JP3048798B2 - Reversible electrode and lithium secondary battery comprising the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reversible electrode made of an organic compound used for electrochemical devices such as a battery, an electrochromic display device, a sensor, and a memory, and a lithium secondary battery using the reversible electrode.

[0002]

2. Description of the Related Art U.S. Pat. No. 4,833,048 proposes an organic disulfide compound as a cathode material for a high energy density lithium secondary battery using lithium metal or a lithium alloy as a negative electrode. This compound is most easily represented by R
It is represented by -SSR (R is an aliphatic or aromatic organic group, and S is sulfur). The SS bond is cleaved by electrolytic reduction to form a cation (M ⁺ ) in the electrolytic bath and a salt represented by RS ^- M ⁺ . This salt is converted to the original R-
Return to SSR. A metal-sulfur secondary battery combining a metal M for supplying and trapping cations (M ⁺ ) and a disulfide compound is proposed in the aforementioned U.S. Patent. 150
With an energy density of Wh / kg or more, an energy density comparable to or higher than that of a normal secondary battery can be expected.

The present inventors have proposed a disulfide compound and a π-electron conjugated conductive polymer as an electrode having a high energy density of the disulfide compound and capable of performing a large current electrolysis (charge / discharge) even at room temperature and having excellent reversibility. Have proposed a reversible composite electrode. The π-electron conjugated conductive polymer complexed with the disulfide compound acts as an electrode catalyst during the electrolytic oxidation and reduction of the disulfide compound, and reduces the potential difference between the oxidation reaction and the reduction reaction of the disulfide compound alone, which was 1 volt or more, to 0. 0.1 volts or less to promote electrode reactions and enable high-current electrolysis (charge / discharge) even at room temperature. In electrolytic oxidation, first, a π-electron conjugated conductive polymer (hereinafter referred to as PAI)
Undergoes oxidation, and the oxidized form of PAI oxidizes the disulfide compound (hereinafter referred to as SS) and returns to the original reduced form,
An oxidized product in which SS is polymerized is formed. In electrolytic reduction, PAI undergoes reduction first, and the reduced form of PAI reduces the oxidized form of SS which has been polymerized and returns to the oxidized form.
A reduced form (monomer) of S is generated.

[0004]

However, if such a redox process of SS is repeated, the oxidized and polymerized SS aggregates to form an insulating mass, and the polymerized oxidant is reduced. Even the original monomer is 100
% Could not be returned, and the problem of gradually becoming inactive came out. When such a composite electrode is used as the positive electrode of a battery, there is a problem that the discharge capacity gradually decreases when the battery is repeatedly charged and discharged. The inventors of the present invention have made intensive studies on such a problem, and have found that the easiness and difficulty of forming the insulating lump vary greatly depending on the type of SS used. The present invention has been made based on the results of such studies.

[0005]

According to the present invention, 7-methyl-2,6,8-trimercaptopurine, which is a specific SS compound in which oxidized and polymerized SS hardly aggregates to form an insulating mass, is converted to π. A reversible electrode is formed by complexing with an electron conjugated conductive polymer.

Further, the lithium secondary battery of the present invention has a positive electrode layer comprising a composition containing a copolymer of 7-methyl-2,6,8-trimercaptopurine, polyaniline and acrylonitrile, and a composition containing an organic electrolyte; It has an organic electrolyte layer containing a coalescence and a negative electrode layer using lithium as an active material.

[0007]

As shown in Chemical formula 1, 7-methyl-2,6,8-trimercaptopurine (hereinafter referred to as MTMcP)
One molecule has three amino groups, and by the action of the amino groups, even when polymerized, good solubility in the electrolyte is maintained, and formation of an insulating mass is suppressed.

[0008]

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The π-electron conjugated conductive polymer complexed with MTMcP is a highly reversible oxidation-reduction reaction at 0 to ± 1.0 volts with respect to an Ag / AgCl electrode such as polyaniline, polypyrrole, polythiophene, and polyacene. Is effectively used. In particular, MTMcP having three amino groups has a very good affinity for polyaniline having a quinone diimine structure and a phenylenediamine structure. Due to the good properties, the formation of insulating lumps is further reduced.

As the metal ions when the polymerized MTMcP is reduced to form a salt, protons can be used in addition to alkali metal ions and alkaline earth metal ions. When a proton is used, a metal hydride such as LaNi ₅ is used as an electrode for supplying and capturing the proton, and an electrolyte that conducts the proton is used, a battery having a voltage of 1 to 2 volts can be formed. When lithium ions are used as the alkali metal ions, if the lithium ions are supplied and trapped using lithium metal or a lithium alloy such as lithium-aluminum and an electrolyte that conducts lithium ions is used, the voltage becomes 3 to
A 4 volt battery can be configured. Furthermore, as an electrolyte,
A copolymer of acrylonitrile and methyl acrylate or methyl methacrylate, a lithium salt, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane,
By using a solid electrolyte composition containing at least one solvent selected from the group consisting of sulfolane and methylsulfolane, formation of an insulating mass can be further reduced.

A reversible composite electrode is obtained by dispersing and mixing a composite electrode comprising MTMcP and polyaniline in a solid electrolyte composition. In such a reversible composite electrode, MTMcP or the MTMcP polymer is partially or entirely dissolved in the solid electrolyte composition,
Since the dissolved monomer or polymer is well fixed on or in solid polyaniline by interaction with polyaniline, MTMcP or MTMcP
The present inventors believe that the polymer is in a very uniformly dispersed state, and that the formation of insulating lumps is effectively suppressed.

The compounding of MTMcP and the π-electron conjugated conductive polymer can be performed by a known method such as mixing, impregnation, eutectoid deposition, and recoating. For example, a composite electrode can be obtained by forming a fibril layer of a conductive polymer on a stainless steel substrate by electrolytic polymerization, impregnating the fibril layer with a solution in which a salt of MTMcP is dissolved, and then removing the solvent. it can. Further, by dispersing the MTMcP particles in a solvent in which the π-electron conjugated conductive polymer is dissolved, and removing the solvent, a layer of the π-electron conjugated conductive polymer may be formed on the surface of the MTMcP particles to form a composite. . Compounding can also be achieved by mixing the π-electron conjugated conductive polymer powder obtained by chemical polymerization or electrolytic polymerization with MTMcP powder. Further, in addition to the above components, a metal powder and a carbon powder can be added to the reversible composite electrode as a conductive agent, if necessary.

[0013]

The present invention will be described below in more detail with reference to examples. [Example 1] lithium borofluoride (LiBF ₄₎ 2.3
3 g, 10.47 g of propylene carbonate and 7.86 g of ethylene carbonate are mixed and heated to 120 ° C. to obtain a homogeneous solution. This solution was mixed with 3 g of a copolymer powder of acrylonitrile having a molecular weight of 50,000 and methyl acrylate at a molar ratio of 20: 1, and heated to 150 ° C. in a sealed 100 ml Erlenmeyer flask to complete the copolymer powder. To obtain a viscous transparent liquid. 30 g of acetonitrile is added to this liquid to obtain an electrolyte solution.

MTMcP powder (Aldrich) 1.5
g, and HBF ₄ -doped polyaniline powder (1.0 g) obtained by chemically polymerizing aniline using ammonium persulfate in borofluoric acid (HBF ₄ ) acid were mixed in a mortar. Electrolyte solution 10
g to obtain an electrode slurry. The electrode slurry is cast on a glass Petri dish having a diameter of 90 mm, dried for 1 hour in a dry argon gas stream at 60 ° C., and further vacuum-dried at 60 ° C. for 1 hour to obtain a flexible sheet having a thickness of about 160 μm. A reversible composite electrode A is obtained. Separately, only 10 g of the electrolyte solution was cast on a glass Petri dish having a diameter of 90 mm, and was similarly dried at 60 ° C.
A μm gel electrolyte membrane A is obtained.

A reversible composite electrode A, a gel electrolyte membrane A, and a metal lithium foil having a thickness of 80 μm were punched into a disk having a diameter of 13 mm and inserted into a Teflon cylinder having an inner diameter of 13 mm to obtain a battery having the structure shown in FIG. Form. In FIG.
Reference numeral 1 denotes a lithium metal negative electrode, 2 denotes a gel electrolyte membrane, and 3 denotes a reversible composite electrode A acting as a positive electrode.

Comparative Example 1 As a disulfide compound, M
Commercially available special grade reagent 2,5 dimercapto-1,3,4-thiadiazole (hereinafter referred to as DMcT) in place of TMcP
A reversible composite electrode B having a thickness of 170 μm was prepared in the same manner as in Example 1 except that を was used. Further, in the same manner as in Example 1, a disc-shaped battery B having a diameter of 13 mm constituted by the gel electrolyte membrane A and the metal lithium anode was used with the reversible composite electrode B as the positive electrode.

The batteries A and B were subjected to a charge / discharge test at 20 ° C. within a range of 4.05 to 1.5 V at a constant current of 0.27 mA. FIG. 2 shows the relationship between the value of the discharge capacity in each cycle and the number of charge / discharge cycles when the discharge capacity in the first cycle is 100%. As is clear from FIG. 2, the battery A using MTMcP as the disulfide compound according to the present invention can obtain a capacity of 80% or more in the first cycle even after 50 cycles. On the other hand, the battery B of the comparative example using DMcT as the disulfide compound gives only about 55% of the capacity in the first cycle after 50 cycles.

Example 2 In a polyethylene triol solution obtained by dissolving 1 part by weight of polyethylene triol having a molecular weight of 3000 in 20 parts by weight of methyl ethyl ketone, a molar ratio of ethylene oxide (EO) to lithium (EO / L) was used.
After dissolving lithium perchlorate so that i) becomes 8/1, 1 part by weight of artificial graphite powder having an average particle size of 0.3 μm;
4 parts by weight of a polyaniline powder obtained by reducing and dedoping the polyaniline obtained by the chemical polymerization used in Example 1,
4 parts by weight of a polysulfide powder obtained by neutralizing TMcP with lithium hydroxide and then oxidatively polymerizing with iodine are dispersed. To this dispersion, an equimolar amount of tolylene diisocyanate was added and mixed with polyethylene triol.
After 2 hours of reaction at ℃, cast on a Petri dish 90mm in diameter,
160 μm thick by holding at 80 ° C. in vacuum for 24 hours
To obtain a reversible composite electrode C of

Further, an equimolar amount of tolylene diisocyanate is added to a polyethylene triol solution in which lithium perchlorate is dissolved, and the mixture is reacted at 80 ° C. for 2 hours.
The mixture is cast on a Petri dish having a thickness of 300 mm and kept at 80 ° C. for 24 hours in a vacuum to obtain a polymer electrolyte having a thickness of about 300 μm.
A battery C was prepared in which the electrode composition thus obtained was used as a positive electrode, the polymer electrolyte was used as an electrolyte membrane, and lithium metal having a structure similar to that shown in FIG. 1 was used as a negative electrode.

[Comparative Example 2] DMcT instead of MTMcP
A reversible composite electrode D having a thickness of 170 μm was prepared in the same manner as in Example 2 except that was used, and a battery D was formed in the same manner as in Example 2. These batteries C and D were subjected to a charge / discharge test at 100 ° C. within a range of 4.05 to 1.5 V at a constant current of 0.27 mA. FIG. 3 shows the relationship between the value of the discharge capacity in each cycle and the number of charge / discharge cycles when the discharge capacity in the first cycle is 100%. As is clear from FIG. 3, the battery C using the polymer of MTMcP as the disulfide compound according to the present invention can obtain a capacity of 80% or more in the first cycle even after 50 cycles. On the other hand, as a disulfide compound, DMcT
The battery D of the comparative example using the polymer of the formula (1) gives only about 45% of the capacity in the first cycle after 50 cycles.

[0021]

According to the present invention, a composite electrode obtained by using 7-methyl-2,6,8-trimercaptopurine as a disulfide compound and compounding it with a π-electron conjugated conductive polymer has excellent reversibility, especially lithium. When used for the positive electrode of a secondary battery, extremely excellent charge / discharge cycle characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a lithium secondary battery according to an embodiment of the present invention.

FIG. 2 is a diagram showing the relationship between the relative value of the discharge capacity and the number of charge / discharge cycles of the lithium secondary batteries of one example and a comparative example of the present invention.

FIG. 3 is a diagram showing the relationship between the relative value of the discharge capacity and the number of charge / discharge cycles of the lithium secondary batteries of other examples and comparative examples of the present invention.

[Explanation of symbols]

 1 negative electrode 2 electrolyte 3 positive electrode

────────────────────────────────────────────────── (5) References JP-A-4-359866 (JP, A) JP-A-5-6708 (JP, A) US Patent 4833048 (US, A) Katsuhiko Naoi et al., "Disulfide Compounds" Polymerization and Depolymerization Electrode Reactions and Application to Cathode Materials for Lithium Secondary Batteries, ”Proceedings of the 32nd Battery Symposium, September 10, 1991, pp. 245-246 (58) Field (Int.Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/60 H01M 10/40

Claims (2)

(57) [Claims] 1. A reversible electrode comprising 7-methyl-2,6,8-trimercaptopurine and a π-electron conjugated conductive polymer. 2. A positive electrode layer composed of a composition containing a copolymer of 7-methyl-2,6,8-trimercaptopurine, polyaniline and acrylonitrile, and an organic electrolyte layer containing a copolymer of acrylonitrile and lithium. A lithium secondary battery comprising a negative electrode layer containing: