JP3052761B2 - Non-aqueous 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 surface treatment of a non-aqueous electrolyte secondary battery, particularly a carbon material used for its negative electrode.

[0002]

2. Description of the Related Art A non-aqueous electrolyte secondary battery using lithium or a lithium compound as a negative electrode is expected to have a high voltage and a high energy density, and has been actively studied.

Heretofore, 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. Recently, higher energy density 4
Li as a positive electrode active material for volt class non-aqueous electrolyte secondary batteries
Research on Mn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , LiFeO _{2 and the} like is being 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 instead of metallic lithium in view of safety and rate characteristics. In particular, graphite powder having a high degree of graphitization has the characteristics of high capacity, a discharge potential of about 0.1 V noble compared to lithium metal, and a small decrease in battery voltage, and has been actively studied. ing.

[0005]

However, carbon materials, particularly graphite powder having a high degree of graphitization, absorb and release lithium during charge and discharge, and generate gas from the surface of the powder particles. Regarding 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 point, and an electrolytic solution, an organic binder used for the negative electrode, and the like are decomposed, and gas is generated at this time.

The generated gas species are mainly hydrogen and carbon monoxide, and a small amount of acetylene, methane or the like is detected. In particular, hydrogen gas accounts for 90% by volume of the whole.

[0007] The gas generation causes liquid leakage due to an increase in the internal pressure of the battery, deformation and expansion of the battery, and a rise in internal resistance and a decrease in charge / discharge performance due to the formation of a gap between the electrodes due to the gas pressure. Was happening. The present invention has been made to solve such a problem, and in a nonaqueous electrolyte secondary battery using a carbon material for a negative electrode, preventing generation of gas due to decomposition of an electrolyte or the like on the surface of the carbon material. It prevents leakage of electrolyte and expansion of the battery due to an increase in battery internal pressure.

[0008]

In order to solve the above-mentioned problems, a non-aqueous electrolyte secondary battery according to the present invention comprises an alkylating agent added to a carbon material used for a negative electrode, and the surface of the carbon material is alkylated. Things. Examples of the method of adding the alkylating agent to the carbon material include a method of mixing and heat-treating the alkylating agent and the carbon material, a method of applying the carbon material to a metal foil, immersing the carbon material in the alkylating agent, and drying and heating. There is. When the alkylating agent is added to the carbon material, the alkylating agent reacts with an OH group or the like existing on the surface of the carbon material, and the surface of the carbon material is chemically modified. After the surface treatment with such an alkylating agent, when heat treatment is performed under predetermined conditions, a dehydration condensation reaction occurs and the carbon material surface is alkylated.

[0009]

In this configuration, by treating the carbon material with an alkylating agent, the OH group or the like existing on the surface of the carbon material is alkylated, and the carbon material surface can be covered with an inert alkyl group. As a result, gas generation due to a decomposition reaction of the electrolyte solution, the binder, and the like during 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 according to the following procedure. First,
A graphite powder and an alkylating agent as shown in Table 1 were mixed at a weight ratio of 100: 10, heated and dried at 120 ° C., washed with water, dried and pulverized at 100 ° C. And Then, the powder and the acrylic binder are mixed at a weight ratio of 100: 5, distilled water and ethanol are added thereto, mixed well by a wet method, and dried with a hot air dryer at 100 ° C. A negative electrode mixture was obtained. This mixture was press-molded into a predetermined shape to obtain a negative electrode.

[0012]

[Table 1]

The positive electrode was prepared by mixing carbon black as a conductive agent with LiCoO ₂ as a positive electrode active material at a weight ratio of 100: 3, further adding distilled water and ethanol, and wet-mixing. To this was added an aqueous dispersion of polytetrafluoroethylene as a binder, further mixed, and dried with a hot air drier 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 is a cross-sectional view of a battery manufactured using these positive and negative electrodes. A porous polypropylene separator 3 was disposed between the positive electrode 1 and the negative electrode 4. Further, as a non-aqueous electrolyte, a solution in which 1 mol / l of lithium hexafluorophosphate is dissolved in an isobaric mixed solvent of ethylene carbonate and diethyl carbonate is injected, and sealed with a sealing plate 5 to form a coin-shaped battery. did. Then, as shown in (Table 1), batteries A to E were made corresponding to each alkylating agent.

As a comparative example, the same procedure as described above was carried out except that an alkylating agent was not added to the graphite powder, and a graphite powder and an acrylic binder were mixed at a weight ratio of 100: 5. A battery was produced and this was designated as Battery F.

Next, a charge / discharge cycle life test was performed using these batteries A to F. Charge / discharge conditions are 2 mA current
To a voltage of 4.1 V, and a voltage of 3.0 V at a current of 2 mA.
The discharge was carried out.

The change in thickness of the battery before and after charging and discharging was measured, and the results are shown in Table 1. FIG. 2 shows a discharge curve at the tenth cycle of each battery.

As shown in Table 1, in battery F using graphite which had not been subjected to an alkylation treatment, gas was generated during charging, and the thickness of the battery expanded by about 0.56 mm. On the other hand, in the batteries A to E, the expansion of the thickness was 0.07 to 0.11 m.
m, and gas generation due to decomposition of the electrolytic solution on the graphite surface could be suppressed.

Further, 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 was accumulated between the electrode plates, thereby increasing the internal resistance of the battery and decreasing the discharge characteristics, whereas the batteries A to E did not generate gas, so the discharge characteristics were improved. it is conceivable that.

Next, the amount of the alkylating agent added was examined. A battery was prepared using trimethylmethoxysilane as an alkylating agent and the amount added was in the range of 1 to 60 parts with respect to 100 parts of the graphite material, and the same charge / discharge test was performed as described above.

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 tenth cycle. When the amount of the alkylating agent added to the graphite material is 5% or more, the expansion of the battery after the first charge is about 0.08 m.
m, and it was found that it hardly expanded. The average discharge voltage of the battery is as low as about 3.53 V when the addition amount is 0%, increases as the addition amount increases, and reaches 3.6 V at 5%. However, if 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, a case where a system in which 1 mol / l of lithium hexafluorophosphate is dissolved in an equal volume mixed solvent of ethylene carbonate and diethyl carbonate is used as the non-aqueous electrolyte is described. In addition to the solvent, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, carbonates such as ethyl methyl carbonate, gamma-butyrolactone, esters such as methyl acetate alone or a mixture of one or more selected from these. Use
Similar results were obtained when using lithium perchlorate, lithium borofluoride, and lithium hexafluorophosphate as solutes.

In this embodiment, a coin-shaped battery has been described. However, a cylindrical battery or a square battery could be similarly improved in discharge characteristics and prevented from being deformed.

The carbon material may be natural graphite, coke and graphite using petroleum pitch or coal pitch as a raw material, or carbon material or graphite using a polymer resin as a raw material.

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

[0026]

As described above, in the non-aqueous electrolyte secondary battery of the present invention, since the surface of the carbon material is alkylated by adding an alkylating agent to the carbon material used for the negative electrode, the non-aqueous electrolyte is present on the surface of the carbon material. An active OH group or the like can be alkylated to cover the surface with an inert alkyl group. For this reason, generation of gas due to decomposition of the electrolytic solution or the like at the active point on the surface of the carbon material can be suppressed, and an increase in the internal pressure and an increase in the internal resistance of the battery can be prevented.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a coin-type non-aqueous electrolyte secondary battery used in the present embodiment.

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

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

[Explanation of symbols]

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

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Akikatsu Morita 1006 Kazuma Kadoma, Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-5-114421 (JP, A) JP-A-6-106 168721 (JP, A) JP-A-3-167766 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/58 H01M 10/40

Claims (3)

(57) [Claims] 1. A negative electrode using a carbon material capable of inserting and extracting lithium, a positive electrode, and a non-aqueous electrolyte,
Silane or titanate organic compound
Non-aqueous electrolyte secondary battery that has been alkylated with a substance . 2. An alkylating agent is used in an amount of 5 to 3 with respect to the carbon material.
2. The non-aqueous electrolyte secondary battery according to claim 1, wherein 0% by weight is added. 3. An organic compound based on silane or titanate.
The product is trimethylmethoxysilane, aminopropyltri
Methoxysilane, aminoethyltrimethoxysilane, a
Sopropyl triacryl titanate, isopropyl tri
One or more selected from the group of methacryl titanates
The non-aqueous electrolyte secondary battery according to claim 1.