JPH0340380A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To improve the charging/discharging characteristics as well as the strong load characteristic at low temps. by using a mixture liquid of specific compounds as organic solvent for electrolyte. CONSTITUTION:As organic solvent for electrolyte, a mixture liquid is used consisting of propylene carbonate(PC), ethylene carbonate(EC), dimetoxyethane(CME), and 2-methyltetrahydrofuran. Generally a solvent with higher dielectric factor provides a higher degree of ion dissolution for the solute, which heightens the electroconductivity of the electrolytic solution. If attention is paid to only this point, it is considered that the EC/DME type using EC with a high dielectric factor is more favorable than the PC/DME type. However, the EC/DME type is not serviceable at a low temp. because the EC solidifies. Thus PC, EC, and DME are added to provide serviceability even at low temp. Further, adding of 2ME-THF enhances the charging/discharging efficiency.

Description

[Detailed description of the invention] Industrial applications The present invention relates to a lithium secondary battery.

Conventional technology and its problems Conventionally, lithium secondary batteries use molybdenum disulfide (MoS2) to molybdenum dioxide (Mo03) as positive electrode active materials.
, manganese dioxide (Mn02) and vanadium pentoxide (
Inorganic substances such as V2O3), metal lithium and alloys that absorb and release lithium ions as negative electrodes, and propylene carbonate in which lithium salts such as lithium perchlorate, lithium borofluoride, and lithium arsenate hexafluoride are dissolved as electrolytes. Mixed solutions of dimethoxyethane and dimethoxyethane are known.

A large number of configurations are possible depending on the combination of these positive and negative electrodes and electrolytes, but when considering the energy density of the battery, a system using metallic lithium as the negative electrode is considered to be the most advantageous. Batteries that use metallic lithium as the negative electrode - a mixed solution of propylene carbonate and dimethoxyethane in which lithium perchlorate is dissolved, as the electrolyte, quickly decrease in charging and discharging efficiency, which is thought to be caused by lithium dendrites, when charging and discharging. A problem occurred in which the battery capacity decreased.

In addition, batteries using the above electrolyte had insufficient strong load characteristics at low temperatures.

OBJECTS OF THE INVENTION The object of the present invention is to provide a lithium secondary battery with improved Q point Ic#i, charge/discharge characteristics, and heavy load characteristics at low temperatures.

Structure of the Invention The present invention has a negative electrode containing lithium as an active material, a positive electrode, and an organic solvent of an electrolyte in which a lithium salt is dissolved in propylene carbonate (hereinafter referred to as PC). , ethylene carbonate (hereinafter referred to as EC)
, dimethoxyethane (hereinafter referred to as DME), and 2
-Presence of a mixed liquid consisting of methyltetrahydrofuran (hereinafter referred to as 2Me-THF)! This is a lithium secondary battery that uses an electrolyte consisting of a solvent.

action Table 1 shows the properties of various solvents.

Generally, a solvent with a high dielectric constant has a high degree of ionic dissociation of the solute, and therefore has the advantage that the conductivity of the electrolytic solution is also high. If you focus only on this point, PC/DI! The KO/DIE system, which uses EO having a higher dielectric constant than the KO/DIE system, is considered to be more advantageous.

However, the eO/DIE system cannot be used because EO solidifies at low temperatures. Therefore, PO, EC.

Furthermore, by mixing DME, it was proposed that it would maintain its liquid state even at low temperatures, and when it was investigated as a solvent for electrolyte solutions, it was found that the conductivity was even higher than that of the PO/DIE system.
It was found that the heavy load characteristics were improved.

Furthermore, 2Me-
It was found that when THF was added to form a four-type mixture, the charging and discharging efficiency was greatly improved, and the decrease in capacity due to charging and discharging was suppressed compared to the PO/DIE system.

Example Hereinafter, the details of the present invention will be explained with reference to Examples.

FIG. 1 shows a cross-sectional view of a lithium secondary battery using manganese dioxide as a positive electrode, metallic lithium as a negative electrode, and an electrolytic solution consisting of a nonaqueous solvent. 1 is a case that also serves as a positive electrode terminal, 2 is a sealing plate that forms a negative w1 terminal, 3 is a polypropylene gasket that insulates the case and the sealing plate, and 4 is a positive electrode, which is made of 85 parts by weight of manganese dioxide and a conductive material. 10.1 parts by weight of a certain acetylene powder and 5 parts by weight of polytethofluoroethylene as a binder were kneaded and formed into a sheet with a thickness of 0.711 m, which was then punched out to a diameter of 15.0 m.

Thereafter, it was vacuum dried at high temperature, and the positive electrode current collector 5tC, which had been welded to case 1 in advance, was crimped. Reference numeral 6 denotes metallic lithium, which was crimped onto the negative electrode current collector 7 with a thickness of 0.4 m and a diameter of 16 m.

8 is a separator made of a microporous membrane made of polypropylene.

The electrolytic solution was a mixture of four types: PC%EC, DIE, and 2Me-THF, and the mixing ratio was 1 + 2:172.

These contain lithium peroxide (LiO104) as a solute.
) was dissolved at % 1 mo/L//1.

The following 2m type test was conducted on the battery thus produced.

Cycle test Test temperature: 25°C Charging = constant current 0.5*mm, final voltage 3.5v Discharging:
Constant current 1.0*mm, final voltage 2.4v Low temperature discharge test Test temperature: 0°C Release t: Constant current 10.0*mu Comparative example The solvent of the electrolyte was PC and DIE, and the mixing ratio was 1=1. Battery B was manufactured in the same manner as in Example except for the above, and the test was conducted in the same manner.

Figure 2 shows the results of the cycle test. As is clear from the figure, it can be seen that the effect of the present invention is exerted in Battery M with less decrease in capacity than Battery B.

FIG. 5 shows an investigation of the voltage drop during discharge of battery M and B in ot:tc. Battery M has a smaller voltage drop than Battery B at low temperatures. This shows that the present invention is reliable under heavy loads at low temperatures.

Effects of the Invention As described above, the present invention can provide a lithium secondary battery with improved charge/discharge characteristics and heavy load characteristics at low temperatures, and therefore has extremely great industrial value.

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view of a lithium secondary battery according to an embodiment of the present invention, and FIGS. 2 and 5 are characteristic comparison diagrams of a battery of the present invention and a conventional battery. 1... Case 7-2... Sealing plate 3... Gasket 4... Positive electrode 5... Positive electrode current collector 6... Negative electrode

Claims (1)

[Claims] A mixture comprising a negative electrode containing lithium as an active material, a positive electrode, and an electrolytic solution consisting of an organic solvent in which a lithium salt is dissolved, and the organic solvent of the electrolytic solution is composed of propylene carbonate, ethylene carbonate, dimethoxyethane, and 2-methyltetrahydrofuran. A lithium secondary battery characterized by using a liquid.