discharged to examine the discharge characteristics.

Charging is performed at a constant current of 0.5 mA.
After charging to 2V, charging was further performed at a constant voltage of 4.2V for 2 hours, and discharging was performed at a constant current of 0.5 mA and a final voltage of 2.5V. The discharge characteristics of the cell of Example 1 and the cell of Comparative Example 1 obtained under the above conditions are also shown in FIG.

As can be seen from FIG. 1, LiMn ₂ O ₄ having a spinel crystal structure and LiMnO ₂ having a layer structure.
The cell of Example 1 using the mixed oxide of Example 1 as a positive electrode active material has a larger discharge capacity than the cell of Comparative Example 1 using LiMn ₂ O ₄ alone.

From this fact, LiMn ₂ O ₄ having a spinel crystal structure and Li having a layer structure were used as the positive electrode active material.
It can be seen that the use of the mixed oxide of MnO ₂ is effective in compensating for Li that is irreversibly incorporated into the carbon material and obtaining a large discharge capacity.

The carbon material used as the negative electrode active material in this example had a Li desorption / insertion ratio η of 0.75. The desorption amount / insertion amount ratio η of Li was in the range of 0.6 to 0.998 in the carbon material with which a large negative electrode capacity was obtained. Therefore, it is appropriate to mix LiMnO ₂ with an amount such that the molar ratio is 0.2 to 40% with respect to LiMn ₂ O ₄ .

[0055]

As is clear from the above description, in the non-aqueous electrolyte secondary battery of the present invention, a carbon material is used as the negative electrode active material and a LiMn ₂ O ₄ chemical compound is used as the positive electrode active material. Since the mixed oxide of the lithium-containing manganese oxide having the spinel crystal structure and the lithium-containing manganese oxide having the layer structure represented by the chemical formula LiMnO ₂ is used, it is irreversible in the negative electrode active material during the first charging process. Li taken in by is supplemented by Li desorbed during the phase change of LiMnO ₂ , and it is possible to obtain a large discharge capacity in the subsequent charge and discharge process. Further, both LiMn ₂ O ₄ and LiMnO _{2 which} are the positive electrode active materials can be manufactured by a simple method which does not require a special operation.

Therefore, according to the present invention, it is possible to obtain a high performance non-aqueous electrolyte secondary battery without lowering the productivity of the battery.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing discharge characteristics of a battery.

Claims (2)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a negative electrode using a carbon material capable of doping / dedoping lithium as a negative electrode active material, and a positive electrode and a non-aqueous electrolyte solution. The constituent positive electrode active material is a mixed oxidation of a lithium-containing manganese oxide having a spinel crystal structure represented by the chemical formula LiMn ₂ O ₄ and a lithium-containing manganese oxide having a layer structure represented by the chemical formula LiMnO _2. A non-aqueous electrolyte secondary battery characterized by being a product. _2. The mixed amount of LiMnO ₂ in the mixed oxide is 0.2 to 40% in molar ratio with respect to the amount of LiMn ₂ O _4.
The non-aqueous electrolyte secondary battery according to claim 1, wherein