JPH07153486A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To improve the high-rate discharge characteristic at a low temperature by using a lithium secondary battery added with gamma butyrolactone of a specific range % (volume &) to a mixture of ethylene carbonate and dimethyl carbonate at the specific volume ratio. CONSTITUTION:The dielectric constant of an electrolyte added and mixed with gamma butyrolactone to a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) at the volume ratio of 1:1 at -20 deg.C is higher than that of the conventional mixture of EC+DMC+DEC (2:2:1). LiCoO2 having the average grain size of 6mum, acetylene black powder, and polyvinylidene fluoride are mixed, N-methyl pyrrolidone is added into a paste shape, and it is coated with an Al foil to form a positive electrode plate, for example. Artificial graphite and spherical graphite are mixed, polyvinylidene is mixed, N-methyl pyrrolidone is added, and it is applied to a copper foil to form a negative electrode plate. Both electrode plates are stored in a container, and the electrolyte is injected to obtain a battery having a high-rate discharge characteristic at a low temperature.

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.

[0002]

2. Description of the Related Art A lithium secondary battery is a battery having a high voltage and a high energy density, but has a problem in safety. A lithium secondary battery using a carbon material for the negative electrode has significantly higher safety than a conventional battery using metallic lithium for the negative electrode. In particular, a lithium secondary battery using graphite as a carbon material is excellent because it has a high energy density. However, it was found that the charge / discharge characteristics of graphite are significantly reduced depending on the type of electrolyte. That is, it was found that the graphite negative electrode has a poor compatibility with propylene carbonate (PC) and ethylene carbonate (EC) is good. However, EC has a problem in that it has a high freezing point (solid at room temperature).

As a result of examination of additives which lower the freezing point of EC and do not lower the electrical conductivity and stability, it is found that ethylene carbonate and dimethyl carbonate (D
MC: 1: 1 mixture (volume ratio) with diethyl carbonate (DEC) EC: DMC: DEC = 2: 2: 1
The mixture is used so as to have a (volume ratio).

DMC has the effect of lowering the viscosity of the electrolyte and improving the low temperature conductivity. EC and DMC 1:
The reason for mixing at 1 is that the freezing point is the lowest at this time. Addition of DEC further lowers the freezing point and improves low temperature characteristics.

However, since the lithium secondary battery uses an organic electrolyte solution, its characteristics are remarkably deteriorated at low temperatures, and it is always an important subject to improve the characteristics. That is,
It is important to improve the conductivity at low temperatures such as -20 ° C. However, high temperature stability should not be sacrificed at this time.

[0006]

SUMMARY OF THE INVENTION The present invention is a 1: 1 (volume ratio) of ethylene carbonate and dimethyl carbonate.
Gamma-butyrolactone in the mixture is 0.5% or more and 50
% Or less (volume fraction) is added to solve the above-mentioned problems by using a lithium secondary battery.

[0007]

The inventor has tried to improve low temperature conductivity by mixing various solvents into a 1: 1 mixture of EC and DMC. As a result, it was found that excellent effects can be obtained by mixing gamma-butyrolactone (GBL). However,
It was also found that if the amount of GBL added is increased too much, the conductivity rather decreases. It is considered that the reason why the conductivity is improved by adding GBL is that the freezing point is further lowered and that GBL has high solute solubility.

In addition, compared with the conventional DME-added product, GB
It was also found that the L-added product was more excellent in stability at high temperatures exceeding 60 ° C. This is GB with a cyclic structure
It is considered that L is more excellent in stability than DEC having a linear structure. DMC is also considered to be inferior in stability because it has a straight-chain structure, but since there is no other suitable solvent that has the effect of lowering the viscosity of the electrolytic solution, it must be used at this time. .

[0009]

EXAMPLES Examples will be shown below.

The electrical conductivity at -20 ° C. of an electrolytic solution prepared by adding and mixing gammabutyrolactone (GBL) to a 1: 1 (volume ratio) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) is the conventional EC + DMC + DEC.
A comparison with the conductivity of the (2: 2: 1) electrolyte is shown in FIG. As can be seen from the figure, GBL was increased from 0.5% to 50
The electrolyte solution added up to 10% shows a high conductivity equal to or higher than that of the conventional electrolyte solution.

Next, the lithium secondary battery of the present invention was manufactured as follows.

86: 5: LiCoO ₂ having an average particle size of 6 μm, acetylene black powder and polyvinylidene fluoride were mixed.
9 (weight ratio) and N-methylpyrrolidone were added to form a paste, which was applied to an aluminum foil having a thickness of 20 microns. This is dried, rolled, and then cut to reduce the thickness.
A strip-shaped positive electrode plate having a length of 0.150 mm, a width of 40 mm, and a length of 28 mm, and a strip-shaped positive electrode plate having a thickness of 0.150 mm, a width of 40 mm, and a length of 300 mm were prototyped.

Next, artificial graphite having an average particle size of 25 microns and spherical graphite having an average particle size of 20 microns are mixed in a weight ratio of 3:
The graphite mixture mixed with 1 and polyvinylidene fluoride were mixed at 86:14 (weight ratio), N-methylpyrrolidone was added to form a paste, and the paste was applied to a copper foil having a thickness of 18 microns. This was dried, rolled, and then cut to fabricate a strip negative electrode plate having a thickness of 0.110 mm, a width of 41 mm, and a length of 430 mm.

With the strip-shaped positive electrode plate as a core, the strip-shaped negative electrode plate is wound through a microporous polyethylene separator having a thickness of 25 μm, and then the strip-shaped positive electrode plate is wound.
A battery power generation element with an elliptical winding surface was prototyped.

The above power-generating element was sealed in a square container and a container lid made of a 0.22-mm-thick steel plate having a 70-micron PP film adhered to the inner surface and an 8-micron coating film on the outer surface, and provided on the container lid. The positive electrode plate and the negative electrode plate were electrically connected to the stainless steel rivet terminals (two pieces). The rivet terminal is insulated and airtight by a gasket made of methylpentene copolymer (TPX). In addition, the container lid and the container are hermetically sealed by a double-winding sealing port.

EC + DMC + GBL (4
0:40:10) The lithium secondary battery (A) of the present invention into which an electrolytic solution has been injected and EC + DMC + DEC (40: 40: 2).
0) was injected and a battery (a) for comparison was manufactured as a trial.
Charge these batteries up to 4.1V at 200mA at room temperature to reach 500mA
Discharged to 2.75V. Subsequently, the battery was charged and discharged in the same manner at -20 ° C. Table 1 shows a comparison of the discharge capacities at this time.

[0017]

[Table 1] As is clear from Table 1, the battery of the present invention shows less decrease in discharge capacity at low temperatures than the conventional battery.

[0018]

According to the present invention, a lithium secondary battery having excellent high rate discharge characteristics at low temperature can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing electric conductivity at −20 ° C.

Claims (1)

[Claims] 1. An electrolytic solution comprising a 1: 1 (volume ratio) mixture of ethylene carbonate and dimethyl carbonate to which gamma-butyrolactone is added in an amount of 0.5% or more and 50% or less (volume fraction). And a lithium secondary battery.