JPH1131525A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To heighten the dielectric constant of an EV battery using carbon material for a negative electrode, and suppress the increase of an irreversible capacity without spoiling cold resistance by causing the solvent of organic electrolyte to contain γ-butyro-lactone as a main component and at least ethylene carbonate as an accessory component. SOLUTION: Besides ethylene carbonate, other organic solvent may be allowed to be contained as an accessory component. A composition ratio of γ- butyro-lactone as a main component in organic solvent is decided based on the relative relationship with ethylene carbonate and the other accessory components, as long as the organic solvent can effectively maintain high electric conductivity and a non-coagulation property, it is not always required to occupy the major amount or more of the whole organic solvent. The lower limit of the ethylene carbonate in the organic solvent is required to have at least such amount as the decomposing reaction of the γ-butyro-lactone and graphite at a charging time can be avoided. According to an experimental sample, the amount is 15 vol.% or more and less than 35 vol.% of the whole organic solvent.

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, and more particularly, to a lithium secondary battery which can be particularly suitably used as a battery for an electric vehicle (hereinafter referred to as "EV"). About.

There is a strict requirement for an organic electrolyte used for an EV battery to stably withstand use in a wide temperature range of -30 ° C. to 60 ° C. and to have high conductivity. Further, it is considered that a graphite-based material is desirable for the negative electrode of the EV battery because of its good voltage flatness and high current performance.

[0003]

2. Description of the Related Art Conventionally, characteristics of a lithium secondary battery have been evaluated by using ethylene carbonate (hereinafter referred to as "EC") and diethyl carbonate (hereinafter referred to as "DEC"). An organic electrolyte using a mixed solvent of (1) and (2) is widely used. The reason is that this organic electrolytic solution can be applied to a battery using either a graphite-based material or a non-graphite-based material as a negative electrode, or has a low irreversible capacity and a high charge / discharge efficiency.

However, in applying this organic electrolytic solution to an EV battery, the strict requirements for the EV battery described above are a bottleneck.

That is, for example, when the composition ratio of EC: DEC is 1:
In the case of 1, the conductivity is good, but -10 ° C
It does not withstand use because it solidifies as soon as possible at a nearby temperature. However, when the composition ratio of DEC is increased, the freezing point decreases. For example, when the composition ratio of EC: DEC is 1: 4, −
Although an electrolyte solution that does not coagulate even at 30 ° C. is obtained, in this case, the conductivity is extremely low, and it is not usable. In other words, even if the composition ratio of EC: DEC is variously changed, there is a trade-off relationship in which excellent cold resistance (non-solidification) and high conductivity are incompatible.

[0006] For this reason, an organic electrolyte using a mixed solvent of EC and DEC is considered to be an excellent organic electrolyte in general, but severe use conditions must be assumed for use in an EV battery. It is said that it is difficult to apply it.

On the other hand, an organic electrolyte using γ-butyrolactone (hereinafter referred to as “GBL” in the present specification) as a solvent has a relatively high conductivity even at −30 ° C., for example.
It is known that it does not solidify. Therefore, this is
It is also possible to consider whether to use it for the organic electrolyte of the V battery.

However, when a GBL-based organic electrolytic solution is applied to a secondary battery using a graphite-based material as a negative electrode, the GBB is charged during charging.
It has been pointed out that L and graphite react and decompose, resulting in an increase in irreversible capacity and deterioration in charge / discharge efficiency (for example, the “Battery Handbook-Supplemented Edition” published by Maruzen in 1995) Description on page 541).

For this reason, any conventional battery using a GBL-based organic electrolytic solution does not use a graphite-based material as an electrode. For example, in the invention of the organic electrolyte battery according to JP-A-3-110765, LiCoO ₂
And a non-aqueous electrolyte battery according to Japanese Patent Application Laid-Open No. 5-217602 discloses an electrode of Li transition metal oxide / hard carbon (non-graphite carbon material). Configuration.

[0010]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a lithium secondary battery capable of satisfying the above strict requirements for an EV battery on the premise that a graphite-based material is used for the negative electrode. Should be technical issues.

[0011]

The present inventor studied the means for solving the above-mentioned problems, and found the following findings in connection with the increase in the irreversible capacity due to the reaction between the GBL-based organic electrolyte and the negative electrode graphite-based material. I got

That is, in a lithium secondary battery using a GBL-based organic electrolyte and a graphite-based negative electrode, Li ions solvated by GBL move to the negative electrode during charging, and accordingly, the negative-graphite-based material The decomposition reaction between GBL and graphite takes place on the surface of, and the irreversible capacity increases.

However, if the organic electrolyte contains an organic solvent A having a higher dielectric constant in addition to GBL, the Li ions solvated with the organic solvent A during charging first It moves to the negative electrode, and the organic solvent A and graphite react preferentially. Therefore, in this case, the increasing range of the irreversible capacity is GB
It is determined not by L but by the organic solvent A.

From the above findings, the inventor of the present application has conceived that searching for an organic solvent satisfying the following conditions leads to means for solving the problem.
Knowing that it was C, the present invention was completed. (1) The dielectric constant is higher than GBL, (2) the increase in the irreversible capacity due to the reaction with graphite is smaller than GBL, and (3) GB is a ratio that does not impair the cold resistance of the GBL.
Compositions with L are possible.

[0015]

Means for Solving the Problems According to the present invention (the invention described in the claims) for solving the above problems, a lithium transition metal oxide is used for a positive electrode, a graphite-based material is used for a negative electrode, and an electrolyte is lithium. In a lithium secondary battery that is an organic electrolyte in which a salt is dissolved, a solvent of the organic electrolyte is GBL (γ-
This is a lithium secondary battery having a composition containing (butyrolactone) as a main component and at least EC (ethylene carbonate) as a subcomponent.

[0016]

EC is an organic solvent satisfying the above conditions (1) to (3). For this reason, in the lithium secondary battery of the present invention, during charging, Li ions solvated by GBL also move to the negative electrode, but Li ions solvated by EC having a dielectric constant higher than that of GBL quickly shift to the negative electrode. Therefore, the decomposition reaction between EC and graphite occurs preferentially on the surface of the negative electrode graphite-based material, and the decomposition reaction between GBL and graphite is substantially avoided.

Accordingly, the irreversible capacity due to the decomposition reaction between the organic solvent and graphite is determined by EC, and the increase in the irreversible capacity is effectively reduced as compared with the case where GBL is involved in the reaction.

Therefore, a GBL-based organic electrolyte can be applied to a secondary battery using a graphite-based material as a negative electrode without greatly reducing the irreversible capacity.

On the other hand, as long as the composition of the organic electrolyte does not employ an extreme composition ratio such that EC is considered to be the main component, the excellent cold resistance and high conductivity of the above-described GBL are maintained.

Therefore, the lithium secondary battery of the present invention has
Strict use conditions for the V battery can be satisfied.

[0021]

Next, an embodiment of the present invention will be described.

[Lithium Secondary Battery] Although the lithium secondary battery of the present invention has an action and effect particularly suitable for an EV battery, it goes without saying that its use or method of use is not limited to an EV battery. It can be used as a lithium secondary battery for any purpose.

Therefore, the physical configuration of the secondary battery is not limited to that of an EV battery, but can be configured as a small or portable battery that can be incorporated into various electric appliances and the like.

[Positive electrode of lithium secondary battery] A lithium transition metal oxide is used for a positive electrode of a lithium secondary battery. There is no particular limitation on the type, and in short, any type can be used as long as lithium ions can be reversibly and electrically inserted and removed. Representative examples include LiCoO ₂ , LiNiO ₂ ,
LiMn ₂ O _{4 and the} like can be exemplified.

The shape and form of the positive electrode as an electrode, the method of manufacturing the same, and the like are not limited at all.

[Negative electrode of lithium secondary battery] A graphite material is used for the negative electrode of the lithium secondary battery. The term "graphite-based material" used herein includes not only artificial graphite, natural graphite and other graphite materials that may be used as electrodes, but also graphite materials blended with non-graphite-based materials such as hard carbon and heat-treated coke. It is a concept.

[Organic Electrolyte] The organic electrolyte contains at least an organic solvent and a supporting electrolyte, and the organic solvent has a composition containing GBL as a main component and at least EC as a subcomponent. It is permissible to include further organic solvents as accessory components.

The composition ratio of GBL as a main component in an organic solvent is such that ECB and other subcomponents can be used as long as the organic solvent can effectively maintain high conductivity and non-coagulability even at −30 ° C. It is determined by a relative relationship, and does not necessarily have to account for a majority or more of the entire organic solvent.

Regarding the lower limit of the composition ratio of EC as an auxiliary component in an organic solvent, as described in the section of “Functions and Effects of the Invention”, at least the decomposition reaction between GBL and graphite at the time of charging is substantially reduced. It is required that the amount be avoided. The specific numerical value may vary depending on the type and composition ratio of the other subcomponents, and it is difficult to determine uniformly. However, according to one experimental example, the specific numerical value is 15% by volume or more of the entire organic solvent. The upper limit of the composition ratio of EC is 35% of the entire organic solvent.
% By volume. This is because if EC occupies 35% by volume or more, solidification of EC occurs at a temperature of -30 ° C or higher, and the electric conductivity of the organic electrolyte solution is not preferable.

As the supporting electrolyte for the organic electrolyte, one or more of various well-known or known lithium compounds which may be used for this kind of application may be arbitrarily used. Typical examples are LiPF ₆ , LiB
_{F 4, LiClO 4, LiAsF 6} , but LiCF ₃ SO ₃ and the like, without limitation.

[0031]

Next, an embodiment of the present invention will be described.

Example 1 The mixing ratio of GBL was 0 to 100.
%, 20 kinds of mixed solvents with the balance being EC are prepared,
High purity LiBF ₄ manufactured by Toyama Pharmaceutical Co., Ltd.
By dissolving them in a mole / liter, 20 kinds of organic electrolytes were prepared.

Next, using a glass cell provided with two platinum electrodes, the conductivity (measured at 10 kHz) of each of the above-mentioned organic electrolytes at −30 ° C. and the presence or absence of solidification were examined. The conductivity was measured with an impedance analyzer (4192A, Hewlett-Packard Company).

As a result, in each of the organic electrolytes having an EC ratio of less than 35%, the conductivity at -30 ° C. was practically high and did not solidify.

[Example 2] LiM manufactured by Honjo Chemical Industry
n ₂ O ₄ 18.5 parts, 1.5 parts of acetylene black manufactured by Tokai Carbon, Kureha Chemical Co. of polyvinylidene fluoride (hereinafter, referred to herein as "PVDF".) 8 parts of the powder, pure Wako 72 parts of medicinal N-methylpyrrolidone,
Mix well to obtain a slurry. Thick this slurry 2
A positive electrode material was obtained by coating on a 0 μm aluminum foil.

On the other hand, raw coke produced in China was heat-treated at 900 ° C., and 65 parts of the heat-treated product was mixed with 35 parts of Chuetsu natural graphite, and 100 parts of the mixture was mixed with P
N-methylpyrrolidone 100 as above with 10 parts of VDF
To obtain a slurry. This slurry was applied on a copper foil to obtain a negative electrode material.

Next, each of the positive electrode material and the negative electrode material was punched into a disk having a diameter of 17 mm.

Separately, GBL: EC: DE by volume ratio
C = 4: 1: 1 mixed solution (EC composition ratio is 16% by volume)
An organic electrolyte solution obtained by dissolving LiBF ₄ in the same manner as described above to a concentration of 1 mol / liter is impregnated in a polyethylene separator made by Tonen Chemical Co., Ltd. having a diameter of 19 mm, and this separator is placed between the disc-shaped electrodes. The lithium secondary battery according to the present example was configured with the.

The battery of this example was charged to 4.2 V at a constant current of 1 mA / cm ² , and then further charged at a constant voltage of 4.2 V. Charged battery is 0.5mA / c
The battery was discharged at a constant current of m ² , and the initial discharge capacity and irreversible capacity were determined. The evaluation results are shown in Table 1 at the end of the sentence.

Example 3 The composition ratio of the solvent of the organic electrolyte was GBL: EC: DEC = 2: 1: 1 (EC
Was carried out in the same manner as in Example 2 except that the composition ratio was 25% by volume. Table 1 at the end of the sentence
Shown in

Example 4 The composition ratio of the solvent of the organic electrolyte was GBL: EC: DEC = 1: 1: 1 (EC
Was carried out in the same manner as in Example 2, except that the composition ratio was 33% by volume. Table 1 at the end of the sentence
Shown in

Comparative Example 1 Evaluation was performed in the same manner as in Example 2 except that the composition ratio of the solvent in the organic electrolyte was EC: DEC = 1: 1 (excluding GBL) by volume. did. The evaluation results are shown in Table 1 at the end of the sentence.

Comparative Example 2 The same procedure as in Example 2 was carried out except that the solvent of the organic electrolyte was composed of only GBL, and the evaluation was performed. The evaluation results are shown in Table 1 at the end of the sentence.

[Evaluation of Examples 2 to 4 and Comparative Examples 1 and 2] As is apparent from Table 1, the lithium secondary batteries of the respective examples did not solidify their organic electrolytes at -30 ° C. It shows high conductivity and has excellent charge / discharge efficiency of the battery. In Comparative Example 1, the organic electrolyte solution solidified. In Comparative Example 2, the irreversible capacity was considerably large.

Example 5 Example 2 was repeated except that graphite MCMB25-28 manufactured by Osaka Gas was used as the negative electrode material.
The same was performed and evaluated. The evaluation results are shown in Table 2 at the end of the sentence.

FIG. 1 shows a current density-time curve at the time of charging when charging / discharging is repeated in the lithium secondary battery of this example. In FIG. 1, the solid line is the curve at the first charge, the dotted line is the curve at the second charge, and the dashed line is the curve at the time of the fourth charge. From the results of FIG. 1, the first charge is E compared to FIGS. 2 and 3 described below.
Li ion solvated with C, G in the second and fourth
It can be seen that this is done with Li ions solvated in BL respectively.

Example 6 The same procedure as in Example 3 was carried out except that the same negative electrode as in Example 5 was used, and the evaluation was performed. The evaluation results are shown in Table 2 at the end of the sentence.

Comparative Example 3 The same operation as in Comparative Example 2 was performed except that the same negative electrode as in Example 5 was used. The evaluation results are shown in Table 2 at the end of the sentence.

FIG. 2 shows a current density-time curve at the time of charging when charging / discharging is repeated in the lithium secondary battery of this example. In FIG. 2, the solid line is the curve at the first charge, the dotted line is the curve at the second charge, and the dashed line is the curve at the time of the fourth charge. From the results shown in FIG. 2, it can be seen that, in the electrolytic solution using GBL, the time decay of the current density under a constant voltage of 4.2 V occurs slowly.

Comparative Example 4 The same operation as in Comparative Example 1 was performed except that the same negative electrode as in Example 5 was used. The evaluation results are shown in Table 2 at the end of the sentence.

FIG. 3 shows a current density-time curve at the time of charging when charging / discharging is repeated in the lithium secondary battery of this example. In FIG. 3, the solid line is the curve at the time of the first charge, the dotted line is the curve at the second time, and the chain line is the curve at the time of the fourth charge. From the results in FIG. 3, it can be seen that in the electrolytic solution using EC / DEC, the time decay of the current density under a constant voltage of 4.2 V is sharp.

[Evaluation of Examples 5 to 6 and Comparative Examples 3 and 4] As is clear from Table 2, the lithium secondary batteries of the respective examples did not solidify at -30 ° C in their organic electrolytes. It shows high conductivity and has excellent charge / discharge efficiency of the battery. In Comparative Example 3, the irreversible capacity was considerably large, and the initial discharge capacity was low. In Comparative Example 4, the organic electrolyte was solidified.

FIG. 4 summarizes the charge / discharge cycle characteristics of the lithium secondary batteries of Example 5, Example 6, Comparative Example 3 and Comparative Example 4.

[0054]

[Table 1]

[0055]

[Table 2]

[Brief description of the drawings]

FIG. 1 is a diagram showing a current density-time curve of a lithium secondary battery of Example 5.

FIG. 2 is a diagram showing a current density-time curve of a lithium secondary battery of Comparative Example 3.

FIG. 3 is a diagram showing a current density-time curve of a lithium secondary battery of Comparative Example 4.

FIG. 4 is a diagram showing charge / discharge cycle characteristics of lithium secondary batteries of Example 5, Example 6, Comparative Example 3 and Comparative Example 4.

Claims (1)

[Claims] 1. A lithium secondary battery in which a lithium transition metal oxide is used for a positive electrode and a graphite-based material is used for a negative electrode, and an electrolytic solution is an organic electrolytic solution in which a lithium salt is dissolved. A lithium secondary battery comprising butyrolactone as a main component and at least ethylene carbonate as an auxiliary component.