JP6699052B2 - Positive electrode for sodium secondary battery and sodium secondary battery - Google Patents

Description

  The present invention relates to a sodium secondary battery positive electrode and a sodium secondary battery.

  The lithium secondary battery is a high energy density secondary battery and has already been put to practical use as a small power source for mobile phones, notebook computers, and the like. Further, the lithium secondary battery has already been put to practical use as a small power source for electric vehicles, hybrid vehicles and the like. Further, since the lithium secondary battery can be used as a large-scale power source such as a power source for automobiles such as electric vehicles and hybrid vehicles and a power source for distributed power storage, the demand thereof is increasing.

  However, since a rare metal element such as lithium is used in the lithium secondary battery, when the demand for the lithium secondary battery increases, there is a concern that the supply of the rare metal element is unstable.

  In order to solve the above-mentioned problem of supply concern, research on sodium secondary batteries is under way. As a positive electrode active material for a sodium secondary battery, sodium, which has abundant resources and is inexpensive, is used instead of expensive lithium. Therefore, if the sodium secondary battery can be put to practical use, the problem of unstable supply can be solved.

  By the way, as a positive electrode active material for a sodium secondary battery, a complex oxide of Na and a transition metal such as Cr, Mn, Fe, Co or Ni is used. Among these complex metal oxides, those not containing Co, which is a rare metal element, can contribute to the reduction of the production cost of the sodium secondary battery and can meet the increased demand of the sodium secondary battery.

  Conventionally, as a binder for the positive electrode material, an organic solvent-based PVDF binder having excellent oxidation resistance is often used. On the other hand, in the negative electrode of a lithium secondary battery, styrene-butadiene rubber (SBR) is used as an aqueous binder from the viewpoints of cost reduction, environmental load reduction, and battery characteristic improvement. However, since SBR has poor oxidation resistance, it cannot be used as a positive electrode binder.

JP, 2007-287661, A JP 2012-182087 A JP2012-201588A

  The performance of the sodium secondary battery using the positive electrode active material composed of the composite metal oxide having the P2 type structure disclosed in Patent Documents 1 to 3 is currently put into practical use, particularly in terms of charge/discharge cycle characteristics. It is not sufficient compared with the performance of the lithium secondary battery. Further, the sodium-containing mixed metal oxide having a layered structure tends to react with moisture in the atmosphere to be deteriorated, so that an aqueous binder cannot be used.

  The present invention has been made to solve the above problems, and its purpose is to reduce the cost, reduce the environmental load, from the viewpoint of improving the charge-discharge cycle characteristics, a positive electrode prepared using an aqueous binder, It is to provide a sodium secondary battery including a positive electrode.

The present inventors have conducted extensive studies to solve the above problems. As a result, at least Na _2/3 Ni _1/3 Mn _2/3 O ₂ having a P2 type or P3 type crystal structure and Na _x MO ₂ (M is Mn, Fe, Co, or At least one transition metal selected from Ni, 0<x≦1, and part of the M is substituted with at least one element selected from Li, Mg, Al, and Zn.) If an electrode mixture layer containing a positive electrode active material containing any one selected from the complex metal oxides and a binder containing polycarboxylic acid and/or polycarboxylic acid alkali metal salt is used, The inventors have found that the above can be solved and completed the present invention. More specifically, the present invention provides the following.

(1) Na _2/3 Ni _1/3 Mn _2/3 O ₂ having at least a P2 type or P3 type crystal structure, and Na _x MO ₂ having a P3 type crystal structure (M is Mn, Fe, Co, At least one transition metal selected from Ni, 0<x≦1, and part of the M is substituted with at least one element selected from Li, Mg, Al, and Zn.) A positive electrode active material selected from the group consisting of complex metal oxides, and an electrode mixture layer containing a binder containing a polycarboxylic acid and/or an alkali metal salt of a polycarboxylic acid. Positive electrode for sodium secondary batteries.
(2) The polycarboxylic acid and/or polycarboxylic acid alkali metal salt contains at least one selected from the group consisting of polyacrylic acid, polyacrylic acid alkali metal salt, carboxymethyl cellulose and carboxymethyl cellulose metal salt. The positive electrode for a sodium secondary battery according to (1).
(3) The positive electrode for a sodium secondary battery according to (1) or (2), wherein the content of the binder in the electrode mixture layer is 15% by mass or less.
(4) A sodium secondary battery having the positive electrode for a sodium secondary battery according to any one of (1) to (3).

  The positive electrode of the present invention can realize cost reduction and environmental load reduction by using the water-based binder, and at the same time, the battery performance of the sodium secondary battery is the same as that of the conventional sodium secondary battery. It can be improved in comparison.

It is a figure which shows the result of the powder X-ray-diffraction measurement of the composite metal oxide used in Examples 1 and 2 and the comparative example 1. It is a figure which shows the result of the powder X-ray-diffraction measurement of the composite metal oxide used in Examples 3, 4 and the comparative example 2. 7 is a diagram showing the result of powder X-ray diffraction measurement of the composite metal oxide used in Comparative Example 3. FIG. 7 is a diagram showing the results of powder X-ray diffraction measurement of the composite metal oxides used in Example 5 and Comparative Example 4. FIG. 7 is a diagram showing the results of powder X-ray diffraction measurement of the composite metal oxides used in Example 6 and Comparative Example 5. FIG. FIG. 7 is a graph showing the results of powder X-ray diffraction measurement of the composite metal oxides used in Example 7 and Comparative Example 6. 6 is a surface photograph of the electrode sheet after coating and drying produced in Comparative Example 3 (drawing substitute photograph).

  Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the embodiments below.

<Composite metal oxide>
The positive electrode active material used in the positive electrode of the present invention contains a composite metal oxide, and one embodiment of the composite metal oxide (hereinafter, may be abbreviated as “composite metal oxide 1 according to an embodiment”). , Na _2/3 Ni _1/3 Mn _2/3 O ₂ having at least a P2 type or P3 type crystal structure. In addition, a composite metal oxide (hereinafter, may be abbreviated as “composite metal oxide 2 according to an embodiment”), which is another embodiment of the metal composite oxide, is Na _x MO ₂ (having a P3 type crystal structure). M is at least one kind of transition metal selected from Mn, Fe, Co and Ni, and 0<x≦1, and a part of the M is at least one kind selected from Li, Mg, Al and Zn. Substituted with an element)).
The present inventors have been conducting research on a sodium secondary battery capable of stably repeating charge and discharge, and reacting with moisture in the atmosphere in a sodium-containing composite oxide having a layered structure as a positive electrode active material. I have found that there are things that change. Such a complex metal oxide requires strict water content control in the process of manufacturing a sodium secondary battery, and cannot use an aqueous binder.
Then, the present inventors have found that, among the sodium-containing composite oxides, the above-mentioned composite metal oxide 1 and composite metal oxide 2 suppress the alteration due to the reaction with water, and can apply the water-based binder. It was.

The composite metal oxide 1 and the composite metal oxide 2 according to the embodiment are characterized by having a P2 or P3 type crystal structure, and the P2 and P3 type structures will be described in more detail. A typical crystal system having a P2-type structure is assigned to the space group P63/mmc due to its symmetry. A typical crystal system having a P3 type structure is assigned to the space group R3m or R-3m due to its symmetry. In addition, when the symmetry is further reduced, it may be possible to belong to the orthorhombic system such as Cmcm or the monoclinic system space group such as C2/m, but the basic structure is classified into P2 or P3 type. It has a layered structure. However, a P2 or P3 type stacking fault may occur with the P2 or P3 type structure as a parent structure. Whether or not the composite metal oxide is an oxide having a P2 or P3 type structure can be confirmed by X-ray diffraction. Specifically, it can be confirmed by the method described in Examples.
The composite metal oxide 1 and the composite metal oxide 2 according to the embodiment are not particularly limited in details such as the position of a peak in X-ray diffraction as long as they have a P2 or P3 type crystal structure, but the X-ray source In powder X-ray diffraction measurement using CuKα ray, it is preferable that no peak is observed in the range of 18° to 20°. By not observing a peak in the range of 18° to 20°, it can be determined that the content of the oxide other than the P2 or P3 type crystal structure is small and the composite metal oxide is a good quality. The phrase “no peak is observed” means that no peak is substantially observed, and a peak that can be determined as noise is not included.

The mixed metal oxide 2 according to the embodiment is Na _x MO ₂ (M is at least one transition metal selected from Mn, Fe, Co, and Ni and has a P3 type crystal structure, and 0<x≦1. And a part of M is substituted with at least one element selected from Li, Mg, Al, and Zn), and a specific numerical value of x satisfies the above condition. It is not particularly limited.
x is preferably 1/3 or more, more preferably 1/2 or more, preferably 5/6 or less, more preferably 2/3 or less. Since mobile sodium ions increase as x increases, the reversible capacity increases, which is preferable. However, when x exceeds 5/6, a structure other than the P2 or P3 type structure tends to appear, and direct synthesis by the solid phase method tends to occur. Since it becomes difficult, it is preferably 5/6 or less.

Li, Mg, Al, and Zn are elements capable of substituting a transition metal, and are characterized in that the valence does not change even when charging and discharging. By substituting these elements for M in the above formula, the volume change of the material is suppressed and the material becomes stable against repeated charge and discharge due to the pillar effect due to the fact that Na ions do not completely escape at the charging end. Is estimated. If the total substitution amount exceeds 30 mol %, the capacity is greatly reduced, which is not preferable. Further, if the substitution amount is 1 mol% or less, the substitution effect is small, which is not preferable.

  The value of x can be adjusted by controlling the amount of raw material used, manufacturing conditions, and the like. Further, the substitution amounts of Li, Mg, Al, and Zn elements can be adjusted by controlling the amount of raw materials used, manufacturing conditions, and the like. Details will be described later.

(Average primary particle size)
The average diameter (average primary particle diameter) of the composite metal oxide 1 and the composite metal oxide 2 according to the embodiment is not particularly limited, but the lower limit is preferably 0.1 μm or more, more preferably 0.2 μm or more, The most preferable value is 0.3 μm or more, and the upper limit is preferably 5 μm or less, more preferably 3 μm or less, further preferably 2 μm or less, and most preferably 1.5 μm or less. If the average primary particle diameter exceeds the above upper limit, the specific surface area is reduced, which may increase the possibility that the battery characteristics such as rate characteristics and output characteristics are deteriorated. Below the lower limit, there is a possibility that problems such as poor reversibility of charge and discharge due to undeveloped crystals.
In addition, the average primary particle diameter in the present invention is an average diameter observed with a scanning electron microscope (SEM), and using an SEM image of 30,000 times, as an average value of about 10 to 30 primary particle diameters. You can ask.

(Median diameter (secondary particle))
The median diameter (50% cumulative diameter (D50)) of the secondary particles of the composite metal oxide 1 and the composite metal oxide 2 according to the embodiment is not particularly limited, but is usually 2 μm or more, preferably 2.5 μm or more, and most preferably. Is 4 μm or more, usually 20 μm or less, preferably 18 μm or less, and most preferably 15 μm or less. If the median diameter is less than this lower limit, there may be a problem in coatability at the time of forming the positive electrode active material layer, and if it exceeds the upper limit, battery performance may be deteriorated.

(BET specific surface area)
The BET specific surface areas of the composite metal oxide 1 and the composite metal oxide 2 according to the embodiment are not particularly limited, but are usually 0.1 m ² /g or more, preferably 0.2 m ² /g or more, most preferably 0. It is 3 m ² /g or more and is usually 10 m ² /g or less, preferably 5 m ² /g or less, more preferably 3 m ² /g or less, further preferably 2 m ² /g or less, most preferably 1 m ² /g or less. ..
If the BET specific surface area is smaller than this range, the battery performance tends to deteriorate, and if it is large, the bulk density tends to be difficult to increase, and there is a possibility that problems may occur in the coatability during formation of the positive electrode active material layer.

(Tap density)
The tap density of the composite metal oxide 1 and the composite metal oxide 2 according to the embodiment is not particularly limited, but is usually 0.8 g/cc or more, preferably 1 g/cc or more, more preferably 1.4 g/cc or more, It is most preferably 1.5 g/cc or more, usually 3.0 g/cc or less, preferably 2.8 g/cc or less, and most preferably 2.5 g/cc or less. When the tap density exceeds the upper limit, it is preferable for improving the powder filling property and the electrode density, but the specific surface area may be too low and the battery performance may be deteriorated. If the tap density is below this lower limit, the powder filling property and the positive electrode preparation may be adversely affected.
The tap density in the present invention is determined as the powder packing density when 5 to 10 g of the composite metal oxide powder is put in a 10 ml graduated cylinder and tapped 200 times with a stroke of 20 mm.

  The composite metal oxide 1 and the composite metal oxide 2 according to the embodiment are not particularly limited as long as the conditions described above are satisfied.

(Method for producing complex oxide)
The composite metal oxide 1 and the composite metal oxide 2 according to the embodiment can be produced by firing a mixture of metal-containing compounds as described below, but are not limited to the composite metal oxide produced by such a production method. Needless to say.

  Specifically, it can be produced by weighing and mixing a metal-containing compound containing a corresponding metal element so as to have a predetermined composition, and then firing the obtained mixture.

For example, a complex oxide having a metal element ratio represented by Na:Ni:Mn=2/3:1/3:2/3 is sodium carbonate (Na ₂ CO ₃ ) and nickel hydroxide (Ni(OH)). ₂ ) and dimanganese trioxide (Mn ₂ O ₃ ) raw materials were weighed so that the molar ratio of Na:Ni:Mn was 7/10 (5% excess): 1/3:2/3, It can manufacture by mixing them and baking the obtained mixture.

Examples of the metal-containing compound that can be used to produce the composite metal oxide include oxides, hydroxides, oxyhydroxides, hydrogen carbonates, carbonates, nitrates, sulfates, halides, oxalates, etc. The organic acid salt of can be used. As the sodium compound, Na ₂ CO ₃ , NaHCO ₃ , and Na ₂ O ₂ are preferable, and Na ₂ CO ₃ is more preferable from the viewpoint of handleability. The manganese compound is preferably MnO ₂ , Mn ₂ O ₃ or Mn ₃ O ₄ , and the nickel compound is preferably NiCO ₃ , Ni(OH) ₂ , NiOOH or NiO. Further, as examples of the raw material compound of the substitution element, LiOH and Li ₂ CO ₃ are preferable in the case of a lithium compound, MgO and MgCO ₃ are preferable in the case of a magnesium compound, and AlOOH, Al ₂ O ₃ and Al are the case in the case of an aluminum compound. (OH) ₃ is preferable, and in the case of a zinc compound, ZnO, basic zinc carbonate and zinc acetate are preferable. Further, these metal-containing compounds may be hydrates.

  For the mixing of the metal-containing compound, an apparatus which is commonly used in industry, such as a ball mill, a V-type mixer, a stirrer, or a dyno mill, can be used. The mixing at this time may be dry mixing or wet mixing. Also, a mixture of metal-containing compounds having a predetermined composition may be obtained by a crystallization method. Further, a co-precipitation method may be used to obtain a composite metal carbonate or hydroxide having a predetermined composition and then obtain a mixture with a sodium compound.

  The composite metal oxide can be obtained by firing the mixture of the metal-containing compounds obtained as described above. The firing conditions are not particularly limited, but it is preferable to set the firing temperature in the range of 600 to 1000° C. and the firing time in the range of 2 to 24 hours. If the firing temperature is 700° C. or higher, it is preferable because the generation of excessive stacking faults is suppressed, and if the firing temperature is 900° C. or lower, it is preferable because the primary particle size is reduced. Further, it is preferable that the firing time be 12 hours or more because a uniform chemical composition of a single particle is obtained, and if the firing time is 24 hours or less, crystal growth can be performed at a low temperature while maintaining stacking faults. It is preferable because it becomes possible.

Examples of the atmosphere during firing include an inert atmosphere such as nitrogen and argon: an oxidizing atmosphere such as air, oxygen, oxygen-containing nitrogen, and oxygen-containing argon: and hydrogen-containing nitrogen containing 0.1 to 10% by volume of hydrogen. , A reducing atmosphere such as hydrogen-containing argon containing 0.1 to 10% by volume of hydrogen may be used. In order to perform firing in a strongly reducing atmosphere, an appropriate amount of carbon may be included in the mixture of metal-containing compounds and fired. As the atmosphere during firing, an oxidizing atmosphere such as air is preferable.

  When a compound capable of decomposing and/or oxidizing at a high temperature, for example, a hydroxide, a carbonate, a nitrate, a sulfate, a halide, or an oxalate is used as the metal-containing compound as a raw material, before performing the above firing, The metal-containing compound may be calcined in the temperature range of 200 to 500° C. to form an oxide, or water of crystallization may be removed. The atmosphere in which calcination is performed may be an inert gas atmosphere, an oxidizing atmosphere, or a reducing atmosphere. Further, the calcined product after calcination may be crushed and used.

  In addition, it may be preferable to adjust the particle size of the composite metal oxide obtained as described above by optionally performing pulverization, classification or the like using a ball mill, a jet mill or the like. Further, the firing may be performed twice or more. Further, surface treatment such as coating the surface of the particles of the composite metal oxide with an inorganic substance containing W, Mo, Zr, Si, Y, B or the like may be performed. Moreover, it is preferable that the crystal structure of the composite oxide is not a tunnel structure.

<Polycarboxylic acid, polycarboxylic acid alkali metal salt>
The polycarboxylic acid is a polymer in which a carboxyl group is directly or indirectly bonded to an average of 10% or more of constitutional units (for example, a monomer) of the polymer. Further, the polycarboxylic acid alkali metal salt is a polymer in which H ⁺ of at least a part of the carboxyl groups of the polycarboxylic acid is replaced with an alkali metal ion. The alkali metal is not limited to Na, and may be another alkali metal such as Li or K. Further, the polymer main chain in the polycarboxylic acid or the like (in some cases, a side chain) is a substituted or unsubstituted aliphatic hydrocarbon group (eg, methylene group), a substituted or unsubstituted alicyclic hydrocarbon group (eg, β-glucose). ), a substituted or unsubstituted aromatic hydrocarbon group (for example, a phenylene group), or the like as a structural unit can be applied. The polycarboxylic acid and the alkali metal salt of polycarboxylic acid may be a homopolymer or a copolymer. In the case of the copolymer, it may be any of a random copolymer, an alternating copolymer, a block copolymer and a graft copolymer.
These may be used alone or in combination of two or more. For example, preferred examples of the polycarboxylic acid include carboxymethyl cellulose and polyacrylic acid, and preferred examples of the polycarboxylic acid alkali metal salt include sodium carboxymethyl cellulose and sodium polyacrylate. When these are applied, excellent cycle characteristics can be realized.

<Sodium secondary battery>
A sodium secondary battery, which is one mode of the present invention, is a binder containing a positive electrode active material composed of the composite metal oxide 1 or 2 of the present embodiment and a polycarboxylic acid and/or a polycarboxylic acid alkali metal salt. Other materials are not particularly limited as long as they include a positive electrode having an electrode mixture layer containing an agent, and known materials and techniques used for sodium secondary batteries can be appropriately adopted. In addition to the specific materials used for the sodium secondary battery of the present invention and the manufacturing method thereof, the contents described in, for example, JP 2011-236117 A are appropriately adopted in addition to those described below. be able to.

[Positive electrode]
The positive electrode which is one mode of the present invention includes a current collector and an electrode mixture layer (hereinafter also referred to as a positive electrode active material layer) formed on the surface of the current collector, and the positive electrode active material layer is a positive electrode active material. Contains substances, conductive materials and binders.

  The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably 80 to 95% by mass.

  Examples of the conductive material that can be used in the present invention include carbon materials such as natural graphite, artificial graphite, cokes, and carbon black. The content of the conductive material in the positive electrode active material layer is not particularly limited, but is preferably 5 to 15% by mass.

  The binder that can be used in the present invention is characterized by containing a polycarboxylic acid and/or a polycarboxylic acid alkali metal salt as described above. These may be used alone or in combination of two or more. The content of the binder in the positive electrode active material layer is usually 15% by mass or less, preferably 10% by mass or less, and more preferably 8% by mass or less. The lower limit is not particularly limited, but is usually 1% by mass or more.

  Examples of the current collector that can be used in the present invention include foils, meshes, expanded grids (expanded metals), punched metals, and the like that use conductive materials such as nickel, aluminum, and stainless (SUS). The mesh openings, wire diameter, number of meshes, etc. are not particularly limited, and conventionally known meshes can be used. A typical thickness of the current collector is 5 to 30 μm. However, a current collector having a thickness outside this range may be used.

  The size of the current collector is determined according to the intended use of the battery. If a large electrode used for a large battery is manufactured, a current collector having a large area is used. When manufacturing a small electrode, a current collector having a small area is used.

  As a method for producing a positive electrode, first, a positive electrode active material, a conductive material, a binder, and distilled water are mixed to prepare a positive electrode active material slurry. Since the composite metal oxide of the present invention has high stability to water, it is possible to produce an electrode without changing the material even in a slurry using water as a solvent.

  Next, the positive electrode active material slurry is applied onto the positive electrode current collector, dried, and pressed to fix the positive electrode current collector. Here, examples of the method for applying the positive electrode active material slurry on the positive electrode current collector include a slit die coating method, a screen coating method, a curtain coating method, a knife coating method, a gravure coating method, and an electrostatic spray method. be able to.

  As a method of forming the positive electrode active material layer on the positive electrode current collector, in addition to the above method, a mixture of the positive electrode active material, the conductive material, and the binder is placed on the positive electrode current collector and pressure molding is performed. The method of doing is also good.

[Negative electrode]
The negative electrode includes a current collector and a negative electrode active material layer formed on the surface of the current collector, and the negative electrode active material layer includes a negative electrode active material and a binder. Further, as the negative electrode, an electrode in which a negative electrode mixture containing a negative electrode active material is supported on a negative electrode current collector, or an electrode capable of inserting and extracting sodium ions such as sodium metal or sodium alloy can be used.

  As the negative electrode active material, there are carbon materials such as natural graphite, artificial graphite, cokes, hard carbon, carbon black, pyrolytic carbons, carbon fibers, and organic polymer compound fired bodies that can occlude/desorb sodium. Can be mentioned. The shape of the carbon material may be, for example, a flaky shape such as natural graphite, a spherical shape such as mesocarbon microbeads, a fibrous shape such as graphitized carbon fiber, or an aggregate of fine powder. Here, the carbon material may also serve as a conductive material.

  As described above, the negative electrode active material in the present invention is not limited to a particular one, but it is preferable to use hard carbon. By using hard carbon as the negative electrode active material, it is possible to suppress deterioration in battery performance caused by the negative electrode active material.

  Hard carbon is a carbon material whose stacking order hardly changes even when heat-treated at a high temperature of 2000° C. or higher, and is also called non-graphitizable carbon. As the hard carbon, carbon fibers obtained by carbonizing the infusible yarn which is an intermediate product in the production process of carbon fibers at about 1000 to 1400° C., and air-oxidizing an organic compound at about 150 to 300° C., and then about 1000 to 1400° C. An example is a carbon material carbonized in. The method for producing hard carbon is not particularly limited, and hard carbon produced by a conventionally known method can be used.

  The average particle diameter of the hard carbon, the true density, the spacing between the (002) planes, etc. are not particularly limited, and a preferable one can be selected and implemented.

  The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but it is preferably 80 to 95% by weight.

As the binder, polyvinylidene fluoride (hereinafter sometimes referred to as PVDF), polytetrafluoroethylene (hereinafter sometimes referred to as PTFE), tetrafluoroethylene/propylene hexafluoride/vinylidene fluoride copolymer Examples thereof include coalesce, propylene hexafluoride/vinylidene fluoride copolymer, tetrafluoroethylene/perfluorovinyl ether copolymer, and the like. These may be used alone or in combination of two or more. Other examples of the binder include starch, methyl cellulose, carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl hydroxyethyl cellulose, nitrocellulose, polyacrylic acid, sodium polyacrylate, polyacrylonitrile and the like. Examples include polysaccharides and derivatives thereof. In addition, as the usable binder, inorganic fine particles such as colloidal silica can be used. The content of the binder in the negative electrode active material layer is not particularly limited, but it is preferably 5 to 10% by mass.
A conductive material such as nickel, aluminum, copper, or stainless (SUS) is used as the current collector. The current collector is made of foil, mesh, expanded grid (expanded metal), punched metal, or the like, like the current collector for the positive electrode.

  As a method of forming the negative electrode active material layer on the current collector, the same method as the method of forming the positive electrode active material layer on the current collector can be adopted.

[Electrolytes]
The electrolyte is not particularly limited, and either a general electrolytic solution or a solid electrolyte can be used. Examples of the electrolytic solution include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, isopropylmethyl carbonate, vinylene carbonate, fluoroethylene carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, 1, Carbonates such as 2-di(methoxycarbonyloxy)ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran , Ethers such as 2-methyltetrahydrofuran; esters such as methyl formate, methyl acetate, γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; amides such as N,N-dimethylformamide, N,N-dimethylacetamide; Carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, and 1,3-propanesultone; or those obtained by further introducing a fluorine substituent into the above organic solvent can be used. Usually, two or more of these are mixed and used as the organic solvent.

Among the above electrolytic solutions, it is preferable to employ a substantially non-aqueous solvent consisting of a saturated cyclic carbonate (however, excluding the single use of ethylene carbonate) or a mixed solvent of a saturated cyclic carbonate and a chain carbonate. In particular, when any one of these non-aqueous solvents is used and hard carbon is used as the negative electrode active material, the sodium secondary battery has excellent charge/discharge efficiency and charge/discharge characteristics. Known additives used in lithium-ion batteries may be used if necessary. Particularly, fluoroethylene carbonate is preferable as the additive.

  Here, "substantially" means a non-aqueous solvent consisting only of a saturated cyclic carbonate (however, excluding the single use of ethylene carbonate), a non-aqueous solvent consisting of a mixed solvent of a saturated cyclic carbonate and a chain carbonate. It also means that a solvent containing another solvent in the non-aqueous solvent used in the present invention is included within a range that does not affect the performance of the sodium secondary battery such as charge/discharge characteristics.

  Among the saturated cyclic carbonates, it is preferable to use propylene carbonate. Further, among the mixed solvents, it is preferable to use a mixed solvent of ethylene carbonate and diethyl carbonate or a mixed solvent of ethylene carbonate and propylene carbonate.

  The electrolyte salt that can be used when an electrolytic solution is used as the electrolyte is not particularly limited, and an electrolyte salt that is commonly used in sodium secondary batteries can be used.

Examples of electrolyte salts generally used in sodium secondary batteries include NaClO ₄ , NaPF ₆ , NaBF ₄ , CF ₃ SO ₃ Na, NaAsF ₆ , NaB(C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Na, _{NaN (SO 2 CF 3) 2} , NaN (SO 2 C 2 F 5) 2, NaC (SO 2 CF 3) 3, NaN (SO 3 CF 3) may be mentioned _2. Further, as the electrolyte salt, a Li salt may be used in addition to the Na salt. One of these electrolytes may be used, or two or more may be used in combination.

  The concentration of the electrolyte salt in the electrolytic solution is not particularly limited, but the concentration of the electrolyte salt is preferably 3 to 0.1 mol/l, more preferably 1.5 to 0.5 mol/l. .

As the solid electrolyte, for example, an organic solid electrolyte such as a polyethylene oxide polymer compound or a polymer compound containing at least one polyorganosiloxane chain or polyoxyalkylene chain can be used. Further, a so-called gel type in which a non-aqueous electrolyte solution is held in a polymer compound can also be used. _{Further, Na 2 S-SiS 2,} Na 2 S-GeS 2, NaTi 2 (PO 4) 3, NaFe 2 (PO 4) 3, Na 2 (SO 4) 3, Fe 2 (SO 4) 2 (PO 4 ), Fe ₂ (MoO ₄ ) _{3 and} other inorganic solid electrolytes may be used. Since the composite metal oxide having the P2 type and P3 type layered structure of the present invention is softer than the composite metal oxide having the O3 type layered structure, the effect of excellent adhesion to the solid electrolyte can be expected.

[Structure of sodium secondary battery]
The structure of the sodium secondary battery of the present invention is not particularly limited, and any of the conventionally known forms such as a laminated type (flat type) battery and a wound type (cylindrical type) battery can be distinguished when the form and structure are distinguished. -It can be applied to the structure. When viewed in terms of the electrical connection form (battery structure) in the sodium secondary battery, it can be applied to both (internal parallel connection type) battery and bipolar (internal series connection type) battery.

  Hereinafter, the present invention will be described in detail with reference to examples. The present invention is not limited to the examples shown below.

(Example 1)
Na ₂ CO ₃ , Ni(OH) ₂ and Mn ₂ O ₃ were weighed so that the molar ratio of Na:Ni:Mn was 7/10 (5% excess): 1/3: 2/3 and ball milled. For 12 hours to obtain a mixture of metal-containing compounds. The obtained mixture was pelletized, filled in an alumina boat, and baked in an electric furnace in an air atmosphere at 900° C. for 24 hours to give a mixed metal oxide 1 (P2-Na _2/3 Ni). _1/3 Mn _2/3 O ₂ ) was obtained.

  A positive electrode active material composed of the composite metal oxide 1, acetylene black as a conductive material, and carboxymethyl cellulose (CMC) as a binder were used as positive electrode active material:conductive material:binder=80:10:2 (mass ratio). An electrode was produced by the following procedure so as to have the composition of ). First, the positive electrode active material and the electrically conductive material were thoroughly mixed in an agate mortar, and a binder and distilled water were added to this mixture and the mixture was then mixed uniformly to form a slurry. Then, the obtained positive electrode active material slurry was applied on an aluminum foil having a thickness of 20 μm serving as a current collector with an applicator so as to have a thickness of 40 μm, and put in a dryer to remove distilled water. While sufficiently drying, an electrode sheet was obtained. This electrode sheet was punched with an electrode punching machine to a diameter of 1.0 cm to obtain a positive electrode.

A coin-type sodium secondary battery was manufactured using a negative electrode manufactured by using metallic sodium as a counter electrode and a positive electrode manufactured by using a composite metal oxide as a working electrode. As the electrolytic solution, a solution prepared by dissolving 1M electrolyte salt (NaPF ₆ ) in a non-aqueous solvent (propylene carbonate) was used. A glass filter was used as the separator. Further, the sodium secondary battery was manufactured in a glove box filled with argon.

(Example 2)
A coin-type sodium secondary battery of Example 2 was produced in the same manner as in Example 1 except that sodium polyacrylate (PAANA) was used as the binder.

(Example 3)
Na ₂ CO ₃ , Ni(OH) ₂ and Mn ₂ O ₃ were weighed so that the molar ratio of Na:Ni:Mn was 7/10 (5% excess): 1/3: 2/3 and ball milled. For 12 hours to obtain a mixture of metal-containing compounds. The obtained mixture was pelletized, filled in an alumina boat, and fired under the conditions of an electric furnace at 700° C. for 24 hours in an air atmosphere to obtain a mixed metal oxide 2 (P3-Na _2/3 Ni). _1/3 Mn _2/3 O ₂ ) was obtained.

  Example 3 was performed in the same manner as in Example 1 except that the composite metal oxide 2 was used as the positive electrode active material, and the composition of the positive electrode active material:conductive material:binder=80:10:5 (mass ratio) was used. A coin-type sodium secondary battery was manufactured.

(Example 4)
A coin-type sodium secondary battery of Example 4 was produced in the same manner as in Example 3 except that sodium polyacrylate (PAANA) was used as the binder.

(Example 5)
The _{Na 2 CO 3, Ni (OH} ) 2, Mn 2 O 3, MgO, Na: Ni: Mn: molar ratio of Mg is 7/10 (5% excess): 5/18: 2/3: 1/18 And were mixed for 12 hours with a ball mill to obtain a mixture of metal-containing compounds. The obtained mixture is pelletized, filled in an alumina boat, and baked in an electric furnace in an air atmosphere at 700° C. for 24 hours to obtain a mixed metal oxide 4 (P3-Na _2/3 [. Mg _1/18 Ni _5/18 Mn _2/3 ]O ₂ ) was obtained.

  A coin-type sodium secondary battery of Example 5 was manufactured in the same manner as in Example 1.

(Example 6)
The _{Na 2 CO 3, Ni (OH} ) 2, Mn 2 O 3, ZnO, Na: Ni: Mn: molar ratio of Mg is 7/10 (5% excess): 5/18: 2/3: 1/18 And were mixed for 12 hours with a ball mill to obtain a mixture of metal-containing compounds. The obtained mixture was pelletized, filled in an alumina boat, and fired under the condition of an electric furnace at 700° C. for 48 hours in an air atmosphere to obtain a mixed metal oxide 5 (P3-Na _2/3 [ Zn _1/18 Ni _5/18 Mn _2/3 ]O ₂ ) was obtained.

  A coin-type sodium secondary battery of Example 6 was produced in the same manner as in Example 1.

(Example 7)
The _{Na 2 CO 3, Ni (OH} ) 2, Mn 2 O 3, Al 2 O 3, Na: Ni: Mn: molar ratio of Mg is 7/10 (5% excess): 11/36: 23/36: The mixture was weighed to 1/18 and mixed with a ball mill for 12 hours to obtain a mixture of metal-containing compounds. The obtained mixture is pelletized, filled in an alumina boat, and baked in an electric furnace in an air atmosphere at 700° C. for 24 hours to give a mixed metal oxide 6 (P3-Na _2/3 [. Al _1/18 Ni _11/36 Mn _23/36 ]O ₂ ) was obtained.

  A coin-type sodium secondary battery of Example 7 was manufactured in the same manner as in Example 1.

(Comparative Example 1)
A coin-type sodium secondary battery of Comparative Example 1 was produced in the same manner as in Example 1 except that polyvinylidene fluoride (PVDF) was used as the binder and N-methylpyrrolidone was used instead of distilled water.

(Comparative example 2)
A coin-type sodium secondary battery of Comparative Example 2 was produced in the same manner as in Example 3 except that polyvinylidene fluoride (PVDF) was used as the binder and N-methylpyrrolidone was used instead of distilled water.

(Comparative example 3)
Na ₂ CO ₃ , Ni(OH) ₂ and Mn ₂ O ₃ were weighed so that the molar ratio of Na:Ni:Mn was 21/20 (5% excess): 1/2: 1/2, and ball milled. For 12 hours to obtain a mixture of metal-containing compounds. The obtained mixture was pelletized, filled in an alumina boat, and baked in an electric furnace in the air atmosphere at 800° C. for 24 hours to give a mixed metal oxide 3 (O3-NaNi _1/2 Mn). _1/2 O ₂ ) was obtained.

  A positive electrode active material composed of the composite metal oxide 3, acetylene black as a conductive material, and carboxymethyl cellulose (CMC) as a binder were mixed in a positive electrode active material: conductive material: binder = 80:10:2 (mass ratio). An electrode was produced by the following procedure so as to have the composition of ). First, the positive electrode active material and the electrically conductive material were thoroughly mixed in an agate mortar, and a binder and distilled water were added to this mixture and the mixture was then mixed uniformly to form a slurry. Next, the obtained positive electrode active material slurry was applied on an aluminum foil having a thickness of 20 μm to be a current collector with an applicator so as to have a thickness of 40 μm and put in a dryer to remove pure water. While sufficiently drying, an electrode sheet was obtained. FIG. 2 shows a photograph of the surface of the electrode sheet. From FIG. 2, it can be observed that the surface of the electrode sheet has been fired. This is because the reaction between the composite metal oxide 3 and distilled water raised the pH, corroded the Al current collector, and generated gas, which made it difficult to produce an electrode with a slurry using water as a solvent. Show.

(Comparative example 4)
A coin-type sodium secondary battery of Comparative Example 4 was produced in the same manner as in Example 5 except that polyvinylidene fluoride (PVDF) was used as the binder and N-methylpyrrolidone was used instead of distilled water.

(Comparative Example 5)
A coin-type sodium secondary battery of Comparative Example 5 was produced in the same manner as in Example 6 except that polyvinylidene fluoride (PVDF) was used as the binder and N-methylpyrrolidone was used instead of distilled water.

(Comparative example 6)
A coin-type sodium secondary battery of Comparative Example 6 was produced in the same manner as in Example 7, except that polyvinylidene fluoride (PVDF) was used as the binder and N-methylpyrrolidone was used instead of distilled water.

  It has been confirmed that there are two types of reactions regarding the pH increase due to the reaction with water. The reaction formula and reaction mechanism are described below.

The first reaction is an ion exchange reaction between Na ⁺ and H ⁺ , which is found in a composite metal oxide having an O3 type layered structure, and can be described by (Formula 1) for the material of (Formula 1). The metal oxide material in which H ⁺ has been inserted undergoes a dehydration process associated with drying, leading to structural destruction. It has been confirmed that 80% or more of Na is eluted.
(Formula 1) NaFe ^(III) _0.4 Ni ^(II) _0.3 Mn ^(IV) _0.3 O ₂ +H ₂ O
→NaOH+Fe ^(III) _0.4 Ni ^(II) _0.3 Mn ^(IV) _0.3 OOH

The second reaction is a reductive decomposition reaction of water by trivalent Mn in the layered structure. The material of (Formula 2) can be described by (Formula 2). A reaction in which trivalent Mn is oxidized to tetravalent with hydrogen generation proceeds. It was confirmed that the ion exchange reaction did not occur in the P2 type or P3 type layered structure metal oxide and the reaction stopped when trivalent Mn was consumed, and the P2 type or P3 type layered structure was maintained. ing.
(Formula 2) Na _2/3 Fe ^(III) _1/2 Mn ^(III) _1/6 Mn ^(IV) _1/3 O ₂ +H ₂ O
→ 1/6 NaOH+1/12H ₂ +Na _1/2 Fe ^(III) _1/2 Mn ^(IV) _1/2 O ₂

The ion exchange reaction of the layered composite metal oxide has a large correlation with the shape of the occupied site of Na ⁺ . When Na ⁺ is present in the octahedral site between layers, as in the case of the O3 type layered composite metal oxide, H ⁺ is likely to be stably arranged at the adjacent vacant 4-coordination site, and ion exchange proceeds. On the other hand, when Na ⁺ exists at the prismatic site between layers like the composite metal oxide having the P2 type layered structure or the P3 type layered structure, H ⁺ cannot be stably arranged at the adjacent empty prismatic site. Therefore, it is presumed that ion exchange will not proceed.

(Evaluation 1) Identification of crystal structure by XRD measurement Powder X-ray diffraction measurement was performed on the composite metal oxides of Examples and Comparative Examples. The measurement was performed using a powder X-ray diffraction measuring device MultiFlex manufactured by Rigaku under the following conditions.
X-ray: CuKα
Voltage-current: 40kV-20mA
Measurement angle range: 2θ=10 to 70°
Step: 0.02°
Scan speed: 6°/min

  The measurement results of the composite metal oxide 1 used in Examples 1 and 2 and Comparative Example 1 are shown in FIG. 1a, and the measurement results of the composite metal oxide 2 used in Examples 3 and 4 and Comparative Example 2 are shown in FIG. 1b. The measurement results of the composite metal oxide 3 used in Comparative Example 3 are shown in FIG. 1c, the measurement results of the composite metal oxide 4 used in Example 5 and Comparative Example 4 are shown in FIG. 1d, and the measurement results of Example 6 and Comparative Example 5 are shown. The measurement results of the composite metal compound 5 used are shown in FIG. 1e, and the measurement results of the composite metal oxide 6 used in Example 7 and Comparative Example 6 are shown in FIG. 1f.

  1a to 1f, the composite metal oxides used in Examples 1 and 2 and Comparative Example 1 have P2 type layered structures, and Examples 3, 4, 5, 6, 7 and Comparative Examples 2, 4, 5 It can be seen that the composite metal oxides used in Examples 6 and 7 have a P3 type layered structure, and the composite metal oxide used in Comparative Example 3 has an O3 type layered structure in a substantially single phase.

(Evaluation 2) Charge/Discharge Evaluation The sodium secondary battery of Example 1 was subjected to charge/discharge evaluation. The current density was set to 13 mA/g for each positive electrode material, and constant current charging was performed up to 4.5 V (charging voltage). After charging, the current density was set to a current of 13 mA/g, and constant current discharge was performed up to 2.0 V (discharge voltage). This charge/discharge was performed 10 cycles, and the capacity retention rate after 10 cycles was 94.0%, which was confirmed to be a high cycle retention rate. The charging/discharging was performed at a temperature of 25°C.

  The charge/discharge evaluation of the sodium secondary battery of Example 2 was performed by the same method as the charge/discharge evaluation of the sodium secondary battery of Example 1. The capacity retention rate after 10 cycles was 92.8%, confirming a high cycle retention rate.

  The charge/discharge evaluation of the sodium secondary battery of Comparative Example 1 was performed by the same method as the charge/discharge evaluation of the sodium secondary battery of Example 1. The capacity retention rate after 10 cycles was 85.6%, which was confirmed to be a low cycle retention rate as compared with the case where a specific binder was used.

  The charge/discharge evaluation of the sodium secondary battery of Example 3 was performed by the same method as the charge/discharge evaluation of the sodium secondary battery of Example 1. The capacity retention rate after 10 cycles was 77.0%, confirming a high cycle retention rate.

  The charge/discharge evaluation of the sodium secondary battery of Example 4 was performed by the same method as the charge/discharge evaluation of the sodium secondary battery of Example 1. The capacity retention rate after 10 cycles was 74.4%, which was confirmed to be a high cycle retention rate.

  The charge/discharge evaluation of the sodium secondary battery of Comparative Example 2 was performed by the same method as the charge/discharge evaluation of the sodium secondary battery of Example 1. The capacity retention rate after 10 cycles was 69.5%, which was confirmed to be a low cycle retention rate as compared with the case where a specific binder was used.

  The charge/discharge evaluation of the sodium secondary battery of Example 5 was performed by the same method as the charge/discharge evaluation of the sodium secondary battery of Example 1. The capacity retention rate after 10 cycles was 94.3%, confirming a high cycle retention rate.

  The charge/discharge evaluation of the sodium secondary battery of Comparative Example 4 was performed by the same method as the charge/discharge evaluation of the sodium secondary battery of Example 1. The capacity retention rate after 10 cycles was 88.6%, which was confirmed to be a low cycle retention rate as compared with the case where a specific binder was used.

The charge/discharge evaluation of the sodium secondary battery of Example 6 was performed by the same method as the charge/discharge evaluation of the sodium secondary battery of Example 1. The capacity retention rate after 10 cycles was 103.4%, confirming a high cycle retention rate.

  The charge/discharge evaluation of the sodium secondary battery of Comparative Example 5 was performed by the same method as the charge/discharge evaluation of the sodium secondary battery of Example 1. The capacity retention rate after 10 cycles was 89.6%, which was confirmed to be a low cycle retention rate as compared with the case where a specific binder was used.

  The charge/discharge evaluation of the sodium secondary battery of Example 7 was performed by the same method as the charge/discharge evaluation of the sodium secondary battery of Example 1. The capacity retention rate after 10 cycles was 78.4%, confirming a high cycle retention rate.

  The charge/discharge evaluation of the sodium secondary battery of Comparative Example 6 was performed by the same method as the charge/discharge evaluation of the sodium secondary battery of Example 1. The capacity retention rate after 10 cycles was 72.5%, which was confirmed to be a low cycle retention rate as compared with the case where a specific binder was used.

  The results of Examples 1-7 and Comparative Examples 1-6 are summarized in Table 1. Since the composite metal oxide of this example is stable even in the water slurry, it can be seen that the positive electrode can be prepared with the aqueous binder and that the composite metal oxide has good cycle characteristics.

  The application of the sodium secondary battery provided with the positive electrode active material for a sodium secondary battery of the present invention is not particularly limited, and it can be used for various known applications. Specific examples include laptop computers, pen input computers, mobile computers, e-book players, mobile phones, mobile faxes, mobile copiers, mobile printers, headphone stereos, video movies, LCD TVs, handy cleaners, portable CDs, minidisks, transceivers. , Electronic notebook, calculator, memory card, portable tape recorder, radio, backup power supply, stationary power supply, motor, lighting equipment, toys, game machines, clocks, strobes, cameras, pacemakers, power tools, power sources for bicycles/bikes, Examples thereof include a power source for automobiles, a power source for orbital vehicles, and a power source for artificial satellites.

Claims (4)

Na _2/3 Ni _1/3 Mn _2/3 O ₂ having at least a P2 type or P3 type crystal structure and Na _x MO ₂ having a P3 type crystal structure (M is selected from Mn, Fe, Co and Ni) 1/3≦ x≦ 5 /6 , wherein a part of the M is substituted with at least one element selected from Li, Mg, Al and Zn. A) a positive electrode active material containing any one selected from the complex metal oxides described above and an electrode mixture layer containing a binder containing a polycarboxylic acid and/or a polycarboxylic acid alkali metal salt. Positive electrode for sodium secondary battery.   The polycarboxylic acid and/or polycarboxylic acid alkali metal salt contains at least one selected from the group consisting of polyacrylic acid, polyacrylic acid alkali metal salt, carboxymethyl cellulose and carboxymethyl cellulose metal salt. Item 1. A positive electrode for a sodium secondary battery according to Item 1.   Content of the binder in the said electrode mixture layer is 15 mass% or less, The positive electrode for sodium secondary batteries of Claim 1 or 2 characterized by the above-mentioned.   A sodium secondary battery comprising the positive electrode for a sodium secondary battery according to claim 1.