JPH0982313A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the charging efficiency and discharge capacity of a battery in the first cycle and stabilize the charge/discharge cycle life by forming a silicon compound film at least on a part of a carbon material for forming a negative electrode. SOLUTION: A coin type battery is sealed by caulking inward the opening end part of a positive case 1 and fastening the outer circumference of a sealing plate 2 through a gasket 6. A negative electrode 4 is formed by covering a carbon material with a silicon compound, crushing, forming paste together with a binder, then applying the paste to a current collector. Or, the pasty mixture of the carbon material and the binder, or the pasty mixture of the carbon material, the binder, and an additive for enhancing current collecting effect is applied to the current collector, dried, then the silicon compound film is formed on the surface of the carbon material. The silicon compound decreases irreversible reaction, absorbs a nonaqueous electrolyte to swell, and covers the surface of the carbon negative electrode 4 as a lithium ion conductive film, without disturbing electrode reaction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery using a carbon material as a negative electrode, and an improvement of the negative electrode for the purpose of improving initial Coulombic efficiency and discharge capacity and stabilizing charge / discharge cycle life. Regarding

[0002]

2. Description of the Related Art Secondary non-aqueous electrolytes using lithium as a negative electrode active material, metal chalcogenides and metal oxides as a positive electrode active material, and various salts dissolved in an aprotic solvent as an electrolyte. Batteries have attracted attention as secondary batteries with high energy density and are being actively researched.

However, in a conventional non-aqueous electrolyte secondary battery, lithium as a negative electrode active material is deposited as dendrite-like lithium during repeated charging / discharging, which causes deterioration due to an internal short circuit or a side reaction between lithium and the electrolyte. Have problems such as.

In recent years, a carbon material has come to be used in a non-aqueous electrolyte secondary battery as a negative electrode with less defects as described above, and further, as a positive electrode active material, LiCoO ₂ or LiNi.
The use of O ₂ , LiMn ₂ O ₄ and composite oxides in which Co, Ni and Mn of these active materials are partially replaced with other metal elements are considered.

A non-aqueous electrolyte secondary battery using such a lithium-containing composite metal oxide such as LiCoO ₂ as a positive electrode and a carbon material capable of inserting and extracting lithium ions as a negative electrode has a high voltage of 4V. It can be discharged and has a high energy density.

These batteries can be discharged by assembling and charging them in a discharged state to release lithium ions from the lithium-containing composite metal oxide of the positive electrode and occlude them in the carbon material of the negative electrode.

[0007]

However, when a carbon material is used for the negative electrode and charging / discharging, the plateau of the potential around 0.8 V with respect to the lithium potential observed at the beginning of the first cycle charging. Due to the irreversible reaction, the charging efficiency in the first cycle is poor and the lithium ions of the positive electrode cannot be effectively used. Therefore, the energy density of the battery is lowered, and the generation of gas due to this irreversible reaction is observed, which adversely affects the charge / discharge cycle life.

[0008] From many studies, it is becoming clear that this irreversible reaction occurs because a film is formed on the surface of the carbon negative electrode by the decomposition of the solvent. However, it has not been clarified by what reaction the passive film is formed and what kind of chemical composition or structure it has.

On the other hand, it is considered that this reaction is caused by the reaction between the organic functional group (functional group such as --OH or --COOH) existing on the surface of the carbon material and lithium, and the organic residue is removed by heat treatment, reduction treatment or the like. Research is also being conducted.
(Kikuchi et al., Proc. Of the 33rd Battery Symposium, p193,
1993) (Komazawa et al., Proceedings of Autumn Meeting of the Electrochemical Society of Japan,
Therefore, the present invention provides a non-aqueous electrolyte solution comprising such a conventional chargeable / dischargeable positive electrode containing lithium ions and a negative electrode mainly composed of a carbon material that absorbs / releases lithium ions. It is an object of the present invention to solve the above problems of the secondary battery and to provide a non-aqueous electrolyte secondary battery in which the charging efficiency and the discharge capacity in the first cycle are improved and the charge / discharge cycle life is stable.

[0010]

A non-aqueous electrolyte secondary battery according to the first aspect of the present invention comprises a chargeable / dischargeable positive electrode containing lithium ions, and a negative electrode made of a carbon material that absorbs and releases lithium ions. A non-aqueous electrolyte secondary battery according to a second invention is characterized in that a film of a silicon compound is formed on at least a part of the carbon material. And a negative electrode made of a carbon material that absorbs and desorbs lithium ions, and a coating of a silicon compound is formed on at least a part of the negative electrode.

The present inventor has conducted studies using various carbon materials in order to clarify the irreversible reaction caused by the carbon negative electrode, and has revealed that the irreversible reaction is proportional to the specific surface area of the carbon negative electrode. Therefore, various resins were added to the negative electrode to cover the surface of the carbon material to reduce the specific surface area, and an attempt was made to solve the above problems.

As a result, in order to solve the above problems, it was found that a silicon compound is excellent as a coating material for a carbon negative electrode, and a non-aqueous electrolysis with stable initial Coulombic efficiency, improved discharge capacity, and stable charge / discharge cycle life. It has become possible to provide a liquid secondary battery.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The non-aqueous electrolyte battery according to the present invention comprises:
By covering the entire carbon material of the negative electrode with a silicon compound or by forming a film on a part thereof, and by forming a film of the silicon compound on the whole or a part of the surface of the negative electrode active material, the first cycle Charge efficiency and discharge capacity can be increased, and gas generation is reduced.

In addition, since peeling from the substrate does not occur even after the number of charge / discharge cycles has elapsed, stable cycle characteristics can be obtained.

In the examples, a carbon material is coated with a silicon compound, the carbon material is lightly crushed, and then made into a paste with a binder, and the paste is applied to a current collector to form a negative electrode.
Apply the paste-like carbon material and the binder or the carbon material and the binder and the additive for improving the current collecting effect to the current collector,
After drying, a silicon compound film may be formed on the surface of the carbon material.

Further, this method has an advantage that steps such as heat treatment and reduction treatment are not required, and the electrode can be manufactured very easily.

Conventionally, polyvinylidene fluoride, polyolefin, polyvinylpyrrolidone, various elastomers, etc., which have been used as a binder for carbon materials, have a high insulating property, and when used in a large amount to coat the surface of the carbon material, it becomes rather conductive. Deteriorates and charging / discharging becomes impossible.

However, unlike the conventional binders, the one in which a film is formed of a silicon compound as in the present invention does not deteriorate in conductivity and cannot be charged and discharged.

This is because the silicon compound reduces the irreversible reaction without hindering the electrode reaction, so that it absorbs the non-aqueous electrolyte and swells to coat the surface of the carbon negative electrode as a lithium ion conductive coating. It is considered that

The present invention will be described below with reference to preferred examples and comparative examples.

Example 1 FIG. 1 is a sectional view of a coin type non-aqueous electrolyte secondary battery according to one embodiment of the present invention.

In the figure, 1 is a positive electrode case which also functions as a positive electrode terminal made by punching out a stainless steel (SUS316) steel plate, 2
Is a sealing plate that also functions as a negative electrode terminal made by stamping a stainless steel (SUS316) steel plate.

Reference numeral 3 is a positive electrode, 4 is a negative electrode, and 5 is a separator made of microporous polypropylene impregnated with a non-aqueous electrolyte.

As the non-aqueous electrolyte, 150 ml of a solution prepared by dissolving lithium hexafluorophosphate at a concentration of 1 mol / liter in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 was poured. .

The coin-shaped battery is hermetically sealed by crimping the open end of the positive electrode case 1 inward and tightening the outer periphery of the sealing plate 2 via the gasket 6. The battery has a diameter of 20 mm and a height of 2.0 mm.

The positive electrode is a lithium cobalt composite oxide (L
iCoO ₂ ) and carbon powder as a conductive agent and fluororesin powder as a binder are sufficiently mixed in a weight ratio of 90: 3: 7, and N-methyl-2-pyrrolidone (NMP) is added in an appropriate amount to form a paste. Then, the thickness is about 2 on the stainless steel foil.
It is applied in a thickness of 50 μm, vacuum dried at a temperature of 200 ° C., rolled, and punched into a disc shape having a diameter of 16 mm. The electric capacity of the positive electrode used here was set to have a capacity sufficiently larger than the electric capacity of the negative electrode in order to perform a complete charge / discharge cycle test of the negative electrode.

For the negative electrode, artificial graphite powder was used as the carbon material. 2 parts by weight (2 parts by weight per 100 parts by weight of artificial graphite powder)
%) Allyltrimethylsilane (SiC ₆ H ₁₄ ) was added, and N, N dimethylformamide was added in an appropriate amount to form a paste, which was vacuum dried at a temperature of 150 ° C. After drying, it was lightly ground in a mortar to obtain a carbon material coated with allyltrimethylsilane.

To 102 parts of the carbon material coated with allyltrimethylsilane, 16 parts of polyvinylidene fluoride (PVdF) was mixed as a binder, and NMP was added in an appropriate amount to form a paste. It was applied to a thickness. After vacuum drying at 150 ° C., it was rolled and punched into a disk shape with a diameter of 16 mm to obtain a negative electrode. A battery using this negative electrode was designated as Battery A of the present invention.

On the other hand, as a conventional example to be compared with the examples of the present invention, a negative electrode was prepared in which a carbon material not treated with a coating agent was used and 16 parts of PVdF was used as a binder for 100 parts of the carbon material. . Other than that, the battery was constructed exactly as in the example of the present invention, and the battery G of the conventional example was obtained.

Example 2 A battery was prepared in the same manner as in Test Battery A of Example 1 except that chloromethyldimethylchlorosilane (SiC ₃ H ₈ Cl ₂ ) was used instead of allyltrimethylsilane as the coating material for the carbon material. Configured. The battery of Example 2 is referred to as test battery B.

[Example 3] Decamethylcyclopentasiloxane (Si ₅ C ₁₀ H ₃₀ O ₅ ) as a coating material for a carbon material
A battery was constructed in the same manner as in Example 1 except that was used.
The battery of Example 3 is referred to as test battery C.

[Example 4] A battery was constructed in the same manner as in Example 1 except that hexamethyldisilazane (Si ₂ C ₆ H ₁₉ N) was used as a coating material for the carbon material. The battery of Example 4 is referred to as test battery D.

[Example 5] 3-as a coating material for carbon materials
Isocyanatopropyltriethoxysilane (SiC ₁₀
A battery was constructed in the same manner as in Example 1 except that H ₂₁ NO ₄ ) was used. The battery of Example 5 is referred to as test battery E.

[Example 6] As a coating material for a carbon material (3
-Mercaptopropyl) methyldimethoxysilane (Si
A battery was constructed in the same manner as in Example 1 except that C ₆ H ₁₆ O ₂ S) was used. The battery of Example 6 is designated as test battery F.

The batteries A to G prepared in Examples 1 to 6 were charged to a terminal voltage of 4.1 V with a constant current of 1 mA, and then discharged to a terminal voltage of 2.75 V to determine the initial characteristics of the batteries. .

Table 1 shows the charge capacities, discharge capacities and initial Coulombic efficiencies at the first cycle for the batteries A to G. Here, the initial Coulombic efficiency was defined as the ratio of the discharge capacity to the charge capacity in the first cycle. The charge capacity and discharge capacity were converted to the capacity per unit weight of the carbon material used for the negative electrode and displayed.

[0037]

[Table 1]

Allyltrimethylsilane, decamethylcyclopentasiloxane, hexamethyldisilane,
3-isocyanatopropyltriethoxysilane, (3
-Mercaptopropyl) methyldimethoxysilane improves both discharge capacity and initial Coulombic efficiency,
It can be seen that the irreversible reaction is suppressed.

Next, the batteries A, B, C, D according to the present invention,
3. Using E, F and conventional battery G, at a current of 1 mA.
A charging / discharging cycle test of charging to 1 V and discharging to 2.75 V at a current of 1 mA was performed 100 times.
Table 2 shows the test results.

[0040]

[Table 2]

Each of the batteries according to the present invention had a lower internal resistance than the conventional battery G, little increase in the battery thickness, and stable charge / discharge characteristics were obtained.

On the other hand, in the conventional battery G, gas is generated due to decomposition of the non-aqueous electrolyte, and the thickness of the battery is increased due to increase in internal pressure. In addition, the contact inside the battery deteriorated, the internal resistance increased, and stable charge / discharge characteristics could not be obtained.

Example 7 For the negative electrode, artificial graphite powder was used as the carbon material. 0. 0 for 100 parts of artificial graphite powder.
05 to 20 parts (0.05 to 20%) of allyltrimethylsilane (SiC ₆ H ₁₄ ) was added, and N, N dimethylformamide was added in an appropriate amount to form a paste, which was vacuum dried at a temperature of 150 ° C. After drying, it was lightly ground in a mortar to obtain a carbon material coated with allyltrimethylsilane of various concentrations.

16 parts by weight of polyvinylidene fluoride (PVdF) was mixed as a binder with 100 parts by weight of the carbon material only, excluding the weight of allyltrimethylsilane.
Add an appropriate amount of NMP to make a paste, and apply about 200μ on the copper foil.
m. After vacuum drying at 150 ° C., it was rolled and punched into a disk shape with a diameter of 16 mm to obtain a negative electrode.

A button type battery was manufactured using the same positive electrode and battery structure as in Example 1 except for the negative electrode.

These batteries were charged with a constant current of 1 mA to a terminal voltage of 4.1 V and then discharged to a terminal voltage of 2.75 V to determine the initial characteristics of the batteries. Table 3 shows the charge capacity, discharge capacity, and initial Coulombic efficiency of the various batteries in the first cycle. The charge capacity and discharge capacity were converted to the capacity per unit weight of the carbon material used for the negative electrode and displayed.

[0047]

[Table 3]

When the amount of allyltrimethylsilane used in the coating material was 0.1% or more, both the discharge capacity and the initial Coulombic efficiency were improved, and the irreversible reaction of the negative electrode was suppressed. The effect is expected even if the addition amount of allyltrimethylsilane is 20% or more, but if the addition amount is large, the electrode volume increases and the energy density per volume decreases, so 20% or less is considered sufficient.

[0049]

The non-aqueous electrolyte secondary battery according to the first invention comprises a chargeable / dischargeable positive electrode containing lithium ions, and a negative electrode made of a carbon material that absorbs and releases lithium ions. A non-aqueous electrolyte secondary battery according to a second invention is characterized in that a silicon compound film is formed on at least a part of the carbon material, and a lithium ion-containing chargeable / dischargeable positive electrode and a lithium ion And a negative electrode made of a carbon material for occluding and releasing carbon dioxide, wherein a film of a silicon compound is formed on at least a part of the negative electrode.

Further, the silicon compound is at least 1 represented by Chemical formula 1, Chemical formula 2, Chemical formula 3, Chemical formula 4, Chemical formula 5, or Chemical formula 6.
It is a seed.

[0051]

[Chemical 1]

[0052]

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[0053]

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[0054]

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[0055]

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[0056]

However, in the above chemical formula, 1 ≦ A ≦ 5, 1
≦ B ≦ 48, 1 ≦ C ≦ 46, 1 ≦ D ≦ 4, 1 ≦ E ≦ 6,
F = 1 and 1 ≦ G ≦ 4.

According to this, it is possible to provide a non-aqueous electrolyte secondary battery having higher initial Coulomb efficiency and discharge capacity and less cycle deterioration than the conventional one.

In the examples of the present invention, allyltrimethylsilane, decamethylcyclopentasiloxane, hexamethyldisilazane, 3-isocyanatopropyltriethoxysilane and (3-mercaptopropyl) methyldimethoxysilane were used as the coating agent. , Ethynyltrimethylsilane (SiC ₅ H ₁₀ ), phenyldimethylsilane (SiC ₈ H ₁₂ ), phenylsilane (SiC ₆
H ₈ ), triethylsilane (SiC ₆ H ₁₆ ), trimethylvinylsilane (SiC ₅ H ₁₂ ), n based on Chemical formula 2
- butyldimethylchlorosilane (SiC ₆ H ₁₅ Cl),
n-butyltrichlorosilane (SiC ₄ H ₉ Cl ₃ ),
N-butyltrimethoxysilane (Si
C ₇ H ₁₈ O ₃ ) and dimethyldimethoxysilane (SiC ₄
H ₁₂ O ₂ ), 3-mercaptopropyltrimethoxysilane (SiC ₆ H ₁₆ O ₃ S) based on Chemical formula 4, N-trimethylsilylimidazole (SiC ₆ based on Chemical formula 5)
H ₁₂ N ₂ ), or N-phenylaminomethyltrimethoxysilane (SiC ₁₂ H ₁₇ NO ₃ ) based on Chemical formula 6 can be expected to have similar effects. It goes without saying that the compounds shown in Chemical formula 4 and Chemical formula 5 or Chemical formula 6 are not limited to the above silicon compounds. In addition, the same effect can be expected even with a mixture.

In the embodiment of the present invention, LiCoO ₂ is used for the positive electrode.
Was used, but lithium composite oxides such as LiNiO ₂ , LiMn ₂ O ₄ , LiTiS ₂ , LiV ₂ O ₅ , and LiM were used.
Similar effects can be obtained even when oO ₃ or the like is used.

As the carbon material for the negative electrode, artificial graphite heat-treated at a high temperature was used, but the carbon material is not limited to this, and various carbon materials such as low-temperature fired carbon and carbon fiber, or a mixture thereof may be used. Good.

The batteries in the examples are all coin type batteries, but the present invention is also applicable to cylindrical, prismatic or paper type batteries.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a coin type non-aqueous electrolyte secondary battery according to an embodiment of the present invention.

[Explanation of symbols]

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

Claims (3)

[Claims] 1. A chargeable / dischargeable positive electrode containing lithium ions, and a negative electrode made of a carbon material, which absorbs and releases lithium ions, and a silicon compound film is formed on at least a part of the carbon material. A non-aqueous electrolyte secondary battery characterized by the above. 2. A chargeable / dischargeable positive electrode containing lithium ions, and a negative electrode made of a carbon material that occludes / releases lithium ions, wherein a film of a silicon compound is formed on at least a part of the negative electrode. And a non-aqueous electrolyte secondary battery. 3. The silicon compound according to claim 1, wherein the silicon compound is at least one represented by the following chemical formula 1, chemical formula 2, chemical formula 3, chemical formula 4, chemical formula 5, or chemical formula 6. Non-aqueous electrolyte secondary battery. Embedded image Embedded image Embedded image Embedded image Embedded image [Chemical 6] However, in the above chemical formula, 1 ≦ A ≦ 5, 1 ≦ B ≦ 4
8, 1 ≦ C ≦ 46, 1 ≦ D ≦ 4, 1 ≦ E ≦ 6, F = 1,
1 ≦ G ≦ 4.