JPS59205167A - Solvent for lithium secondary battery - Google Patents

Abstract

PURPOSE:To improve the charge-and-discharge property by using a mixture of ethylene glycol dialkyl ether having asymmetric construction and a specific organic solvent. CONSTITUTION:The present invention can be expressed by a general formula ( I ) R1-O-CH2-CH2-O-R2... ( I ) (provided that R1 and R2 are respectivey alkyl radicals having different constructions) and when using a mixture of ethylene glycol dialkyl ether having asymmetric construction and either of propylene carbonate, ethylene carbonate, gamma-butyrolactone, tetrahydrofuran, 1,3-diolasorane, 2-methyl tetrahydrofuran and sulforan, the polarization of the negative pole of a lithium secondary battery can be remarkably reduced. A metal compound or the like such as niobium, molybdenum and tantalum can be used as a positive active material in addition to MnO2, (CF)n, TiS2, TiO2, V2O5 and WS2. Further, electrolyte salt containing Li such as LiClO4, LiBF4, LiPF6 and LiAsF6 can be used as inorganic electrolyte salt while being combined with the solvent of the present invention.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to improvements in electrolytes used in lithium secondary batteries, and more particularly to solvents used in electrolytes.

By using the solvent of the present invention, the charge/discharge cycle characteristics of the negative electrode can be significantly improved.

The conversion of non-aqueous batteries with high energy density, such as prior art lithium batteries, into secondary batteries is highly desirable, and research is being actively conducted to put them into practical use.

There are many problems in the practical application of lithium secondary batteries, but one of the major issues in realizing such secondary batteries is the selection of an electrolyte system with good charge/discharge efficiency and charge/discharge cycle life. .

Prior art related to this electrolyte system, especially the solvent, includes the addition of a crown ether compound (%
No. 878), use of alkyl-γ-lactone (
(see JP-A-57-152683), addition of cyclic ether (see JP-A-57-152,684), use of a mixture of propylene carbonate and ethylene carbonate (see JP-A-57-170463), propylene carbonate , sulfolane, and tetrahydrofuran (see JP-A-57-187878), addition of a cryptand compound (I!f!i JP-A-57-187878);
210575), the use of a mixture of ethylene carbonate, propylene carbonate, and tetrahydrofuran (see JP-A-57-210576), and the use of diethyl ether and a co-solvent (see JP-A-57-80679). .

However, none of these have satisfactory charge/discharge characteristics as a secondary battery, and for this reason, the electrode portion has been improved (Japanese Unexamined Patent Publications No. 57-11476, No. 57-152669, No. 57-2).
08079 (see Japanese Patent Laid-Open Publication No. 1987-141879, etc.) and optimization of the resolution quality (see Japanese Patent Application Laid-open No. 141879/1983), but these are still not sufficient.

Purpose of the Invention The present inventors aimed to improve the charging and discharging characteristics of the negative electrode of lithium secondary batteries, especially the charging and discharging cycle characteristics. The inventors have completed the present invention by discovering that the charge-discharge cycle characteristics can be significantly improved when a mixture of ethylene glycol dialkyl ether having an asymmetric structure and a specific organic solvent is used as a solvent.

That is, the present invention provides a solvent for use in a lithium secondary battery comprising a negative electrode made of metallic lithium, a positive electrode, and an electrolyte, the solvent being a mixed solvent containing at least one of the following solvents: A solvent for a lithium secondary battery, characterized by: Solvent (2) General formula (r): Ethylene glycol dialkyl ether & -OCHs -CH2-0-R2...
- [1] (However, R1 and R2 are alkyl groups that do not have the same structure.)

The present invention has the general formula (1), R1-0-CHs -CH2-OR2...
- (1) (However, RtXR2 is an alkyl group that does not have the same structure), characterized by using a mixed solvent containing at least one ethylene glycol dialkyl ether (solvent (A)) whose structure is asymmetric However, specific examples of R1 or Mo2 in the above general formula (1) include an alkyl group having 1 or more carbon atoms, such as a methyl group,
Ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, sec-(trrt-)butyl group, etc. are used. Furthermore, specific examples of ethylene glycol dialkyl ether represented by general formula (1) include methoxyethoxyethane, methoxypropoxyethane, methoxyisopropoxyethane, methoxybutoxyethane, methoxyimbutoxyethane, methoxy-tert-methoxyethane, and ethoxypropoxy. Examples include ethane, ethoxyisoprofoxyethane, and the like.

In the ethylene glycol dialkyl ether having an asymmetric structure represented by the general formula [1], a preferable structure of R1 and R2 is such that the total number of carbon atoms in R and R2 is 3 to 8, particularly preferably 3 to 5. Although it is possible to use a solvent having a total number of carbon atoms exceeding 8, the viscosity of the solvent increases, which is not preferable.

In the present invention, a mixed solvent containing at least one of the above-mentioned solvents (4) is used, and examples of the solvent used in combination with the above-mentioned solvent (A) include propylene carbonate, ethylene carbonate, γ-butyrolactone, and tetrahydrofuran. The above-mentioned solvent is mixed with one or more selected from the group consisting of dioxolane, 1,3-dioxolane, 2-methyltetrahydrofuran, sulfolane, and the like. The polarization of the negative electrode of a lithium secondary battery can be significantly reduced only by using a mixture of this solvent (1) and the solvent (B).

The above solvent ■ and solvent (B),! The mixing ratio of L can be used within a wide range, but preferably solvent (A)/Q3
) is used as a capacity ratio of 571 to 115.

The organic solvent battery in which the solvent of the present invention is used is a lithium secondary battery using lithium as a negative electrode.

As positive electrode active materials for lithium secondary batteries, MnO2, (
CF)n, TiS2, Ti0z, V2O5, WS2
In addition, metal compounds such as niobium, molybdenum, tantalum, etc. can be used. In addition, examples of the inorganic electrolyte salt used in combination with the solvent of the present invention include LIct04, LiB
Electrolyte salts containing ]1, i such as F4, LiPF6, LIA BFe, etc. can be used.

, Examples 1 to 4, and Comparative Example 1 Experiments were conducted using a Teflon cell shown in FIG. 1 with an inner diameter of 13 mm and an internal capacity of about 2 mm. Both the test electrode 1 and the counter electrode 2 were disk-shaped (diameter 13), and these were manufactured by pressing a thin lithium layer (thickness 0.2+o+) onto 200 mesh nickel steel. The lithium electrode used as the reference electrode 3 was made by pressing a lithium piece onto nickel steel (2×2×2). Silicone rubber 6.7 was filled in the holes at the top and bottom of the cell that were opened to take lead wire 4.5. Further, nickel steel was used for the current collector 8.9 of each electrode, and a polyglopylene nonwoven fabric 10 was used as a separator to prevent short circuit between the test electrode 1, counter electrode 2, and reference electrode 3 during the experiment.

Methoxyethoxyethane (Example 1) and methoxyinpropoxyethane (Example 2) were added to the above cell as solvent prisoners.
, methoxybutoxyethane (Example 3), methoxyin butoxyethane (Example 4), and dimethoxyethane (Comparative Example 1), respectively, and using propylene carbonate as solvent CB), solvent (A)/(B) ) to make 50,750 mixed solvents with a volume ratio of 1.0 m
The charge-discharge characteristics of the negative electrode were tested using LICtO4 as an electrolyte with a concentration of ol/dm3.

The charging and discharging characteristics of the lithium negative electrode were measured by repeating charging and discharging operations for 10 minutes at a constant current of 1.0 ffM (0,075 A/dm2) using a galvanostat.

Assembly of the cell and measurement of charge/discharge characteristics were performed at 18 to 20°C in a dry box.

Table 1 shows the voltage between the electrodes 10 minutes after the start of discharging or charging, divided into the 1st, 10th, 20th, and 30th times of charging and discharging.
It shows. Regarding Example 2, FIG. 2 shows changes in the electrode potential of the test electrode relative to the reference electrode during discharge (noble side) and charge (shore side).

From the results in Table 1 and Figure 2, it can be seen that when the solvent of the present invention is used, the fluctuation in the electrode potential of the test electrode is extremely small during charge and discharge cycles, and stable charge and discharge cycles are obtained. It is clear that the polarization is also significantly reduced.

Table 1

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a cell used in the charge/discharge characteristics test. 1・・・・・・・・・Test electrode, 2・・・・・・・・・Counter electrode 3・・・・・・Reference electrode, 4.5・・・・・・・・・
・・Lead wire 8.9・・・・・Current collector, 10
・・・・・・・・・Separator 11・・・・・・・・・
Teflon spacer Figure 2 is a diagram showing the change in electrode potential over time of charging and discharging, which represents the result of the charging and discharging characteristics test conducted in Example 2, and the numbers attached to the curves indicate the number of charging and discharging cycles. represents. Patent applicant Mitsubishi Yuka Co., Ltd. Patent applicant Mitsubishi Yuka Fine Co., Ltd. Agent Patent attorney Hidetoshi Furukawa Agent Patent attorney Masatoshi Hase Hisatsu I C Figure 2 Charge/discharge time ('5'f)

Claims (1)

[Scope of Claims] (a) A solvent used in a lithium secondary battery comprising a negative electrode made of metallic lithium, a positive electrode, and an electrolyte, the solvent being a mixture containing at least one of the following solvents. A solvent for lithium secondary batteries characterized by being a solvent, solvent (1), ethylene glycol dialkyl ether represented by the general formula (11)
) (However, R1 and R2 are alkyl groups that do not each have the same structure).