JP2001243982A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery that has a high discharge voltage (4.2 V or more). SOLUTION: The lithium secondary battery comprises a positive electrode and a negative electrode which occlude and discharge lithium ion and an electrolyte containing the above lithium ion. The above positive electrode is composed of a positive electrode active material, a conductive material, a binder and a current collector, and the above conductive material is made of amorphous graphite, and the above electrolyte contains at least one or more kinds of a compound as expressed in the formula (1) or formula (2) (wherein, R1, R2 shows alkyl radical of carbon number 1 to 10 and R1 and R2 may be same.) and has an average discharge voltage of 4.2 V or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery suitable for use in portable equipment such as a portable telephone or a notebook personal computer, a driving power supply for an electric vehicle, and a power storage power supply.

[0002]

2. Description of the Related Art As a conventional technique for improving the characteristics of a secondary battery, Japanese Patent Application Laid-Open No. 11-317241 or Japanese Patent Application Laid-Open No.
No. 666 discloses that a sulfur-containing compound such as dimethyl sulfoxide, sulfolane, dimethyl sulfite, diethyl sulfite and the like can be used in the electrolytic solution.

[0003] In Japanese Patent Application Laid-Open No. 11-162511, a sulfur-containing compound is added to an electrolytic solution.
It is disclosed that low-temperature characteristics, long-term stability, cycle characteristics, and the like can be improved. JP-A-9-245833 discloses that the cycle characteristics of a negative electrode can be improved by adding a dialkyl sulfate to an electrolytic solution.

As described above, it is disclosed that a sulfur-containing compound is added as a solvent or an additive to an electrolytic solution in order to improve the characteristics of a secondary battery.

[0005] However, the average discharge voltage of the above-mentioned conventional lithium secondary battery is 3.6 V to 3.8 V. In recent years, a higher voltage battery (4.2 V or more) has been required. It has become.

Japanese Patent Application Laid-Open No. 9-167635 discloses that a sulfite compound is added to improve the withstand voltage of an electrolytic solution, but this is not always sufficient.

[0007]

At present, in a commercially available lithium secondary battery using LiCoO ₂ as a positive electrode active material, the upper limit voltage of charging is set to 4.2 V, and the average voltage in discharging is 3 V. The range is from 0.6V to 3.8V.

On the other hand, for a battery that generates a high voltage with an average voltage of 4.2 V or more, increasing the voltage can reduce the number of batteries used in a device using a plurality of batteries. There are excellent merits such as realizing high energy.

However, in order to increase the average voltage, it is, of course, necessary to make the upper limit voltage of charging higher than before. Therefore, the present inventors set the voltage of 4.5V.
Attempted to increase the charging voltage to the range of 5.2 V to 5.2 V. However, if the charging voltage is set as high as described above, the electrolytic solution is decomposed on the surface of the positive electrode active material or the conductive agent constituting the positive electrode. It was found that sufficient cycle characteristics could not be obtained.

An object of the present invention is to provide a high average discharge voltage (4.2 V or higher) suitable as a power source for mobile devices such as the above-mentioned mobile phones and electric vehicles using a large number of batteries.
An object of the present invention is to provide a lithium secondary battery.

[0011]

The gist of the present invention that can achieve the above object is as follows.

[0012] In a lithium secondary battery having a positive electrode and a negative electrode that occlude and release lithium ions, and an electrolyte solution containing the lithium ions, the positive electrode includes a positive electrode active material, a conductive agent, a binder, and a current collector. Wherein the conductive agent is amorphous carbon, and the electrolyte is of the formula (1) or (2)

[0013]

Embedded image

Wherein R ₁ and R ₂ each represent an alkyl group having 1 to 10 carbon atoms, and R ₁ and R ₂ may be the same, and have an average discharge voltage of 4 .2V
This is the feature of the lithium secondary battery.

The concentration of the compound represented by the formula (1) or (2) contained in the electrolyte solution is 0.05 to 0.5 mol.
1 / dm ³ .

As the positive electrode active material, a chemical formula LiNi
_x Mn _2-x O ₄ (where 0.4 ≦ x ≦ 0.6) or a chemical formula LiNi _x Q _y Mn _2-xy O ₄ (where Q is the periodic table IIa
Group, IIIa group, IVa group, 4th period transition metal, Zn, A
at least one selected from l, Ga, Si, and Ge;
0.4 ≦ x ≦ 0.6, 0 <y ≦ 0.1), and the average particle size of the positive electrode active material is 3 to 20 μm.
Wherein particles having a maximum particle size of 50 μm or less have a volume fraction of 9
Desirably, it is present at 0% or more.

In the case of LiNi _x Mn _2-x O ₄ , when x ≧ 0.5, the internal resistance of the positive electrode increases, but as a different element, the periodic table IIa group, IIIa group, IVa
Group, transition metal of the fourth period, Zn, Al, Ga, Si, G
LiNi _x M to which at least one member selected from the group consisting of
By using n _2-x O _4, it can reduce the internal resistance.

The conductive agent has a graphite interlayer distance of 0.1 mm.
Amorphous carbon having a thickness of 344 nm or more is used. As the amorphous carbon, carbon black is desirable, and its specific surface area is 5
It is desirably 0 to 1000 m ² / g.

Carbon black is roughly classified into two types produced by a pyrolysis method and an incomplete combustion method, and any type of carbon black can be used. Among those carbon blacks, aggregates that are the smallest aggregates of carbon black are connected in a complicatedly branched shape,
It is desirable that the structure is developed.

[0020] This is because, when the aggregates extend like branches and leaves, the number of paths in the electric network increases, and a sufficient current collection performance can be obtained. Examples of such carbon black include acetylene black and Ketjen black.

In addition, a fibrous amorphous carbon material is desirable. In the present invention, a vapor grown carbon fiber produced by a pyrolysis method, a carbon nanotube produced by discharging a carbon material, a carbon fiber obtained by spinning pitch and carbonizing, and the like can be used. Since these fibrous carbons are also advantageous for forming an electrical network path, sufficient current collection performance can be obtained.

The above is based on various experimental results.
When graphite having an interlayer distance of less than 0.344 nm is used as the conductive agent, if the charging voltage is increased to 4.5 V or more, anions present in the electrolytic solution are occluded between the graphite layers, and at this time, the electrolytic solution is decomposed. It turned out to cause. Therefore, sufficient cycle characteristics were not obtained, and graphite could not be used. In the case of amorphous carbon, no anion was absorbed between the layers, indicating that the electrolyte was difficult to decompose.

The interlayer distance of the carbon material can be determined by disassembling the battery in the discharged state, taking out the positive electrode, and performing X-ray diffraction.

On the other hand, as the negative electrode active material, a lithium metal, a lithium alloy, and a carbon material such as graphite or amorphous carbon are used.

The main solvent of the electrolytic solution is at least one of ethylene carbonate, propylene carbonate, γ-butyrolactone and sulfolane, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, methyl propionate, propionate A mixed solvent to which one or more of ethyl, dimethoxyethane and 2-methyltetrahydrofuran are added is used.

The solutes of the electrolyte include LiPF ₆ , L
iBF ₄ , LiClO ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiN
(C ₂ F ₅ SO ₂ ) ₂ is used.

The compound represented by the above formula (1) or (2) is added to the electrolyte prepared by using these solvents and solutes at a concentration of 0.05 to 0.5 mol / dm ³ .

As the compound represented by the above formula (1) or (2), dimethyl sulfate, diethyl sulfate, dimethyl sulfone or diethyl sulfone is preferable.

Although the details are unknown, these compounds represented by the formulas (1) and (2) have -SO ₂ -as a common double bond between S and O as a common structure, The -SO _2- portion adsorbs (coordinates) on the surface of the positive electrode active material at a high potential, and covers the surface of the positive electrode active material with the compound represented by the formula (1) or (2). It is thought that it acts to suppress the decomposition reaction of the solvent.

As described above, in the present invention, first, amorphous carbon is used as the conductive agent to suppress the solvent decomposition reaction due to occlusion of the conductive agent by anion, and second, the -SO ₂ -structure By combining the two techniques of adding a compound having the formula (1) to suppress the solvent decomposition reaction on the positive electrode surface, a lithium secondary battery capable of high-voltage charging can be provided.

The case where only one of the above was adopted was examined, but it was found that the effect of suppressing the decomposition of the electrolytic solution was insufficient.

[0032]

[Embodiment 1] The average particle size is 10 μm.
m, the proportion of particles having a particle size of 50 μm or less is 9 by volume fraction.
Using 5% of LiNi _0.4 Mn _1.6 O ₄ as a positive electrode active material, a positive electrode was produced by the following method.

After acetylene black having a graphite interlayer distance of 0.365 nm is added to the positive electrode active material as a conductive agent and mixed well, polyvinylidene fluoride dissolved in n-methyl-2-pyrrolidone (NMP) is used as a binder. PVd
F) to give a paste. The ratio of the positive electrode active material, the conductive agent and the binder was 90: 4: 6 by weight. This paste was applied to an aluminum foil as a current collector, and NMP was dried and then pressed and formed to obtain a positive electrode.

As the electrolytic solution, a solution obtained by dissolving LiPF ₆ at a ratio of 1 mol / dm ³ in a mixed solvent of ethylene carbonate and dimethyl carbonate at a volume ratio of 1: 2, and further 0.1 mol of dimethyl sulfone were used. / D
The solution to which m ^{3 was} added was used.

A lithium secondary battery of the present invention shown in FIG. 1 was prepared by using the above positive electrode, electrolytic solution and lithium metal as the negative electrode. In the figure, 11 is a positive electrode, 12 is a metallic lithium negative electrode, 13 is a separator, 14 is a positive electrode can (made of Al), 1
Reference numeral 5 denotes a negative electrode can (made of Ni), 16 denotes a gasket, and 17 denotes a positive electrode current collector (made of Al).

Comparative Example 1 A lithium secondary battery was manufactured in the same manner as in Example 1, except that graphite having a graphite interlayer distance of 0.337 nm was used as the conductive agent.

COMPARATIVE EXAMPLE 2 LiPF ₆ was added at 1 mol / d to a mixed solvent of ethylene carbonate and dimethyl carbonate in a volume ratio of 1: 2 without adding dimethyl sulfone.
A lithium secondary battery was produced in the same manner as in Example 1 using the electrolytic solution in which m ^{3 was} dissolved.

Using the lithium secondary batteries of Example 1 and Comparative Examples 1 and 2, the end-of-charge voltage was 5.0 V, the end-of-discharge voltage was 3.5 V, and the charge / discharge current was 3 hours (0.33 C). Constant current charge / discharge was repeated with a corresponding current. FIG. 2 shows the change in the capacity retention ratio of the battery at that time.

It can be seen that the lithium secondary battery of Example 1 has better cycle characteristics than Comparative Examples 1 and 2. It can be seen that in Comparative Example 1 using graphite as the conductive agent or Comparative Example 2 in which dimethyl sulfone was not added, the effect was insufficient.

Example 2 The addition amount of dimethyl sulfone was 0.01, 0.05, 0.2, 0.5, 1 mol / dm ^3.
And a lithium secondary battery was fabricated in the same manner as in Example 1.

With respect to the lithium secondary battery of Example 2, the end-of-charge voltage was 5.0 V, the end-of-discharge voltage was 3.5 V, and the charge / discharge current was a constant corresponding to a 3-hour rate (0.33 C). Current charging / discharging was repeated. FIG. 3 shows the change in the capacity retention ratio of the battery at that time.

When the concentration of dimethyl sulfone is 0.05 to 0.5
Good cycle characteristics were obtained in the range of mol / dm ³ .

On the other hand, the dimethyl sulfone concentration was 0.01 m
It can be seen that when the addition amount is small as ol / dm ³ , the effect is hardly exhibited. Conversely, 1 mol / dm
^When it was as large as ³ , it was found that dimethyl sulfone itself was also oxidatively decomposed and the cycle characteristics were poor. From the above results, the additive amount of the additive dimethyl sulfone was 0.1%.
It has been found that a range of from 0.5 to 0.5 mol / dm ³ is desirable.

Example 3 Instead of dimethyl sulfone, diethyl sulfone, dimethyl sulfate, diethyl sulfate,
Using dimethyl sulfoxide and dimethyl sulfite as additives, a lithium secondary battery was produced in the same manner as in Example 1. Table 1 shows the structural formulas of these additives.

In dimethyl sulfone, diethyl sulfone, dimethyl sulfate and diethyl sulfate, a —SO ₂ — structure exists as a common structure.

[0046]

[Table 1]

Next, with respect to the lithium secondary battery of Example 3, the end-of-charge voltage was 5.0 V and the end-of-discharge voltage was 3.5.
V, a constant current charge / discharge was repeated at a charge / discharge current of a current corresponding to a 3-hour rate (0.33 C). FIG. 4 shows the change in the capacity retention rate of the battery at that time.

Good cycle characteristics can be obtained only when diethyl sulfone, dimethyl sulfate or diethyl sulfate having a -SO ₂ -structure is added, and a structure different from the -SO ₂ -structure such as dimethyl sulfoxide or dimethyl sulfite is obtained. Shows that there is almost no effect as an additive.

[0049]

An amorphous carbon conductive agent according to the present invention, the formula in the electrolyte (1), -SO ₂ shown in (2) - by the addition of a compound having the structure, 4.5V～5.2V Since the decomposition reaction of the electrolyte at a high potential is suppressed, a lithium secondary battery having a high battery voltage can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a lithium secondary battery used in Examples of the present invention and Comparative Examples.

FIG. 2 is a graph showing changes in the capacity retention of the lithium secondary batteries of Example 1 and Comparative Examples 1 and 2.

FIG. 3 is a graph showing a change in the capacity retention of the lithium secondary battery of Example 2.

FIG. 4 is a graph showing a change in a capacity retention ratio of a lithium secondary battery of Example 3.

[Explanation of symbols]

11 positive electrode, 12 negative electrode, 13 separator, 14 positive electrode can, 15 negative electrode can, 16 gasket, 17 positive electrode current collector.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Ren Muranaka 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture F-term in Hitachi Research Laboratory, Hitachi, Ltd. 5H029 AJ02 AK03 AL12 AM02 AM03 AM04 AM05 AM07 DJ08 DJ18 EJ04 EJ11 HJ02 HJ07 HJ10 HJ13 5H050 AA02 BA17 CA08 CB12 DA02 DA10 EA09 FA20 HA02 HA07 HA10 HA13

Claims (5)

[Claims] 1. A lithium secondary battery having a positive electrode and a negative electrode that occlude and release lithium ions, and an electrolyte solution containing the lithium ions, wherein the positive electrode is a positive electrode active material, a conductive agent, a binder, and a current collector. Wherein the conductive agent is amorphous carbon, and
The electrolytic solution has the formula (1) or the formula (2) Wherein R ₁ and R ₂ each represent an alkyl group having 1 to 10 carbon atoms, and R ₁ and R ₂ may be the same, and have an average discharge voltage of 4.2 V or more. A lithium secondary battery characterized by the following. 2. The concentration of the compound represented by the formula (1) or the formula (2) contained in the electrolytic solution is 0.05 to 0.5 m.
2. The lithium secondary battery according to claim 1, wherein the ratio is ol / dm ³ . 3. The compound represented by the formula (1) or (2) is dimethyl sulfate, diethyl sulfate, dimethyl sulfone or diethyl sulfone.
4. The lithium secondary battery according to 1. 4. The conductive agent has a graphite interlayer distance of 0.34.
4. The lithium secondary battery according to claim 1, wherein the carbon is amorphous carbon having a thickness of 4 nm or more, and the specific surface area of the amorphous carbon is 50 to 1000 m ² / g. 5. 5. The method according to claim 1, wherein the positive electrode active material has a chemical formula of LiNi _x M
n _2-x O ₄ (where 0.4 ≦ x ≦ 0.6) or a chemical formula L
iNi _x Q _y Mn _2-xy O ₄ (where Q is group IIa of the periodic table,
IIIa group, IVa group, fourth period transition metal, Zn, Al,
At least one selected from Ga, Si and Ge, 0.4
≤ x ≤ 0.6 and 0 <y ≤ 0.1). The lithium secondary battery according to any one of claims 1 to 4, wherein