JPH08162152A - Lithium secondary battery - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a lithium secondary battery having a high ionic conductivity of a non-aqueous electrolyte, a high rate discharge property, and a high safety. [Constitution] In a lithium secondary battery composed of a negative electrode 1 made of a carbon material capable of inserting and extracting lithium ions, a non-aqueous electrolyte solution, and a positive electrode 2 made of a lithium-containing oxide, a solvent in the non-aqueous electrolyte solution is used. A chain ester having the following characteristics (a) and (b) is mixed in 30% or less of the total volume of the solvent. (A) Viscosity is 0.9 cP or less, preferably 0.6 or less. (B) Dielectric constant of 4.7 or more, preferably 5.7 or more.
The chain ester is, for example, propyl acetate, isobutyl formate, or butyl propionate.

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, and more particularly to a solvent for a non-aqueous electrolyte of a lithium secondary battery.

[0002]

2. Description of the Related Art Since a lithium battery using lithium as a negative electrode active material has high energy, it has been attempted to be used as a secondary battery in various fields. When pure metal lithium is used as the negative electrode active material, needle-like deposition of negative electrode lithium with repeated charging and discharging, that is, generation of so-called dendrites is a problem. That is, needle-like precipitated lithium pierces the separator and reaches the positive electrode, causing a short circuit inside the battery, resulting in a significant decrease in battery performance, and an abnormal rise in temperature due to an excessive current flowing due to the internal short circuit. At this point, the organic electrolyte volatilizes, causing the battery to explode or explode in the worst case, which is a safety issue. Particularly in terms of safety, when the battery ruptures, chemically active and highly reactive metallic lithium reacts with moisture in the air, resulting in “Li + H ₂ O → LiOH +
The generation of hydrogen gas and reaction heat due to the reaction of "1/2 H ₂ "
Further reduces safety.

In order to solve such a problem caused by the generation of dendrites, it has been proposed to use a carbon material capable of inserting and extracting lithium ions in a negative electrode as it is charged and discharged. Then, as the solvent of the electrolytic solution, a cyclic carbonic acid ester (for example, ethylene carbonate, propylene carbonate, γ-butyrolactone) excellent in charge / discharge efficiency and oxidation resistance is noted, and as the electrolyte, LiClO ₄ , LiBF _{4 is used.} ,
LiPF ₆ and LiSO ₃ CF ₃ are used.
Since cyclic carbonic acid ester such as ethylene carbonate has a high dielectric constant and a large ionic solubility, an electrolytic solution having a high ionic conductivity can be obtained by using this as a solvent.
However, its melting point is as high as 36.4 ° C., and it is a solid at room temperature. Moreover, since it has a high viscosity, it cannot be used alone. Therefore, in order to allow the electrolytic solution to exist as a liquid at −20 ° C. to 60 ° C., which is generally regarded as the operating temperature range of a lithium secondary battery, a cyclic carbonic acid ester is added to a chain carbonic acid ester (for example, dimethyl carbonate, diethyl carbonate, It has been proposed to mix (methyl ethyl carbonate). In addition, in order to improve the ionic conductivity or charge / discharge efficiency of the electrolytic solution, the general formula RCOOR '(wherein R is
Is an alkyl group having 3 or more carbon atoms, R'is 1 or 2 carbon atoms
It is proposed to mix a carboxylic acid ester represented by

[0004]

However, the above general formula R
When the carbon number of R in COOR 'is increased, the dielectric constant decreases and the viscosity increases. Therefore, when the carboxylic acid ester represented by the above general formula RCOOR 'is mixed in a mixed solvent of cyclic carbonic acid ester and chain carbonic acid ester, the low viscosity characteristic of chain carbonic acid ester is impaired, and the ionic conductivity of these mixed solvent systems is impaired. However, there is a problem that is not sufficiently improved. The problem to be solved by the present invention is to provide a lithium secondary battery in which the ionic conductivity of the non-aqueous electrolyte is high, the high rate discharge is excellent, and the safety is high.

[0005]

In order to solve the above problems, a lithium secondary battery according to the present invention comprises a negative electrode made of a carbon material capable of inserting and extracting lithium ions, a non-aqueous electrolyte, and a lithium-containing oxide. In a lithium secondary battery composed of a positive electrode made of a material, as a solvent of the non-aqueous electrolyte, one or more kinds of chain ester having the following characteristics (a) and (b) are contained, and the chain ester It is characterized in that the content is 30% or less of the volume of the whole solvent of the non-aqueous electrolyte. (A) Viscosity of 0.9 cP (20 ° C.) or less (b) Dielectric constant of 4.7 (20 ° C.) or more The chain ester having the characteristics (a) and (b) described above preferably has the following ( It has the characteristics of c) and (d). (C) Viscosity of 0.6 cP (20 ° C.) or less (d) Dielectric constant of 5.7 (20 ° C.) or more Chain ester having the above-mentioned characteristics (a) and (b) to (c) and (d) The main solvent of the non-aqueous electrolytic solution containing is preferably composed mainly of cyclic carbonic acid ester and chain carbonic acid ester.

[0006]

When a chain ester having the above characteristics (a) and (b) is focused on as a solvent for a non-aqueous electrolyte, it has the advantages of low viscosity and low melting point, but has the disadvantage of low boiling point and low flash point. There is also. When used as a solvent for the non-aqueous electrolyte, the amount of the solvent mixed in the solvent must be 30% or less of the total volume of the solvent to ensure safety. A chain ester having the above properties (a) and (b) is 3
Even when used in an amount of 0% or less, it is possible to reduce the viscosity of a mixed solvent of a highly viscous cyclic ester carbonate and a chain ester carbonate,
Moreover, the feature of the high dielectric constant of the cyclic carbonic acid ester is not significantly impaired. Therefore, the ionic conductivity of the non-aqueous electrolyte is improved, and good high rate discharge characteristics can be obtained. When the chain ester has the above-mentioned properties (c) and (d), the effect becomes more remarkable.

[0007]

EXAMPLE Ethylene carbonate was used as the cyclic ester carbonate, and dimethyl carbonate and diethyl carbonate were used as the chain ester carbonate, and the mixing ratio was 2: 1: 1. The characteristics of this mixed solvent shown in Table 1 (dielectric constant and viscosity at 20 ° C.)
The various chain-like esters having the above were mixed at the ratio shown in Table 1 (volume% in all the solvents) to prepare a solvent for the non-aqueous electrolytic solution. LiClO ₄ is used as the electrolyte and its concentration is 1
M. The operation of adding the electrolyte to the solvent was performed in an argon atmosphere. Table 1 also shows the ionic conductivity of the prepared non-aqueous electrolyte. The ionic conductivity was measured with a conductivity meter (“DS-14” manufactured by Horiba) after the temperature of the electrolytic solution reached 25 ° C. From Table 1, the ionic conductivity of the electrolytic solution according to the example of the present invention is higher than that of ethylene carbonate having a high ionic conductivity alone (Comparative Example 1, dimethyl carbonate, diethyl carbonate and chain ester are not included). You can see that it has become. Among them, it is found that the ionic conductivity is particularly high in Example 1 (when the viscosity of the chain ester is 0.6 cP or less and the dielectric constant is 5.7 or more). The electrolytes other than Comparative Example 1 were liquid at -20 ° C to 60 ° C, which is a general specification temperature range of lithium secondary batteries.

[0008]

[Table 1]

Cylindrical lithium secondary batteries in which the non-aqueous electrolyte was the above-mentioned various electrolytes were prepared and their high rate discharge characteristics were examined.
The structure of this lithium secondary battery is shown in FIG. 1 is a negative electrode, and its manufacturing method is as follows. First, artificial graphite powder (“JSP” manufactured by Nippon Graphite Co., Ltd.) and polyvinylidene fluoride as a binder are weighed at a weight ratio of 90:10, and N-methylpyrrolidone (NMP) is added and wet-mixed. . Apply this to both sides of the current collector copper foil,
It was prepared by drying at 120 ° C. for 30 minutes. 2 is a positive electrode,
The manufacturing method is as follows. First, LiCoO ₂ and graphite powder as a conductive additive and a binder were mixed in a weight ratio of 85:10:
Weigh in a ratio of 5, NMP is added and wet mixed. Apply this to both sides of the current collector aluminum foil,
It was prepared by drying at 0 ° C. for 30 minutes. Negative electrode 1 and positive electrode 2
Is vacuum dried at 200 ° C. for 4 hours, and the positive electrode 2, the separator 3 made of a polypropylene microporous film, the negative electrode 1, and the separator 3 are layered and wound in this order in a dry atmosphere.
This was housed in Case 4 to prepare a battery having a nominal capacity of 400 mAh. The positive electrode 2 was connected to the positive electrode terminal 5 also serving as a lid, and the negative electrode 1 was connected to the case 4 by ultrasonic welding.

After charging these lithium secondary batteries for 5 hours at a constant voltage of 4.15V (limit current 100mA), 1A
High-rate discharge was performed at a final voltage of 3 V with the current of. The result is shown in FIG. From FIG. 2, it is understood that the lithium secondary battery according to the example of the present invention has a high capacity at high rate discharge. This is a result of the high ionic conductivity of the electrolytic solution. Among them, the lithium secondary battery of Example 1 (when the chain ester has a viscosity of 0.6 cP or less and a dielectric constant of 5.7 or more) has excellent high-rate discharge characteristics.

In the above embodiment, propylene carbonate or γ- is used as the main solvent of the electrolytic solution instead of ethylene carbonate.
When butyrolactone is used, LiClO is used as the electrolyte.
₄ of LiBF instead _4, LiPF ₆ or LiSO ₃ C
Even when a lithium salt such as F ₃ was used, almost the same results as in the above example were obtained.

[0012]

As described above, the lithium secondary battery according to the present invention has a viscosity of 0.9 cP as a solvent of the non-aqueous electrolyte.
Since a chain ester having a dielectric constant of 4.7 or more is mixed below, the ionic conductivity of the electrolytic solution is improved and excellent high rate discharge characteristics are exhibited, as compared with the conventional lithium secondary battery. . If a chain ester having a viscosity of 0.6 cP or less and a dielectric constant of 5.7 or more is mixed, the effect becomes more remarkable. Further, since the amount of the chain ester mixed is 30% or less of the volume of the whole solvent, the safety is high.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a configuration of a lithium secondary battery according to the present invention.

FIG. 2 is a high rate discharge characteristic diagram of lithium secondary batteries according to an example of the present invention and a comparative example.

[Explanation of symbols]

 1 is negative electrode 2 is positive electrode 3 is separator 4 is case 5 is positive electrode terminal

Claims (3)

[Claims] 1. A lithium secondary battery comprising a negative electrode made of a carbon material capable of inserting and extracting lithium ions, a non-aqueous electrolyte, and a positive electrode made of a lithium-containing oxide, wherein the non-aqueous electrolyte is used as a solvent. , (A) and (b) below
One or more kinds of chain ester having the characteristics of 1) is contained, and the content of the chain ester is 3
A lithium secondary battery characterized by being 0% or less. (A) Viscosity of 0.9 cP (20 ° C) or less (b) Dielectric constant of 4.7 (20 ° C) or more 2. A chain ester having the following (c) and (d):
The lithium secondary battery according to claim 1, having the characteristics of: (C) Viscosity of 0.6 cP (20 ° C) or less (d) Dielectric constant of 5.7 (20 ° C) or more 3. The lithium secondary battery according to claim 1, wherein the main solvent of the non-aqueous electrolyte is a cyclic carbonate and a chain carbonate as main components.