WO2001056099A1

WO2001056099A1 - Electrode for lithium cell and lithium secondary cell

Info

Publication number: WO2001056099A1
Application number: PCT/JP2001/000397
Authority: WO
Inventors: Yoichi Domoto; Hiromasa Yagi; Hisaki Tarui; Hiroaki Ikeda; Masahisa Fujimoto; Hiroshi Kurokawa; Shin Fujitani; Shigeki Matsuta
Original assignee: Sanyo Electric Co., Ltd.
Priority date: 2000-01-25
Filing date: 2001-01-22
Publication date: 2001-08-02
Also published as: AU2001227085A1; JP2005235397A

Abstract

An electrode for a lithium cell containing an active material which occludes and releases lithium, characterized in that an amorphous material such as an amorphous silicon containing at least one impurity selected from among carbon, oxygen, nitrogen, argon and fluorine is used as the active material.

Description

Description Electrode for lithium battery and lithium secondary battery Technical field

The present invention relates to a lithium battery electrode, a lithium battery using the same, and a lithium secondary battery. Background art

In recent years, lithium secondary batteries, which are being actively researched and developed, greatly affect battery characteristics such as charge / discharge voltage, charge / discharge cycle life characteristics, and storage characteristics depending on the electrodes used. For this reason, the battery characteristics are being improved by improving the electrode active material.

When lithium metal is used as the negative electrode active material, a battery having a high energy density per weight and per volume can be formed.However, there is a problem that lithium is deposited in a dendrite shape during charging and causes an internal short circuit. there were

On the other hand, lithium secondary batteries that use aluminum, silicon, tin, etc., which are electrochemically alloyed with lithium during charging, as electrodes have been reported (Solid State Ionics, 1). 13-115, p57 (1998)). Of these, silicon is particularly promising as a negative electrode for batteries that has a large theoretical capacity and a high capacity, and various secondary batteries using this as a negative electrode have been proposed (Japanese Patent Laid-Open No. 10-25). No. 768 publication). However, in this type of alloy negative electrode, sufficient cycle characteristics have not been obtained because the alloy itself, which is an electrode active material, is pulverized by charging and discharging and the current collecting characteristics are deteriorated.

The present applicant has formed on a metal foil such as a copper foil by a plasma CVD method or the like. By using the amorphous silicon thin film as the negative electrode for a lithium secondary battery, it has been found that a lithium secondary battery having a high charge / discharge capacity and excellent charge / discharge cycle characteristics can be obtained (Japanese Patent Application No. Hei 1 (1994)). 13 0 1 6 7 9). Disclosure of the invention

An object of the present invention is to provide an electrode for a lithium battery using an amorphous material as an active material, and a lithium secondary battery having excellent charge / discharge cycle characteristics when used as an electrode for a secondary battery. An object of the present invention is to provide a battery electrode and a lithium battery and a lithium secondary battery using the same. The lithium battery electrode of the present invention is a lithium battery electrode including an active material that inserts and extracts lithium. It is characterized in that an amorphous material containing at least one kind of impurity selected from oxygen, nitrogen, argon, and fluorine is used as an active material.

In the present invention, the amorphous material preferably contains at least silicon, and more preferably, is microcrystalline silicon or amorphous silicon. Microcrystalline silicon, in Raman spectroscopic analysis, the crystal region corresponding 5 2 0 cm - ¹ and the vicinity of the peak, 4 8 0 cm corresponding to the amorphous region - ¹ both peaks of near neighbor is substantially The silicon to be detected. In addition, in the amorphous silicon, a peak near 520 cm ¹ corresponding to the crystalline region was not substantially detected in the Raman spectroscopic analysis, and a peak near 480 cm− ¹ corresponding to the amorphous region was not detected. Silicon whose peak is substantially detected.

In the present invention, the carbon concentration in the silicon is preferably less than 50 atomic%, more preferably 3.0 atomic% or less, and further preferably 2.0 atomic% or less. When carbon is contained, the lower limit of the carbon concentration is preferably 0.0002 atomic% or more. Also, the oxygen concentration Is preferably less than 67 atomic%, more preferably 50 atoms. /. The content is more preferably 20 atomic% or less. When oxygen is contained, the lower limit of the oxygen concentration is preferably 0.002 at% or more. The nitrogen concentration is 57 atoms. /. Is preferably less than 2, more preferably 2 atoms. /. Hereinafter, more preferably 0.1 atom. /. It is as follows. When oxygen is contained, the lower limit of the oxygen concentration is preferably not less than 0.0000 atomic%. The argon concentration in the silicon is ⁰ / atom. Preferably, the fluorine concentration is 0.01 atom. Les, preferably less than / ₀ . The concentrations of carbon, oxygen, nitrogen, argon, and fluorine can be measured by secondary ion mass spectrometry (SIMS). The number of atoms measured by SIMS by a dividing by 5 X 1 0 ²² can be converted to atomic percent.

In the present invention, the microcrystalline silicon or the amorphous silicon is preferably a silicon thin film. As a method for incorporating the above-described impurities into such a silicon thin film, a method similar to the method of doping impurities into a general semiconductor thin film can be used. For example, an impurity source gas may be mixed with a silicon thin film source gas such as a silane gas, and a silicon thin film may be formed by a CVD method such as a plasma CVD method, and impurities may be contained in the silicon thin film. After the formation of the silicon thin film, an impurity may be contained by a method such as an ion implantation method. Alternatively, a silicon thin film containing impurities may be formed by sputtering or the like, using silicon containing impurities in advance as a target or the like.

The silicon thin film is preferably provided on a current collector. As a method of providing a silicon thin film on a current collector, a current collector is used as a substrate, and a silicon thin film is formed on the current collector by a thin film forming method such as a CVD method, a sputtering method, a vacuum evaporation method, or a thermal spraying method. Is formed. As the current collector, it is preferable to use a current collector made of at least one selected from copper, nickel, iron, stainless steel, molybdenum, tungsten, and tantalum. As a form of the current collector, a metal foil is preferable. For example, a copper foil such as a rolled copper foil and an electrolytic copper foil can be used as the current collector.From the viewpoint of increasing the adhesion of the silicon thin film to the current collector, a copper foil having a large surface roughness Ra is used. Certain electrolytic copper foils are preferably used.

In the present invention, the term "lithium battery" includes a lithium primary battery and a lithium secondary battery. Therefore, the electrode of the present invention may be used for a lithium primary battery and a lithium secondary battery. it can..,

A lithium battery of the present invention includes a negative electrode comprising the above-described electrode for a lithium battery of the present invention, a positive electrode, and an electrolyte.

A lithium secondary battery of the present invention includes a negative electrode comprising the above-described electrode for a lithium battery of the present invention, a positive electrode, and a non-aqueous electrolyte.

Solvents for the electrolyte used in the lithium secondary battery of the present invention are not particularly limited, but cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, and dimethyl carbonate, methyl ethyl carbonate, and getyl carbonate. And a mixed solvent of the cyclic carbonate and an ether-based solvent such as 1,2-dimethoxetane and 1,2-dietoxetane. As the solute of the electrolyte, L i PF, L i BF 4, L i CF 3 S 0 3, L i N (CF 3 S_〇 _{₂₎ 2,} L i N

(C ₂ F ₅ S0 ₂ ) ₂ , L i N (CFS〇 ₂ ) (C ₄ F _¾ S 0 ₂ ), L i C (CFS 0 ₂ ) ₃ , L i C; (C ₂ F ₅ S) And mixtures thereof. Furthermore, examples of the electrolyte include a gel-like polymer electrolyte obtained by impregnating an electrolyte with a polymer electrolyte such as polyethylene oxide and polyacrylonitrile, and an inorganic solid electrolyte such as LiI and LiN. Is shown. The electrolyte of the lithium secondary battery of the present invention can be used as long as the Li compound as a solvent that develops ionic conductivity and the solvent that dissolves and retains the Li compound do not decompose during charging or discharging or storage of the battery. It can be used without any restrictions.

As the positive electrode active material of the lithium secondary battery of the present invention, L i C O_〇 _2, L i N i 〇 _{_2,} L i M n _2, and i Mn_〇 _{_{2, L i C o os N}} i o. 5 O ₂ , L i N i. CO o M n o. 0 ₂ lithium-containing transition metal oxides such as and, metal oxides containing no lithium such as Mn 0 ₂ are exemplified. In addition, any other substance capable of electrochemically inserting and removing lithium can be used without limitation. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram showing a distribution of an impurity concentration in a depth direction in a negative electrode manufactured in Example 1 according to the present invention.

FIG. 2 is a diagram showing a distribution of an impurity concentration in a depth direction in a negative electrode manufactured in Example 2 according to the present invention.

FIG. 3 is a diagram showing a distribution of an impurity concentration in a depth direction in a negative electrode manufactured in Example 3 according to the present invention.

FIG. 4 is a diagram showing a distribution of an impurity concentration in a depth direction in a negative electrode manufactured in Example 4 according to the present invention.

FIG. 5 is a perspective view showing a lithium secondary battery produced in an example according to the present invention.

FIG. 6 is a schematic cross-sectional view showing a lithium secondary battery produced in an example according to the present invention. BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited to the following Examples at all, and can be implemented with appropriate modifications within the scope of the gist of the present invention. ,.,

(Experiment 1)

(Preparation of electrode)

A copper foil (thickness: 18 μπι) was used as a substrate, and a plasma CVD method was used to measure the carbon, oxygen, nitrogen, argon, or fluorine concentrations shown in Tables 1 to 5 on this copper foil. A silicon thin film was formed. It is a silicon thin film formed of the material gas, a silane (S i Η ₄₎ gas, - is the carrier gas using a hydrogen gas, during the formation of the silicon thin film to contain carbon, When forming a silicon thin film in which CH ₄ gas is mixed in the source gas and oxygen is present, co ₂ gas is mixed in the source gas and in forming a silicon thin film containing nitrogen, the raw material gas is mixed. N ₂ gas _c also is mixed, at the time of formation of the silicon thin film to contain argon, the raw material gas is mixed a r gas, during the formation of the silicon thin film to contain fluorine, the raw material gas Was mixed with CF ₄ gas.

The film forming conditions, S i the gas flow rate: 1 0~ 1 0 0 sccm, H 2 gas flow rate: 0~ 2 0 0 sccm, substrate temperature: 1 8 0 ^e C, reaction pressure: 4 0 P a, the high-frequency power : 1 0 0 and 5 5 5 W, CH ₄ gas flow rate: () ~ 1 0 0 sccm , C 0 2 gas flow rate: 0 1 0 0 sccm, N ₂ gas flow rate: 0: 1 0 0 sccm, Ar gas flow rate: 0 to 100 sccm, CF ₄ gas flow rate: 0 to 100 sccm, and impurity concentrations as shown in Tables] to 5 were obtained. An amorphous silicon thin film was deposited on copper foil under the above conditions until the film thickness was 2 μm:

Each copper foil on which a silicon thin film was formed as described above was cut into a size of 2 cm × 2 cm to prepare an electrode. [Measurement of charge / discharge characteristics S:]

Using each of the above electrodes as a working electrode, a test cell was fabricated in which the counter electrode and the reference electrode were metallized. As an electrolytic solution, an equal volume mixed solvent of ethylene carbonate and di E chill carbonate used was the L i PF ₆ dissolved 1 mol / l. In a unipolar test cell, the reduction of the working electrode is charged and the oxidation is discharge.

Each of the above test cells was charged at a constant current of 0.5 mA at 25 ° C until the potential with respect to the reference electrode reached 0 V, and then discharged until the potential reached 2 V . This was defined as one cycle of charge and discharge, and the charge and discharge efficiency at the first cycle and at the fifth cycle was measured. The results are shown in Tables 1 to 5. table 1

Sample Ash degree ^ Charge / discharge efficiency (%)

No. (atomic%) 1st cycle 5th cycle

1 0.4 85 97

2 0.8 8 97

3 1 .0 89 99

4 1 .0 94 93

5 2.0 87 99

6 50 40

Table 2

Table 3

Sample Nitrogen concentration Charge / discharge efficiency (%)

No. (atomic%) 1st cycle 5th cycle

1 3 0.02 95 97

1 4 0.02 94 93

1 5 0.08 85 97

1 6 0.08 89 99

1 7 0. 1 0 87 99

1 8 57 5

Table 4

Table 5

As shown in Table 1, when the carbon concentration is less than 50 atomic%, good charge / discharge characteristics are obtained, and particularly when the carbon concentration is 2.0 atomic% or less, good results are obtained.

Further, as shown in Table 2, when the oxygen concentration was less than 67%, good charge / discharge characteristics were obtained, and particularly when the oxygen concentration was 50 atomic% or less, good results were obtained.

Further, as shown in Table 3, when the nitrogen concentration is less than 57%, good charge / discharge characteristics are obtained. In particular, when the nitrogen concentration is 0.1 atomic% or less, good results are obtained.

In addition, as shown in Table 4, good charge / discharge was achieved at an Ar concentration of 1 atomic% or less. Electrical characteristics are obtained.

Further, as shown in Table 5, good charge / discharge characteristics were obtained at a fluorine concentration of 0.01 atomic% or less.

In the fifth cycle of Sample No. 6 and Sample No. 18, charging could not be performed, and thus charge / discharge efficiency was not determined.

(Experiment 2)

(Preparation of negative electrode)

Next, as a current collector, a rolled copper foil (thickness: 26 μπι) whose surface was roughened by electrolytically depositing copper on the surface was used. A silicon thin film was formed by using a die-type RF sputtering apparatus: The silicon thin film was formed by using only argon gas as an atmosphere gas for sputtering and changing the flow rate of argon gas. Other forming conditions were as shown in Table 6. As targets, the negative electrodes of Examples 1 to 4 were manufactured using a 99.99% silicon single crystal. The thickness of the silicon thin film was set at about 6 to 1 l / m. In Example 4, the thin film formation process was started at the ultimate pressure of 10 _ ^{: i} Torr, whereas in Examples 1 to 3, vacuum evacuation was performed to 1 () ^—H Torr. went.

As a result of Raman spectroscopic analysis of the formed silicon thin film, a peak near 520 cm 1 'was not observed, and a broad peak was observed near 480 cm- ^1. It was confirmed that it was a silicon thin film.

Furthermore, the impurity concentration (carbon, nitrogen, oxygen) of each sample was measured by SIMS. The measurement results are shown in Figs. From these figures, it can be seen that each impurity has the highest concentration near the current collector surface and gradually decreases toward the surface in the silicon thin film. Also the atom. /. Table 6 shows the converted values. From these results, extremely large amounts were obtained under these conditions. It can be seen that the impurities of (a) are not mixed in the silicon thin layer.

The silicon thin film was limitedly formed in a 2.5 cm × 2.5 cm area on the copper foil using a mask. After the thin film was formed, the negative electrode tab was mounted on the copper foil area where the silicon thin film was not formed, and the negative electrode was completed.

(Preparation of positive electrode)

L i C o 0 ₂ powder 9 0 parts by weight, and the artificial graphite powder 5 by weight of a conductive material, polytetramethylene full O b ethylene of 5% by weight, including 5 parts by weight N- Mechirupirori Don as a binder It was mixed with an aqueous solution to obtain a positive electrode mixture slurry. This slurry was applied on a 2 cm × 2 cm area of an aluminum foil (thickness: 18 ^ m) as a positive electrode current collector by a doctor blade method, and then dried to form a positive electrode active material layer. A positive electrode tab was attached to the area of the aluminum foil on which the positive electrode active material layer was not applied to complete the positive electrode.

(Preparation of electrolyte solution)

In an equal volume mixed solvent of ethylene carbonate and dimethyl carbonate,

L i PF _B was dissolved 1 mole Z l to prepare an electrolytic solution, using the same in the production of the following cell.

(Production of battery)

FIG. 5 is a perspective view showing the manufactured lithium secondary battery. FIG. 6 is a schematic cross-sectional view showing the manufactured lithium secondary battery. As shown in FIG. 5, a positive electrode and a negative electrode are inserted into an exterior body 10 made of an aluminum laminate film. On the negative electrode current collector 11, a silicon thin film 12 as a negative electrode active material is provided, and on the positive electrode current collector 13, a positive electrode active material layer 14 is provided. The silicon thin film 12 and the positive electrode active material layer 14 are arranged to face each other with the separator 15 interposed therebetween. The electrolytic solution 16 described above is injected into the exterior body 10. The end of the exterior body 10 is sealed by welding. The negative electrode tab 17 attached to the negative electrode current collector 11 having the sealing portion 10a formed therein is taken out through the sealing portion 10a. Although not shown in FIG. 6, the positive electrode tab 18 attached to the positive electrode current collector 13 is also taken out through the sealing portion 10a similarly.

[Charge / discharge cycle test]

A charge / discharge cycle test was performed on the lithium secondary battery manufactured as described above. The charge and discharge conditions are as follows: charge until the charge current reaches 9 mAh, then discharge at a discharge current of 9 mA until the discharge end voltage reaches 2.75 V, which is defined as one cycle of charge and discharge. The discharge capacity and charge / discharge efficiency at the 1st, 5th, and 20th cycles were determined for each battery. Table 6 shows the results.

As is evident from the results shown in Table 6, under these conditions, a high discharge capacity and good charge / discharge efficiency were obtained. Therefore, with the impurity concentration in this example, no decrease in the discharge capacity and charge / discharge efficiency was observed, indicating that the electrode has characteristics as an electrode for a lithium secondary battery.

Table 6

The invention's effect

According to the present invention, a lithium secondary battery having excellent charge / discharge cycle characteristics can be obtained.

Three

Claims

The scope of the claims

I. In a lithium battery electrode containing an active material that absorbs and releases lithium, an amorphous material containing at least one impurity selected from carbon, oxygen, nitrogen, argon, and fluorine is used as the active material. An electrode for lithium batteries that features

2. The electrode for a lithium battery according to claim 1, wherein the amorphous material contains at least silicon.

3. The electrode for a lithium battery according to claim 1, wherein the amorphous material is microcrystalline silicon or amorphous silicon.

4. The carbon concentration is 50 atoms. /. The electrode for a lithium battery according to any one of claims 1 to 3, wherein

5. The electrode for a lithium battery according to any one of claims 1 to 4, wherein the oxygen concentration is less than 67 atomic%.

6. The electrode for a lithium battery according to any one of claims 1 to 5, wherein the nitrogen concentration is less than 57 atomic%.

7. The electrode for a lithium battery according to any one of claims 1 to 6, wherein the argon concentration is 1 atomic% or less.

8. The lithium battery electrode according to any one of claims 1 to 7, wherein the fluorine concentration is 0.01 atomic% or less.

9. The electrode for a lithium battery according to any one of claims 3 to 8, wherein the microcrystalline silicon or the amorphous silicon is a silicon thin film.

10. The electrode for a lithium battery according to claim 9, wherein the silicon thin film is provided on a current collector.

II. The silicon thin film is formed by plasma CVD, shattering, The electrode for a lithium battery according to claim 10, wherein the electrode is formed by a vacuum evaporation method or a thermal spraying method.

12. The lithium battery according to claim 10 or 11, wherein the current collector is at least one selected from copper, nickel, iron, stainless steel, molybdenum, tungsten, and tantalum. Electrodes.

13. A lithium battery comprising a negative electrode comprising the electrode according to any one of claims 1 to 12, a positive electrode, and an electrolyte.

14. A lithium secondary battery comprising a negative electrode comprising the electrode according to any one of claims 1 to 12, a positive electrode, and a non-aqueous electrolyte.