JPH10199528A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable charging/discharging characteristics of a secondary battery to be improved by forming positive active material particles in a complex body consisting of LiMn2 O4 and Li2 Mn2 O4 and restricting elution of Mn out of Mn oxide of positive active material during charging and crystal structure destruction of spinel type LiMn2 O4 during discharging. SOLUTION: A mixture of molar ratio 95:5 between LiMn2 O4 powder and LiOH powder is subjected to heat treatment at 600 deg.C for 6 hours in vacuum so as to form a complex whose surface is covered by Li2 Mn2 O4 . In this complex powder 90 pts.wt. carbon black of conductive material 5 pts.wt., and 5 pts.wt. of solid component of binding agent of polyvinylidene fluoride dissolved in N-methyl-2-pyrrolidone are mixed together to form positive black mixture. Hereby, the surface of positive active material LiMn2 O4 particles surfaces are covered by Li2 Mn2 O4 particles and during a charging period elution of Mn included in LiMn2 O4 can be restricted. In addition, the crystal structure of LiMn2 O4 particles can be stabilized also against storage/release of Li ions at repetitive charging/discharging, so that cycle characteristic may be improved.

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 using manganese oxide as a positive electrode active material, and more particularly to an improvement in cycle characteristics and an increase in capacity.

[0002]

2. Description of the Related Art Conventionally, titanium disulfide, vanadium pentoxide, manganese oxide and the like have been proposed as a positive electrode active material of a nonaqueous electrolyte secondary battery. In particular, a manganese oxide represented by LiMn ₂ O ₄ has attracted particular attention because of its ability to extract a high voltage, being rich in resources, and being inexpensive.

It is known that the LiMn ₂ O ₄ particles have a spinel structure, and that lithium ions can be doped and de-doped in the crystal, and that excellent charge / discharge characteristics can be obtained. .

However, conventionally, in a non-aqueous electrolyte secondary battery using a manganese oxide such as LiMn ₂ O ₄ particles, manganese is eluted at the time of charging when the positive electrode has a high potential. In this case, there is a problem that the discharge capacity is significantly reduced.

In order to solve the above problem, Japanese Patent Application Laid-Open No. HEI 6-111819 discloses LiMn ₂ O ₄ and LiMn.
_It has been proposed to use a composite powder of ₂ MnO ₃ .

[0006] However, in the above publication, the surface of the spinel type LiMn ₂ O ₄ particles is made of LiMn ₂ O ₄ particles having no spinel skeleton.
Since it is covered with a ₂ MnO ₃ layer, LiMn ₂ O ₄ particles and L
The interface lattice irregularity at the interface of i ₂ MnO ₃ particles is large,
At the time of discharge at which the positive electrode has a low potential, there is a problem that the crystal structure at the interface collapses due to the occlusion of Li, and the charge / discharge cycle characteristics deteriorate.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, and an object of the present invention is to elute manganese from manganese oxide as a positive electrode active material during charging and spinel type LiMn ₂ O ₄ during discharging. It is an object of the present invention to provide a nonaqueous electrolyte battery having excellent charge / discharge characteristics by suppressing the collapse of the crystal structure of the nonaqueous electrolyte solution.

[0008]

According to the present invention, there is provided a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode active material particles are LiMn ₂ O ₄ and Li ₂ Mn. ₂ O
And a complex with ₄ .

[0009] The complex referred to here is simply LiMn ₂ O ₄
Particles, rather than the mixture obtained by mixing the Li ₂ Mn ₂ O ₄ particles, refers to the state in which the LiMn ₂ O ₄ particles and Li ₂ Mn ₂ O ₄ particles are chemically bonded to each other.

In order to obtain such a composite, for example, a mixture of LiMn ₂ O ₄ and lithium hydroxide is mixed in a vacuum or in a non-oxidizing gas atmosphere such as nitrogen gas, argon gas or the like for 600 minutes. It is obtained by baking at a temperature of about 5 ° C. for 5 hours or more.

The form of the composite thus obtained is as follows:
At least a part of the surface of the LiMn ₂ O ₄ particles is Li ₂ M
It is formed of an n ₂ O ₄ layer.

Thus, LiMn_TwoO_FourLi on the surface of the particles
_TwoMn_TwoO_FourWhen a layer is formed, the high potential during charging
LiMn_TwoO_FourSome of the manganese in the
And can be suppressed. The reason is that Li_TwoMn_TwoO
_FourIs LiMn_TwoO_FourRelatively stable to high potentials
Because there is Li_TwoMn_TwoO_FourLiMn coated with layer_TwoO _Four
The particles are also believed to be stable to high potentials.

Further, at the time of discharging at a low potential, it is possible to prevent the crystal structure of LiMn ₂ O _{4 from} collapsing due to occlusion of lithium.

The reason is that the Li ₂ Mn ₂ O ₄ particles to be composited with the spinel type LiMn ₂ O ₄ particles also have a spinel structure, so that even if both are compounded, the interface is a particle having a spinel structure. It is considered that, since they are both, the crystal structures of both do not collapse even when lithium ions move.

The Li ₂ Mn ₂ O ₄ layer is composed of LiMn ₂ O ₄
The effect can be further obtained by covering the entire surface of the particle.

Further, it is preferable that the thickness of the Li ₂ Mn ₂ O ₄ layer is 20 nm or more and 100 nm or less.

The reason is that the film thickness of the Li ₂ Mn ₂ O ₄ layer is 2
If the thickness is less than 0 nm, the effect of forming a composite of LiMn ₂ O ₄ and Li ₂ Mn ₂ O ₄ cannot be obtained. This is because the amount of LiMn ₂ O ₄ due to the positive electrode capacity decreases, which causes a problem that the discharge capacity of the battery decreases. Further, the film thickness of Li ₂ Mn ₂ O ₄ is particularly 40
It is more preferably from 70 nm to 70 nm.

The solvent for the non-aqueous electrolyte of the non-aqueous electrolyte secondary battery of the present invention may be a solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and 1,2-dimethoxyethane. Or a mixed solvent thereof. As the solute, LiPF ₆ , LiBF ₄ , L
iClO ₄ , LiCF ₃ SO _{3 and the} like.

Further, as the negative electrode of the non-aqueous electrolyte secondary battery of the present invention, lithium metal, a lithium alloy and lithium ions are stored. Releasable carbon materials are included.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION

Example 1 [Preparation of Positive Electrode] Molar ratio of LiMn ₂ O ₄ powder having a spinel structure to lithium hydroxide (LiOH) powder 95: 5
Is heated in a vacuum at 600 ° C. for 6 hours,
A composite in which the surface of LiMn ₂ O ₄ was coated with Li ₂ Mn ₂ O ₄ was synthesized.

The composite was subjected to X-ray diffraction measurement, and the obtained diffraction pattern was compared with a JCPDS card. As a result, it was confirmed that the composite was composed of LiMn ₂ O ₄ and Li ₂ Mn ₂ O ₄ . When the above composite was observed with a transmission electron microscope at a high resolution, the composite particles were found to be LiMn ₂
O ₄ particles, and 50 nm of Li ₂ Mn ₂
It was confirmed that an O ₄ film was formed.

Next, 90 parts by weight of the composite powder, 5 parts by weight of carbon black as a conductive agent, and 5 parts by weight of polyvinylidene fluoride dissolved in N-methyl-2-pyrrolidone as a binder as a solid content. The mixture was mixed as described above to obtain a positive electrode mixture.

The positive electrode mixture is coated on both sides of an aluminum foil core, dried, and then rolled by a roller press.
An aluminum lead was ultrasonically welded to the end, followed by drying treatment to produce a positive electrode.

[Preparation of Negative Electrode]
95 parts by weight of 5 μm natural graphite powder and N-methyl-2-
Polyvinylidene fluoride dissolved in pyrrolidone was mixed to a solid content of 5 parts by weight to prepare a negative electrode mixture.

This negative electrode mixture was coated on both sides of a copper foil, dried, and then rolled by a roller press. A nickel lead was ultrasonically welded to an end portion, followed by drying treatment to prepare a negative electrode.

[Adjustment of electrolyte solution] LiPF ₆ was added to a mixed solvent having a volume mixing ratio of ethylene carbonate and dimethyl carbonate of 1: 1 so as to have a concentration of 1 mol / dm ^3.
Was dissolved to prepare a non-aqueous electrolyte.

[Preparation of Battery] The above positive electrode and negative electrode were spirally wound through a 25-μm-thick porous polypropylene separator to produce a spiral electrode body.

After inserting this spiral electrode body into a nickel-plated iron battery can, the above-mentioned electrolytic solution was injected.

Next, a sealed cylindrical battery was manufactured by a sealing body with a gasket interposed at the opening of the battery can. This sealed cylindrical battery is referred to as Battery A1 of the present invention.

Comparative Example 1 LiMn ₂ O ₄ powder and LiOH
Powder and a molar ratio of 1: 0.02, and mixed in air at 37.degree.
5 ° C. 20 hours of heat treatment is made in, except for using LiMn ₂ O ₄ on the surface of the powder Li ₂ MnO ₃ layer is formed complexes, the battery was fabricated in the same manner as in Example 1.

The battery fabricated in this manner was used as a comparative battery X
Let it be 1.

Comparative Example 2 The surface of LiMn ₂ O ₄ powder was L
A battery was produced in the same manner as in Example 1, except that only the LiMn ₂ O ₄ powder having a spinel structure was used instead of the composite having the i ₂ Mn ₂ O ₄ layer formed thereon.

The battery fabricated in this manner was used as a comparative battery X
Let it be 2.

Comparative Example 3 The surface of LiMn ₂ O ₄ powder was L
Except for using a mixture of a LiMn ₂ O ₄ powder having a spinel structure and a Li ₂ Mn ₂ O ₄ powder (molar ratio = 5: 1) instead of the composite having the i ₂ Mn ₂ O ₄ layer formed thereon, Example 1
In the same manner as in the above, a battery was produced.

The battery manufactured in this manner was used as a comparative battery X
3 is assumed.

(Experiment 1) The battery A1 of the present invention and the comparative batteries X1 to X3 were used to examine the discharge characteristics of each battery. The results are shown in FIG. The experimental conditions were as follows.
At 240 mA, the battery was charged until the battery voltage reached 4.2 V, and then the capacity was measured when the battery was discharged to a discharge end voltage of 3.0 V at a discharge current of 0.2 C (240 mA). 1, the vertical axis represents the battery voltage, and the horizontal axis represents the positive electrode active material 1.
The discharge capacity per g was shown.

As is clear from FIG. 1, the battery A1 of the present invention
Indicates that the discharge capacity is improved as compared with the comparative batteries X2 to X3. This is because the surface of the LiMn ₂ O ₄ particles as the positive electrode active material of the battery A1 of the present invention is coated with the Li ₂ Mn ₂ O ₄ particles, so that the manganese in the LiMn ₂ O ₄ elutes into the electrolyte during charging. It is thought that it is because it was able to suppress that. As in Comparative Example 3, LiMn ₂ O ₄ particles and Li ₂
Simply mixing the Mn ₂ O ₄ particles with the Mn ₂ O ₄ particles results in LiMn ₂ O ₄
It is considered that elution of manganese from the particles cannot be suppressed and the discharge capacity decreases.

(Experiment 2) Using the battery A1 of the present invention and the comparative batteries X1 to X3, the cycle characteristics of each battery were examined. The results are shown in FIG. The experimental conditions were as follows.
After charging until the battery voltage reaches 4.2 V at 2 C (240 mA), a series of operations of discharging to a discharge end voltage of 3.0 V with a discharge current of 0.2 C (240 mA) is repeated. The vertical axis in FIG. 2 shows the relative ratio to the initial discharge capacity, and the horizontal axis shows the number of cycles.

As is apparent from FIG. 2, the battery A1 of the present invention
Indicates that the cycle characteristics are improved as compared with the comparative batteries X1 to X3. It is considered that this is because the LiMn ₂ O ₄ particles, which are the positive electrode active materials of the comparative batteries X1 to X3, had their crystal structure collapsed by repeated charge and discharge, and their cycle characteristics deteriorated.

As can be seen from the results of Experiments 1 and 2, when the battery A1 of the present invention is compared with the comparative battery X1, both of the discharge characteristics are equivalent, but the battery A1 of the present invention is superior in cycle characteristics. It turns out that there is.

This is because, in the discharge characteristics, both can suppress the elution of manganese at a high potential, but in the cycle characteristics, the comparative battery X1 has Li ₂ having no spinel skeleton.
It is considered that since the LiMn ₂ O ₄ was covered with the MnO ₃ layer, the crystal structure of LiMn ₂ O ₄ was broken by the occlusion of Li at a low potential, and the cycle characteristics were degraded.

On the other hand, by forming at least a part of the surface of the spinel type LiMn ₂ O ₄ particles with a Li ₂ Mn ₂ O ₄ layer having a spinel structure as in the battery A1 of the present invention, The crystal structure of the LiMn ₂ O ₄ particles can be maintained more stably even with respect to insertion and extraction of lithium ions due to repeated charge and discharge, and cycle characteristics can be improved.

[0043]

According to the non-aqueous electrolyte secondary battery of the present invention, since the composite of LiMn ₂ O ₄ and Li ₂ Mn ₂ O ₄ is used for the positive electrode active material particles, the elution of manganese during charging is prevented. It is possible to suppress the collapse of the crystal structure of the LiMn ₂ O ₄ particles at the time of discharge and also to suppress the collapse of the crystal structure of the LiMn ₂ O ₄ particles.

[Brief description of the drawings]

FIG. 1 is a diagram showing the discharge characteristics of Invention A1 of the present application and Comparative Batteries X1 to X3.

FIG. 2 is a view showing cycle characteristics of the invention A1 of the present application and comparative batteries X1 to X3.

Claims (4)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode active material particles are L
A non-aqueous electrolyte secondary battery, which is a composite of iMn ₂ O ₄ and Li ₂ Mn ₂ O ₄ . 2. The non-aqueous electrolyte solution according to claim 1, wherein the positive electrode active material particles have a Li ₂ Mn ₂ O ₄ layer formed on at least a part of the surface of the LiMn ₂ O ₄ particles. Next battery. 3. The Li ₂ Mn ₂ O ₄ layer covers the entire surface of the LiMn ₂ O ₄ particles.
The non-aqueous electrolyte secondary battery according to the above. _4. The film thickness of the Li ₂ Mn ₂ O ₄ layer is 20 nm.
The non-aqueous electrolyte secondary battery according to claim 3, wherein the thickness is at least 100 nm.