JP2007234512A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery capable of conducting reversible charge discharge and obtaining high initial discharge capacity density. <P>SOLUTION: A negative active material contains easily graphitizable carbon having a crystallite size in the direction of c axis Lc of 72.5 nm or less as a single component or the main component. A negative electrode material is obtained by mixing the negative active material and polyacrylonitrile of a binder in a weight ratio of 97:3. Next, N-methyl-2-pyrolidone is added to the negative electrode material, and they are kneaded to prepare negative mix slurry. The slurry is applied to both surfaces of copper foil which is a negative current collector by a doctor blade process, and dried to form a negative active material layer. The copper foil formed with the negative active material layer is cut in a prescribed size, and a negative tab is fixed to produce a negative electrode. <P>COPYRIGHT: (C)2007,JPO&INPIT

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 as a secondary battery with a high energy density, for example, charge and discharge by moving lithium ions between the positive electrode and the negative electrode are widely used. Yes.

In such a non-aqueous electrolyte secondary battery, a lithium transition metal composite oxide having a layered structure such as lithium nickelate (LiNiO ₂ ) and lithium cobaltate (LiCoO ₂ ) is generally used as a positive electrode, and lithium as a negative electrode. Carbon materials, lithium metal, lithium alloys and the like that can be occluded and released are used (see, for example, Patent Document 1).

  By using the non-aqueous electrolyte secondary battery, a discharge capacity of 150 to 180 mAh / g, a potential of about 4 V, and a theoretical capacity of about 260 mAh / g can be obtained.

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. ing.

  On the other hand, recently, research on non-aqueous electrolyte secondary batteries using sodium ions instead of lithium ions has been started.

There are very few research examples of the non-aqueous electrolyte secondary battery using this sodium ion, sodium perchlorite (NaFeO ₂ ) is used for the positive electrode, and sodium perchlorate (NaClO ₄ ) which is a peroxide for the non-aqueous electrolyte. Has been proposed (see Non-Patent Document 1, for example).
JP 2003-151549 A Summary of the 45th Battery Symposium p268-p269

  However, since the charge / discharge reaction of the conventional nonaqueous electrolyte secondary battery is performed by dissolution and precipitation of sodium ions, good initial discharge capacity density, charge / discharge efficiency, and charge / discharge characteristics (charge / discharge cycle characteristics) are obtained. Is difficult.

  Moreover, when charging and discharging are repeated, dendritic precipitates (dendrites) are likely to be generated in the nonaqueous electrolyte. Therefore, an internal short circuit may occur due to the dendrite, and it is difficult to ensure sufficient safety.

  Furthermore, in general, non-aqueous electrolyte secondary batteries using lithium ions use highly practical negative electrodes made of carbon or silicon that can occlude and release lithium ions, but use sodium ions. In the nonaqueous electrolyte secondary battery, when a negative electrode made of carbon (graphite) is used, a small amount of sodium ions are injected into the negative electrode. As a result, good charge / discharge characteristics cannot be obtained.

  When a negative electrode made of silicon is used, the negative electrode does not occlude sodium ions. As a result, reversible charging / discharging cannot be performed.

  An object of the present invention is to provide a non-aqueous electrolyte secondary battery that can perform reversible charging / discharging and obtain a high initial discharge capacity density.

  The non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte including sodium ions. The negative electrode active material has a crystallite size Lc in the c-axis direction. It contains graphitizable carbon of 72.5 nm or less as a single component or main component.

  In the nonaqueous electrolyte secondary battery according to the present invention, sodium ions are sufficiently occluded and released from the negative electrode active material containing graphitizable carbon as a single component or main component. Thereby, reversible charging / discharging can be performed and a high initial discharge capacity density can be obtained.

  The crystallite size Lc in the c-axis direction of the graphitizable carbon may be 15.5 nm or less.

  In this case, a higher initial discharge capacity density can be obtained.

  The nonaqueous electrolyte secondary battery may further include a current collector made of a metal foil, and the negative electrode active material may be formed on the current collector.

  In this case, the negative electrode active material is easily formed on the current collector made of the metal foil.

  The non-aqueous electrolyte may include sodium hexafluorophosphate. In this case, safety is improved.

  The non-aqueous electrolyte may contain one or more selected from the group consisting of cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles and amides.

  In this case, the cost can be reduced and the safety is improved.

  According to the nonaqueous electrolyte secondary battery according to the present invention, sodium ions are sufficiently occluded and released from the negative electrode containing carbon (graphitizable carbon). Thereby, reversible charging / discharging can be performed, and a high initial discharge capacity density can be obtained.

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

  The nonaqueous electrolyte secondary battery according to the present embodiment includes a working electrode (hereinafter referred to as a negative electrode), a counter electrode (hereinafter referred to as a positive electrode), and a nonaqueous electrolyte.

  The various materials described below and the thicknesses and concentrations of the materials are not limited to those described below, and can be set as appropriate.

(1) Background of the Invention It is generally known that sodium ions are occluded and released into hard carbon (non-graphitizable carbon) which is a kind of carbon.

  Hard carbon refers to carbon that does not graphitize even when heat-treated at a high temperature, because the heat-treatment temperature is not sufficiently high and graphitization has not progressed, so that it is amorphous in the measurement by the X-ray diffraction method. On the other hand, since the heat treatment temperature is not sufficiently high and graphitization has not progressed, the measurement by X-ray diffraction shows amorphous, but the carbon that is graphitized by heat treatment at a high temperature is soft carbon (easy graphite). Carbon).

  In hard carbon, the crystallites are disordered, so graphitization does not proceed even when heat-treated at high temperatures. However, in soft carbon, microcrystals are arranged in almost the same direction. Graphitization occurs by diffusion.

  It is also generally known that sodium ions are not occluded and released from graphite even with the same carbon.

  In the present embodiment, in order to obtain knowledge about what physical properties of graphite that does not occlude and release sodium ions can be occluded and released, a mechanical milling method is used. A carbon material was obtained by pulverizing graphite (artificial graphite) with

  Then, when a charge / discharge test was performed using this carbon material, it was devised that sodium ions were occluded and released from the negative electrode when the crystallite size Lc in the c-axis direction was below a certain value. .

(2) Each configuration of the nonaqueous electrolyte secondary battery (2-1) Production of negative electrode In the present embodiment, the negative electrode active material described later has a crystallite c-axis size Lc of, for example, 72.5 nm. Hereinafter, a carbon material preferably having a size of 15.5 nm or less is included as a single component or a main component.

  In order to obtain such a negative electrode active material, for example, mechanical milling treatment is performed on artificial graphite for 1 to 6 hours, for example, at a rotation speed of 300 rpm for 5 minutes and repeated for 5 minutes.

  A negative electrode material is obtained by mixing the negative electrode active material obtained by the mechanical milling process and polyacrylonitrile (PAN) as a binder at a weight ratio of 97: 3, for example.

  Next, N-methyl-2-pyrrolidone is added to the negative electrode material and kneaded to prepare a slurry as a negative electrode mixture.

  Next, the slurry is applied to a negative electrode current collector, for example, a 11 μm thick copper foil by a doctor blade method, and then dried to form a negative electrode active material layer.

  Next, the copper foil on which the negative electrode active material layer is formed is cut into a size of 2 cm × 2 cm, and a negative electrode is attached by attaching a negative electrode tab.

(2-2) Production of non-aqueous electrolyte As the non-aqueous electrolyte, a solution obtained by dissolving an electrolyte salt in a non-aqueous solvent can be used.

  Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles, amides, and the like, which are usually used as non-aqueous solvents for batteries. Is mentioned.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate and the like, and those in which some or all of these hydrogen groups are fluorinated can be used. For example, trifluoropropylene carbonate, fluoro Examples include ethyl carbonate.

  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. Some of these hydrogen groups are fluorinated. It is possible to use.

  Examples of the esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and 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-cineol, crown ether and the like.

  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, pentyl phenyl. Ether, methoxytoluene, benzyl ethyl 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, tetraethy Glycol dimethyl and the like.

  Nitriles include acetonitrile and the like, and amides include dimethylformamide and the like.

Examples of electrolyte salts include non-peroxides that are soluble in nonaqueous solvents such as sodium hexafluorophosphate (NaPF ₆ ), sodium tetrafluoroborate (NaBF ₄ ), NaCF ₃ SO ₃ , and NaBeTi. Use expensive ones. In addition, 1 type may be used among said electrolyte salt, and may be used in combination of 2 or more type.

  In the present embodiment, as a nonaqueous electrolyte, a nonaqueous solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 50:50, and sodium hexafluorophosphate as an electrolyte salt to a concentration of 1 mol / l. What was added so that it may become is used.

(2-3) Production of Nonaqueous Electrolyte Secondary Battery A nonaqueous electrolyte secondary battery as shown below is produced using the negative electrode, the nonaqueous electrolyte, the positive electrode and the reference electrode described below.

  FIG. 1 is a schematic explanatory diagram of a test cell of a nonaqueous electrolyte secondary battery according to the present embodiment.

As shown in FIG. 1, in an inert atmosphere, a lead is attached to the negative electrode (working electrode) 1 and a lead is attached to the positive electrode 2 made of sodium metal. Instead of the positive electrode (counter electrode) 2 made of sodium metal, a positive electrode 2 containing a material such as NaMnO ₂ capable of inserting and extracting sodium ions may be used.

  Next, the separator 4 is inserted between the negative electrode 1 and the positive electrode 2, and the negative electrode 1, the positive electrode 2, and the reference electrode 3 made of sodium metal are disposed in the cell container 10. And the nonaqueous electrolyte secondary battery as a test cell is produced by inject | pouring the said nonaqueous electrolyte 5 in the cell container 10. FIG.

(3) Effects in this Embodiment In the nonaqueous electrolyte secondary battery according to this embodiment, a carbon material having a crystallite size Lc in the c-axis direction of 72.5 nm or less is a single component or a main component. By using the negative electrode active material contained as sodium ions, sodium ions are sufficiently occluded and released from the negative electrode 1. Thereby, reversible charging / discharging can be performed and a high initial discharge capacity density can be obtained.

  In addition, by using a carbon material having a c-axis direction crystallite size Lc of 15.5 nm or less as the negative electrode active material, reversible charging / discharging can be performed and a higher initial discharge capacity density can be obtained. It becomes possible.

  The present inventor considers the reason why reversible charging / discharging is possible and the reason why a high initial discharge capacity density can be obtained as follows.

  By applying a physical impact to the artificial graphite by mechanical milling treatment, the layered graphite crystals gradually collapse, so that the graphite layer is finely divided into a carbon material in a disordered state. As a result, the crystallite size Lc in the c-axis direction is considered to be small.

  Such a carbon material has many voids and end face portions therein. It is considered that even sodium ions that are not occluded and released between the graphite layers before mechanical milling are likely to be occluded and released in the voids and end surfaces that have spread by mechanical milling. Therefore, a high initial discharge capacity density can be obtained.

  On the other hand, when the graphite layer is too finely divided and excessively disturbed by the mechanical milling process, the voids and end face portions are reduced. As a result, it is considered that sodium ions are not occluded and released, making it difficult to perform reversible charging / discharging.

  Hereinafter, the nonaqueous electrolyte secondary battery of an Example and a comparative example is demonstrated.

(A) Example 1
The charge / discharge characteristics of the nonaqueous electrolyte secondary battery produced based on the above-described embodiment were examined.

  In this example, artificial milling was performed for 1 hour on artificial graphite.

  The interlayer distance d of the carbon material as the negative electrode active material used in this example was 0.3367 nm, and the crystallite size Lc in the c-axis direction was 72.5 nm. The interlayer distance d and the crystallite size Lc in the c-axis direction are the peak intensity values when the diffraction angle 2θ is in the range of 10 degrees to 40 degrees using an X-ray diffractometer (XRD). Based on the calculation. FIG. 2 shows the measurement results of the XRD measurement when the diffraction angle 2θ is in the range of 10 degrees to 40 degrees.

  In the X-ray diffractometer, Cu (copper) was used as the X-ray source (target), the working voltage and working current were 40 kV and 40 mA, respectively, and the scanning speed was 0.6 degrees / minute.

  In the produced non-aqueous electrolyte secondary battery, charging was performed at a constant current of 0.4 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 0V. Then, the charge / discharge characteristics were examined by discharging at a constant current of 0.4 mA until the potential of the negative electrode 1 with respect to the reference electrode 3 reached 1.0 V.

  As a result, an initial discharge capacity density of 40 mAh / g could be obtained.

(B) Example 2
The charge / discharge characteristics of the nonaqueous electrolyte secondary battery produced based on the above-described embodiment were examined.

  In this example, artificial milling was performed on artificial graphite for 2 hours. FIG. 3 shows the measurement results of the XRD measurement when the diffraction angle 2θ is in the range of 10 degrees to 40 degrees. The interlayer distance d of the carbon material as the negative electrode active material used in this example was 0.3337 nm, and the crystallite size Lc in the c-axis direction was 15.5 nm.

  In the produced non-aqueous electrolyte secondary battery, charging was performed at a constant current of 0.4 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 0V. Then, the charge / discharge characteristics were examined by discharging at a constant current of 0.4 mA until the potential of the negative electrode 1 with respect to the reference electrode 3 reached 1.0 V.

  As a result, an initial discharge capacity density of about 61 mAh / g could be obtained.

(C) Example 3
The charge / discharge characteristics of the nonaqueous electrolyte secondary battery produced based on the above-described embodiment were examined.

  In this example, the artificial graphite was subjected to mechanical milling for 3 hours. FIG. 4 shows the measurement results of the XRD measurement when the diffraction angle 2θ is in the range of 10 degrees to 40 degrees. The interlayer distance d of the carbon material as the negative electrode active material used in this example was 0.3440 nm, and the crystallite size Lc in the c-axis direction was 1.6 nm.

  In the produced non-aqueous electrolyte secondary battery, charging was performed at a constant current of 0.4 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 0V. Then, the charge / discharge characteristics were examined by discharging at a constant current of 0.4 mA until the potential of the negative electrode 1 with respect to the reference electrode 3 reached 1.0 V.

  As a result, an initial discharge capacity density of about 120 mAh / g could be obtained.

(D) Example 4
The charge / discharge characteristics of the nonaqueous electrolyte secondary battery produced based on the above-described embodiment were examined.

  In this example, artificial milling was performed on artificial graphite for 6 hours. FIG. 5 shows the measurement results of the XRD measurement when the diffraction angle 2θ is in the range of 10 degrees to 40 degrees. The interlayer distance d of the carbon material as the negative electrode active material used in this example was 0.3542 nm, and the crystallite size Lc in the c-axis direction was 0.65 nm.

  In the produced non-aqueous electrolyte secondary battery, charging was performed at a constant current of 0.4 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 0V. Then, the charge / discharge characteristics were examined by discharging at a constant current of 0.4 mA until the potential of the negative electrode 1 with respect to the reference electrode 3 reached 1.0 V.

  As a result, an initial discharge capacity density of 107 mAh / g could be obtained.

(E) Comparative Example 1
The charge / discharge characteristics of the nonaqueous electrolyte secondary battery produced based on the above-described embodiment were examined.

  In this comparative example, artificial milling was performed for 0.5 hours on artificial graphite. FIG. 6 shows the measurement results of the XRD measurement when the diffraction angle 2θ is in the range of 10 degrees to 40 degrees. The interlayer distance d of the carbon material as the negative electrode active material used in this comparative example was 0.3366 nm, and the crystallite size Lc in the c-axis direction was 80.6 nm.

  In the produced non-aqueous electrolyte secondary battery, charging was performed at a constant current of 0.4 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 0V. Thereafter, discharge was attempted at a constant current of 0.4 mA until the potential of the negative electrode 1 with respect to the reference electrode 3 reached 1.0 V, but no discharge was possible.

(F) Comparative example 2
The nonaqueous electrolyte secondary battery of this comparative example is different from the nonaqueous electrolyte secondary battery of Comparative Example 1 in the negative electrode.

  In this comparative example, negative electrode material was obtained by mixing artificial graphite not subjected to mechanical milling treatment as a negative electrode active material and polyacrylonitrile (PAN) as a binder at a weight ratio of 97: 3.

  Next, N-methyl-2-pyrrolidone was added to the negative electrode material and kneaded to prepare a slurry as a negative electrode mixture.

  Next, the slurry was applied on a copper foil having a thickness of 11 μm as a negative electrode current collector by a doctor blade method, and then dried to form a negative electrode active material layer.

  Next, the copper foil on which the negative electrode active material layer was formed was cut into a size of 2 cm × 2 cm, and a negative electrode tab was attached to prepare a negative electrode.

  FIG. 7 shows the measurement result of the XRD measurement when the diffraction angle 2θ is in the range of 10 degrees to 40 degrees. The interlayer distance d of the carbon material as the negative electrode active material used in this comparative example was 0.3366 nm, and the crystallite size Lc in the c-axis direction was 80.6 nm.

  In the produced non-aqueous electrolyte secondary battery, charging was performed at a constant current of 0.4 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 0V. Thereafter, discharge was attempted at a constant current of 0.4 mA until the potential of the negative electrode 1 with respect to the reference electrode 3 reached 1.0 V, but no discharge was possible.

(G) Summary FIG. 8 is a graph showing the relationship between the crystallite size Lc in the c-axis direction of the negative electrode active material (carbon material) in Examples 1 to 4 and Comparative Examples 1 and 2 and the initial discharge capacity density. is there. FIG. 9 is a graph showing the relationship between the interlayer distance d of the negative electrode active material (carbon material) and the initial discharge capacity density in Examples 1 to 4 and Comparative Examples 1 and 2.

  It was found that the negative electrode active material can occlude and release sodium ions by applying physical impact to the artificial graphite, which is a negative electrode active material that cannot occlude and release sodium ions, by mechanical milling treatment. It was.

  In this case, it was found that whether or not sodium ions can be occluded and released depends on the time for performing the mechanical milling treatment, and the treatment time is preferably 1 to 6 hours.

  As shown in FIG. 8, when the size Lc of the crystallite in the c-axis direction of the negative electrode active material is 72.5 nm or less, the negative electrode active material can occlude and release sodium ions, which is higher. In order to obtain the initial discharge capacity density, it has been found that the crystallite size Lc in the c-axis direction of the negative electrode active material is preferably 15.5 nm or less.

  Thus, the charge / discharge characteristics of the carbon material as the negative electrode active material obtained by mechanical milling of artificial graphite depend only on Lc, and as shown in FIG. 9, it may not depend on the interlayer distance d. It was revealed. Thereby, it turned out that the carbon material as a negative electrode active material used in the said Example is a thing different from normal hard carbon.

  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 automotive power source.

It is a schematic explanatory drawing of the test cell of the nonaqueous electrolyte secondary battery which concerns on this Embodiment. 3 is a graph showing measurement results of XRD measurement of Example 1. 6 is a graph showing measurement results of XRD measurement of Example 2. 6 is a graph showing measurement results of XRD measurement of Example 3. 6 is a graph showing measurement results of XRD measurement of Example 4. 6 is a graph showing measurement results of XRD measurement in Comparative Example 1. 10 is a graph showing measurement results of XRD measurement of Comparative Example 2. It is a graph which shows the relationship between the magnitude | size Lc of the crystallite of the c-axis direction of the negative electrode active material (carbon material) in Examples 1-4 and Comparative Examples 1 and 2, and initial stage discharge capacity density. It is a graph which shows the relationship between the interlayer distance d of the negative electrode active material (carbon material) in Examples 1-4, and Comparative Examples 1 and 2, and initial stage discharge capacity density.

Explanation of symbols

1 Negative electrode (working electrode)
2 Positive electrode (counter electrode)
3 Reference electrode 4 Separator 5 Nonaqueous electrolyte 10 Cell container

Claims (5)

A positive electrode, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte containing sodium ions,
The non-aqueous electrolyte secondary battery, wherein the negative electrode active material includes graphitizable carbon having a c-axis direction crystallite size Lc of 72.5 nm or less as a single component or a main component. The nonaqueous electrolyte secondary battery according to claim 1, wherein the crystallite size Lc in the c-axis direction of the graphitizable carbon is 15.5 nm or less. A current collector made of metal foil;
The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material is formed on the current collector. The nonaqueous electrolyte secondary battery according to claim 1, wherein the nonaqueous electrolyte contains sodium hexafluorophosphate. The non-aqueous electrolyte includes one or more selected from the group consisting of cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles and amides. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4.

Images