JP4739770B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a positive electrode and a nonaqueous electrolyte secondary battery including the positive electrode, the negative electrode, and a nonaqueous 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.
JP 2003-151549 A

  However, in the conventional non-aqueous electrolyte secondary battery using lithium ions as described above, cobalt (Co) or nickel (Ni) oxide is mainly used as the positive electrode, so that there are limited resources.

  Moreover, when all the lithium ions are released from lithium nickelate or lithium cobaltate in the non-aqueous electrolyte secondary battery, the crystal structure of lithium nickelate or lithium cobaltate collapses. As a result, oxygen is released from lithium nickelate or lithium cobaltate, and there is a concern about safety. Therefore, the above discharge capacity cannot be further improved.

  On the other hand, manganese (Mn), which is abundant in resources, may be used instead of nickel or cobalt. In this case, the capacity of the nonaqueous electrolyte secondary battery is halved.

In the case of using manganese, it is difficult to produce lithium manganate (LiMnO ₂ ) having a layered structure for improving the mobility of lithium ions. Thereby, generally lithium manganate (LiMn ₂ O ₄ ) having a spinel structure is used. In the LiMn ₂ O ₄ , the state of MnO ₂ is maintained even when all lithium ions are released. Manganese is stable in the tetravalent state, so it does not release oxygen and has excellent safety.

However, when LiMn ₂ O ₄ is used, a potential of 4 V can be obtained, but only a discharge capacity of 100 to 120 mAh / g can be obtained.

Although attempts have been made to produce LiMnO ₂ having a layered structure, the potential decreases to about 3 V, and when the charge / discharge cycle is repeated, the LiMnO ₂ changes to LiMn ₂ O ₄ having a spinel structure. End up. The reason why LiMnO ₂ having a layered structure is not chemically stable is that the radius of lithium ions is small.

  An object of the present invention is to provide a positive electrode made of an inexpensive material and capable of sufficiently occluding and releasing ions.

  Another object of the present invention is to provide an inexpensive non-aqueous electrolyte secondary battery that can be reversibly charged and discharged.

  The nonaqueous electrolyte secondary battery according to the present invention is a nonaqueous electrolyte secondary battery comprising a positive electrode made of an oxide containing sodium and manganese, a negative electrode, and a nonaqueous electrolyte containing sodium ions. Contains sodium hexafluorophosphate, the oxide being Na _x MnO _{2 + y} Wherein x is greater than 0 and less than or equal to 1, and y is greater than −0.1 and less than 0.1.

  In the present invention, since the positive electrode is made of an oxide containing sodium and manganese, sodium ions are sufficiently occluded and released from the positive electrode. Further, the cost can be reduced by using abundant sodium.

  The crystal system of the oxide may be hexagonal, orthorhombic, monoclinic or tetragonal. In this case, sodium ions are efficiently occluded and released from the positive electrode.

In the nonaqueous electrolyte secondary battery according to the present invention, by using the positive electrode , reversible charging / discharging can be performed and cost reduction can be achieved.

  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 of the present invention, reversible charging / discharging can be performed by using the positive electrode, and the cost can be reduced by using resource-rich sodium.

  Hereinafter, the positive electrode according to the present embodiment and the nonaqueous electrolyte secondary battery using the positive electrode will be described.

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

(Production of working electrode)
For example, 85 parts by weight of sodium manganate (Na _x MnO _{2 + y} ) (for example, 0 <x ≦ 1, −0.1 <y <0.1) powder as a positive electrode active material, 10 parts by weight of a conductive agent A material (hereinafter referred to as a positive electrode material) containing ketjen black, which is a carbon black powder, and 5 parts by weight of polyvinylidene fluoride as a binder is prepared. For example, Na _0.7 MnO _{2 + y} where x is 0.7 is used as the sodium manganate of the positive electrode active material.

  In the present embodiment, as the sodium manganate, the crystal system (crystal structure) in JCPDS (Joint Committee on Powder Diffraction Standards) in which X-ray diffraction data of about 6000 kinds of inorganic compounds and organic compounds is recorded is hexagonal. The system uses sodium manganate with card number 270751.

  Instead of the sodium manganate having the card number 270751, the crystal system is orthorhombic (SG Pmmn), card numbers 250844, 720415 and 720831, sodium manganate and orthorhombic (SG C) card number 270747. And 277072 sodium manganate, orthorhombic card number 380965 sodium manganate, monoclinic (b-axis) card number 250845 and 270749 sodium manganate, monoclinic (b-axis) (SG C2 / m) card 720830 sodium manganate and tetragonal (SGI) card 270747 sodium manganate.

  The positive electrode material is mixed with, for example, a 10% by weight N-methylpyrrolidone solution with respect to the positive electrode material to prepare a slurry as a positive electrode mixture.

  Next, the slurry is applied by a doctor blade method onto a 3 cm × 3 cm region of, for example, an aluminum foil having a thickness of 18 μm, which is a positive electrode current collector, and then dried to form a positive electrode active material layer.

  Subsequently, a working electrode is produced by attaching a positive electrode tab on the area | region of the aluminum foil which does not form a positive electrode active material layer.

  In addition, as a binder for the positive electrode material, instead of polyvinylidene fluoride, polytetrafluoroethylene, polyethylene oxide, polyvinyl acetate, polymethacrylate, polyacrylate, polyacrylonitrile, polyvinyl alcohol, styrene-butadiene rubber, carboxymethylcellulose, etc. At least one selected from can be used.

  Note that when the amount of the binder is large, the ratio of the positive electrode active material contained in the positive electrode material is reduced, so that a high energy density cannot be obtained. Therefore, the amount of the binder is in the range of 0 to 30% by weight of the whole positive electrode material, preferably in the range of 0 to 20% by weight, and more preferably in the range of 0 to 10% by weight.

  Further, as the conductive agent of the positive electrode material, other carbon materials such as acetylene black and graphite can be used instead of ketjen black. If the addition amount of the conductive agent is small, the conductivity in the positive electrode material cannot be sufficiently improved. On the other hand, if the addition amount is too large, the proportion of the positive electrode active material contained in the positive electrode material decreases and high energy is increased. Density cannot be obtained. Therefore, the amount of the conductive agent is in the range of 0 to 30% by weight of the whole positive electrode material, preferably in the range of 0 to 20% by weight, and more preferably in the range of 0 to 10% by weight.

  Furthermore, as the positive electrode current collector, foamed aluminum, foamed nickel, or the like can be used in order to increase electronic conductivity.

(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. What was added so that it may become is used.

(Preparation of non-aqueous electrolyte secondary battery)
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 working electrode 1, and a lead is attached to a counter electrode 2 made of, for example, sodium metal. Instead of the counter electrode 2 made of sodium metal, the counter electrode 2 made of another material such as a carbon material capable of inserting and extracting sodium ions may be used.

  Next, the separator 4 is inserted between the working electrode 1 and the counter electrode 2, and the working electrode 1, the counter electrode 2, and the reference electrode 3 made of, for example, sodium metal are disposed in the cell container 10. Then, the test cell is manufactured by injecting the nonaqueous electrolyte 5 into the cell container 10.

  By using the positive electrode according to the present embodiment, sodium ions are sufficiently occluded and released. Further, the cost can be reduced by using abundant sodium.

  Furthermore, in the present embodiment, by using the positive electrode as described above for a non-aqueous electrolyte secondary battery, it is possible to perform reversible charging / discharging and provide an inexpensive non-aqueous electrolyte secondary battery. can do.

(Example and its evaluation)
As shown below, the charge / discharge characteristics of the non-aqueous electrolyte secondary battery were examined using the test cell prepared based on the above embodiment.

  FIG. 2 is a graph showing the charge / discharge characteristics of the nonaqueous electrolyte secondary battery.

  In the manufactured test cell, charging was performed at a constant current of 0.15 mA until the potential of the working electrode 1 with respect to the reference electrode 3 reached 4.2V.

  Thereafter, the charge / discharge characteristics were examined by performing discharge at a constant current of 0.15 mA until the potential of the working electrode 1 based on the reference electrode 3 reached 1.5V.

  As a result, the discharge capacity density per 1 g of the positive electrode active material was about 151 mAh / g, and it was found that charging / discharging was performed satisfactorily.

  That is, it was revealed that sodium ions were reversibly occluded and released from the working electrode 1. As a result, the effectiveness of the new non-aqueous electrolyte secondary battery replacing the conventional non-aqueous electrolyte secondary battery using lithium ions could be confirmed.

  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 of the nonaqueous electrolyte secondary battery which concerns on this Embodiment. It is the graph which showed the charging / discharging characteristic of the nonaqueous electrolyte secondary battery.

Explanation of symbols

1 Working Electrode 2 Counter Electrode 3 Reference Electrode 4 Separator 5 Nonaqueous Electrolyte 10 Cell Container

Claims (3)

In a non-aqueous electrolyte secondary battery comprising a positive electrode made of an oxide containing sodium and manganese, a negative electrode, and a non-aqueous electrolyte containing sodium ions,
The non-aqueous electrolyte includes sodium hexafluorophosphate,
The oxide is Na _x MnO _{2 + y} X is greater than 0 and less than or equal to 1, and y is greater than -0.1 and less than 0.1. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the crystal system of the oxide is hexagonal, orthorhombic, monoclinic or tetragonal . 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 claim 1 or 2 .

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