JPH08180865A - Nonaqueous electrolytic secondary battery - Google Patents

Abstract

PURPOSE: To provide a nonaqueous electrolytic secondary battery in which the generation of a gas on the surface of a carbon material is prevented to prevent the leakage of an electrolyte due to the rising of the battery internal pressure and the expansion of the battery by using the carbon material whose surface is alkylated for a negative electrode. CONSTITUTION: In a nonaqueous electrolytic secondary battery formed of a negative electrode using a carbon material capable of storing and releasing lithium, a positive electrode and a nonaqueous electrolyte, the surface of the carbon material is alkylated. This alkylating treatment is preferably performed by adding 5-30wt.% of an alkylating agent such as trimethyl methoxysilane to the carbon material such as graphite and then thermally treating it under a prescribed condition. The surface of the carbon material is covered with the inert alkyl group and the gas generation by the decomposing reaction of the electrolyte or a binder at charging is suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface treatment of a carbon material used for a non-aqueous electrolyte secondary battery, especially for its negative electrode.

[0002]

2. Description of the Related Art Non-aqueous electrolyte secondary batteries using lithium or a lithium compound as a negative electrode are expected to have a high voltage and a high energy density, and have been actively studied.

So far, V ₂ O ₅ , Cr ₂ O ₅ , MnO ₂ , TiS _{2 and the} like have been known as positive electrode active materials for non-aqueous electrolyte secondary batteries. 4 with higher energy density in recent years
Li as a positive electrode active material for a bolt-class non-aqueous electrolyte secondary battery
Researches on Mn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , LiFeO _2, etc. have been actively conducted. In particular, LiMn ₂ O ₄ ,
LiNiO ₂ and LiFeO ₂ are attracting attention because of their low cost and stable supply of raw materials.

On the other hand, as a negative electrode active material, a carbon material capable of inserting and extracting lithium has been attracting attention in place of metallic lithium in terms of safety and rate characteristics. In particular, graphite powder with a high degree of graphitization has the characteristics of high capacity, discharge potential of about 0.1 V, and less decrease in battery voltage compared to metallic lithium, and has been actively studied. ing.

[0005]

However, a carbon material, particularly graphite powder having a high degree of graphitization, absorbs and releases lithium during charging and discharging, and gas is generated from the surface of the powder particles. With regard to this gas generation mechanism, it is considered that a functional group such as an OH group on the surface of the carbon material becomes an active site, the organic binder used in the electrolytic solution or the negative electrode is decomposed, and a gas is generated at this time.

The generated gas species are mainly hydrogen and carbon monoxide, and trace amounts of acetylene, methane and the like are also detected. Particularly, hydrogen gas occupies 90% by volume of the whole.

[0007] The gas generation causes leakage of liquid due to an increase in internal pressure of the battery, deformation and expansion of the battery, and an increase in internal resistance and deterioration of charge / discharge performance caused by a gap between the electrodes due to the pressure of the gas. It was happening. The present invention is to solve such a problem, in a non-aqueous electrolyte secondary battery using a carbon material in the negative electrode, to prevent the generation of gas by decomposition of the electrolyte solution or the like on the surface of the carbon material It is intended to prevent electrolyte leakage and battery expansion due to increase in battery internal pressure.

[0008]

In order to solve the above problems, in the non-aqueous electrolyte secondary battery of the present invention, an alkylating agent is added to the carbon material used for the negative electrode, and the surface of the carbon material is alkylated. It is a thing. Examples of the method for adding the alkylating agent to the carbon material include, for example, a method in which the alkylating agent and the carbon material are mixed and heat treated, a method in which the carbon material is applied to a metal foil, and then immersed in the alkylating agent and dried and heated. There is. When the alkylating agent is added to the carbon material, the alkylating agent reacts with the OH group or the like existing on the surface of the carbon material to chemically modify the surface of the carbon material. After the surface treatment with such an alkylating agent, a heat treatment is performed under predetermined conditions to cause a dehydration condensation reaction and alkylate the surface of the carbon material.

[0009]

In this structure, by treating the carbon material with the alkylating agent, the OH groups and the like existing on the surface of the carbon material can be alkylated to cover the surface of the carbon material with the inactive alkyl group. Then, by this, gas generation due to decomposition reaction of the electrolytic solution, the binder and the like at the time of charging is suppressed, and an increase in battery internal pressure can be prevented.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

A battery was manufactured by the following procedure. First,
Graphite powder and an alkylating agent as shown in (Table 1) were mixed at a weight ratio of 100: 10, dried by heating at 120 ° C, washed with water, dried at 100 ° C and pulverized to obtain a powder. And Then, the powder and the acrylic binder are mixed at a weight ratio of 100: 5, distilled water and ethanol are added to the mixture, the mixture is thoroughly wet-mixed, and dried in a warm air dryer at 100 ° C., A negative electrode mixture was obtained. This mixture was press-molded into a predetermined shape to give a negative electrode.

[0012]

[Table 1]

For the positive electrode, LiCoO _{2 as the} positive electrode active material was mixed with carbon black as a conductive agent at a weight ratio of 100: 3, and distilled water and ethanol were further added and wet mixed. An aqueous dispersion of polytetrafluoroethylene was added to this as a binder, and the mixture was further mixed and dried in a hot air dryer at 100 ° C. to obtain a positive electrode mixture. This positive electrode mixture was press-molded into a predetermined shape to obtain a positive electrode.

FIG. 1 shows a sectional view of a battery manufactured by using these positive and negative electrodes. A porous polypropylene separator 3 was arranged between the positive electrode 1 and the negative electrode 4. In addition, as a non-aqueous electrolyte, a solution prepared by dissolving 1 mol / liter of lithium hexafluorophosphate in a solvent mixture of ethylene carbonate and diethyl carbonate in an equal volume ratio is injected, and sealed with a sealing plate 5 to form a coin-type battery. did. Then, as shown in (Table 1), batteries A to E were made corresponding to each alkylating agent.

Further, as a comparative example, the same as above except that the alkylating agent was not added to the graphite powder and a negative electrode in which the graphite powder and the acrylic binder were mixed at a weight ratio of 100: 5 was used. A battery was produced and designated as Battery F.

Next, a charge / discharge cycle life test was conducted using these batteries A to F. Charge / discharge condition is current 2mA
Charged to a voltage of 4.1V with a current of 2mA and a voltage of 3.0V
It went to discharge.

The change in battery thickness before and after charge and discharge was measured, and the results are shown in (Table 1). Further, FIG. 2 shows a discharge curve of the 10th cycle of each battery.

As shown in (Table 1), in the battery F using graphite which was not subjected to the alkylation treatment, gas was generated during charging, and the battery thickness expanded by about 0.56 mm. On the other hand, in batteries A to E, the expansion of the thickness is 0.07 to 0.11 m.
It was in the range of m, and it was possible to suppress gas generation due to decomposition of the electrolytic solution on the graphite surface.

In addition, as shown in FIG.
In the discharge curve at the cycle, the average discharge voltage was about 0.05 V higher than that of the comparative battery F, and the discharge capacity was about 20% larger. This is because in the comparative battery, gas is accumulated between the electrode plates, which increases the internal resistance of the battery and deteriorates the discharge characteristics, whereas in the batteries A to E, the discharge characteristics are improved because no gas is generated. it is conceivable that.

Next, the amount of the alkylating agent added was examined. Trimethylmethoxysilane was used as the alkylating agent, and the amount of addition was in the range of 1 to 60 parts with respect to 100 parts of the graphite material, and a battery was prepared and the same charge and discharge test as above was performed.

FIG. 3 shows the relationship between the amount of trimethylmethoxysilane added, the change in battery thickness and the internal resistance of the battery at the 10th cycle. When the amount of alkylating agent added to the graphite material is 5% or more, the expansion of the battery after initial charging is about 0.08 m.
It was found that there was almost no expansion. Further, the average discharge voltage of the battery was as low as about 3.53V when the amount of addition was 0%, increased with the increase of the amount of addition, and reached 3.6V at 5%. However, when it exceeds 30%, the average discharge voltage gradually decreases again. Therefore, the addition amount of the alkylating agent is preferably in the range of 5% to 30%.

In this embodiment, the case where a system in which 1 mol / l lithium hexafluorophosphate is dissolved in an equal volume mixed solvent of ethylene carbonate and diethyl carbonate is used as the non-aqueous electrolyte has been described. As the solvent, other than these, carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, esters such as gamma-butyrolactone and methyl acetate may be used alone or by mixing one or more selected from them. Used,
Similar results were obtained when lithium perchlorate, lithium borofluoride, or lithium hexafluorophosphate was used as the solute.

In this embodiment, the coin type battery has been explained, but the cylindrical battery and the prismatic battery can also improve the discharge characteristics and prevent the deformation of the battery.

The carbon material may be natural graphite, coke or graphite made from petroleum pitch or coal pitch, or carbon material or graphite made from polymer resin.

Further, the alkylating agent may be other silane-based or titanate-based alkylating agents such as aminopropyltriethoxysilane and N-methylaminopropyltrimethoxysilane.

[0026]

As described above, in the non-aqueous electrolyte secondary battery of the present invention, an alkylating agent is added to the carbon material used for the negative electrode to alkylate the surface of the carbon material, so that it exists on the surface of the carbon material. An active OH group or the like can be alkylated to cover the surface with an inactive alkyl group. Therefore, it is possible to prevent the electrolytic solution and the like from being decomposed at the active sites on the surface of the carbon material to generate gas, and it is possible to prevent the internal pressure and internal resistance of the battery from increasing.

[Brief description of drawings]

FIG. 1 is a sectional view of a coin-shaped non-aqueous electrolyte secondary battery used in this example.

FIG. 2 is a diagram showing a discharge curve at the 10th cycle of a battery.

FIG. 3 is a graph showing the relationship between the amount of alkylating agent added, the battery thickness, and the average discharge voltage.

[Explanation of symbols]

 1 Positive electrode 2 Case 3 Separator 4 Negative electrode 5 Sealing plate 6 Gasket

Front page continuation (72) Inventor Akikatsu Morita 1006, Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (2)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a negative electrode using a carbon material capable of inserting and extracting lithium, a positive electrode, and a non-aqueous electrolyte, and having a carbon material surface alkylated. 2. An alkylating agent is added to the carbon material in an amount of 5 to 30.
The non-aqueous electrolyte secondary battery according to claim 1, which is added by weight%.