JP4963806B2 - Nonaqueous electrolyte secondary battery - Google Patents

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 ) or lithium cobaltate (LiCoO ₂ ) is generally used as a positive electrode, and lithium is occluded as a negative electrode. In addition, carbon materials that can be released, lithium metal, lithium alloys, and the like are used (for example, see 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 non-aqueous 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 ferrite (NaFeO ₂ ) is used for the positive electrode, and sodium perchlorate (NaClO ₄ ), which is a peroxide, is used 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, charge / discharge efficiency and charge / discharge characteristics are not good.

  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. In addition, 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 capable of performing reversible charge / discharge and capable of obtaining good charge / discharge characteristics.

  A nonaqueous electrolyte secondary battery according to the present invention includes a positive electrode capable of inserting and extracting sodium, a negative electrode including a negative electrode active material capable of inserting and extracting sodium, and a nonaqueous electrolyte in which sodium ions are dissolved. The active material is composed of a carbon material having an interlayer distance d (002) of 0.377 nm or more by an X-ray diffraction method and a crystallite size Lc of c axis direction of 1.29 nm or less as a single component or a main component. Is included.

  In the nonaqueous electrolyte secondary battery according to the present invention, sodium ions are sufficiently occluded and released from the negative electrode including the carbon material. Accordingly, reversible charge / discharge can be performed, and a high initial discharge capacity density and good charge / discharge characteristics can be obtained.

  A current collector made of a metal foil may be further provided, 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.

  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 including the carbon material. Thereby, reversible charge / discharge can be performed, and a high initial discharge capacity density and good charge / discharge characteristics 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.

(First embodiment)
[Preparation of negative electrode]
In the present embodiment, the negative electrode active material described later has an interplanar spacing d (002) of 0.383 nm, for example, used as a quantitative measure for measuring the degree of crystallite development of the negative electrode active material, and is in the c-axis direction. A carbon material having a crystallite size Lc of, for example, 1.14 nm is included as a single component or a main component.

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

  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.

[Preparation of non-aqueous electrolyte]
As the non-aqueous electrolyte, an electrolyte salt dissolved 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, etc., 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 the chain carbonic acid ester 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, etc. are mentioned.

  As chain ethers, 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. The one added so as to be used.

[Preparation of non-aqueous electrolyte secondary battery]
Using the negative electrode, nonaqueous electrolyte, positive electrode and reference electrode prepared as described above, a nonaqueous electrolyte secondary battery as shown below is prepared.

  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. In place 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.

(Second Embodiment)
In the present embodiment, as the negative electrode active material, a carbon material having an interplanar spacing d (002) of, for example, 0.377 nm and a c-axis direction crystallite size Lc of, for example, 1.29 nm is used. Similarly to the first embodiment, the negative electrode 1 and the nonaqueous electrolyte secondary battery are manufactured.

(Effects of the first and second embodiments)
In the first and second embodiments, sodium ions are sufficiently occluded and released from the negative electrode 1 containing a carbon material.

  Further, as a negative electrode active material, a carbon material having a face spacing d (002) of 0.377 nm or more and a crystallite size Lc in the c-axis direction of 1.29 nm or less is used, so that reversible charging is achieved. Discharge can be performed, and high initial discharge capacity density and good charge / discharge characteristics can be obtained.

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

  In the produced non-aqueous electrolyte secondary battery, charging was performed at a constant current of 0.6 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 0V. Thereafter, discharging was performed at a constant current of 0.6 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 1.0 V, and thereby the charge / discharge characteristics were examined.

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

  In the produced non-aqueous electrolyte secondary battery, charging was performed at a constant current of 0.6 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 0V. Thereafter, discharging was performed at a constant current of 0.6 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 1.0 V, and thereby the charge / discharge characteristics were examined.

(Comparative Example 1)
In Comparative Example 1, the above example was used except that a carbon material having an interplanar spacing d (002) of 0.348 nm and a c-axis direction crystallite size Lc of 1.20 nm was used as the negative electrode active material. In the same manner as in Example 1, a negative electrode 1 and a nonaqueous electrolyte secondary battery were produced.

  In the produced non-aqueous electrolyte secondary battery, charging was performed at a constant current of 0.6 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 0V. Thereafter, discharging was performed at a constant current of 0.6 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 1.0 V, and thereby the charge / discharge characteristics were examined.

(Comparative Example 2)
In Comparative Example 2, the above example was used except that a carbon material having an interplanar spacing d (002) of 0.346 nm and a crystallite size Lc of 4.67 nm in the c-axis direction was used as the negative electrode active material. In the same manner as in Example 1, a negative electrode 1 and a nonaqueous electrolyte secondary battery were produced.

  In the produced non-aqueous electrolyte secondary battery, charging was performed at a constant current of 0.6 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 0V. Thereafter, discharging was performed at a constant current of 0.6 mA until the potential of the negative electrode 1 based on the reference electrode 3 reached 1.0 V, and thereby the charge / discharge characteristics were examined.

(Evaluation of Examples 1 and 2 and Comparative Examples 1 and 2)
FIG. 2 is a graph showing initial discharge characteristics of the nonaqueous electrolyte secondary batteries of Example 1, Example 2, and Comparative Example 2. In addition, the interplanar spacing d (002) of the negative electrode active material in Examples 1 and 2 and Comparative Examples 1 and 2, the crystallite size Lc in the c-axis direction, and the initial discharge capacity density of each nonaqueous electrolyte secondary battery Table 1 shows.

  As shown in FIG. 2 and Table 1, in the nonaqueous electrolyte secondary batteries of Example 1 and Example 2, high initial discharge capacity densities of about 234 mAh / g and about 190 mAh / g are obtained, respectively. I was able to. Moreover, reversible charging / discharging was possible after 2 cycles, and good charging / discharging characteristics could be obtained.

  On the other hand, the nonaqueous electrolyte secondary battery of Comparative Example 1 could not be discharged at all. Moreover, in the nonaqueous electrolyte secondary battery of Comparative Example 2, the initial discharge capacity density was about 63 mA, which was a low initial discharge capacity density.

  From these results, it is possible to reversibly use a carbon material having an interplanar spacing d (002) of 0.377 nm or more and a crystallite size Lc in the c-axis direction of 1.29 nm or less as the negative electrode active material. It was found that a high initial discharge capacity density and good charge / discharge characteristics can 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 automotive power source.

It is a schematic explanatory drawing of the test cell of the nonaqueous electrolyte secondary battery which concerns on this Embodiment. 6 is a graph showing initial discharge characteristics of the nonaqueous electrolyte secondary batteries of Example 1, Example 2, and Comparative Example 2.

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 (4)

A positive electrode capable of occluding and releasing sodium;
A negative electrode containing a negative electrode active material capable of occluding and releasing sodium;
A non-aqueous electrolyte in which sodium ions are dissolved,
The negative electrode active material is a single component or a main component of a carbon material having an interlayer distance d (002) by an X-ray diffraction method of 0.377 nm or more and a crystallite size Lc in the c-axis direction of 1.29 nm or less. A non-aqueous electrolyte secondary battery comprising as a component. 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 or 2, 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 3.

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