JP4932243B2 - Positive electrode for lithium battery and secondary battery using the same - Google Patents

Description

  The present invention relates to a positive electrode for a lithium battery and a secondary battery using the same.

Since vanadium pentoxide (V ₂ O ₅ ) has lithium ion intercalation characteristics, attempts to apply it to a positive electrode material of a lithium secondary battery have been made for a long time. For example, a secondary using this positive electrode material The battery is commercially available as a backup power source. However, vanadium pentoxide has been found to cause irreversible capacity degradation when lithium ions are occluded in an equimolar amount or more, and repeated charge / discharge with a high charge / discharge capacity gradually reduces the capacity that can be removed. It has caused the problem of end.

  According to Non-Patent Document 1 (page 86), generally, when vanadium pentoxide incorporates equimolar or more of lithium ions, the battery performance of the lithium secondary battery is deteriorated between the layers of vanadium pentoxide layered crystals. This is because the weak V—O bond is cleaved to form a bronze body between vanadium pentoxide and lithium, resulting in an irreversible change in the crystal structure. As a measure for solving the problems caused by the structural anisotropy of vanadium pentoxide, a method of making vanadium pentoxide amorphous is employed.

For example, for the purpose of suppressing structural fluctuation due to lithium occlusion and achieving high charge / discharge characteristics, Patent Document 1 discloses that a compound such as MoO ₃ or B ₂ O ₃ is used as an amorphizing agent in vanadium pentoxide. A method for adding vanadium and amorphizing vanadium pentoxide by heat treatment has been reported.

Moreover, it is thought that the same effect can be acquired also about the xerogel (page 93-94 of a nonpatent literature 1) which took in water in the vanadium pentoxide layer, and the composite_body | complex which took in the conductive polymer. In particular, Non-Patent Documents 2 and 3 oxidize and polymerize 3,4-ethylenedioxythiophene (EDOT) in the presence of layered vanadium pentoxide, and poly 3,4-ethylenedioxythiophene between the vanadium pentoxide layers. It has been reported that the charge / discharge capacity is improved by inserting (PEDOT). In Patent Document 2, a polythiophene derivative is combined with an intercalating material such as a transition metal oxide.
Japanese Patent No. 2849490 Japanese Patent No. 3429543 Supervised by Zenichiro Takehara, "High-density lithium secondary battery", Techno System Co., Ltd., March 14, 1998 J. Mater. Chem., 2001, 11, 2470-2475 Electrochemistry Communications 4 (2002) 384-387

  However, in any case, satisfactory charge / discharge capacity has not been obtained. The reason for this is not necessarily clear, but in the case of Patent Document 1, it is considered that the movement of lithium ions does not proceed rapidly only by adding an amorphizing agent to vanadium pentoxide. In the case of Non-Patent Documents 2 and 3, and Patent Document 2, it is considered that there is a problem that a high charge / discharge capacity cannot be exhibited because lithium ion incorporation is limited.

  Therefore, an object of this invention is to provide the positive electrode for lithium batteries which can obtain high charging / discharging capacity, and a secondary battery using the same.

  The inventors of the present invention have intensively studied the development of a high energy density positive electrode material capable of stably incorporating lithium ions in an equimolar amount or more into a layered transition metal oxide such as vanadium pentoxide. It has been found that by allowing an oxidation polymer of a 4-ethylenedioxythiophene compound to be present between the layered compounds, a positive electrode material can be obtained that maintains a high discharge capacity even when equimolar amounts of lithium ions are incorporated. The present invention is based on such knowledge.

That is, according to one aspect of the present invention, a conductive substrate, a layer of cathode material formed on the surface of the conductive substrate, wherein the cathode material is a transition metal oxide having a layered as an active material It contains vanadium pentoxide and a conductive organic sulfur compound, and the conductive organic sulfur compound has the following formula (1):

(Wherein, R represents C ₁ -C ₂₀ alkyl group) oxide polymer saw including alkyl-substituted 3,4-ethylenedioxy thiophene compound represented by, the conductive organic sulfur compound layers of the vanadium pentoxide A positive electrode for a lithium battery is provided.

  According to another aspect of the present invention, a positive electrode, a negative electrode, and an electrolyte layer disposed between the positive electrode and the negative electrode are provided, and the positive electrode includes the positive electrode for a lithium battery according to the present invention. A secondary battery is provided.

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode for lithium batteries which can achieve a high charge / discharge capacity, and a secondary battery using the same can be obtained.

  Hereinafter, the present invention will be described in more detail.

  The positive electrode for a lithium battery of the present invention comprises a conductive substrate and a layer of a positive electrode material formed on the surface thereof. The positive electrode material contains, as active materials, a layered transition metal oxide containing vanadium and a conductive organic sulfur compound.

The conductive organic sulfur compound used in the present invention is represented by the following formula (1):

(Wherein R is an alkyl group) and an oxidized polymer of an alkyl-substituted 3,4-ethylenedioxythiophene compound. Hereinafter, the compound represented by the formula (1) is abbreviated as EDOT-R. Here, R is synonymous with R in Formula (1). For example, the compound of formula (1) where R is a methyl group is represented as EDOT-CH ₃ (or methyl). Alkyl group represented by R is preferably a _C 1 _{-C 20} alkyl, more preferably a _C 8 _{-C 16} alkyl. The alkyl group is preferably linear, but may be a branched or cyclic alkyl group.

  In the present invention, the transition metal oxide contains vanadium and is usually a layered compound that intercalates lithium ions. As the transition metal oxide, vanadium pentoxide is preferable.

  The active material of the positive electrode for a lithium battery of the present invention is obtained by suspending a layered transition metal oxide containing vanadium and EDOT-R in a solvent (for example, water), and stirring the suspension at 100 ° C. for 3 hours to 100 hours. Can be manufactured. The oxidizable layered transition metal oxide undergoes redox interaction with EDOT-R (monomer) under the EDOT-R polymerization conditions. As a result, the monomer undergoes oxidative polymerization between the layers to form an oxidized polymer (hereinafter referred to as PEDOT-R). Abbreviated as).

  The active material of the positive electrode for a lithium battery of the present invention preferably contains PEDOT-R in a proportion of 0.1 to 25% by weight based on the weight of the transition metal oxide. When the amount of PEDOT-R is less than 0.1% by weight, conductivity tends to be not maintained. On the other hand, when the amount of PEDOT-R exceeds 20% by weight, PEDOT-R cannot be taken in between the layers of the layered compound, and the capacity tends to decrease.

  The positive electrode active material for a lithium battery thus obtained is dried and then mixed with a binder such as polyvinylidene fluoride and preferably with conductive particles to form a positive electrode material, which is applied onto a conductive substrate to form a positive electrode Can be produced.

  The conductive particles improve the conductivity of the positive electrode active material of the present invention. Examples of conductive particles include conductive carbon (conductive carbon black such as ketjen black), copper, iron, silver, nickel, palladium, gold, platinum, indium, tungsten and other metals, indium oxide, oxidation Conductive metal oxides such as tin. These conductive particles are preferably contained in a proportion of 1 to 30% by weight of the total amount of the layered transition metal oxide and the conductive organic sulfur compound.

  In the present invention, the substrate (current collector) that supports the layer of the positive electrode material is a conductive substrate that exhibits conductivity at least on the surface in contact with the layer of the positive electrode material of the present invention. The substrate can be formed of a conductive material such as metal, conductive metal oxide, or conductive carbon, but is preferably formed of copper, gold, aluminum, an alloy thereof, or conductive carbon. Alternatively, the substrate can also be formed by coating a substrate body formed of a nonconductive material with these conductive materials.

  In the present invention, the layer of the positive electrode material for a lithium battery preferably has a thickness of 10 to 200 μm.

  The secondary battery of the present invention includes the positive electrode of the present invention, a negative electrode, and an electrolyte layer disposed between the positive electrode and the negative electrode. The secondary battery of the present invention is preferably a lithium secondary battery.

  In the lithium secondary battery, the negative electrode is preferably formed of a lithium material that occludes lithium. Examples of such lithium-based materials include lithium-based metal materials such as metallic lithium and lithium alloys (for example, Li-Al alloys), intermetallic compound materials of metals and lithium such as tin and silicon, and lithium nitride. A lithium compound or a lithium intercalation carbon material can be exemplified. The lithium-based metal material is preferably used in the form of a foil in terms of reducing the weight of the battery.

In the lithium secondary battery, as an electrolyte, CF ₃ SO ₃ Li, C ₄ F ₉ SO ₈ Li, (CF ₃ SO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₃ CLi, LiBF ₄ , LiPF ₆ , LiClO ₄ Lithium salts such as LiC ₄ O ₈ B can be used. The solvent for dissolving these electrolytes is preferably a non-aqueous solvent. Non-aqueous solvents include chain carbonates, cyclic carbonates, cyclic esters, nitrile compounds, acid anhydrides, amide compounds, phosphate compounds, amine compounds, and the like. Specific examples of the non-aqueous solvent include ethylene carbonate, diethyl carbonate (DEC), propylene carbonate, dimethoxyethane, γ-butyrolactone, n-methylpyrrolidinone, N, N′-dimethylacetamide, acetonitrile, or propylene carbonate and dimethoxyethane. And a mixture of sulfolane and tetrahydrofuran. The electrolyte layer interposed between the positive electrode and the negative electrode may be a solution of the above electrolyte in a non-aqueous solvent or a polymer gel (polymer gel electrolyte) containing this electrolyte solution.

  In the positive electrode for lithium batteries of the present invention, it has been found that an active material containing PEDOT-R and vanadium pentoxide significantly increases the doping amount of lithium ions. Although this effect has not yet been elucidated, it is speculated that it may be due to the following two reasons.

  First, EDOT is the most cationic in the EDOT skeleton, and the electronic state of hydrogen on the 2nd carbon, which is the polymerization reaction site, was calculated by molecular orbital calculation optimized by CAChe's MM / PM5 geometry. When an alkyl group is introduced onto the ethylene chain, the total positive charge density of hydrogen on the 2-position carbon decreases. Thereby, the ionic interaction between polymerized PEDOT-R terminal H and vanadium pentoxide is reduced, and the amount of lithium ions taken up is increased.

  Second, the presence of an alkyl group introduced on EDOT inhibits the stacking of vanadium pentoxide in the c-axis direction, lowers the crystallinity of vanadium pentoxide, and increases the amount of lithium ion deinsertion.

In the positive electrode for a lithium battery according to the present invention, since the doping amount of the lithium ion of the active material is greatly increased as described above, a high charge / discharge capacity can be obtained. In particular, the lithium secondary battery having the positive electrode of the present invention has a high discharge capacity in the initial stage. However, as described in Synthetic Metals 118 (2001) 105-109, PEDOT-C ₆ H ₁₃ is somewhat inferior in conductivity, or a secondary battery using the PEDOT-C ₆ H ₁₃ has a little long-term charge / discharge stability. There is an inconvenience.

EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited to this.
In addition, all the alkyl groups shown by R of EDOT-R used in a present Example are linear alkyl groups.

Example 1
0.086 g of EDOT-CH ₃ synthesized according to the previous report and 2 g of vanadium pentoxide xerogel (water content: 13.1 wt%) were mixed, 100 ml of an aqueous solvent was added, and the mixture was stirred at 100 ° C. for 12 hours. After stirring, the green gel material was separated by filtration and dried in vacuo at 60 to 80 ° C. overnight to obtain an active material. In the differential thermal analysis, the amount of the organic component obtained by determining all exothermic components at 150 ° C. or higher as organic matter, that is, PEDOT-CH ₃ , was calculated to be 5.1% by weight. As a result of X-ray analysis, a diffraction line similar to vanadium pentoxide xerogel was shown. When the bottom diffraction width of the 001 plane was determined, it was calculated to be 12.6 mm. The results are shown in Table 1.

Example 2
An active material was synthesized in the same manner as in Example 1 except that 0.102 g of EDOT-C ₃ H ₇ was used. In the differential thermal analysis, the amount of the organic component determined by assuming that all exothermic components at 150 ° C. or higher were organic, that is, PEDOT-C ₃ H ₇ , was calculated to be 5.2% by weight. As a result of X-ray analysis, a diffraction line similar to vanadium pentoxide xerogel was shown. When the bottom diffraction width of the 001 plane was determined, it was calculated to be 12.5 mm. The results are shown in Table 1.

Example 3
An active material was synthesized in the same manner as in Example 1 except that 0.081 g of EDOT-C ₆ H ₁₃ was used. In the differential thermal analysis, the amount of the organic matter component obtained by assuming that all exothermic components at 150 ° C. or higher were organic matter, that is, PEDOT-C ₆ H ₁₃ was calculated to be 5.6% by weight. As a result of X-ray analysis, a diffraction line similar to vanadium pentoxide xerogel was shown. When the bottom diffraction width of the 001 plane was determined, it was calculated to be 12.6 mm. The results are shown in Table 1.

Example 4
An active material was synthesized in the same manner as in Example 1 except that 0.092 g of EDOT-C ₈ H ₁₇ was used. In the differential thermal analysis, the amount of the organic component determined by assuming that all exothermic components at 150 ° C. or higher were organic, that is, PEDOT-C ₈ H ₁₇ , was calculated to be 4.6% by weight. As a result of X-ray analysis, a diffraction line similar to vanadium pentoxide xerogel was shown. When the bottom diffraction width of the 001 plane was determined, it was calculated to be 12.6 mm. The results are shown in Table 1.

Example 5
An active material was synthesized in the same manner as in Example 1 except that 0.117 g of EDOT-C ₁₀ H ₂₁ was used. In the differential thermal analysis, the amount of the organic component determined by assuming that all exothermic components at 150 ° C. or higher were organic, that is, PEDOT-C ₁₀ H ₂₁ was calculated to be 3.4% by weight. As a result of X-ray analysis, a diffraction line similar to vanadium pentoxide xerogel was shown. When the bottom diffraction width of the 001 plane was determined, it was calculated to be 12.4 mm. The results are shown in Table 1.

Example 6
An active material was synthesized in the same manner as in Example 1 except that 0.122 g of EDOT-C ₁₄ H ₂₉ was used. In the differential thermal analysis, the amount of the organic component determined by assuming that all exothermic components at 150 ° C. or higher are organic, that is, PEDOT-C ₁₄ H ₂₉ , was calculated to be 3.4% by weight. As a result of X-ray analysis, a diffraction line similar to vanadium pentoxide xerogel was shown. When the bottom diffraction width of the 001 plane was determined, it was calculated to be 12.4 mm. The results are shown in Table 1.

Comparative Example 1
Vanadium pentoxide xerogel having a water content of 13.1% by weight and an X-ray diffraction 001 plane bottom surface diffraction width of 11.6 mm was used as an active material.

Comparative Example 2
An active material was synthesized in the same manner as in Example 1 except that 0.785 g of EDOT was used. In the differential thermal analysis, the amount of the organic component obtained by determining all exothermic components at 150 ° C. or higher as organic matter, that is, PEDOT was calculated to be 6.0% by weight. As a result of X-ray analysis, a diffraction line similar to vanadium pentoxide xerogel was shown. When the bottom diffraction width of the 001 plane was determined, it was calculated to be 13.6 mm. The results are shown in Table 1.

In each of the active materials obtained in Examples 1 to 6 and Comparative Examples 1 and 2, conductive particles (carbon black) and a binder (polyvinylidene fluoride) are used. An active material: conductive particles: a binder is used. The mixture was mixed at a weight ratio of 70: 25: 5, diluted with n-methylpyrrolidinone to prepare a paste, and this paste was uniformly applied on an aluminum foil current collector. After drying under reduced pressure at 150 ° C. to remove excess solvent, pressing was performed to obtain a positive electrode material layer having a coating amount of 2 mg / cm ² and a thickness of 30 μm.

Each obtained positive electrode was cut into 20 × 20 mm, and an aluminum terminal was welded. Further, a lithium metal foil (25 × 25 mm) having a thickness of 100 μm was brought into close contact with the nickel mesh metal, and a nickel terminal was further welded to form a counter electrode. A battery structure was formed by laminating a positive electrode and a negative electrode through a polyolefin microporous membrane (separator), and packaged with an aluminum wrapping material having a resin film laminated on the inner surface, leaving an opening. An electrolytic solution made of a mixture of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 3 in which 1 mol / L of lithium borofluoride (LiBF ₄ ) was dissolved was placed in an aluminum wrapping material so that the battery structure was immersed therein. Thereafter, the opening was sealed to obtain a battery.

The batteries produced as described above are referred to as invention batteries 1 to 6 and comparative batteries 1 and 2, respectively. A charge / discharge cycle test was conducted using this battery. The test conditions were assumed to be 400 mAh / g (active material), the first discharge of 1.5V cut was performed, the actually obtained capacity was 1 ImAt, and the current amount equivalent to 0.1 ImAt was used for the following cycle test. Set in. Charging during the cycle was 20 hours in a 4.2V constant current-constant voltage charging (CC-CV) method, and discharging was a 1.5V cut constant current discharging (CC) method. The discharge capacity at the second cycle and the discharge capacity at the 20th cycle are summarized in Table 2.

  As is apparent from the numerical values in Table 2, when discharged to 1.5 V, the initial discharge capacities of the present batteries 1 to 6 using the positive electrode of the present invention are higher than those of the comparative batteries 1 and 2. It was. Also, the discharge capacity after 20 cycles showed that the battery of the present invention had a higher capacity than the comparative battery.

Claims (6)

A conductive substrate and a layer of a positive electrode material formed on a surface of the conductive substrate, the positive electrode material including vanadium pentoxide , which is a layered transition metal oxide, and a conductive organic sulfur compound as an active material; The conductive organic sulfur compound is represented by the following formula (1):

(Wherein, R represents C ₁ -C ₂₀ alkyl group) oxide polymer saw including alkyl-substituted 3,4-ethylenedioxy thiophene compound represented by, the conductive organic sulfur compound layers of the vanadium pentoxide A positive electrode for a lithium battery, Wherein R is a positive electrode for a lithium battery according to claim 1, characterized in that the C ₈ -C ₁₆ alkyl. The positive electrode for a lithium battery according to claim 1 or 2 , wherein the active material contains the oxidation polymer in a proportion of 0.1 to 25% by weight based on the weight of the transition metal oxide. The said positive electrode material contains electroconductive particle and a binder, The positive electrode for lithium batteries as described in any one of Claims 1-3 characterized by the above-mentioned. A positive electrode, a negative electrode, and an electrolyte layer disposed between the positive electrode and the negative electrode are provided, and the positive electrode includes the positive electrode for a lithium battery according to any one of claims 1 to 4. Next battery. The secondary battery according to claim 5 , wherein the secondary battery is a lithium secondary battery.