JP2006092971A - Lithium battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium battery having high safety, a long shelf life, and excellent heavy current characteristics. <P>SOLUTION: In this lithium battery equipped with electrolyte using ionic liquid containing an organic cation expressed by formula 1, (in the formula, R, R' are 1-4C alkyl groups, and R'' is halogen or an 1-3C alkyl group), a material expressed by M<SP>a+</SP>X<SP>a-</SP>(M is a metal ion, X is an anion, and (a) is the number of electrons), and reduced to M in a discharge reaction is used as a positive electrode active material. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a lithium battery using an ionic liquid.

  At present, lithium batteries using an organic electrolyte in which a lithium salt is dissolved in an organic solvent are widely used as a memory backup or driving power source for portable devices. Since these batteries use an organic solvent for the organic electrolyte, it is necessary to consider that the vapor pressure becomes high at high temperatures and that the safety is enhanced because of the flammability. In particular, when considering use as an emergency power source, it may be impossible to use a combustible organic electrolyte.

  Therefore, an ionic liquid that is liquid at room temperature has attracted attention as a solid electrolyte made of an inorganic solid or a nonflammable non-aqueous electrolyte. For example, an ionic liquid electrolyte is disclosed in JP-A-4-349365 (Patent Document 1).

  Since the ionic liquid electrolyte has a higher viscosity than the organic electrolytic solution, there is a problem that the penetration into the positive electrode active material having a relatively high density is poor, so that the high current discharge characteristics cannot be improved.

On the other hand, various ionic liquids have been recently developed. For example, an ionic liquid containing a cyano group as an anion as disclosed in JP-A-2004-165131 (Patent Document 2) is effective for an electrochemical device because of low viscosity and high conductivity. Therefore, there is a problem that it cannot be used for a lithium battery.
JP-A-4-349365 JP 2004-165131 A

  In conventional lithium batteries using ionic liquids, ionic liquid electrolytes have a higher viscosity than organic electrolytes, so the penetration into relatively high-density positive electrode active materials is poor, so high current discharge characteristics cannot be improved. There was a problem. Some ionic liquids have high reactivity with lithium metal as a negative electrode, so that some storage properties are very poor and cannot be used.

  The present invention has been made in view of the above problems, and provides a lithium battery having high safety and excellent storage characteristics and large current performance.

（但し、Ｒ、Ｒ’はＣ1〜Ｃ4のアルキル基、Ｒ”はハロゲンもしくはＣ1〜Ｃ3のアルキル基）
請求項２のリチウム電池は、請求項１において、前記正極活物質がＣｕ4Ｏ（ＰＯ4）2であることを特徴とする。 In order to solve the above-mentioned problems, a lithium battery according to claim 1 is composed of a positive electrode mixture comprising a positive electrode active material, a conductive auxiliary agent, and a binder, and a lithium metal or lithium alloy disposed opposite to the positive electrode mixture. In a lithium battery comprising: a negative electrode containing at least one of the above; and an electrolyte using an ionic liquid containing an organic cation represented by the following chemical formula (1) interposed between the negative electrode and the positive electrode mixture: The positive electrode active material is represented by M ^{a +} X ^a- (M is a metal ion, X is an anion, a is the number of electrons), and is reduced to M in a discharge reaction, and the density is 1.6 to Lithium battery characterized by being in a range of 2.4 g / cm 3.

(Where R and R ′ are C1 to C4 alkyl groups, R ″ is halogen or C1 to C3 alkyl groups)
A lithium battery of claim 2 is characterized in that, in claim 1, the positive electrode active material is Cu4O (PO4) 2.

  According to a third aspect of the present invention, there is provided a lithium battery according to the first aspect, wherein the positive electrode active material is FeS or FeS2.

  The lithium battery of claim 4 is characterized in that the positive electrode active material has a particle size in the range of 1 to 50 μm.

  A lithium battery according to a fifth aspect is characterized in that, in the first aspect, the content of the conductive additive in the positive electrode mixture is 1 wt% to 5 wt%.

  A lithium battery according to a sixth aspect is characterized in that, in the first aspect, the electrolytic solution contains a bisoxalatoborate anion or vinylene carbonate.

  By using the battery as described above, it is possible to provide a lithium battery that is highly safe and excellent in long-term storage and large current characteristics.

  1M-LiTFSI / using an ionic liquid having an organic cation represented by (Chemical formula 1) having excellent conductivity and high electrochemical stability to the Li / MnO2 system, which is the most popular battery system using metallic lithium as a negative electrode When EMITFSI was applied as an electrolytic solution, it was confirmed that the long-term storage characteristics were very inferior compared to the organic electrolytic solution. When the deteriorated battery was examined, it was confirmed that EMI + was inserted into the channel of MnO2. In other words, the organic cation represented by (Chemical Formula 1) is taken into the channel into which Li ions enter the electrode active material that inserts and desorbs Li ions, and as a result, no discharge occurs. It is thought that the active material capacity will decrease.

Therefore, as a result of diligent research to solve this problem, the positive electrode active material is expressed by M ^{a +} X ^a− (M is a metal ion, X is an anion, a is the number of electrons), and is reduced to M in the discharge reaction. As a result, it has been found that even when an ionic liquid containing an organic cation represented by (Chemical Formula 1) is used, the battery has excellent long-term storage characteristics. It has also been found that the positive electrode mixture density needs to be in the range of 1.6 to 2.4 g / cm @ 3 in order to improve the large current characteristics. If the mixture density is less than 1.6 g / cm 3, the electrode becomes too sparse and sufficient conductivity cannot be obtained, which is disadvantageous for large current characteristics. If it exceeds 2.4 g / cm 3, the impregnation property of the ionic liquid is poor. The falling electrode resistance increases and the large current characteristics deteriorate. That is, according to the present invention, a positive electrode mixture comprising a positive electrode active material, a conductive agent and a binder, a negative electrode containing at least one of lithium metal or lithium alloy, and an organic cation represented by (Chemical Formula 1) (provided that R) , R ′ is C1-C4 alkyl, R ″ is halogen or C1-C3 alkyl), and the positive electrode active material is M ^{a +} X ^a− (M Is a metal ion, X is an anion, and a is the number of electrons), and is reduced to M in the discharge reaction, and the positive electrode mixture density is within the range of 1.6 to 2.4 g / cm 3. Therefore, a lithium battery having high safety and excellent in both long-term storage characteristics and large current characteristics can be obtained.

  Hereinafter, the lithium battery according to the present invention will be described for each member.

1) Positive electrode The positive electrode active material is represented by M ^{a +} X ^a- (M is a metal ion, X is an anion, a is the number of electrons), and is reduced to M in a discharge reaction.

  Examples of the positive electrode active material include iron disulfide (FeS2), iron sulfide (FeS), copper oxide (CuO), copper oxyphosphate (Cu4O (PO4) 2), silver chromate (Ag2Cr2O4), and copper sulfide (CuS). ), Copper fluoride (CuF2), copper chloride (CuCl2), trinickel disulfide (Ni3S2), silver chloride (AgCl), dibismuth trioxide (Bi2O3), and the like.

  Of these, copper oxyphosphate is preferred. Since the organic cation represented by (Chemical Formula 1) has lower electrochemical stability than conventional organic solvents, the Li / Cu4O (PO4) 2 system has a theoretical voltage of 2.7 V and an operating voltage of 2.3 V. This combination is advantageous for long-term storage characteristics with little damage to the electrolyte.

  FeS and FeS2 are also preferred positive electrodes. When these positive electrodes are used in an organic electrolyte, Li2S, which is a reactive organism, has the disadvantage that it dissolves into the electrolyte, but when an ionic liquid containing an organic cation represented by (Chemical Formula 1) is used, this Li2S This is advantageous because elution can be prevented.

  The particle size of the positive electrode active material is preferably 1-50 μm. If it is smaller than 1 μm, the reaction with the electrolyte solution unrelated to the battery reaction becomes remarkable, and if it is larger than 50 μm, the resistance increases, which is disadvantageous for large discharge characteristics. In an ordinary organic solvent, using such an active material having a small particle diameter is disadvantageous from the viewpoint of side reaction with the electrolyte solution. However, by using a chemically stable ionic liquid, It becomes possible to use an active material having a small particle size.

Examples of the conductive assistant include graphite, acetylene black, and ketjen black. The specific surface area of the conductive agent is preferably 120 m2 / g or more. If it is less than 120 m @ 2 / g, the resistance in the electrode increases and the role as a conductive aid becomes insufficient. By the way, in the present invention, the positive electrode active material is represented by M ^{a +} X ^a− (M is a metal ion, X is an anion, a is the number of electrons), and is reduced to M in the discharge reaction. When such a positive electrode is used, M having conductivity is generated as the discharge reaction proceeds, and the resistance in the electrode is reduced. Therefore, the amount of the conductive auxiliary agent can be reduced, the amount of the positive electrode active material in the positive electrode mixture can be increased, and the battery capacity is increased. The amount of the conductive assistant is preferably 1-5 wt% of the total weight of the positive electrode mixture. This is because if it is less than 1 wt%, the effect as a conductive additive is insufficient, and if it exceeds 5 wt%, the battery capacity decreases.

  Examples of the binder include polytetrafluoroethylene and polyvinylidene fluoride.

2) Negative electrode The negative electrode includes one of lithium metal or lithium alloy.

  Examples of the lithium alloy include alloys of Li and at least one element selected from Si, Sn, Al, B, Ga, In, Pb, Bi, and Sb.

  Examples of the other negative electrode include a nitride containing Li and Co or Ni, a mixture of a compound containing Li and Si and carbon, and the like.

  Among these, lithium metal is preferable.

3) Electrolytic Solution The electrolytic solution contains an ionic liquid containing at least an organic cation represented by (Chemical Formula 1) as a solvent. Among these cations, 1-ethyl-3-methylimidazolium cation, 1-butyl-3-methylimidazolium cation, and 1,2-dimethyl-3-propylimidazolium cation are preferable because of high conductivity. Examples of anions include PF6-, BF4-, Cl-, AlCl4-, AsF6-, 2.3HF-, C4F9SO3-, CF3SO3-, (CF3SO2) 2N-, (C2F5SO2) 2N-, (CF3SO2) 3C- and the like. It is done. The amount of the ionic liquid in the electrolytic solution is preferably 50-100 wt%. If it is less than 50 wt%, the expected nonflammability effect cannot be obtained sufficiently, and a highly safe product cannot be realized. Here, among the ionic liquids mentioned as examples, those in which the anion is BF4- have high reactivity with lithium metal and poor storage properties. However, this can be used by adding a bisoxalatoborate anion or vinylene carbonate to the electrolyte so that a film that suppresses the reaction between the ionic liquid and lithium can be formed on the negative electrode surface. .

  Moreover, you may contain organic solvents other than an ionic liquid as a solvent. Organic solvents other than ionic liquids include ethylene carbonate, propylene carbonate, butylene carbonate, dimethoxyethane, γ-butyrolactone, dimethyl carbonate, methyl ethyl carbonate, acetonitrile, tetrahydrofuran, vinylene carbonate, vinylene acetate, vinyl ethylene carbonate, phenyl ethylene carbonate And catechol carbonate. These solvents may be contained in one kind or two or more kinds.

  Examples of the lithium salt as the electrolyte salt include LiBF4, LiPF6, LiClO4, LiAsF6, LiSbF6, LiCF3SO3, Li (CF3SO3) 2N, Li (C2F5SO3) 2N, LiC4F9SO3, Li (CF3SO2) 3C. One type or two or more types of these Li salts may be contained. However, the total concentration of the Li salt is preferably 0.5M to 3.5M. When the Li salt concentration is less than 0.5M, sufficient lithium ion conductivity cannot be obtained. On the other hand, when the Li salt concentration is more than 3.5M, the viscosity of the electrolytic solution increases so that the conductivity is lowered.

4) Separator As the separator, an insulating thin film having excellent ion permeability and mechanical strength can be used. Sheets made from polyolefins such as polypropylene and polyethylene, polyethylene terephthalate, polyvinylene terephthalate, polyimide, polyamide, glass fiber, polyvinylidene fluoride, polytetrafluoroethylene, cellulose, etc., microporous membranes and non-woven fabrics from organic solvent resistance Used.

  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 as long as the gist of the invention is not exceeded.

Example 1
<Preparation of positive electrode>
Copper oxyphosphate having a particle size of 40 μm is used as the positive electrode active material, acetylene black and graphite are used as the conductive assistant, and polytetrafluoroethylene is used as the binder, and these are in a weight ratio of 100: 2.5: 2.5: 2. Then, the mixture was uniformly mixed and pressure-molded to produce a pellet-shaped positive electrode mixture having a density of 2.2 g / cm 3, a thickness of 5 mm, and a diameter of 16.0 mm.

<Production of negative electrode>
Lithium metal punched into a thickness of 5 mm and a diameter of 16.0 mm was used as the negative electrode.

<Preparation of electrolyte>
1M bistrifluoromethylsulfonylimide lithium (LiTFSI) was dissolved in bistrifluoromethylsulfonylimide 1-ethyl-3-methylimidazolium (EMITFSI) to obtain an electrolytic solution.

<Battery assembly>
The positive electrode mixture is housed in a positive electrode can made of stainless steel, the negative electrode is housed in a negative electrode can made of stainless steel, and a separator made of a polypropylene non-woven fabric is disposed between the positive electrode mixture and the negative electrode. A coin-type lithium battery having the structure shown in FIG. 1 was assembled by caulking and fixing the negative electrode can through an insulating gasket. 1 is a negative electrode can, 2 is a negative electrode, 3 is a gasket, 4 is a positive electrode can, 5 is a positive electrode mixture, and 6 is a separator.

  That is, a pellet-shaped positive electrode mixture 5 is accommodated in a bottomed cylindrical positive electrode can 4. On the other hand, a negative electrode 2 is accommodated in a bottomed cylindrical negative electrode can 1. The separator 6 is disposed between the positive electrode mixture 5 and the negative electrode 2. The electrolytic solution is impregnated in the positive electrode mixture 5, the separator 6, and the negative electrode 2. The negative electrode can 1 is caulked and fixed to the positive electrode can 4 via an insulating gasket.

(Example 2)
Example 1 except that 1M lithium tetrafluoroborate (LiBF4) and 5 wt% lithium bisoxalatoborate were dissolved in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) to obtain an electrolytic solution. The same lithium battery as 1 was produced.

(Example 3)
Same as Example 1 except that 1M lithium tetrafluoroborate (LiBF4) and 5 wt% vinylene carbonate were dissolved in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) to obtain an electrolyte solution. A lithium battery was produced.

Example 4
The same lithium battery as in Example 1 was produced except that 1M bispentafluoroethylsulfonylimide lithium (LiBETI) was dissolved in 1-ethyl-3-methylimidazolium (EMIBETI) to obtain an electrolyte solution. did.

(Example 5)
A lithium battery was prepared in the same manner as in Example 1 except that 1M bistrifluoromethylsulfonylimide lithium (LiTFSI) was dissolved in bistrifluoromethylsulfonylimide 1-butyl-3-methylimidazolium (BMITSI) to obtain an electrolyte.

(Example 6)
The same lithium battery as in Example 1 was prepared except that 1M bistrifluoromethylsulfonylimide lithium (LiTFSI) was dissolved in bistrifluoromethylsulfonylimide 1,2-dimethyl-3-propylimidazolium (DMPITFSI) to obtain an electrolyte. did.

(Example 7)
The same lithium battery as in Example 1 was produced except that the positive electrode density was 1.6 g / cm 3.

(Example 8)
The same lithium battery as in Example 1 was prepared except that the positive electrode density was 2.4 g / cm 3.

Example 9
The same lithium battery as in Example 1 was produced except that the particle diameter of the positive electrode active material was 1 μm.

(Example 10)
The same lithium battery as in Example 1 was produced except that the particle diameter of the positive electrode active material was 50 μm.

(Example 11)
The same lithium battery as in Example 1 was produced except that the positive electrode active material was FeS.

(Example 12)
The same lithium battery as in Example 1 was produced except that the positive electrode active material was FeS2.

(Comparative Example 1)
The same lithium battery as in Example 1 was produced except that the positive electrode active material was MnO2.

(Comparative Example 2)
The same lithium battery as in Example 1 was produced except that the positive electrode active material was (CF) n.

(Comparative Example 3)
The same lithium battery as in Example 1 was produced except that the positive electrode active material was V2O5.

(Comparative Example 4)
The same lithium battery as in Example 1 was produced except that the positive electrode density was 1.2 g / cm 3.

(Comparative Example 5)
The same lithium battery as in Example 1 was produced except that the positive electrode density was 2.8 g / cm 3.

(Comparative Example 6)
The same lithium battery as in Example 1 was produced except that the particle diameter of the positive electrode active material was 0.5 μm.

(Comparative Example 7)
The same lithium battery as in Example 1 was produced except that the particle diameter of the positive electrode active material was set to 70 μm.

(Comparative Example 8)
A lithium battery was prepared in the same manner as in Example 1 except that 1M lithium tetrafluoroborate (LiBF4) was dissolved in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) to obtain an electrolytic solution.

尚、表１において、大電流放電時および長期保存後のデータは、それぞれの電池作製後６時間に２０℃において０．５ｍＡ／ｃｍ２で直流放電を行い、電池電圧が０．８Ｖに達した際の容量を１００とした場合の相対値である。 The batteries of Examples 1 to 12 and Comparative Examples 1 to 8 were subjected to DC discharge at 0.5 mA / cm 2 at 20 ° C. for 6 hours after production, and the capacity when the battery voltage reached 0.8 V (this was expressed as 100 Measured). Next, the batteries of Examples 1 to 12 and Comparative Examples 1 to 8 were subjected to direct current discharge at 5 mA / cm 2 at 20 ° C. for 6 hours after production, and the capacity when the battery voltage reached 0.8 V was measured. The current characteristics were investigated. In addition, after the batteries of Examples 1 to 12 and Comparative Examples 1 to 8 were prepared, they were left in a constant temperature layer at 85 ° C. for 1 week and then subjected to direct current discharge at 20 ° C. at 0.5 mA / cm 2 to reach a battery voltage of 0.8V The capacity was measured and the long-term storage characteristics were examined. The results are summarized in Table 1.

In Table 1, the data at the time of large current discharge and after long-term storage are obtained when DC discharge is performed at 0.5 mA / cm 2 at 20 ° C. for 6 hours after each battery is produced, and the battery voltage reaches 0.8V. The relative value when the capacity of 100 is 100.

By comparing the Examples and Comparative Examples 1 to 3, the positive electrode active material is expressed by M ^{a +} X ^a− (M is a metal ion, X is an anion, a is the number of electrons), and is reduced to M in the discharge reaction. It can be seen that the long-term storage characteristics have been dramatically improved by using those. Further, by comparing the Examples with Comparative Examples 4 and 5, it is understood that the density of the positive electrode mixture must be in the range of 1.6 to 2.4 g / cm 3 in order to be excellent in large current characteristics. . By comparing the Examples with Comparative Examples 6 and 7, it is found that the positive electrode active material particle size must be in the range of 1 to 50 μm in order to achieve both large current characteristics and long-term storage characteristics.

  Further, by comparing Examples 2 and 3 with Comparative Example 8, it is found that when BF4- is used as the anion, a bisoxalatoborate anion or vinylene carbonate is necessary.

Sectional drawing explaining the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Negative electrode can 2 Negative electrode 3 Gasket 4 Positive electrode can 5 Positive electrode mixture 6 Separator

Claims (6)

A positive electrode mixture comprising a positive electrode active material, a conductive additive, and a binder, a negative electrode disposed opposite to the positive electrode mixture and containing at least one of lithium metal or lithium alloy, and the negative electrode and the positive electrode mixture And an electrolyte using an ionic liquid containing an organic cation represented by the following chemical formula (1), wherein the positive electrode active material is M ^{a +} X ^a− (M is a metal Lithium battery characterized in that it is represented by ions, X is an anion, and a is the number of electrons), and is reduced to M in the discharge reaction, and has a density in the range of 1.6 to 2.4 g / cm 3. .

(Where R and R ′ are C1 to C4 alkyl groups, R ″ is halogen or C1 to C3 alkyl groups)   The lithium battery according to claim 1, wherein the positive electrode active material is Cu 4 O (PO 4) 2.   The lithium battery according to claim 1, wherein the positive electrode active material is FeS or FeS 2.   The lithium battery according to claim 1, wherein a particle diameter of the positive electrode active material is in a range of 1 to 50 μm.   2. The lithium battery according to claim 1, wherein the content of the conductive additive in the positive electrode mixture is 1 wt% to 5 wt%. The lithium battery according to claim 1, wherein the electrolytic solution contains a bisoxalatoborate anion or vinylene carbonate.

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