JP2006164810A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery capable of providing high volume capacity density. <P>SOLUTION: Lithium metal, a lithium alloy or a carbon material such as graphite or the like capable of storing and releasing lithium is used for a positive electrode. A negative electrode includes a collector and an active material. The collector of the negative electrode is formed of a fiber containing carbon, and holes (voids) formed by the fiber containing carbon are filled with carbon being the active material of the negative electrode. In order to hold the active material of the negative electrode, the size of the holes is preferably 1-50 μm and further preferably 3-30 μm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte.

  Currently, non-aqueous electrolyte secondary batteries that use a non-aqueous electrolyte and charge and discharge by moving lithium ions between a positive electrode and a negative electrode are used as secondary batteries with high energy density.

In such a nonaqueous electrolyte secondary battery, a lithium transition metal composite oxide such as lithium cobaltate (LiCoO ₂ ) is generally used as a positive electrode, and a carbon material capable of occluding and releasing lithium as a negative electrode, lithium metal, lithium An alloy or the like is used. In addition, a nonaqueous electrolyte in which an electrolyte salt such as lithium tetrafluoroborate (LiBF ₄ ) or lithium hexafluorophosphate (LiPF ₆ ) is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate is used. Yes.

  In recent years, such non-aqueous electrolyte secondary batteries have been used as power sources for various portable devices, etc., but with the increase in power consumption due to multifunctional portable devices, non-aqueous electrolytes with higher energy density Secondary batteries are desired.

The current collector for the negative electrode must be excellent in conductivity, be a thin film, and be electrochemically stable. Therefore, copper foil is used as a current collector for the negative electrode, and carbon materials such as graphite and acetylene black are used as the active material for the negative electrode (see, for example, Patent Document 1).
JP 2004-311429 A

  However, the copper foil used as the current collector for the negative electrode is not directly involved in charging / discharging and has a very heavy weight of 8.96 g per cubic centimeter. This copper foil is the second most heavy component after the positive electrode active material, the electrolytic solution as the nonaqueous solvent, and the outer can of the nonaqueous electrolyte secondary battery.

  It is difficult to reduce the amount of the positive electrode active material and the electrolyte because it is directly related to the capacity of the nonaqueous electrolyte secondary battery. In addition, if the thickness of the outer can is reduced, the reliability of the nonaqueous electrolyte secondary battery may be reduced.

  An object of the present invention is to provide a nonaqueous electrolyte secondary battery capable of obtaining a high volume capacity density.

  A non-aqueous electrolyte secondary battery according to the present invention includes a negative electrode that can occlude and release lithium and includes a current collector, a positive electrode that can occlude and release lithium, and a non-aqueous electrolyte. The body is composed of carbon-containing fibers.

  In the non-aqueous electrolyte secondary battery according to the present invention, the current collector of the negative electrode is composed of carbon-containing fibers, so that the carbon-containing fibers themselves act as active materials, and the carbon-containing fiber pores It is also possible to fill the active material. Thereby, a high volume capacity density is obtained. Moreover, since the fiber containing carbon is lighter than a metal, the weight of the negative electrode can be reduced.

  The negative electrode may further include an active material, and the active material may be filled in pores formed by fibers containing carbon. Thereby, a higher volume capacity density is obtained.

  The packing density of the active material in the negative electrode may be 1.66 g / ml or more. Thereby, a high volume capacity density can be obtained as compared with the case where a current collector made of metal is used.

  According to the present invention, a high volume capacity density can be obtained and the current collector can be reduced in weight. Further, the current collector itself can be involved in charging / discharging.

  Hereinafter, the nonaqueous electrolyte secondary battery according to the present embodiment will be described.

  The nonaqueous electrolyte secondary battery according to the present embodiment includes a negative electrode, a positive electrode, and a nonaqueous electrolyte.

  As the positive electrode, a lithium metal capable of inserting and extracting lithium (Li), a lithium alloy, a carbon material such as graphite, or the like is used.

  As the binder for the positive electrode, at least one selected from polytetrafluoroethylene, polyvinylidene fluoride, polyethylene oxide, polyvinyl acetate, polymethacrylate, polyacrylate, polyacrylonitrile, polyvinyl alcohol, styrene-butadiene rubber, carboxymethyl cellulose, or the like is used. Can be used.

  When the amount of the binder added is large, the ratio of the active material contained in the positive electrode becomes small, so that a high capacity cannot be obtained. Therefore, the addition amount of the binder is in the range of 0.1 to 30% by weight of the whole positive electrode, preferably in the range of 0.1 to 20% by weight, more preferably in the range of 0.1 to 10% by weight. To do.

  The negative electrode includes a current collector and an active material (hereinafter referred to as a negative electrode active material). The current collector of the negative electrode is made of carbon-containing fibers (hereinafter simply referred to as carbon fibers), and pores (voids) formed by the carbon fibers are filled with carbon as the negative electrode active material. In this embodiment, carbon fiber paper (carbon fiber sheet) is used as the negative electrode current collector.

  The conditions required for the current collector of the negative electrode include that electrons can move smoothly between the negative electrode active material, that is, the conductivity is excellent, the electrochemical stability is stable, and the volume is small and low. It is mentioned that it is weight.

  Examples of the nonaqueous solvent for the nonaqueous electrolyte in the present embodiment include cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, and nitriles that are usually used as nonaqueous solvents for batteries. Or amides etc. are mentioned, At least 1 sort (s) selected from these can be used.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and the like, and trifluoropropylene carbonate or fluoroethyl carbonate in which a part or all of these hydrogen groups are fluorinated.

  Examples of chain carbonic acid esters include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, and the like, and some or all of these hydrogens are fluorinated. Also mentioned.

  Examples of esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate or γ-butyrolactone.

  Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,4-dioxane, 1,3 , 5-trioxane, furan, 2-methylfuran, 1,8-cineole or crown ether.

  Examples of chain ethers include 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, Pentylphenyl ether, methoxytoluene, benzylethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1 , 1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether or the La ethylene glycol dimethyl and the like.

  Examples of nitriles include acetonitrile, and examples of amides include dimethylformamide.

As an example of the lithium salt added to the nonaqueous solvent of the nonaqueous electrolyte, a lithium salt generally used as an electrolyte in a conventional nonaqueous electrolyte secondary battery can be used. For example, LiBF ₄ , LiPF ₆ , LiCF ₃ SO ₃ , LiAsF ₆ , LiN (C _l F _{21 + 1} SO ₂ ) (C _m F _{2m + 1} SO ₂ ) (where l and m are integers of 1 or more), LiC (C _p F _{2p + 1} SO ₂ ) At least one selected from (C _q F _{2q + 1} SO ₂ ) (C _r F _{2r + 1} SO ₂ ) (where p, q, and r are integers of 1 or more) can be used, or two The above may be used in combination. The lithium salt is used after being dissolved in the non-aqueous solvent at a concentration of 0.1 to 1.5 mol / l, preferably 0.5 to 1.5 mol / l.

  In the present embodiment, carbon fiber paper made of carbon fiber is used as a current collector for the negative electrode, and pores formed by the carbon fiber are filled with, for example, the following materials as the negative electrode active material.

In other words, any negative electrode active material that can be impregnated into carbon fiber paper may be used. For example, graphitizable carbon such as natural graphite or artificial graphite, non-graphitizable carbon such as carbon black or amorphous carbon, lithium-aluminum alloy, ferric oxide (Fe ₂ O ₃ ) or tungsten oxide (WO ₂ ) Oxides such as tin compounds, silicon-based materials, germanium-based materials, nitrides such as Li ₇ MnN ₄ or Li ₃ FeN ₂ can be used as the negative electrode active material.

  As described above, in the present embodiment, since the current collector of the negative electrode is composed of carbon-containing fibers, the carbon-containing fibers themselves act as the negative electrode active material, and the carbon-containing fiber pores It is also possible to fill the active material for the negative electrode. Thereby, a high volume capacity density is obtained.

  However, in order to allow the negative electrode active material to be soaked into the carbon fiber paper, it is necessary that the negative electrode active material have a particle size that can penetrate between the carbon fibers constituting the carbon fiber paper.

Further, as a current collector for the negative electrode, carbon fiber paper is remarkably used without using a copper foil having a density of 8.96 g / cm ³ and having a weight about 4 times that of carbon as the negative electrode active material. Weight reduction can be achieved.

  Furthermore, by using carbon fiber paper containing carbon fibers that can occlude and release lithium as the current collector of the negative electrode, a higher volume capacity density than when using a material that cannot occlude and release lithium as the current collector Can be obtained.

  In addition, in order to hold | maintain the active material for negative electrodes, it is preferable that the said hole is 1 micrometer-50 micrometers, and it is more preferable that it is 3 micrometers-30 micrometers.

  Hereinafter, the nonaqueous electrolyte secondary battery of an Example is demonstrated. In addition, the nonaqueous electrolyte secondary battery in this invention is not limited to what was shown in the following Example, It can implement by changing suitably in the range which does not change the summary.

  FIG. 1 is a schematic explanatory diagram of test cells prepared in preliminary experiments, examples and comparative examples described later.

  As shown in FIG. 1, a carbon electrode was used for the negative electrode 1 in an inert atmosphere. In addition, a lithium metal electrode was used for the counter electrode 2, lithium metal was used for the reference electrode 3, and the nonaqueous electrolyte 5 was injected into the cell container 10. A separator 4 is interposed between the negative electrode 1 and the counter electrode 2. In this example, lithium metal is used for the counter electrode 2. However, in the original nonaqueous electrolyte secondary battery, the counter electrode 2 is used as a positive electrode.

(Preliminary experiment)
A negative electrode 1 was obtained by cutting only carbon fiber paper into a size of 2 × 2 cm. The porosity of the carbon fiber paper is 96.4%. In addition, the porosity is a ratio of the pores in the carbon fiber paper.

  As the nonaqueous electrolyte 5, a solution obtained by dissolving lithium hexafluorophosphate as a solute at a ratio of 1 mol / l in a mixed solvent of ethylene carbonate and diethyl carbonate was used. The volume ratio of ethylene carbonate to diethyl carbonate is 50:50.

  FIG. 2 is a graph showing the discharge characteristics of the test cell produced in the preliminary experiment.

  As shown in FIG. 2, a discharge capacity density (mass discharge capacity density) of 62 mAh / g (volume discharge capacity density per electrode: 4.9 mAh / ml) could be obtained.

(Example)
In this example, the negative electrode 1 was produced as shown below.

  Mixing scaly graphite having a particle diameter of 4 μm as an active material for a negative electrode, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a thickener in a weight ratio of 98: 1: 1, respectively. Thus, a mixture was prepared. And the slurry was produced by adding a pure water to this mixture.

  The carbon fiber paper as the current collector of the negative electrode 1 was sufficiently impregnated with the above slurry, dried, and then cut into a size of 2 × 2 cm to be the negative electrode 1.

  The weight of the negative electrode 1 was 136 mg. In addition, the porosity of the carbon fiber paper used in this example is 96.4%, and the discharge capacity density of the carbon fiber paper alone is 62 mAh / g (volume discharge capacity density per electrode: 4.9 mAh / ml). Met. The filling density of the negative electrode active material in the negative electrode 1 including the current collector was 1.66 g / ml.

  A test cell having the configuration shown in FIG. 1 was produced using the negative electrode 1 produced as described above, the counter electrode 2 and the reference electrode 3 made of lithium metal.

  As the nonaqueous electrolyte 5, a solution obtained by dissolving lithium hexafluorophosphate as a solute at a ratio of 1 mol / l in a mixed solvent of ethylene carbonate and diethyl carbonate was used. The volume ratio of ethylene carbonate to diethyl carbonate is 50:50.

  FIG. 3 is a graph showing the discharge characteristics of the test cells produced in the examples.

  As shown in FIG. 3, a discharge capacity density of 278.5 mAh / g (volume discharge capacity density per electrode: 462.8 mAh / ml) could be obtained.

  FIG. 4 is an enlarged photograph of the carbon fiber paper surface by a scanning electron microscope (SEM).

  As shown in FIG. 4, it was found that the particle size of the negative electrode active material is preferably 30 μm or less, and more preferably 20 μm or less.

(Comparative example)
In this comparative example, the negative electrode 1 was produced as shown below.

  Mixing scaly graphite having a particle diameter of 4 μm as an active material for a negative electrode, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a thickener in a weight ratio of 98: 1: 1, respectively. Thus, a mixture was prepared. And the slurry was produced by adding a pure water to this mixture.

  A negative electrode 1 was obtained by applying the prepared slurry to a 2 × 2 cm copper foil as a current collector of the negative electrode 1. The copper foil has a thickness of 11 μm.

  The weight of the negative electrode 1 was 48.6 mg, and the packing density of the negative electrode active material in the negative electrode 1 including the current collector was 1.16 g / ml.

  A test cell having the configuration shown in FIG. 1 was produced using the negative electrode 1 produced as described above, the counter electrode 2 and the reference electrode 3 made of lithium metal.

  As the nonaqueous electrolyte 5, a solution obtained by dissolving lithium hexafluorophosphate as a solute at a ratio of 1 mol / l in a mixed solvent of ethylene carbonate and diethyl carbonate was used. The volume ratio of ethylene carbonate to diethyl carbonate is 50:50.

  FIG. 5 is a graph showing the discharge characteristics of the test cell produced in the comparative example.

  As shown in FIG. 5, a discharge capacity density of 212.9 mAh / g (volume discharge capacity density per electrode: 251.6 mAh / ml) could be obtained.

(Evaluation)
By using carbon fiber paper that can occlude and release lithium and can be highly filled with an active material for a negative electrode instead of a current collector made of metal (for example, copper foil) as a negative electrode current collector, the negative electrode As compared with the case where copper foil was used as the current collector, the volume discharge capacity density per electrode could be increased by about 1.8 times.

  Moreover, when this volume discharge capacity density (mAh / ml) is converted into mass discharge capacity density (mAh / g), the increase is approximately equal to the discharge capacity density (62 mAh / g) of carbon fiber paper. It becomes density. Thus, it was found that not only the negative electrode active material but also the current collector itself was involved in charging and discharging.

  Further, in the examples, the negative electrode active material was filled in the pores between the carbon fibers of the carbon fiber paper. Thereby, a packing density of 1.66 g / ml, which greatly exceeds the packing density of the negative electrode active material of 1.16 g / ml in the comparative example, could be obtained.

  The nonaqueous electrolyte secondary battery according to the present invention can be used as various power sources such as a portable power source and an automobile power source.

It is a schematic explanatory drawing of the test cell produced in the preliminary experiment, the Example, and the comparative example. It is the graph which showed the discharge characteristic of the test cell produced by the preliminary experiment. It is the graph which showed the discharge characteristic of the test cell produced in the Example. It is an enlarged photograph of the carbon fiber paper surface by a scanning electron microscope. It is the graph which showed the discharge characteristic of the test cell produced by the comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Negative electrode 2 Counter electrode 3 Reference electrode 4 Separator 5 Nonaqueous electrolyte 10 Cell container

Claims (3)

A negative electrode capable of inserting and extracting lithium and including a current collector; a positive electrode capable of inserting and extracting lithium; and a non-aqueous electrolyte.
The non-aqueous electrolyte secondary battery, wherein the negative electrode current collector is composed of a fiber containing carbon. The negative electrode further includes an active material,
The non-aqueous electrolyte secondary battery according to claim 1, wherein the active material is filled in pores formed by the carbon-containing fibers. The nonaqueous electrolyte secondary battery according to claim 1, wherein a filling density of the active material in the negative electrode is 1.66 g / ml or more.

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