JP2705529B2 - Organic electrolyte secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-energy-density and high-safety organic electrolyte secondary battery as a power supply for driving electronic equipment or a power supply for holding a memory.

[0002]

2. Description of the Related Art With the rapid reduction in size and weight of electronic equipment, the development of secondary batteries that are small, lightweight, have a high energy density, and can be repeatedly charged and discharged with respect to the battery that is the power source of the electronic equipment has been developed. Demands are growing. As a secondary battery satisfying these requirements, an organic electrolyte secondary battery is most promising.

As the positive electrode active material of the organic electrolyte secondary battery, various substances such as titanium disulfide, lithium cobalt composite oxide, spinel type lithium manganese oxide, vanadium pentoxide and molybdenum trioxide have been studied. ing. Among them, lithium cobalt composite oxide, spinel type lithium manganese oxide and the like are 4V (vs.
Since the battery is charged and discharged at a very noble potential of Li / Li ⁺ ) or more, a battery having a high discharge voltage can be realized by using it as a positive electrode.

[0004] As the negative electrode active material of the organic electrolyte secondary battery, various materials such as lithium metal and a Li-Al alloy capable of occluding and releasing lithium and carbon materials have been studied. Has an advantage that a battery having high safety and a long cycle life can be obtained.

Since an electrolyte for an organic electrolyte secondary battery is required to have a wide operating temperature range (-20 to 60 ° C.) and a high ionic conductivity, a high dielectric constant such as ethylene carbonate or propylene carbonate is generally required. A low-viscosity solvent such as 2-dimethoxyethane or dimethyl carbonate is mixed with 1:
An organic solvent mixed at a mixing ratio of 1 is used.

However, a lithium-cobalt composite oxide and a spinel-type lithium manganese oxide (LixMn ₂ O ₄ ) are used for the positive electrode.
In a secondary battery using a carbon material for the negative electrode, the electrolyte is exposed to a severe oxidation-reduction atmosphere, so that the electrolyte deteriorates with the progress of the charge / discharge cycle, and the discharge capacity of the battery decreases. was there.

Therefore, there has been a demand for the development of an electrolyte having excellent electrochemical stability.

[0008]

SUMMARY OF THE INVENTION The present invention relates to a secondary battery comprising a positive electrode made of a material capable of storing and releasing lithium ions, a negative electrode formed of a carbon material capable of storing and releasing lithium ions, and an organic electrolyte. The electrolyte is ethylene carbonate (EC) and dimethyl carbonate (DMC)
And methyl ethyl carbonate (MEC), and the composition ratio of EC, DMC and MEC is 30 to 50 vol%, 10
An object of the present invention is to solve the above problems by providing an organic electrolyte secondary battery characterized by being at 50 vol% and at 50 vol%.

[0009]

The organic electrolyte secondary battery of the present invention has an effect that the discharge capacity retention characteristics after repeated charge / discharge cycles are superior to the conventional organic electrolyte secondary battery. This is considered to be due to the improved electrochemical stability of the new organic solvent used in the organic electrolyte secondary battery of the present invention.

[0010]

The present invention will be described below with reference to preferred embodiments.

As the organic solvent, ethylene carbonate (E
C), dimethyl carbonate (DMC) and methyl ethyl carbonate (MEC). The electrolyte is
By changing the composition ratio (volume ratio) of EC, DMC, MEC,
Each of them as a solute is lithium hexafluorophosphate (LiP
F ₆ ) was dissolved at a rate of 1 mol / l.

FIG. 1 shows the results of measuring the freezing points of the above mixed electrolytes having various compositions. Since the electrolyte is required not to coagulate at −20 ° C., it can be seen that the composition range of the mixed solvent that can be used for the battery is naturally limited. EC and DM
In a two-component mixed system with C, the composition ratio of EC is 40 to 50.
vol%, and E in a binary mixture of EC and MEC.
The composition ratio of C is 10 to 50 vol%.

FIG. 2 shows the results of measuring the ionic conductivity at a temperature of 25 ° C. for an electrolyte having a composition range of less than the freezing point of −20 ° C. EC and M, which have the highest ionic conductivity of the two-component mixed electrolyte of EC and DMC and have a high MEC composition ratio,
The binary electrolyte mixed with EC shows the lowest ionic conductivity. Since the high-rate discharge performance of a battery is greatly affected by the ionic conductivity, it is expected that a battery using a two-component mixed electrolyte of EC and DMC will have excellent high-rate discharge performance.

Next, a test using a battery was performed. FIG.
FIG. 3 is a longitudinal sectional view of an organic electrolyte secondary battery using LiCoO _{2 for} a positive electrode and a carbon material for a negative electrode. In the figure, reference numeral 1 denotes a case also serving as a positive electrode terminal punched out of a stainless steel sheet having resistance to organic electrolyte by a press, and reference numeral 2 denotes a sealing plate serving also as a negative electrode terminal punched out of the same material. Negative electrode 3 on the inner wall
Is abutted. The negative electrode was manufactured as follows. An appropriate amount of polyvinylidene fluoride (8 parts by weight) and N-methyl-2-pyrrolidone as a solvent were added to 92 parts by weight of carbon powder (pyrolyzed carbon) and kneaded well to prepare a negative electrode mixture paste. This paste was coated with a 100-mesh copper wire mesh (wire diameter
1mm), hot air drying at 85 ° C for 10 hours, and vacuum drying at 250 ° C for 3 hours.
A negative electrode plate was prototyped by punching into a circular plate. The charge / discharge capacity of this electrode is about 15 mAh. Reference numeral 5 denotes a separator made of polypropylene impregnated with an organic electrolyte, and reference numeral 6 denotes a positive electrode, which was produced as follows. To 82 parts by weight of LiCoO _2, 6.5 parts by weight of polyvinylidene fluoride, 10 parts by weight of graphite (SFG6 manufactured by Lonza), 1.5 parts by weight of Ketjen black and an appropriate amount of N-methyl-2-pyrrolidone as a solvent were added. And kneaded well to prepare a positive electrode mixture paste. This paste was uniformly applied to a 100-mesh aluminum wire mesh (wire diameter: 0.1 mm), dried with hot air at a temperature of 85 ° C. for 10 hours, and then vacuum-dried at a temperature of 250 ° C. for 3 hours.
A lithium-cobalt composite oxide electrode was fabricated by punching it into an mm disk. The theoretical capacity of this electrode is about 15 mAh, assuming that 0.5 mol of lithium is inserted and extracted per 1 mol of the active material. Numeral 1 denotes a case also serving as a positive electrode terminal, which is hermetically sealed by caulking an opening end inward and tightening an inner periphery of a sealing plate 2 also serving as a negative electrode terminal via a gasket 4. The solvent of the electrolyte solution has a freezing point of -20.
Those having a composition range of less than ° C were used.

Next, these batteries are charged at a constant current of 2.0 mA until the terminal voltage reaches 4.2 V, and subsequently, at a constant current of 2.0 mA, the terminal voltage is increased to 2.7 V. Charge-discharge cycle life test (temperature 60
° C). FIG. 4 shows the discharge capacity retention ratio after the 100-cycle charge / discharge test with respect to the initial capacity. The number of test batteries was three for each condition, and the results were average values.

A two-component mixed electrolyte of EC and DMC or EC
Of the two-component mixed electrolyte of MEC and MEC greatly deteriorates with the progress of the charge / discharge cycle, but the electrolyte of the present invention in which EC, DMC and MEC are mixed has a small decrease in capacity with the progress of the charge / discharge cycle. . Especially the composition is EC40-50
vol%, DMC20-30vol%, MEC20-3
The battery using the electrolyte in the range of 0 vol% exhibited excellent charge / discharge cycle characteristics. It is not clear why the battery performance was improved by mixing EC, DMC and MEC at a specific composition ratio, but it is considered that the electrochemical stability was improved by the interaction between EC, DMC and MEC. .

As described above, the electrolyte is EC, DMC and ME.
EC, DMC and ME
The composition ratio of C is 30 to 50 v with respect to the entire solvent.
ol%, 10 to 50 vol%, and 10 to 50 vol%, a battery having excellent charge / discharge cycle characteristics can be provided. In particular, considering the high rate discharge performance of the battery, the preferred composition range is EC 30 to 50 vol%, DMC 20
~ 50 vol% and MEC10 ~ 40vol%, and EC40 ~ 50vol considering the cycle life.
1%, DMC 20-30 vol%, MEC 20-30 v
ol% is a particularly preferred composition range.

In the above embodiment, the case where the lithium-cobalt composite oxide is used as the positive electrode active material has been described.
Various materials such as titanium disulfide, manganese dioxide, spinel type lithium manganese oxide (LixMn ₂ O ₄ ), vanadium pentoxide, and molybdenum trioxide can be used. Although the case where pyrolytic carbon is used as the negative electrode has been described, various carbon materials such as artificial graphite, natural graphite, and pitch-based spherical graphite can be used. Further, in the above embodiment, the case where lithium hexafluorophosphate is used as the electrolyte has been described, but the type and concentration of the electrolyte are not basically limited. For example, LiAsF ₆ , LiBF ₄ ,
LiPF ₆ , LiCF ₃ SO _3, etc., at a concentration of 0.5 to 2
It can be used in the range of about mol / l. The batteries according to the above embodiments are all button-type batteries,
Similar effects can be obtained by applying the present invention to a cylindrical, square or paper battery.

[0019]

As described above, in the organic electrolyte secondary battery of the present invention, the decrease in the discharge capacity with the progress of the charge / discharge cycle is small.

[Brief description of the drawings]

FIG. 1 is a diagram showing a freezing point of a mixed electrolytic solution of EC, DMC and MEC.

FIG. 2 is a diagram showing the ionic conductivity of a mixed electrolytic solution of EC, DEC, and MEC.

FIG. 3 is a diagram showing an internal structure of a button battery as an example of an organic electrolyte secondary battery.

FIG. 4 is a diagram showing a capacity retention ratio of a test battery after a 100-cycle charge / discharge test (at a temperature of 60 ° C.).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Negative electrode 4 Gasket 5 Separator 6 Positive electrode

Claims (1)

(57) [Claims] 1. An organic electrolyte secondary battery comprising: a positive electrode made of a substance capable of storing and releasing lithium ions; a negative electrode formed of a carbon material storing and releasing lithium ions; and an organic electrolyte. Is ethylene carbonate (EC), dimethyl carbonate (DMC) and methyl ethyl carbonate (ME
C), and the composition ratio of EC, DMC and MEC is 30 to 50 vol% and 10 to 50 v, respectively, based on the whole solvent.
ol% and 10 to 50 vol%.