JP4989049B2 - 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.

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

  Further, when all lithium ions are released from lithium nickelate or lithium cobaltate, the crystal structure of lithium nickelate or lithium cobaltate collapses. As a result, oxygen is released from lithium nickelate or lithium cobaltate, the crystal structure cannot be maintained, and the battery characteristics cannot be maintained. Therefore, the discharge capacity cannot be further improved. In some cases, abundant manganese (Mn) is used instead of nickel or cobalt. In this case, the capacity of the nonaqueous electrolyte secondary battery is halved.

  Furthermore, in the conventional non-aqueous electrolyte secondary battery using sodium ions, the stability of the non-aqueous electrolyte is low, and it is difficult to use the non-aqueous electrolyte for a long time.

  An object of the present invention is to provide a positive electrode and a non-aqueous electrolyte secondary battery that can perform reversible charge / discharge, can obtain good charge / discharge characteristics, and are inexpensive.

The positive electrode of the nonaqueous electrolyte secondary battery of the present invention includes a positive electrode active material, and the positive electrode active material is made of an oxide containing sodium and cobalt.

  In the positive electrode according to the present invention, when the positive electrode active material is made of an oxide containing sodium and cobalt, sodium ions are sufficiently occluded and released from the positive electrode. Further, the cost can be reduced by using abundant sodium. Furthermore, even when sodium ions are released from the oxide, a positive electrode having excellent thermal stability can be obtained because the oxide is excellent in thermal stability.

  Therefore, when the positive electrode according to the present invention is used for a non-aqueous electrolyte secondary battery, reversible charge / discharge can be performed, good charge / discharge characteristics can be obtained, and cost reduction can be realized. it can.

The oxide containing sodium and cobalt contains Na _x Co _y O ₂ , x is greater than 0 and less than or equal to 1.0, and y is less than 1.5 . The Na _x Co _y O ₂ may be NaCoO ₂ .

  In this case, sodium ions are reliably occluded and released from the positive electrode containing the oxide.

  The crystal system of the oxide containing sodium and cobalt may be hexagonal, orthorhombic, or monoclinic.

  In this case, sodium ions are efficiently occluded and released from the positive electrode.

The nonaqueous electrolyte secondary battery according to the present invention includes a positive electrode made of an oxide containing sodium and cobalt as a positive electrode active material , a negative electrode, and a nonaqueous electrolyte.

In the nonaqueous electrolyte secondary battery according to the present invention, by using a positive electrode made of an oxide containing sodium and cobalt as a positive electrode active material , reversible charge / discharge can be performed and good charge / discharge characteristics are obtained. It is possible to realize a non-aqueous electrolyte secondary battery that can be used.

  In addition, even when sodium ions are released from a positive electrode made of an oxide containing sodium and cobalt, a nonaqueous electrolyte secondary battery having excellent thermal stability can be obtained because the oxide has excellent thermal stability. Can do.

  Furthermore, the cost of the nonaqueous electrolyte secondary battery can be reduced by using abundant sodium.

The negative electrode includes germanium alone . In this case, ions are sufficiently occluded and released from the negative electrode containing germanium alone.

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

  The negative electrode may include a current collector made of metal, and the germanium alone may be formed in a thin film on the current collector. In this case, germanium alone is easily formed as a thin film on the current collector.

  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 present invention, reversible charging / discharging can be performed, good charging / discharging characteristics can be obtained, and an inexpensive positive electrode and nonaqueous electrolyte secondary battery 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 positive electrode), a counter electrode (hereinafter referred to as 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.

[Preparation of positive electrode]
For example, 80 parts by weight of sodium cobaltate (NaCoO ₂ ) powder as a positive electrode active material, 10 parts by weight of Ketjen black powder as carbon black powder as a conductive agent, and 10 parts by weight of polytetrafluoroethylene as binder Are prepared (hereinafter referred to as positive electrode materials).

In this embodiment, NaCoO ₂ powder is used as the positive electrode active material, but sodium cobaltate powder represented by Na _x Co _y O ₂ (0 <x ≦ 1.0, y <1.5). May be used.

In this case, X-ray diffraction data of about 6000 kinds of inorganic compounds and organic compounds is recorded as the sodium cobaltate (Na _x Co _y O ₂ (0 <x ≦ 1.0, y <1.5)). It is possible to use sodium cobaltate whose crystal system (crystal structure) in JCPDS (Joint Committee on Powder Diffraction Standards) is hexagonal, monoclinic or orthorhombic.

  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 2 cm × 2 cm region of a 30 μm-thick aluminum foil as a positive electrode current collector, and then dried to form a positive electrode active material layer.

  Subsequently, a positive electrode is produced by attaching a positive electrode tab on the region of the aluminum foil where the positive electrode active material layer is not formed.

  In addition, as the binder for the positive electrode material, instead of polytetrafluoroethylene, polyvinylidene fluoride, 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.

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

[Preparation of negative electrode]
As the negative electrode current collector, for example, a rolled foil having a thickness of 26 μm, for example, made of roughened copper whose surface is formed in an uneven shape by copper being deposited by an electrolytic method is prepared.

  On the negative electrode current collector made of the rolled foil, a negative electrode active material layer made of germanium (Ge) alone having a thickness of 0.5 μm, for example, is deposited as follows using the sputtering apparatus shown in FIG. The deposition conditions are shown in Table 1. The deposited germanium alone is amorphous.

First, after evacuating the chamber 50 to 1 × 10 ⁻⁴ Pa, argon is introduced into the chamber 50 so that the gas pressure in the chamber 50 becomes 1.7 to 1.8 × 10 ⁻¹ Pa. To stabilize the gas pressure.

  Next, in a state where the gas pressure in the chamber 50 is stable, high frequency power is applied to the sputtering source 51 of germanium simple substance by a high frequency power source 52 for a predetermined time. Thereby, a negative electrode active material layer made of germanium alone is deposited on the negative electrode current collector.

  As described above, the present invention is not limited to depositing a negative electrode active material layer made of germanium alone by a sputtering apparatus. For example, a thin film or foil containing germanium alone is formed on a negative electrode current collector as a negative electrode active material layer. Also good.

  Moreover, it is not limited to a germanium simple substance, For example, you may form the negative electrode active material layer which consists of a tin (Sn) simple substance on a negative electrode electrical power collector.

  Next, the negative electrode current collector on which the negative electrode active material layer made of germanium alone is deposited is cut into a size of, for example, 2 cm × 2 cm, and a negative electrode tab is attached to the negative electrode current collector to produce a negative electrode.

  Here, the arithmetic average roughness Ra, which is a parameter representing the surface roughness defined in Japanese Industrial Standards (JIS B 0601-1994) in the roughened rolled foil, is 0.1 μm or more and 10 μm or less. Is preferred. The arithmetic average roughness Ra can be measured by, for example, a stylus type surface roughness meter.

  When an amorphous negative electrode active material layer was deposited on a negative electrode current collector made of a rolled foil having a concavo-convex surface, the surface of the negative electrode active material layer corresponded to the concavo-convex shape on the negative electrode current collector. It becomes a shape.

  When charging / discharging is performed using such a negative electrode active material layer, the stress accompanying the expansion and contraction of the negative electrode active material layer concentrates on the concave and convex portions of the negative electrode active material layer, and cuts are formed in the concave and convex portions of the negative electrode active material layer Is done. The stress generated by charging / discharging is dispersed by this break. Thereby, reversible charge / discharge is easily performed, and excellent charge / discharge characteristics can be obtained.

[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 Glycols 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]
Using the positive electrode, the negative electrode and the non-aqueous electrolyte, a non-aqueous electrolyte secondary battery is produced as shown below.

  FIG. 2 is a schematic explanatory diagram of a test cell of the nonaqueous electrolyte secondary battery according to the present embodiment.

  As shown in FIG. 2, a lead is attached to the positive electrode 1 and a lead is attached to the negative electrode 2 in an inert atmosphere.

  Next, the separator 4 is inserted between the positive electrode 1 and the negative electrode 2, and the positive electrode 1, the negative electrode 2, and the reference electrode 3 made of, for example, 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.

(Effect in this Embodiment)
In the present embodiment, sodium ions are sufficiently occluded and released from the positive electrode containing sodium cobaltate. In addition, sodium ions are sufficiently occluded and released from the negative electrode containing germanium alone.

  Further, by using the positive electrode and the negative electrode as described above, it is possible to realize a non-aqueous electrolyte secondary battery that can be reversibly charged and discharged and that can obtain good charge and discharge characteristics.

  In addition, even when sodium ions are released from sodium cobaltate, a nonaqueous electrolyte secondary battery excellent in thermal stability can be obtained because sodium cobaltate is excellent in thermal stability.

  Furthermore, the cost of the nonaqueous electrolyte secondary battery can be reduced by using abundant sodium.

( Reference Example 1 and its evaluation)
Except for the following points, the charge / discharge characteristics of the nonaqueous electrolyte secondary battery produced based on the above-described embodiment were examined.

The configuration of the nonaqueous electrolyte secondary battery produced in Reference Example 1 is different from the configuration of the nonaqueous electrolyte secondary battery according to the present embodiment in that the negative electrode 2 is made of sodium metal.

FIG. 3 is a graph showing the charge / discharge characteristics of the nonaqueous electrolyte secondary battery of Reference Example 1.

In the produced non-aqueous electrolyte secondary battery, charging was performed at a constant current of 0.025 mA / cm ² until the potential of the positive electrode 1 based on the reference electrode 3 reached 4.3V. Thereafter, discharge was performed at a constant current of 0.025 mA / cm ² until the potential of the positive electrode 1 based on the reference electrode 3 reached 2.5 V, thereby examining the charge / discharge characteristics of three cycles.

  As a result, as shown in FIG. 3, it was found that charging / discharging was performed satisfactorily.

  That is, it became clear that sodium ions were reversibly occluded and released from the positive 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.

(Example 2 and its evaluation)
In Example 2, the charge / discharge characteristics of the nonaqueous electrolyte secondary battery produced based on the above-described embodiment were examined.

  FIG. 4 is a graph showing the charge / discharge characteristics of the nonaqueous electrolyte secondary battery of Example 2.

In the produced non-aqueous electrolyte secondary battery, charging was performed at a constant current of 0.3 mA / cm ² until the potential of the positive electrode 1 based on the reference electrode 3 reached 4.5V. Thereafter, discharging was performed at a constant current of 0.3 mA / cm ² until the potential of the positive electrode 1 based on the reference electrode 3 reached 2.5 V, thereby examining the charge / discharge characteristics of 5 cycles.

  As a result, as shown in FIG. 4, it was found that charging / discharging was performed satisfactorily.

  That is, it was revealed that sodium ions were reversibly occluded and released from the positive electrode 1 and the negative electrode 2. 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.

(Comprehensive evaluation of Reference Example 1 and Example 2)
Table 2 shows the initial charge capacity density, the initial discharge capacity density, the initial charge / discharge efficiency, and the discharge capacity density retention rate of the charge / discharge test in Reference Example 1 and Example 2.

  The initial charge / discharge efficiency is defined by the ratio (%) of the initial discharge capacity density to the initial charge capacity density, and the discharge capacity density maintenance factor is defined by the ratio of the discharge capacity density at a certain cycle to the initial discharge capacity density. The

The discharge capacity density maintenance rate of Reference Example 1 was calculated by the ratio of the discharge capacity density at the time of 3 cycles to the initial discharge capacity density, and the discharge capacity density maintenance rate of Example 2 was the discharge at the time of 5 cycles relative to the initial discharge capacity density It was calculated by the ratio of capacity density.

As shown in Table 2, in both Reference Example 1 and Example 2, it was found that a good discharge capacity density maintenance rate exceeding 90% can be obtained.

( Reference Example 3 and its evaluation)
The nonaqueous electrolyte secondary battery of Reference Example 1 was disassembled, and the positive electrode active material layer made of sodium cobaltate (NaCoO ₂ ) was measured by XRD (X-ray diffractometer). As measurement conditions, CuKα ray (λ = 0.15418 nm) was used as the X-ray source, and the applied voltage and applied current were 40 kV and 40 mA, respectively.

FIG. 5 is a graph showing the measurement results of the XRD measurement of Reference Example 1. The XRD measurement was performed by putting the positive electrode active material layer in a plastic bag and sealing the plastic bag so that the positive electrode active material layer did not come into contact with air. For reference, XRD patterns of sodium cobaltate (NaCoO ₂ ) with a card number of 301833 and sodium cobaltate (Na _0.75 CoO ₂ ) with a card number of 321068 in JCPDS are shown in FIGS. 6 and 7, respectively.

  In FIG. 5, the peak of the X-ray with a diffraction angle 2θ of around 17 ° is that when sodium ions are inserted into the positive electrode.

  As a result of the XRD measurement, it was found that sodium ions were reliably occluded in the positive electrode 1 containing sodium cobaltate.

  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 diagram of a sputtering device. It is a perspective view which shows the nonaqueous electrolyte secondary battery which concerns on this Embodiment. 4 is a graph showing charge / discharge characteristics of the nonaqueous electrolyte secondary battery of Reference Example 1. 6 is a graph showing charge / discharge characteristics of the nonaqueous electrolyte secondary battery of Example 2. FIG. 5 is a graph showing measurement results of XRD measurement in Reference Example 1. It is a graph showing the XRD pattern of the sodium cobaltate card number 301183 in JCPDS (NaCoO _2). It is a graph showing the XRD pattern of the sodium cobaltate card number 321068 in JCPDS (Na _0.75 CoO _2).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Reference electrode 4 Separator 5 Nonaqueous electrolyte 10 Cell container 50 Chamber 51 Sputter source 52 High frequency power supply

Claims (5)

The positive electrode active material comprises a positive electrode made of an oxide containing sodium and cobalt, a negative electrode containing germanium alone, and a non-aqueous electrolyte ,
The oxide containing sodium and cobalt contains Na _x Co _y O ₂ , x is greater than 0 and less than or equal to 1.0, and y is less than 1.5;
The non-aqueous electrolyte includes sodium hexafluorophosphate,
A non-aqueous electrolyte secondary battery. It said Na _x Co _y O ₂ is non-aqueous electrolyte secondary battery according to claim 1, wherein it is NaCoO _2. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein a crystal system of the oxide containing sodium and cobalt is a hexagonal system, an orthorhombic system, or a monoclinic system. 4. The negative electrode includes a current collector made of metal, the germanium alone, non-aqueous electrolyte secondary according to any one of claims 1 to 3, characterized in that it is formed as a thin film on the current collector Next battery. 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 .