JP2000123834A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery capable of maintaining discharge capacity after repeted charge and discharge. SOLUTION: LiCoO2 has a laminated rock-salt structure and Mg or Ca is substituted for Li in LiCoO2, thereby increasing electron conductivity. Thus, a positive electrode active material as a solid solution expressed by Li1-xCoMgxO2 where 0<x<=0.05, or a positive electrode active material as a solid solution expressed by Li1-yCoCayO2 where 0<y<=0.002 is used for a positive electrode. As a result, a nonaqueous electrolyte secondary battery improved in a capacity retention rate and having durability against repeated charge and discharge processes can be provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery using a positive electrode active material having an improved composition.

[0002]

_2. Description of the Related Art Conventionally, as a non-aqueous electrolyte secondary battery, a battery using LiCoO ₂ or LiMn ₂ O ₄ as a positive electrode active material has been put into practical use. As a power source for portable electronic devices.

[0003]

In a portable electronic device, the battery capacity is set to be relatively small, so that the number of times of charging and discharging tends to be large. However, in the above-mentioned conventional nonaqueous electrolyte secondary battery, there is a problem that the discharge capacity tends to decrease when the charge / discharge is repeated many times, and the so-called cycle characteristics are insufficient. .

The present invention has been made in view of the above circumstances, and has as its object to provide a non-aqueous electrolyte secondary battery capable of maintaining a discharge capacity even when charging and discharging are repeated many times.

[0005]

Means for Solving the Problems To achieve the above object,
Therefore, the invention according to claim 1 reversibly converts lithium ions.
Absorbing or releasing material or lithium or lithium alloy
A negative electrode made of gold and Li _1-xCoMg_xO₂Represented by
Use positive electrode active material with 0 <x ≦ 0.05
With the positive electrode and non-aqueous electrolyte
Have.

In the solid solution according to the first aspect,
It is more preferable that the value of the variable x is 0.0005 ≦ x ≦ 0.03 (the invention according to claim 2).

[0007] Further, the invention according to claim 3 provides a negative electrode made of a material or a lithium or lithium alloy that reversibly stores or releases lithium ions, and a Li _1-y CoC
It is characterized by comprising a positive electrode using a positive electrode active material satisfying 0 <y ≦ 0.002 in a solid solution represented by a _y O ₂ and a non-aqueous electrolyte.

Further, in the solid solution according to the third aspect, it is more preferable that the value of the variable y is 0 <x ≦ 0.001 (the invention according to the fourth aspect).

Further, according to a fifth aspect of the present invention, in the non-aqueous electrolyte secondary battery according to any one of the first to fourth aspects, the solid solution of the positive electrode active material contains Fe in a weight percent concentration. 0.1% or less, Cu is 0.1% or less, Na
Is 0.5% or less, Si is 0.5% or less, Ni is 0.5%
The features are as follows.

Further, according to a sixth aspect of the present invention, in the non-aqueous electrolyte secondary battery according to any one of the first to fourth aspects, the solid solution of the positive electrode active material contains Fe in a weight percent concentration. It is characterized in that 0.03% or less, Cu is 0.005% or less, Na is 0.1% or less, Si is 0.1% or less, and Ni is 0.15% or less.

[0011]

The operation and effect of the present invention LiCoO ₂ has a layered rock salt structure, and a structure in which part of Li is replaced by Mg is a solid solution represented by Li _1-x CoMg _x O ₂ . L
The reason why the cycle characteristics are improved by substituting i with Mg is not clear, but is presumed to be due to an increase in electron conductivity. According to the experiment, the cycle characteristics tend to be improved as the substitution amount of Mg increases. However, on the other hand, M
The initial capacity decreases as the replacement amount of g increases.

For this reason, from the viewpoint of improving the cycle characteristics, it is preferable that x is a large value exceeding 0,
From the viewpoint of securing the initial capacity, x is preferably small.
According to experiments, the appropriate range was 0 <x ≦ 0.05 (the invention of claim 1).

By setting 0.005 <x ≦ 0.03, more excellent cycle characteristics can be obtained together with a larger initial capacity (the invention of claim 2).

Further, a structure in which a part of Li of LiCoO ₂ is replaced by Ca is a solid solution represented by Li _1-y CoCa _y O ₂ . The reason why the cycle characteristics are improved when Li is replaced by Ca is still not clear. In addition, since Ca also functions as an impurity for the positive electrode active material, the initial capacity decreases as the replacement amount of Ca increases. An appropriate range for y is experimentally 0 <y ≦ 0.
002 (invention of claim 3), and a more preferable range is 0 <y ≦ 0.001 (invention of claim 4).

Among the impurities contained during the production of the positive electrode active material, Fe, Cu, Na, Si, N
Regarding i, by increasing the purity of the raw material, 0.1% or less, 0.1% or less, 0.5% or less,
It is appropriate that the content is 0.5% or less, and 0.5% or less (the invention of claim 5). If these values are exceeded, the initial capacity decreases. In particular, Fe is 0.03% or less, Cu is 0.005% or less, Na is 0.1% or less, S
Most preferably, i is 0.1% or less and Ni is 0.15% or less (the invention of claim 6).

[0016]

<Example 1> A method for manufacturing a nonaqueous electrolyte secondary battery according to this example is as described below. First, L
A high-purity dry powder sample of i ₂ CO ₃ , Co ₃ O ₄ and MgCO ₃ was mixed well and pre-fired in air at 650 ° C. for 2 hours first for decarboxylation and then 900 hours
Calcination was performed at ℃ for 3 days. Thereafter, by slow _cooling, a solid solution of _{Li 1-x} CoMg x O ₂ was obtained. The composition of solid solution in each of Examples 1-1 to 1-7 (x
Is as shown in Table 1 below. The solid solution thus obtained was used as a positive electrode active material.

[0017]

[Table 1]

The above-mentioned positive electrode active material is mixed with acetylene black as a conductive agent and polyvinylidene fluoride as a binder together with N-methylpyrrolidone as a solvent to form a paste, which is used as a current collector of aluminum foil. After the application, a heat treatment is performed, followed by pressing and molding to produce a positive electrode plate.

A charge / discharge cycle test was performed using a beaker cell with the positive electrode plate as the positive electrode 4. In the beaker cell, as shown in FIG. 1, an electrolytic solution 2 in which 1 mol / l of LiClO ₄ is dissolved in a mixed solution of equal amounts of ethylene carbonate and diethyl carbonate is injected into a beaker 1 which is a glass container, The opening 1 is covered with a lid 3 made of polypropylene. Then, the positive electrode 4 is arranged in the electrolytic solution 2 near the center of the beaker 1, and the counter electrode 5 using a lithium foil and the lithium reference electrode 6 are placed in the electrolytic solution 2 at positions that sandwich the positive electrode 4 at a predetermined interval. It is the structure arranged in.

Using the beaker cell manufactured as described above, a charge / discharge cycle test was performed in a dry box in an argon atmosphere in order to prevent adverse effects due to moisture.
The test was conducted at a current density of 1 mA / cm ^{2 at} a current density of 4.3 V with respect to the lithium potential, and then discharged at a current density of ² mA / cm ² to 3.0 V as one cycle. The capacity was measured after performing 50 cycles, and the capacity retention was obtained from the results.

FIG. 2 shows the measurement results, and FIG. 3 shows the capacity retention. As described above, in the present embodiment, as the value of x increases, the capacity retention rate increases, but on the other hand, the initial capacity gradually decreases. Therefore,
The range of x suitable for actual use as a battery is 0 <x
≦ 0.05. And 0.0005 ≦ x ≦
When it is in the range of 0.03, the battery has a better capacity retention rate and a large initial capacity, so that a good non-aqueous electrolyte secondary battery can be obtained.

In addition, the positive electrode active material is Fe, C
Since the inclusion of the elements u, Na, Si, and Ni causes the initial capacity of the non-aqueous electrolyte secondary battery to decrease, the positive electrode active material according to the present example has a weight percent concentration of Fe Is 0.1% or less, Cu is 0.1% or less, Na
Is 0.5% or less, Si is 0.5% or less, Ni is 0.5%
It is desirable to be as follows. In particular, in this embodiment, the positive electrode active material contained 0.03% by weight of Fe.
% Or less, Cu is 0.005% or less, Na is 0.1% or less, Si is 0.1% or less, and Ni is 0.15% or less, so that the initial capacity of the nonaqueous electrolyte secondary battery is reduced. It became possible to make it larger.

The above-mentioned charge / discharge cycle test was conducted using a beaker cell. A practical non-aqueous electrolyte secondary battery was produced using the above-mentioned positive electrode plate, for example, as shown below. Can be.

The negative electrode plate is prepared by mixing artificial graphite with polyvinylidene fluoride as a binder and N-methylpyrrolidone as a solvent to form a paste. The paste is applied to a copper foil current collector and then heat-treated. It is manufactured by pressing and molding.

The positive electrode plate and the negative electrode plate are spirally wound with a separator interposed therebetween, accommodated in a battery container, formed with a well-known electrode lead-out structure, injected with an electrolytic solution, and sealed to form a non-aqueous electrolytic solution. A liquid secondary battery. The electrolytic solution was 1 mol / l in a mixed solvent of ethylene carbonate, dimethyl carbonate and diethyl carbonate in a ratio of 2: 2: 1.
Used in which LiPF ₆ is dissolved.

By manufacturing a non-electrolyte secondary battery in this manner, it is possible to obtain a good non-aqueous electrolyte secondary battery having a better capacity retention rate and a large initial capacity. .

<Embodiment 2> This embodiment relates to a positive electrode,
Li _1-y CoCa _y O manufactured using a high-purity dry powder sample of Li ₂ CO ₃ , Co ₃ O ₄ and CaCO ₃
Example 1 in that the solid solution of No. ₂ was used as the positive electrode active material.
However, since the other configuration is the same, the description is omitted. Composition (y) in each of Examples 2-1 to 2-5
Are as shown in Table 2 below. Using these, a beaker cell was manufactured and a charge / discharge cycle test was performed in the same manner as in Example 1 above. The measurement results are shown in FIG. 4, and the capacity retention is shown in FIG.

[0028]

[Table 2]

From the results shown in FIG. 5, in this embodiment, when the value of y is 0.0005, the capacity retention ratio shows the best value, and when it exceeds that value, it decreases. On the other hand, as shown in FIG. 4, the initial capacity gradually decreases as the value of y increases. Therefore, the range of y suitable for use as a battery is 0 <y ≦ 0.002. And
In the range of 0 <y ≦ 0.001, it is possible to obtain a good non-aqueous electrolyte secondary battery having both a large capacity retention and an initial capacity.

In this embodiment, as in the case of the first embodiment, the positive electrode active material has a weight percent concentration of Fe of 0.1%.
03% or less, Cu is 0.005% or less, Na is 0.1%
Hereinafter, by setting the content of Si to 0.1% or less and the content of Ni to 0.15% or less, the initial capacity of the nonaqueous electrolyte secondary battery can be increased.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a beaker cell.

FIG. 2 is a graph showing the relationship between the initial capacity and the post-cycle capacity of the nonaqueous electrolyte secondary battery with respect to the value of x of the positive electrode active material according to Example 1.

FIG. 3 is a graph showing the relationship between the value of x of the positive electrode active material according to Example 1 and the capacity retention of the nonaqueous electrolyte secondary battery.

FIG. 4 is a graph showing the relationship between the initial capacity and the capacity after cycling of the nonaqueous electrolyte secondary battery with respect to the value of y of the positive electrode active material according to Example 2.

FIG. 5 is a graph showing the relationship between the value of y of the positive electrode active material according to Example 2 and the capacity retention of the nonaqueous electrolyte secondary battery.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kazuhiro Nakamitsu F-term (reference) 5G029 AJ03 AJ05 AK03 AL12 AM03 AM05 AM07 BJ02 in 5 Kichijoin Nitta Ichidandan-cho, Minami-ku, Kyoto-shi BJ14 HJ01 HJ02

Claims (6)

[Claims] 1. A negative electrode made of a material or a lithium or lithium alloy that reversibly stores or releases lithium ions, and a solid solution represented by Li _1-x CoMg _x O ₂ ,
A positive electrode using a positive electrode active material satisfying 0 <x ≦ 0.05;
A non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the solid solution is 0.0005 ≦ x ≦ 0.03.
A non-aqueous electrolyte secondary battery characterized by being a positive electrode using the positive electrode active material. 3. A negative electrode made of a material or a lithium or lithium alloy that reversibly stores or releases lithium ions, and a solid solution represented by Li _1-y CoCa _y O ₂ ,
A non-aqueous electrolyte secondary battery comprising a positive electrode using a positive electrode active material satisfying 0 <y ≦ 0.002, and a non-aqueous electrolyte. 4. The non-aqueous electrolyte secondary battery according to claim 3, wherein the solid solution is a positive electrode using a positive electrode active material satisfying 0 <y ≦ 0.001. Liquid secondary battery. 5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content of each of the following elements in the solid solution of the positive electrode active material is F in weight percent concentration.
e is 0.1% or less, Cu is 0.1% or less, and Na is 0.5% or less.
% Or less, 0.5% or less of Si, and 0.5% or less of Ni. 6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the following elements in the solid solution of the positive electrode active material is Fe in weight percent concentration.
Is 0.03% or less, Cu is 0.005% or less, Na is 0.1% or less, Si is 0.1% or less, and Ni is 0.15%.
A non-aqueous electrolyte secondary battery characterized by the following.