JP2002141109A - Nonaqueous electrolyte - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte with no performance drop of a secondary battery, high electric conductivity, and high flame retardancy. SOLUTION: In the nonaqueous electrolyte comprising an electrolyte and an electrolyte solvent, γ-butyrolactone of 5-70 wt.% is contained in the electrolyte solvent.

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, and more particularly to a flame-retardant non-aqueous electrolyte used for a lithium secondary battery or a lithium ion secondary battery.

[0002]

2. Description of the Related Art A lithium secondary battery or a lithium ion secondary battery using a non-aqueous electrolyte has a high voltage, a high energy density, and is excellent in reliability such as storage properties. Is widely used as a power source for electronic devices. On the other hand, recently, a lithium secondary battery or a lithium ion secondary battery is required to have a significantly higher energy density and a larger size.

Here, in order to increase the energy density of a lithium secondary battery or a lithium ion secondary battery, it is necessary to increase the amount of inserted and released lithium. In addition, even when the size of the lithium secondary battery or the lithium ion secondary battery is increased, the amount of lithium to be handled is increased because the size of the battery itself is increased. Therefore, when considering a large increase in energy density and size of a lithium secondary battery or a lithium ion secondary battery, the demands on the safety and reliability of the secondary battery become strict, and especially the flame retardancy of the electrolyte With regard to, further improvement in performance is required.

Japanese Patent Application Laid-Open No. 4-184870 discloses that a phosphate ester compound known as a self-digestible compound is added to an electrolytic solution in an appropriate amount in order to solve the above-mentioned problem.

[0005] However, a secondary battery using an electrolytic solution to which a phosphate compound is added has problems in terms of charge / discharge efficiency, power density, energy density, and life. This is considered to be due to the fact that the electric conductivity of the phosphate compound is low, so that the movement of lithium ions becomes slow. In addition, when natural graphite or the like is used for the negative electrode and a phosphate triester is contained in the electrolytic solution, a compound with lithium is generated by a reaction between the acidic functional group on the negative electrode surface and the phosphate triester, It is speculated that one of the causes is that the transfer of lithium ions is hindered by the film adhering to the negative electrode surface.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to improve the performance of a secondary battery such as charge / discharge efficiency, power density, energy density, and life. Another object of the present invention is to provide a flame-retardant non-aqueous electrolyte solution which does not lower the electric conductivity, has high electric conductivity, and is flame-retardant.

[0007]

The non-aqueous electrolyte according to the present invention is, as described in claim 1, a non-aqueous electrolyte comprising an electrolyte and an electrolyte solvent, wherein the electrolyte solvent comprises:
It is a non-aqueous electrolyte containing 5 to 70% by weight of γ-butyrolactone.

Further, the non-aqueous electrolyte according to the present invention, as described in claim 2, is characterized in that, in the invention according to claim 1, a cyclic carbonate and a chain carbonate are contained in the electrolyte solvent. It is a non-aqueous electrolyte contained in a weight ratio of 0.5: 1 to 3: 1.

Further, the non-aqueous electrolyte according to the present invention, as described in claim 3, is the invention according to claim 1 or 2, wherein the electrolyte is LiPF ₆ , LiClO ₄ , or LiBF ₄ . A non-aqueous electrolyte containing a lithium salt containing at least one at a concentration of 0.01 to 4 mol / L.

The present inventor has proposed that in a non-aqueous electrolyte comprising an electrolyte and an electrolyte solvent, a non-aqueous electrolyte containing 5 to 70% by weight of γ-butyrolactone in the electrolyte solvent is used as a secondary battery. The present invention has been completed based on the new finding that the performance of the present invention is not reduced, the electric conductivity is high, and the composition has flame retardancy.

The non-aqueous electrolytic solution of the present invention can contain γ-butyrolactone, a cyclic carbonate, and a chain carbonate, and can contain LiPF ₆ as a lithium salt as an electrolyte. is there.

The content ratio of the cyclic carbonate to the chain carbonate in the electrolyte solution solvent is 0.1% by weight.
It is preferable to contain it in the range of 5: 1 to 3: 1.

As the cyclic carbonate, ethylene carbonate (ethylene carbonate: EC), propylene carbonate (propylene carbonate: PC) and the like can be used. These cyclic carbonates may be used alone or in combination. Is possible.

As the chain carbonate, dimethyl carbonate (dimethyl carbonate: DMC), diethyl carbonate (diethyl carbonate: DEC), ethyl methyl carbonate (ethyl methyl carbonate: EMC), 1,2-dimethoxyethane and the like are used. These chain carbonates can be used alone or as a mixture.

As the electrolyte, LiPF ₆ , LiC
Lithium salts such as 10 ₄ , LiBF ₄ , LiCF ₃ SO ₃ , and LiAsF ₆ can be contained singly or as a mixture.

As the electrolyte, LiPF ₆ , LiCl
O _4, or a lithium salt containing at least one of LiBF _4, it is preferable to be contained at a concentration of 0.01~4mol / L.

Among the above-mentioned electrolytes, LiPF ₆ is most excellent in self-digestibility and is suitable as an electrolyte in an electrolyte.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Table 1 shows the physical properties of organic solvents used in non-aqueous electrolytes. Since cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC) show high dielectric constants, Li
Dissolve well the electrolyte, such as PF _6. However, when using only cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), the electric conductivity of the electrolytic solution decreases due to high viscosity. Therefore, ethylene carbonate (EC) and propylene carbonate (P
C) cyclic carbonates of low viscosity include dimethyl carbonate (DMC) and diethyl carbonate (DE
It is preferable to improve the electric conductivity by mixing C) and a chain carbonate such as ethyl methyl carbonate (EMC).

[0019]

[Table 1]

According to Table 1, the chain carbonates are
Diethyl carbonate (DEC)> ethyl methyl carbonate (EMC)> dimethyl carbonate (DMC) has a lower boiling point and lower flash point in this order. The explosion range is as follows: dimethyl carbonate (DMC)> ethyl methyl carbonate (EMC)> diethyl carbonate (DE
The range is narrowed in the order of C), and it is considered that diethyl carbonate (DEC) is the most preferable chain carbonate ester in consideration of safety. Therefore,
Electrolyte mixed with diethyl carbonate (DEC) seems to be the least flammable.

However, as a result of conducting a combustion experiment with an electrolyte mixed with γ-butyrolactone, the result was different from the above-mentioned expectation. That is, as a result of conducting various combustion experiments on the electrolyte mixed with γ-butyrolactone, it was found that the electrolyte containing dimethyl carbonate (DMC) was the most difficult to burn. That is, an electrolytic solution in which γ-butyrolactone and dimethyl carbonate (DMC) were mixed was put into a glass petri dish, a spark was brought into contact therewith, and the time until ignition was measured. The ignition time was about 5 seconds. On the other hand, an electrolytic solution in which γ-butyrolactone and diethyl carbonate (DEC) were mixed was put into a glass petri dish, a spark was brought into contact therewith, and the time until ignition was measured. The ignition time was about 2 seconds.
In addition, dimethyl carbonate (DMC) has the lowest viscosity among the chain carbonates, and was found to be a chain ester suitable for improving electric conductivity. Therefore, in the non-aqueous electrolytic solution comprising the electrolyte and the electrolytic solution solvent, the non-aqueous electrolytic solution containing γ-butyrolactone and dimethyl carbonate (DMC) in the electrolytic solution solvent is suitable in that it is difficult to burn. .

In a preferred embodiment of the present invention, the content of γ-butyrolactone should be 5 to 70% by weight based on the whole organic solvent used. This is because when the content of γ-butyrolactone is less than 5% by weight, the non-aqueous electrolyte tends to have poor flame retardancy. On the other hand, the content of γ-butyrolactone is 70% by weight.
If the number is larger, the viscosity of the non-aqueous electrolyte becomes high, and therefore, a secondary battery using such a non-aqueous electrolyte may have reduced characteristics such as charge and discharge characteristics. The content of γ-butyrolactone is preferably 15 to 50% by weight based on the whole organic solvent used, and 30 to 40% by weight based on the whole organic solvent used. Is more preferable.

In the non-aqueous electrolyte according to the present invention, γ-
As an electrolytic solution solvent other than butyrolactone, a cyclic carbonate and a chain carbonate can be mixed. In that case, the cyclic carbonate and the chain carbonate can be contained in a weight ratio of 0.5: 1 to 3: 1. If (cyclic carbonate / chain carbonate) is smaller than (0.5 / 1) in weight ratio, the electrolyte may not be sufficiently dissolved in the electrolyte solvent. On the other hand, when (cyclic carbonate / chain carbonate) is greater than (3/1) in weight ratio, the electrical conductivity of the non-aqueous electrolyte may decrease, and such non-aqueous This is because a secondary battery using an electrolytic solution may have a problem in its electric characteristics.

In the present invention, the electrolyte contained in the electrolytic solution is LiPF ₆ , LiClO ₄ , LiBF ₄ ,
_{LiCF 3 SO 3, LiAsF 6,} LiSbF 6, LiSb
A lithium salt such as F ₆ Li can be used. Particularly, from the viewpoint of self-digestibility, LiPF ₆ is the most preferred electrolyte. The above-mentioned electrolyte is 0.01 to 4 mol / L.
It is preferable to dissolve it at a concentration of 0.5 to 1.5 mol / L and adjust the concentration. The electrolyte concentration in the electrolyte is 0.01
If the amount is less than mol / L, the cycle characteristics of a secondary battery using such an electrolyte may decrease. On the other hand, when the electrolyte concentration in the electrolytic solution is higher than 4 mol / L, the electrolyte becomes difficult to dissolve in the electrolytic solution. Therefore, the electrolyte is preferably contained at a concentration of 0.01 to 4 mol / L, but the electrolyte concentration in the electrolyte is not limited to this range.

[0025]

EXAMPLES (Example 1) Ethylene carbonate (E
C): LiPF ₆ was dissolved at a concentration of 1 mol / L in an electrolyte solution solvent prepared at a ratio of 45:25:30 (weight ratio): dimethyl carbonate (DMC): γ-butyrolactone (GBL). An aqueous electrolyte was prepared.

The temperature of the electrolytic solution was adjusted to 25 ° C., and the electric conductivity was measured. As a result, it was 10.93 mS / cm. Further, this electrolyte was allowed to stand in a thermostat set at −25 ° C. for 3 hours or more, and the low-temperature characteristics were observed by observing the state of the electrolyte thereafter. As a result, Example 1
Was in a liquid state. As a method for testing the combustion of the electrolyte, the electrolyte was placed in a glass petri dish, and a spark was brought into contact therewith. The electrolyte was ignited within 3 seconds, and it burned. Example 1
It was determined that the result of the combustion test of the electrolytic solution according to the above did not burn.
The results are shown in Table 2 below.

Example 2 Propylene carbonate (P
C): LiPF ₆ was dissolved at a concentration of 1 mol / L in an electrolyte solvent prepared to have a ratio of dimethyl carbonate (DMC): γ-butyrolactone (GBL) = 46: 24: 30 (weight ratio). An aqueous electrolyte was prepared.

With respect to the electrolyte in Example 2, the electric conductivity and the low-temperature characteristics were measured and a combustion test was performed in the same manner as in Example 1 described above. As a result, the electric conductivity of the electrolytic solution in Example 2 was determined to be 10.01 mS / cm, the low-temperature characteristic was liquid, and the result of the combustion test was determined not to burn. The results are shown in Table 2 below.

Example 3 Ethylene carbonate (E
C): Propylene carbonate (PC): Dimethyl carbonate (DMC): γ-butyrolactone (GBL) =
LiPF ₆ was dissolved in an electrolyte solvent prepared at a ratio of 21: 21: 28: 30 (weight ratio) to 1 mol / L to prepare a non-aqueous electrolyte.

With respect to the electrolytic solution in Example 3, the electric conductivity and the low-temperature characteristics were measured and a combustion test was performed in the same manner as in Example 1 described above. As a result, the electric conductivity of the electrolytic solution in Example 3 was 10.38 mS / cm, the low-temperature property was liquid, and the result of the combustion test was determined not to burn. The results are shown in Table 2 below.

Example 4 Ethylene carbonate (E
C): Ethyl methyl carbonate (EMC): γ-butyrolactone (GBL) = 1 mol / LiPF ₆ in an electrolyte solvent prepared in a ratio of 40:20:40 (weight ratio).
L to prepare a non-aqueous electrolyte solution.

With respect to the electrolytic solution in Example 4, the electric conductivity and the low-temperature characteristics were measured and a combustion test was performed in the same manner as in Example 1 described above. As a result, the electric conductivity of the electrolytic solution in Example 4 was determined to be 11.10 mS / cm, the low-temperature characteristic was liquid, and the result of the combustion test was determined not to burn. The results are shown in Table 2 below.

Example 5 Ethylene carbonate (E
C) LiPF ₆ was dissolved at a concentration of 1 mol / L in an electrolyte solvent prepared at a ratio of dimethyl carbonate (DMC): γ-butyrolactone (GBL) = 50: 20: 30 (weight ratio). An aqueous electrolyte was prepared.

With respect to the electrolytic solution in Example 5, the electric conductivity and the low-temperature characteristics were measured and a combustion test was performed in the same manner as in Example 1 described above. As a result, the electric conductivity of the electrolytic solution in Example 5 was 10.93 mS / cm, the low-temperature property was liquid, and the result of the combustion test was determined not to burn. The results are shown in Table 2 below.

Example 6 Ethylene carbonate (E
C): Ethyl methyl carbonate (EMC): dimethyl carbonate (DMC): γ-butyrolactone (GB
L) LiPF ₆ was dissolved in an electrolyte solvent prepared at a ratio of 35: 20: 20: 25 (weight ratio) to 1 mol / L to prepare a non-aqueous electrolyte.

With respect to the electrolytic solution in Example 6, the electric conductivity and the low-temperature characteristics were measured and a combustion test was performed in the same manner as in Example 1 described above. As a result, the electric conductivity of the electrolytic solution in Example 6 was 11.05 mS / cm, the low-temperature property was liquid, and the result of the combustion test was determined not to burn. The results are shown in Table 2 below.

Comparative Example 1 Ethylene carbonate (E
C): dimethyl carbonate (DMC) = 71: 29
(Weight ratio) to the electrolyte solvent prepared in a ratio of LiPF ₆
Was dissolved to 1 mol / L to prepare a non-aqueous electrolyte solution.

With respect to the electrolytic solution of Comparative Example 1, the electric conductivity and the low-temperature characteristics were measured and a combustion test was performed in the same manner as in Example 1 described above. As a result, the electric conductivity of the electrolytic solution in Comparative Example 1 was 10.15 mS / cm, the low-temperature characteristic was solidification, and the result of the combustion test was determined to be burning. The results are shown in Table 2 below.

Comparative Example 2 Propylene carbonate (P
C): dimethyl carbonate (DMC) = 69: 31
(Weight ratio) to the electrolyte solvent prepared in a ratio of LiPF ₆
Was dissolved to 1 mol / L to prepare a non-aqueous electrolyte solution.

With respect to the electrolytic solution in Comparative Example 2, the electric conductivity and the low-temperature characteristics were measured and a combustion test was performed in the same manner as in Example 1 described above. As a result, the electric conductivity of the electrolytic solution in Comparative Example 2 was 8.98 mS / cm, the low-temperature property was solidification, and the result of the combustion test was determined to be burning. The results are shown in Table 2 below.

Comparative Example 3 Ethylene carbonate (E
C): Propylene carbonate (PC): dimethyl carbonate (DMC) = 1 mol / L of LiPF ₆ in an electrolyte solvent prepared at a ratio of 33:33:34 (weight ratio).
To obtain a non-aqueous electrolyte solution.

With respect to the electrolytic solution of Comparative Example 3, the electric conductivity and the low-temperature characteristics were measured and a combustion test was performed in the same manner as in Example 1 described above. As a result, the electric conductivity of the electrolytic solution in Comparative Example 3 was 9.57 mS / cm, the low-temperature property was solidification, and the result of the combustion test was determined to be burning. The results are shown in Table 2 below.

Comparative Example 4 Ethylene carbonate (E
C) LiPF ₆ was dissolved at a concentration of 1 mol / L in an electrolyte solvent prepared to have a ratio of dimethyl carbonate (DMC): γ-butyrolactone (GBL) = 64: 32: 4 (weight ratio). An aqueous electrolyte was prepared.

With respect to the electrolytic solution in Comparative Example 4, the electric conductivity and the low-temperature characteristics were measured in the same manner as in Example 1 described above, and a combustion test was performed. As a result, the electric conductivity of the electrolytic solution in Comparative Example 4 was 10.18 mS / cm, the low-temperature property was solidification, and the result of the combustion test was determined to be burning. The results are shown in Table 2 below.

Comparative Example 5 Ethylene carbonate (E
C): LiPF ₆ was dissolved at a concentration of 1 mol / L in an electrolyte solution solvent prepared at a ratio of 17: 8: 75 (weight ratio): dimethyl carbonate (DMC): γ-butyrolactone (GBL). An aqueous electrolyte was prepared.

With respect to the electrolytic solution of Comparative Example 5, the electric conductivity and the low-temperature characteristics were measured in the same manner as in Example 1, and a combustion test was performed. As a result, the electric conductivity of the electrolytic solution in Comparative Example 5 was 11.10 mS / cm, the low-temperature characteristics were liquid, and the result of the combustion test was determined to be non-flammable. The results are shown in Table 2 below.

(Evaluation by Battery) LiCoO ₂ , acetylene black as a conductive agent, and polyvinylidene fluoride as a binder were used in a weight ratio of 90: 6: 4 using an N-methylpyrrolidone (NMP) solvent. And mixed to obtain a positive electrode mixture. This positive electrode mixture was applied to an aluminum current collector, dried, and then rolled by a rolling mill to produce a positive electrode.

A negative electrode was prepared by mixing natural graphite or coke-based graphite with polyvinylidene fluoride as a binder at a weight ratio of 95: 5 using an NMP solvent to obtain a negative electrode mixture.

As a separator, # 2400 manufactured by Celgard Co., Ltd. was used. Then, using a positive electrode, a negative electrode, and a separator, a cylindrical battery (diameter 18 mm, length 65 m)
m) was prepared. As the electrolyte, those prepared in Examples 1 to 6 and Comparative Examples 1 to 5 were used. The charging was performed at a charging current of 400 mA and a charging end voltage of 4.18 V. The discharge was performed at a discharge current of 800 mA for 1 hour. A test of cycle characteristics in which these charge / discharge steps were defined as one cycle was performed. When testing the cycle characteristics, the ambient temperature is 2
The capacity after performing 300 cycles at 5 ° C.
When the capacity was reduced to 80% or less of the initial capacity, the evaluation was x, and when not, the evaluation was ○. Is Example 1
Table 2 below shows the test results of the cycle characteristics of the electrolytes in Comparative Examples 1 to 6 and Comparative Examples 1 to 5.

[0050]

[Table 2]

The electrolytes of Examples 1 to 6 did not burn even in the combustion test, and maintained an electric conductivity of 10 mS / cm or more.
Also, it can be seen that the liquid state is maintained even at a low temperature.
In this evaluation, the cycle characteristics were all ○
Was obtained. On the other hand, in the comparative example, the result satisfying all of the evaluations was not obtained.

It should be understood that the embodiments and examples disclosed this time are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

[0053]

According to the present invention, by adding γ-butyrolactone, the electrolyte itself has no flammability, the electric conductivity of the electrolyte is 10 mS / cm or more, and -2.
A non-aqueous flame-retardant electrolytic solution which did not solidify even at 5 ° C. and maintained a liquid was obtained.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Fumio Tachizono 1-1, Sanyocho, Daito-shi, Osaka Sanyo Electronics Co., Ltd. F term (reference) 5H029 AJ12 AK03 AL07 AM03 AM04 AM05 AM06 AM07 HJ01 HJ10

Claims (3)

[Claims] 1. A non-aqueous electrolyte comprising an electrolyte and an electrolyte solvent, wherein the electrolyte solvent contains 5 to 70% by weight of γ-butyrolactone. 2. The method according to claim 1, wherein the cyclic carbonate and the chain carbonate are mixed in a weight ratio of 0.5: 1 to 3:
The non-aqueous electrolyte solution according to claim 1, which is contained in the range of 1. 3. An electrolyte comprising: LiPF ₆ , LiC
The non-aqueous electrolyte according to claim 1, further comprising a lithium salt containing at least one of lO ₄ and LiBF ₄ at a concentration of 0.01 to ₄ mol / L.