JPH06196154A - Lithium electrode for lithium secondary battery and manufacturing method thereof - Google Patents

Abstract

(57) [Abstract] [Purpose] To suppress the non-uniform growth of lithium metal in the charging reaction of a lithium secondary battery, and to enable the practical use of a lithium secondary battery having high energy density and high output density. [Structure] A fluorine-based organic polymer thin film having an electron-donating polar group was formed on a lithium surface whose surface was cleaned by sputtering. The fluorine-based organic polymer thin film can be deposited on the substrate placed in the plasma atmosphere by introducing a gaseous fluorine-based organic compound into the plasma atmosphere.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium electrode for a lithium secondary battery whose surface is modified with a fluorine-based organic polymer thin film and a method for producing the same. More specifically, for the purpose of improving the electrodeposition reaction process of lithium during the charging reaction of the lithium electrode of the lithium secondary battery, the lithium-based organic polymer thin film having lithium ion conductivity is formed on the lithium metal surface. electrode,
And its manufacturing method.

[0002]

2. Description of the Related Art Lithium is a metal that is lightweight and has the most base potential, and is most suitable as a negative electrode active material for high energy density batteries.

Conventionally, in order to improve the charging efficiency of lithium secondary batteries, various methods have been studied for obtaining a smooth electrodeposition surface in the electrodeposition process of lithium metal. Among these, a typical method is a method of mixing an additive in the electrolytic solution. For example, creating an ion conductive film on the electrode surface by a chemical reaction between the additive and lithium,
Adhesives are adsorbed on the electrode surface to suppress the formation of a film having low conductivity on the electrode. However, a smooth electrodeposition surface of lithium can be obtained only when the current density is limited, and thus far, the electrodeposition surface has not been sufficiently controlled.

[0004]

In the charging reaction of a lithium secondary battery, if a part of the lithium in the negative electrode deposited by charging is not used for discharging, not only the Coulombic efficiency lowers but also non-uniform growth occurs. Some of the lithium
There was a drawback that it penetrated through the separator and reached the positive electrode, causing a short circuit of the battery. In order to solve this problem, it is necessary to prevent uneven growth during electrodeposition of the lithium negative electrode.

[0005]

In order to suppress the non-uniform growth of lithium metal in the charging reaction of a lithium secondary battery, the present invention provides a lithium surface whose surface has been cleaned by sputtering with an electron-donating polar group. It is characterized in that a fluorine-based organic polymer thin film having is formed. The fluorine-based organic polymer thin film can be deposited on the substrate placed in the plasma atmosphere by introducing a gaseous fluorine-based organic compound into the plasma atmosphere.

[0006]

When the lithium negative electrode is charged, lithium metal is electrodeposited on the surface thereof. The reason why the electrodeposition occurs in a non-uniform state is that the electrode surface is in a non-uniform state.
Lithium metal is extremely active and reacts with a trace amount of impurities even in an inert atmosphere such as an argon dry box, and its surface is covered with a film of lithium oxide, lithium hydroxide, lithium carbonate or the like. It is considered that the initial process of non-uniform formation that occurs when the lithium negative electrode is charged is due to the non-uniform state of this film present at the electrode interface. Therefore, the problem can be solved by removing this non-uniform coating by sputtering and covering the electrode with a uniform and stable coating. Further, the coating film to be coated needs to have conductivity with respect to lithium ions.

As a surface film which meets the above conditions, there is a polymerized film formed by plasma CVD. Plasma CVD is a method of depositing a gas of an organic compound as a polymer on a substrate in a non-equilibrium plasma, and is suitable for depositing a uniform, pinhole-free, and highly crosslinked organic thin film. A fluorine-based organic polymer film is suitable as a film that is chemically stable against lithium. By introducing a gaseous fluorine-based organic compound into the plasma atmosphere, a fluorine-based organic polymer thin film can be deposited on the substrate by plasma polymerization. The fluorine atom in the fluorine-based organic polymer is a weak electron-donating polar group, and can coordinate with lithium ions to cause ion dissociation.

As the fluorine-based organic compound, 1,1 difluoroethene or vinyl fluoride is preferable. By introducing these alone or a mixture together with plasma assist gas such as argon into a plasma CVD apparatus, it is possible to polymerize and deposit as a fluorine-based organic polymer on a substrate placed in a plasma atmosphere.

[0009]

EXAMPLES Example 1 FIG. 1 shows a schematic view of a plasma CVD apparatus. 1 is an argon dry box, 2 is a glass bell jar, 3 is a vacuum exhaust system, 4 is an introduction system of argon as a starting material and plasma assist gas, 5 is a parallel plate type internal electrode, 6 is a high frequency power supply, and 7 is during plasma CVD. 2 shows the substrate holder of FIG.

First, the oxide film existing on the surface of the lithium foil used as the substrate was removed by sputtering with argon gas introduced. After that, plasma CVD using 1,1-difluoroethene (CH ₂ = CF ₂ ) as a starting material was performed under the following conditions. Monomer flow rate: 10 cm ³ (STP) min ^-1 Discharge power: 20 W Discharge frequency: 13.56 MHz Electrode distance: 6 cm Pressure: 0.5 Torr By this operation, a thin film of a fluorine-based organic polymer was formed on the lithium surface. The fluorine atom of this thin film is a weak electron-donating polar group, which can be ion-dissociated by coordinating with a lithium ion,
Acts as a polymer solid electrolyte.

FIG. 2 shows an FT-IR spectrum of a polymerized thin film obtained by plasma CVD on a lithium substrate. The exact structure of the product is not clear, but the starting material, 1,1-
Comparing with the IR spectrum of difluoroethene, it was found that the olefin part disappeared and the absorption of CH stretching vibration was weakened.

FIG. 3 shows the ESCA (X-ray photoelectron spectroscopy) C _1S spectrum of the polymerized thin film. From the highest binding energy, the peak due to CF ₃ , the peak due to CF ₂ , C = O 2, CF
Due to C-CF _x . C
Due to the small peak areas of F ₃ and CF _2, the large peak areas of CF and C-CF _x , and the desorption of hydrogen in the FT-IR spectrum, the thin film obtained by plasma CVD was not crosslinked. It seems to be progressing. Moreover, when the ratio of fluorine to carbon in the polymerized thin film was obtained by ESCA, the F / C value was 0.66, which was a decrease compared to the F / C value of the monomer of 1. It was also found that some of them were desorbed during plasma polymerization.

The critical surface energy of the thin film, which was determined by the Zisman plot by the droplet method, was 20 dyncm ^-1 , which was an intermediate value between that of the perfluorocarbon polymer and that of the vinyl fluoride polymer. The thin film obtained by plasma CVD is considered to be a fluorine-based organic polymer.

SEM of cross section of polymerized thin film by plasma CVD
When observed with a (scanning electron microscope), the thin film with a polymerization time of 15 minutes had a uniform surface with a thickness of 0.7 μm and no pinholes.

The conductivity of the polymerized thin film prepared by the plasma CVD method was measured. The conductivity of the polymerized thin film was about 3 × 10 ⁻³ S / cm at 30 ° C. in the electrolyte solution of propylene carbonate in which 1M concentration of lithium perchlorate was dissolved.

Lithium was deposited on the lithium electrode on which this thin film was formed, and the morphology of the electrode surface was observed by SEM. In the propylene carbonate electrolyte, 0.
An electric quantity of 1 coulomb / cm ^{2 was} applied at a current of 5 mA / cm ² . When the electrodeposited lithium was observed by SEM photographs, it was shown that the surface of the electrode was uniformly covered with fine mossy lithium, which could prevent non-uniformity of lithium electrolytic deposition. Similarly, an electrodeposition test was conducted using lithium metal without a thin film formed by plasma CVD. In the case of lithium metal without a thin film, lithium was deposited in the form of dendrite, and the surface irregularities became large.

Lithium deposited uniformly in the charging reaction is uniformly dissolved in the discharging reaction, so the lithium electrode of the present invention is most suitable as a lithium electrode for a lithium secondary battery. Example 2 A thin film was prepared according to Example 1 using vinyl fluoride (CH ₂ = CHF) as a starting material under the following plasma CVD conditions. The substrate is lithium metal. Monomer flow rate: 10cm ³ (STP) min ^-1 Discharge power: 10W Discharge frequency: 10MHz Electrode distance: 5cm Pressure: 0.3Torr The polymerized thin film obtained is uniform and pinhole-free,
From FT-IR measurement, it was found that the polymerization proceeded by the plasma-initiated polymerization in which the double bond of the starting material was cleaved. The conductivity of the thus-formed thin film was measured in a 1 M solution of lithium perchlorate in propylene carbonate and found to be 5 × 10 ⁻⁸ S / cm at 25 ° C. In the same manner as in Example 1, using a lithium substrate on which a thin film was formed, lithium was electrodeposited in propylene carbonate at an electric quantity of 1 coulomb / cm ² , and non-uniform electrodeposition was observed on the lithium surface. I couldn't do it.

[0018]

INDUSTRIAL APPLICABILITY When the present invention is applied to the lithium electrode of a lithium secondary battery, the problems described above in the problems to be solved by the invention can be solved. At present, the most problematic point in practical application of a lithium secondary battery is a decrease in charging efficiency and safety due to uneven growth of lithium during charging of a lithium negative electrode, and the present invention solves this problem. As a result, a lithium secondary battery having a high energy density and a high output density can be put into practical use. In the examples, propylene carbonate was used as the electrolytic solution, but similar results can be obtained with other organic electrolytic solutions that conduct lithium ions and battery systems using a solid electrolyte.

[Brief description of drawings]

FIG. 1 is a schematic view of a plasma CVD apparatus used in the present invention.

[Figure 2] FT-IR measurement diagram of plasma polymerized thin film.

FIG. 3 is a C _1S measurement diagram of ESCA of a plasma-polymerized thin film.

[Explanation of symbols] 1 Argon dry box 2 Glass bell jar 3 Vacuum exhaust system 4 Argon introduction system 5 Parallel plate type internal electrode 6 High frequency power supply 7 Substrate holder

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Zenichiro Takehara 6-10-14 Oedashinishinbayashi, Nishikyo-ku, Kyoto (72) Inventor Zenpachi Kokumi 89-3 Shimogamomatsunogi-cho, Sakyo-ku, Kyoto (72) Inventor Uchimoto Kiharu 23-11, Daigo Ohatacho, Fushimi-ku, Kyoto

Claims (3)

[Claims] 1. A lithium electrode for a lithium secondary battery, comprising a fluorine-based organic polymer thin film having an electron-donating polar group on the surface of lithium metal. 2. A surface of lithium metal is cleaned in a gas atmosphere capable of inducing plasma, and then a fluorine organic compound is introduced into the gas atmosphere capable of inducing plasma to induce plasma. A method for producing a lithium electrode for a lithium secondary battery, which comprises exciting a fluorine-based organic compound to form a fluorine-based organic polymer thin film having an electron-donating polar group on the cleaned lithium metal surface. 3. The method for producing a lithium electrode for a lithium secondary battery according to claim 2, wherein the fluorine-based organic compound is 1,1 difluoroethene or vinyl fluoride, and a mixture thereof.