JPH10241666A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an irreversible side reaction on an electrode-electrolyte boundary face in a charging process, so as to obtain a secondary battery superior in a charge and discharge characteristics by providing a negative electrode, which can be doped with and un-doped from light metal ions and a positive electrode, and nonaqueous electrolytic solution whose electrolyte is light metal salt, and coating a positive electrode surface by a thin film. SOLUTION: A negative electrode 3, made of carbon material or the like which can be doped with an off-doped from light metal, lithium ions and a positive electrode 1 made of lithium cobalt composite oxide or the like, are laminated via a separator 6 of porous PP or the like, and the same is impregnated with a nonaqueous electrolytic solution. The nonaqueous electrolytic solution is prepared by dissolving a lithium salt, such as LiClO4 or the like as electrolyte into organic solvent such as propylene carbonate or the like. Next, by assembling via a negative electrode collector 4 and a positive electrode collector 5 using a gasket 7, a nonaqueous electrolyte secondary battery is obtained. The surface of the positive electrode 1 is coated by a thin film 2. The thin film 2 is preferably made of a carbonaceous material, such as a carbon fiber or the like of 0.1nm to 5μm is film thickness which is formed by plasma CVD method.

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 used as a power source for portable electronic equipment, and the like.
In particular, it relates to improvement of a positive electrode.

[0002]

2. Description of the Related Art In recent years, with the reduction in size and weight of various electronic devices such as video tape recorders and communication devices, there has been an increasing demand for secondary batteries having a high energy density as power sources for these devices.

A non-aqueous electrolyte secondary battery using a carbonaceous material as a negative electrode has a high voltage and a high energy density, and is therefore widely used as a power source for consumer electronic devices, and has been actively researched and developed. It has been done. In a non-aqueous electrolyte secondary battery using such a carbonaceous material for the negative electrode, LiC
A 4V secondary battery using a lithium-containing transition metal oxide such as oO ₂ or LiNiO ₂ for a positive electrode has been realized.

[0004] Secondary batteries using a lithium metal or lithium alloy for the negative electrode and a lithium composite oxide for the positive electrode are also being developed as next-generation secondary batteries.

However, at present, the above-mentioned non-aqueous electrolyte secondary battery does not yet have the charging / discharging characteristics that are expected from the theoretical capacity of the electrode. In these non-aqueous electrolyte secondary batteries, further improvement in charge / discharge characteristics is strongly desired with the expansion of portable electronic devices and the like.

[0006]

There are various possible causes for the charging / discharging characteristics not being as expected from the theoretical capacity of the electrode. However, irreversible side reactions at the electrode-electrolyte interface during the charging process are considered. It is considered as one of the causes.

Accordingly, the present invention has been proposed in view of such conventional circumstances, and suppresses irreversible side reactions at the electrode-electrolyte interface during the charging process, and provides a non-aqueous solution having excellent charge / discharge characteristics. An object of the present invention is to provide an electrolyte secondary battery.

[0008]

Means for Solving the Problems The inventors of the present invention have made intensive studies in order to solve the above-mentioned problems, and as a result, by coating the surface of the positive electrode with a thin film, the electrode-electrolyte interface in the charging process has been obtained. It has been found that side reactions are suppressed and charge / discharge characteristics are improved.

That is, the non-aqueous electrolyte secondary battery according to the present invention comprises a negative electrode and a positive electrode made of a material capable of doping and dedoping light metal ions, and a non-aqueous solution obtained by dissolving an electrolyte made of a light metal salt in a non-aqueous solvent. A nonaqueous electrolyte secondary battery comprising an electrolyte, wherein the surface of the positive electrode is covered with a thin film.

In the nonaqueous electrolyte secondary battery according to the present invention, since the positive electrode is coated with a thin film, a side reaction at the electrode-electrolyte interface during the charging process is suppressed, and the battery has excellent charge / discharge characteristics. Becomes

The above thin film has a thickness of 0.1 nm.
It is preferably about 5 μm. When the thickness of the thin film exceeds 5 μm, the effective resistance is large, and the battery characteristics are deteriorated. When the thickness of the thin film is less than 0.1 nm,
The effect of suppressing side reactions at the electrode-electrolyte interface during charging is small.

Preferably, the thin film is made of a carbonaceous material. Further, the thin film is preferably formed by a plasma enhanced chemical vapor deposition method.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a non-aqueous electrolyte secondary battery according to the present invention will be described in detail.

The non-aqueous electrolyte secondary battery according to the present invention comprises a negative electrode and a positive electrode made of a material capable of doping and undoping light metal ions, and a non-aqueous electrolyte obtained by dissolving an electrolyte made of a light metal salt in a non-aqueous solvent. Wherein the surface of the positive electrode is covered with a thin film.

In the nonaqueous electrolyte secondary battery according to the present invention, since the positive electrode is coated with a thin film, side reactions at the electrode-electrolyte interface during charging are suppressed, and the battery has excellent charge / discharge characteristics. Becomes

The thin film has a thickness of 0.1 nm.
It is preferably about 5 μm. When the thickness of the thin film exceeds 5 μm, the effective resistance is large, and the battery characteristics are deteriorated. When the thickness of the thin film is less than 0.1 nm,
The effect of suppressing side reactions at the electrode-electrolyte interface during charging is small. Therefore, the thickness of the thin film is from 0.1 nm to
It is preferably 5 μm, and more preferably about 1 μm from the viewpoint of working efficiency.

The thin film material for coating the positive electrode is not particularly limited, but is preferably an inorganic compound or an organic compound having ionic conductivity and an electronic conductivity of 10 ³ S / cm or less. , Preferably 10 ^-10 S / cm or less. If the electron conductivity is 10 ³ S / cm or more, an electrochemical reaction proceeds on the surface of the thin film, and it does not make sense to modify the surface of the positive electrode.

Examples of the thin film material include carbonaceous materials such as hydrocarbons [(CH) _n ] and fluorocarbons [(CF) _n ], ordinary transition metal oxides, and thionyl chloride [SO
Cl ₂ ]. Also,
Organic compounds having ion conductivity such as polyethylene oxide, polypropylene oxide, polyethylene succinate, polyethylene imine, and polyalkylene sulfide can be used. The same effect can be obtained by using any of them, but it is particularly preferable to use a carbonaceous material.

Further, the thin film is preferably formed by a plasma enhanced chemical vapor deposition method. The thin film obtained by plasma-enhanced chemical vapor deposition is a uniform thin film, a thin film that can be formed well around the unevenness of the substrate, and the thickness and film quality can be easily controlled by controlling the parameters. It is preferably used because it is a thin film that can be formed.

The positive electrode used in the nonaqueous electrolyte secondary battery according to the present invention is not particularly limited as long as it is a material capable of doping and undoping light metal ions.

For example, generally used transition metal oxides such as manganese dioxide and vanadium pentoxide,
Transition metal chalcogenides such as iron sulfide and titanium sulfide, and further Li _x MO ₂ (where M represents a transition metal such as Co, Ni or Mn and 0.5 ≦ x ≦ 1), or LiNi _p M1 _q M2 _r O ₂ (where M1 and M2 are Al,
At least one element selected from Mn, Fe, Ni, Co, Cr, Ti, and Zn, or a nonmetallic element such as P or B may be used. Furthermore, p + q + r = 1. ) Can be used. In particular, high voltage,
It is desirable to use a lithium-cobalt composite oxide or a lithium-nickel composite oxide because high energy density is obtained and cycle characteristics are excellent.

On the other hand, the negative electrode is not particularly limited as long as it is a material capable of doping and undoping a light metal.

For example, a material that can be doped with or dedoped with an alkali metal such as lithium and sodium or an alkali metal such as lithium during a charge / discharge reaction can be used. Examples of the latter include conductive polymers such as polyacetylene and polypyrrole, or pyrolytic carbons, cokes (petroleum coke, pitch coke, coal coke, etc.),
Carbon black (acetylene black, etc.), glassy carbon, organic polymer material fired body (organic polymer material 500
(A material baked in an inert gas stream at a suitable temperature of at least ℃ or in a vacuum) or a carbonaceous material such as carbon fiber. In particular, it is desirable to use a carbonaceous material because of its high energy density per unit volume.

The non-aqueous electrolyte is adjusted by appropriately combining an organic solvent and an electrolyte. Any of these organic solvents and electrolytes can be used as long as they are used for this type of battery.

For example, as the organic solvent, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran , 1,3-dioxolan, 4-methyl-1,3-
Examples include dioxolan, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, anisole, acetate, butyrate, and propioate.

As the electrolyte, LiClO ₄ , LiAs
F ₆ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , CH
₃ SO ₃ Li, CF ₃ SO ₃ Li, LiCl, LiBr and the like.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described based on experimental results.

Embodiment 1 FIG. 1 shows a coin-type secondary battery manufactured in this embodiment.

First, a disk-shaped positive electrode 1 was prepared as follows. Composition ratio Li / Ni of LiOH.H ₂ O and NiO
= 1 and calcined in dry air at 900 ° C for 5 hours to obtain lithium-nickel composite oxide LiNiO
Got _two . 90 parts by weight of this lithium composite oxide as a positive electrode active material, 7 parts by weight of graphite as a conductive agent, and 3 parts by weight of polyvinylidene fluoride as a binder are mixed, kneaded with dimethylformamide as a solvent, and dried to prepare a positive electrode mix. did. Then, 60 mg of this positive electrode mix was pressure-molded together with an aluminum mesh as a current collector, and a diameter of 1 mg was obtained.
A disk-shaped positive electrode 1 of 5.5 mm was obtained.

Thereafter, the surface of the positive electrode 1 was modified as follows. A parallel plate electrode made of stainless steel was placed in a reaction vessel consisting of a bell jar made of glass and a flange made of stainless steel so as to face each other, and the positive electrode 1 was placed on the cathode electrode. Then, 10 ml of ethylene gas was introduced into the reaction vessel.
In a state where the flow rate is set to be minute, while maintaining the pressure in the reactor at 0.08 Torr by exhaust, the acceleration voltage is 0.7 kV,
A direct current power of 7 mA was applied to the parallel plate electrodes, and a film was formed for one minute by a plasma chemical vapor deposition method. Thus, the positive electrode 1 whose surface was covered with the thin film 2 was obtained.

Next, a lithium foil having a thickness of 1.85 mm was punched into a disc shape, and this was used as a negative electrode 3. And the negative electrode 3
To the negative electrode current collector 4. Here, the amount of lithium is several hundred times the maximum charging capacity of the positive electrode 1, and does not limit the electrochemical performance of the positive electrode 1.

Next, the positive electrode 1 was placed on the positive electrode current collector 5, and a separator 6 made of a porous polypropylene film was placed on the positive electrode 1. Then, LiPF ₆ was added to a mixed solvent of propylene carbonate and dimethyl carbonate (volume ratio is 1: 1).
Is injected at a rate of 1 mol / l into the electrolyte,
The negative electrode current collector 4 to which the negative electrode 3 was crimped was mounted, and was caulked with a gasket 7 and sealed. Thereby, diameter 20m
Thus, a coin-type secondary battery (Example Battery 1) having a thickness of 2.5 mm and a thickness of 2.5 mm was obtained.

Example 2 The processing time by the plasma enhanced chemical vapor deposition was set to 2 minutes. Except for this, the coin-type secondary battery (Example Battery 2) was manufactured in the same manner as Example 1.

Example 3 The processing time by the plasma enhanced chemical vapor deposition was set to 5 minutes. Except for this, the coin-type secondary battery (Example Battery 3) was manufactured in the same manner as Example 1.

Example 4 LiOH.H ₂ O and CoO were mixed at a composition ratio of Li / Co = 1 and fired in dry air at 900 ° C. for 5 hours to obtain a lithium-cobalt composite oxide LiCoO _2. I got A coin-type secondary battery (Example Battery 4) was produced in the same manner as in Example 1 except that this was used as the positive electrode active material.

Comparative Example 1 The same positive electrode active material as in Example 1 was used, and the surface of the positive electrode 1 was not modified by plasma enhanced chemical vapor deposition. Except for this, the coin-type secondary battery (Comparative Battery 1) was manufactured in the same manner as in Example 1.

Comparative Example 2 The same positive electrode active material as in Example 4 was used, and the surface modification of the positive electrode 1 by plasma enhanced chemical vapor deposition was not performed. Except for this, the coin-type secondary battery (Comparative Battery 2) was manufactured in the same manner as in Example 4.

As a result of observing the positive electrodes of the batteries of Examples 1 to 4 manufactured as described above with an electron microscope, it was confirmed that all the positive electrodes were coated with a uniform thin film. The thickness of the thin film was 1 μm for Example Battery 1 and Example Battery 4, 2 μm for Example Battery 2, and 5 μm for Example Battery 3. As a result of analysis by X-ray photoelectron spectroscopy, it was confirmed that the thin film had a carbonaceous composition structure. In each case, the electron conductivity was 10 ⁻¹⁰ S / cm or less.

Charge / Discharge Test Conditions The charge / discharge tests of the fabricated Example batteries 1 to 4 and Comparative example batteries 1 and 2 were performed under the following conditions.

Charging is performed at 500 μA (current density 0.26 m
(A / cm ² ), constant-current charging was performed until the battery voltage reached 4.2 V, and then constant-voltage charging at 4.2 V was performed until the total charging time reached 20 hours. Discharging was performed until the battery voltage reached 2.5V.

Examination of Charge / Discharge Characteristics Table 1 shows the thickness, charge / discharge capacity and charge / discharge efficiency of the thin film of each battery obtained under the conditions of the charge / discharge test described above.

[0042]

[Table 1]

From the results shown in Table 1, it is understood that the batteries of Examples 1 to 3 are superior to the battery 1 of Comparative Example in both of the discharge capacity and the charge / discharge efficiency. Similarly, it can be seen that Example Battery 4 is superior to Comparative Example Battery 2 in discharge capacity and charge / discharge efficiency. On the other hand, the batteries of Examples 1 to 4 have a smaller charge capacity than the batteries of Comparative Examples 1 and 2. This is considered to be a result of suppressing the decomposition reaction at the time of charging such as decomposition of the electrolytic solution by coating the thin film. On the other hand, in the batteries 1 to 4 of the present invention, generation of a reactant on the electrode surface during charging is suppressed, and an increase in the internal resistance of the battery is suppressed.
The discharge capacity has been improved. Thus, Example batteries 1 to
In No. 4, the charge / discharge efficiency is improved due to the decrease in the charge capacity and the increase in the discharge capacity.

Therefore, in this coin-type secondary battery, the surface of the positive electrode is coated with a carbonaceous thin film,
Side reactions at the positive electrode-electrolyte interface can be suppressed, and the charge / discharge characteristics can be improved. In addition, Example batteries 1 to 4
It can be seen from the results that the above-mentioned effect is sufficient with a processing time of 1 minute, that is, a film thickness of 1 μm, regardless of the processing time, that is, the thickness of the thin film.

[0045]

As is clear from the above description, since the nonaqueous electrolyte secondary battery according to the present invention has a positive electrode coated with a thin film, a side reaction at the electrode-electrolyte interface at the time of charging is performed. Can be suppressed, and the charge / discharge characteristics can be improved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a configuration of a coin-type secondary battery to which the present invention is applied.

[Explanation of symbols]

1 positive electrode, 2 thin film, 3 negative electrode, 4 negative electrode current collector, 5
Cathode current collector, 6 separator, 7 gasket

Claims (5)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a negative electrode and a positive electrode made of a material capable of doping / dedoping a light metal ion, and a non-aqueous electrolyte obtained by dissolving an electrolyte made of a light metal salt in a non-aqueous solvent. A non-aqueous electrolyte secondary battery, wherein the surface of the positive electrode is covered with a thin film. 2. The thin film has a thickness of 0.1 nm to 5 μm.
The non-aqueous electrolyte secondary battery according to claim 1, wherein 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the thin film is made of a carbonaceous material. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the thin film is formed by a plasma chemical vapor deposition method. 5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the light metal is lithium.