JP2009259601A - Electrode active material for sodium ion secondary battery and its method for manufacturing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode active material capable of providing a non-aqueous electrolyte secondary battery superior in cycle efficiency at the time of repeating charge and discharge, and its method for manufacturing. <P>SOLUTION: The electrode active material for sodium ion secondary battery includes a sodium compound oxide of which crystal structure is a calcium ferrite type crystal structure. This is a method for manufacturing of the electrode active material consisting of a complex oxide as expressed by the following formula: Na<SB>x</SB>Fe<SB>(2-y)</SB>M<SB>y</SB>O<SB>4</SB>in which a mixture composed of compounds including Na compound, Fe compound, and M (M is one kind or more of metal elements excluding Na and Fe) is calcined at 650°C or more and 1,300°C or less under an oxidation atmosphere. In the formula, M expresses one kind or more of metal elements excluding Na and Fe, x is in the range exceeding 0 and less than 2, and y is in the range exceeding 0 and less than 2. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electrode active material for a non-aqueous electrolyte secondary battery and a method for producing the same. In particular, the present invention relates to an electrode active material for a sodium ion secondary battery and a method for producing the same.

  As a non-aqueous electrolyte secondary battery for portable electronic devices, lithium ion secondary batteries have been put into practical use and are widely used. In recent years, lithium ion secondary batteries have been tried to be applied to medium and large-sized applications such as hybrid vehicles and power storage applications. However, the lithium ion secondary battery electrode active material currently used does not have abundant resources, and in particular, the positive electrode active material has a low reserve as a resource and is expensive. Cobalt and nickel are often used. Therefore, an electrode active material for a non-aqueous electrolyte secondary battery that uses abundant and inexpensive elements as resources is demanded.

  Therefore, composite oxides containing abundant reserves of sodium and iron as resources have been proposed as electrode active materials for non-aqueous electrolyte secondary batteries, and are cheaper non-aqueous electrolyte secondary batteries such as sodium ion secondary batteries. It is expected as a battery that can be realized.

As the composite oxide, Non-Patent Document 1 proposes NaFeO ₂ having a layered structure and reports that it is used as an electrode active material.

Materials Research Bulletin (USA), Pergamon Press, 1994, Vol. 29, no. 6, p. 659-666

However, a sodium ion secondary battery using the electrode active material made of NaFeO ₂ above as the positive electrode active material, not sufficient in terms of cycle efficiency of the discharge capacity when charging and discharging are repeated, sodium NaFeO ₂ It cannot be said that it can be sufficiently used as an electrode active material for an ion secondary battery.

  Accordingly, an object of the present invention is to provide an electrode active material that can provide a non-aqueous electrolyte secondary battery in which the cycle efficiency of the discharge capacity when charging and discharging are repeated is increased.

  The inventors of the present invention have intensively studied to solve the above-mentioned problems and have arrived at the present invention. That is, the present invention relates to the following <1> to <12> inventions.

<1> An electrode active material for a sodium ion secondary battery having a sodium composite oxide having a crystal structure of calcium ferrite type crystal structure.

<2> The electrode active material according to <1>, wherein the sodium composite oxide is represented by the following formula (I).
_{Na x Fe (2-y)} M y O 4 (I)
(In the formula, M represents one or more metal elements excluding Na and Fe, x is in the range of more than 0 and less than 2, and y is in the range of more than 0 and 2 or less.)

<3> The electrode active material according to <2>, wherein M is one or more metal elements selected from Group 4, 5, 6 and Group 14 elements.

<4> The electrode active material according to <2> or <3>, wherein M is one or more metal elements selected from Ti, Sn, Mo, and Nb.

<5> The electrode active material according to any one of <2> to <4>, wherein M is composed of Ti and Mo.

<6> A mixture comprising a compound containing Na compound, Fe compound and M (wherein M is one or more metal elements excluding Na and Fe) is fired at 650 ° C. to 1300 ° C. in an oxidizing atmosphere. The manufacturing method of the electrode active material which consists of complex oxide represented by the following formula | equation (I).
_{Na x Fe (2-y)} M y O 4 (I)
(In the formula, M represents one or more metal elements excluding Na and Fe, x is in the range of more than 0 and less than 2, and y is in the range of more than 0 and 2 or less.)

<7> The method for producing an electrode active material according to <6>, wherein M is one or more metal elements selected from Ti, Sn, Mo, and Nb.

<8> The method for producing an electrode active material according to <7>, wherein the M is composed of Ti and Mo.

<9> A sodium ion secondary battery electrode comprising the electrode active material according to any one of <1> to <5>, a conductive material, and a binder.

<10> A sodium ion secondary battery using the electrode according to <9>.

<11> The sodium ion secondary battery according to <10>, further including a separator.

<12> The sodium ion secondary battery according to <11>, wherein the separator is a separator composed of a laminated porous film in which a heat-resistant porous layer containing a heat-resistant resin and a porous film containing a thermoplastic resin are laminated. .

  By using the electrode active material of the present invention, it is possible to produce a nonaqueous electrolyte secondary battery having higher cycle efficiency of discharge capacity when charging and discharging are repeated and excellent characteristics as a secondary battery. And according to the manufacturing method of this invention, the electrode active material of this invention can be manufactured, and this invention is very useful industrially.

Hereinafter, the present invention will be described in detail.
The electrode active material of the present invention has a sodium composite oxide whose crystal structure is a calcium ferrite (CaFe ₂ O ₄ ) type crystal structure (hereinafter sometimes simply referred to as “sodium composite oxide”).
This sodium composite oxide can be used as a positive electrode active material or a negative electrode active material for a sodium ion secondary battery. That is, in the case of using as a positive electrode active material, charging is performed by dedoping Na ions from the sodium composite oxide, and conversely, discharging is performed by doping Na ions into the sodium composite oxide. On the other hand, when used as a negative electrode active material, the sodium composite oxide is charged by Na ions being charged, and conversely, Na ions are dedoped from the sodium composite oxide, and discharge is performed.

  Whether the sodium composite oxide functions as a positive electrode active material or a negative electrode active material depends on the electrode active material combined with the sodium composite oxide as a sodium ion secondary battery. That is, when the electrode active material to be combined can be doped / undoped with sodium ions at a lower potential than the sodium composite oxide according to the present invention, the sodium composite oxide according to the present invention is a positive electrode active material. In the case where the electrode active material that functions as a combination and can be doped / dedoped with sodium ions at a higher potential than the sodium composite oxide according to the present invention, the sodium composite oxide according to the present invention Functions as a negative electrode active material. The electrode active material of the present invention is suitably used as a positive electrode active material, but can also be used as a negative electrode active material.

As the sodium composite oxide in which the crystal structure is a calcium ferrite type crystal structure, a sodium composite oxide represented by the following formula (I) is preferable.
_{Na x Fe (2-y)} M y O 4 (I)
(In the formula, M represents one or more metal elements excluding Na and Fe, x is in the range of more than 0 and less than 2, and y is in the range of more than 0 and 2 or less.)
Here, the metal element M (hereinafter sometimes simply referred to as “M”) is one or more metal elements excluding Na and Fe, and is preferably selected from Groups 4, 5, 6 and 14 One or more metal elements. Among these, when a metal element capable of taking a valence of 4 or more is used, the discharge capacity tends to increase. In particular, at least one selected from the group consisting of Ti, Sn, Mo, and Nb is preferable. It is particularly preferable that it is composed of Mo and Ti that particularly improve the performance when repeated. Moreover, it is preferable that M contains Ti with abundant reserve from a resource viewpoint.

In the formula (I), the value of x can be selected in the range of more than 0 and less than 2. Here, when the electrode active material of the present invention is used as the positive electrode active material, the value of x is in the range of more than 0 to 1 or less, and particularly when the negative electrode not containing sodium is used, the value of x is The closer to 1, the better. Conversely, when used as a negative electrode active material, the value of x is in the range of 1 to less than 2.
In the formula (I), the value of y can be selected in the range of more than 0 and 2 or less, and Fe may not be included (y = 2).

The electrode active material of the present invention can be produced as follows using a Na compound, an iron compound, and a compound containing the above metal element M as starting materials. That is, it can be manufactured by including the following steps (1), (2) and (3) in this order.
(1) A step of mixing the above starting materials to obtain a mixture.
(2) A step of firing the mixture in an oxidizing atmosphere to obtain a fired product.
(3) A step of pulverizing the fired product.

In step (1), at least one sodium compound is used as the sodium raw material. The sodium compound is not particularly limited as long as it contains sodium, and examples thereof include oxides such as Na ₂ O and Na ₂ O ₂ , salts such as Na ₂ CO ₃ , NaNO ₃ and NaHCO ₃ , and NaOH. A hydroxide is mentioned. Among these, Na ₂ CO ₃ is particularly preferable.

In step (1), at least one iron compound is used as the iron raw material. The iron compound is not particularly limited as long as it contains iron. For example, oxides such as Fe ₃ O ₄ , Fe ₂ O ₃ , and FeO, FeCl ₂ , FeCl ₃ , Fe (NO ₃ ) ₂ , Fe Salts such as (NO ₃ ) ₃ , FeSO ₄ , Fe (SO ₄ ) ₃ , sulfides such as FeS, FeS ₂ , Fe ₂ S ₃ , hydroxylation such as Fe (OH) ₂ , Fe (OH) ₃ , FeOOH Thing etc. are mentioned. Among these, Fe ₃ O ₄ is particularly preferable.

  In the step (1), the mixing method may be dry mixing that is usually used industrially. Examples of the dry mixing apparatus include a V-type mixer, a W-type mixer, a ribbon mixer, a drum mixer, and a dry ball mill. Mixing can also be performed by wet mixing.

  In the step (2), the firing temperature is preferably 650 ° C. or higher and 1300 ° C. or lower, and the firing time is usually 2 to 30 hours. Even within this firing temperature range, firing at 900 ° C. or more and 1200 ° C. or less is more preferable. If the firing temperature is less than 650 ° C., crystals are difficult to produce, and if it exceeds 1300 ° C., the raw material compounds and constituent metal elements used may melt. At the time of firing, the firing temperature may be reached as long as the firing container containing the mixture is not damaged. The holding time at the firing temperature is about 0 to 48 hours. Further, an oxidizing atmosphere is desirable as the firing atmosphere. Examples of the oxidizing atmosphere include an atmosphere having an oxygen concentration of 10% by volume (preferably 20% by volume) or more, and may be an air atmosphere.

  Step (3) may be performed as necessary. What is necessary is just to carry out by the grinding | pulverization normally used industrially as a grinding | pulverization method in a process (3). Examples of the pulverizer include pulverizers such as a vibration mill, a jet mill, and a dry ball mill. Moreover, you may perform classification operation, such as a wind classification, as needed. At this time, it is preferable that the primary particles constituting the electrode active material are not damaged. Moreover, about the particle | grains which comprise the obtained electrode active material, 1 or more types of elements chosen from B, Al, Mg, Ga, In, Si, Ge, Sn, Nb, Ta, W, Mo, and a transition metal element are included. You may perform surface treatments, such as making the compound to contain adhere. Among the above elements, one or more selected from B, Al, Mg, Mn, Fe, Co, Ni, Nb, Ta, W, and Mo are preferable, and Al is more preferable from the viewpoint of operability. Examples of the compound include oxides, hydroxides, oxyhydroxides, carbonates, nitrates, organic acid salts, and mixtures of the above elements. Of these, oxides, hydroxides, oxyhydroxides, carbonates or mixtures thereof are preferred. Of these, alumina is more preferable.

Using the electrode active material of the present invention, for example, an electrode can be obtained as follows.
The electrode can be produced by supporting an electrode mixture containing the electrode active material of the present invention, a conductive material and a binder on an electrode current collector. Examples of the conductive material include carbon materials such as natural graphite, artificial graphite, cokes, and carbon black. These conductive materials may be used alone or in combination of two or more, for example, artificial graphite and carbon black are mixed.

  Examples of the binder include thermoplastic resins, and specifically, polyvinylidene fluoride (hereinafter sometimes referred to as PVdF), polytetrafluoroethylene, tetrafluoroethylene, hexafluoropropylene, and fluoride. Fluorine resins such as vinylidene copolymers, propylene hexafluoride / vinylidene fluoride copolymers, tetrafluoroethylene / perfluorovinyl ether copolymers, and polyolefin resins such as polyethylene and polypropylene. These may be used alone or in combination of two or more.

  As the electrode current collector, Al, Ni, stainless steel or the like can be used. As a method of supporting the electrode mixture on the electrode current collector, it is fixed by press molding or pasting using an organic solvent, coating on the electrode current collector, drying and pressing. A method is mentioned. In the case of forming a paste, a slurry made of an electrode active material, a conductive material, a binder, and an organic solvent is prepared.

  Examples of the organic solvent include amines such as N, N-dimethylaminopropylamine and diethyltriamine, ethers such as ethylene oxide and tetrahydrofuran, ketones such as methyl ethyl ketone, esters such as methyl acetate, dimethylacetamide, 1-methyl- Examples include aprotic polar solvents such as 2-pyrrolidone. Examples of the method of applying the electrode mixture to the 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.

As an example of the sodium ion secondary battery having the electrode active material of the present invention, an example in which an electrode using the electrode active material of the present invention is used as a positive electrode will be described below.
That is, the sodium ion secondary battery of the present invention contains an electrolyte in which a group of electrodes obtained by laminating and winding a positive electrode, a separator, and a negative electrode composed of the above-described electrodes is housed in a container such as a battery can. It can be produced by impregnating an electrolytic solution composed of an organic solvent.

  Examples of the shape of the electrode group include a shape in which a cross-section when the electrode group is cut in a direction perpendicular to the winding axis is a circle, an ellipse, a rectangle, a rectangle with rounded corners, or the like. be able to. In addition, examples of the shape of the battery include a paper shape, a coin shape, a cylindrical shape, and a square shape.

  As the negative electrode, it is possible to use a negative electrode mixture containing a negative electrode material capable of doping and dedoping sodium ions on an electrode current collector, sodium metal or sodium alloy, etc. Specific examples include carbon materials such as natural graphite, artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers, and fired organic polymer compounds, which can be doped / undoped with sodium ions. In addition, chalcogen compounds such as oxides and sulfides that can be doped / undoped with sodium ions at a lower potential than the electrode active material of the present invention used for the positive electrode can also be used. The shape of the carbon material may be any of a flake 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.

  The negative electrode mixture may contain a binder as necessary. Examples of the binder include thermoplastic resins, and specific examples include PVdF, thermoplastic polyimide, carboxymethyl cellulose, polyethylene, and polypropylene.

  Examples of the electrode current collector in the negative electrode include Cu, Ni, stainless steel, and the like, and Cu is preferable because it can be easily processed into a thin film. The method of supporting the negative electrode mixture on the electrode current collector is the same as in the case of the positive electrode described above, and is formed into a paste using a method by pressure molding, a solvent, etc., dried on the electrode current collector, and then pressed. And a method of pressure bonding.

  As the separator, for example, a material having a form such as a porous film, a nonwoven fabric, a woven fabric made of a polyolefin resin such as polyethylene and polypropylene, a fluororesin, a nitrogen-containing aromatic polymer, and the like can be used. Moreover, it is good also as a single layer or laminated separator which used 2 or more types of these materials. Examples of the separator include separators described in JP 2000-30686 A, JP 10-324758 A, and the like. The thickness of the separator is preferably as thin as possible as long as the mechanical strength is maintained in that the volume energy density of the battery is increased and the internal resistance is reduced, and is preferably about 5 to 200 μm, more preferably about 5 to 40 μm.

  The separator preferably has a porous film containing a thermoplastic resin. In a non-aqueous electrolyte secondary battery such as a sodium ion secondary battery, when an abnormal current flows in the battery due to a short circuit between the positive electrode and the negative electrode, normally the current is interrupted and an excessive current flows. It is important to prevent (shut down). Therefore, the separator shuts down at the lowest possible temperature when the normal use temperature is exceeded (if the separator has a porous film containing a thermoplastic resin, the pores of the porous film are blocked). In addition, even if the temperature in the battery rises to a certain high temperature after the shutdown, it is required to maintain the shutdown state without breaking the film due to the temperature, in other words, to have high heat resistance. . By using a separator made of a laminated porous film in which a heat-resistant porous layer containing a heat-resistant resin and a porous film containing a thermoplastic resin are laminated as a separator, it becomes possible to further prevent thermal film breakage. . Here, the heat-resistant porous layer may be laminated on both surfaces of the porous film.

  Hereinafter, a separator composed of a laminated porous film in which a heat resistant porous layer containing a heat resistant resin and a porous film containing a thermoplastic resin are laminated will be described. Here, the thickness of this separator is usually 40 μm or less, preferably 20 μm or less. Moreover, when the thickness of the heat resistant porous layer is A (μm) and the thickness of the porous film is B (μm), the value of A / B is preferably 0.1 or more and 1 or less. Furthermore, from the viewpoint of ion permeability, this separator preferably has an air permeability of 50 to 300 seconds / 100 cc, more preferably 50 to 200 seconds / 100 cc in terms of air permeability by the Gurley method. . The porosity of this separator is usually 30 to 80% by volume, preferably 40 to 70% by volume.

  In the laminated porous film, the heat resistant porous layer contains a heat resistant resin. In order to further enhance the ion permeability, the heat-resistant porous layer is preferably a thin heat-resistant porous layer having a thickness of 1 μm to 10 μm, further 1 μm to 5 μm, particularly 1 μm to 4 μm. The heat-resistant porous layer has fine pores, and the size (diameter) of the pores is usually 3 μm or less, preferably 1 μm or less. Furthermore, the heat resistant porous layer can also contain a filler described later.

  Examples of the heat resistant resin contained in the heat resistant porous layer include polyamide, polyimide, polyamideimide, polycarbonate, polyacetal, polysulfone, polyphenylene sulfide, polyether ketone, aromatic polyester, polyether sulfone, and polyetherimide. From the viewpoint of further improving heat resistance, polyamide, polyimide, polyamideimide, polyethersulfone, and polyetherimide are preferable, and polyamide, polyimide, and polyamideimide are more preferable. Even more preferably, the heat-resistant resin is a nitrogen-containing aromatic polymer such as aromatic polyamide (para-oriented aromatic polyamide, meta-oriented aromatic polyamide), aromatic polyimide, aromatic polyamideimide, and particularly preferably aromatic. Polyamide, particularly preferably a para-oriented aromatic polyamide (hereinafter sometimes referred to as “para-aramid”) in terms of production. Further, examples of the heat resistant resin include poly-4-methylpentene-1 and cyclic olefin polymers. By using these heat resistant resins, the heat resistance can be increased, that is, the thermal film breaking temperature can be increased.

  The thermal film breaking temperature depends on the kind of the heat-resistant resin, but the thermal film breaking temperature is usually 160 ° C. or higher. By using the nitrogen-containing aromatic polymer as the heat-resistant resin, the thermal film breaking temperature can be increased to about 400 ° C. at the maximum. Further, when poly-4-methylpentene-1 is used, the thermal film breaking temperature can be increased up to about 250 ° C., and when a cyclic olefin-based polymer is used up to about 300 ° C., respectively.

  The para-aramid is obtained by condensation polymerization of a para-oriented aromatic diamine and a para-oriented aromatic dicarboxylic acid halide, and the amide bond is in the para position of the aromatic ring or an oriented position equivalent thereto (for example, 4,4′- It consists essentially of repeating units bonded at opposite orientations, such as biphenylene, 1,5-naphthalene, 2,6-naphthalene, etc., oriented in the opposite direction coaxially or in parallel. As para-aramid, para-aramid having a para-orientation type or a structure according to para-orientation type, specifically, poly (paraphenylene terephthalamide), poly (parabenzamide), poly (4,4′-benzanilide terephthalamide) , Poly (paraphenylene-4,4′-biphenylenedicarboxylic amide), poly (paraphenylene-2,6-naphthalenedicarboxylic acid amide), poly (2-chloro-paraphenylene terephthalamide), paraphenylene terephthalamide / 2 , 6-dichloroparaphenylene terephthalamide copolymer and the like.

  The aromatic polyimide is preferably a wholly aromatic polyimide produced by condensation polymerization of an aromatic dianhydride and a diamine. Specific examples of the dianhydride include pyromellitic dianhydride, 3,3 ′, 4,4′-diphenylsulfone tetracarboxylic dianhydride, 3,3 ′, 4,4′-benzophenone tetracarboxylic acid And dianhydride, 2,2′-bis (3,4-dicarboxyphenyl) hexafluoropropane, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride, and the like. Examples of the diamine include oxydianiline, paraphenylenediamine, benzophenonediamine, 3,3′-methylenedianiline, 3,3′-diaminobenzophenone, 3,3′-diaminodiphenylsulfone, 1,5′-naphthalenediamine Etc. Moreover, a polyimide soluble in a solvent can be preferably used. An example of such a polyimide is a polycondensate polyimide of 3,3 ′, 4,4′-diphenylsulfonetetracarboxylic dianhydride and an aromatic diamine.

  Examples of the aromatic polyamideimide include those obtained from condensation polymerization using aromatic dicarboxylic acid and aromatic diisocyanate, and those obtained from condensation polymerization using aromatic diacid anhydride and aromatic diisocyanate. Can be mentioned. Specific examples of the aromatic dicarboxylic acid include isophthalic acid and terephthalic acid. Specific examples of the aromatic dianhydride include trimellitic anhydride. Specific examples of the aromatic diisocyanate include 4,4'-diphenylmethane diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, orthotolylane diisocyanate, m-xylene diisocyanate, and the like.

  The filler that may be contained in the heat-resistant porous layer may be selected from any of organic powder, inorganic powder, or a mixture thereof. The particles constituting the filler preferably have an average particle size of 0.01 μm or more and 1 μm or less. Examples of the shape of the filler include a substantially spherical shape, a plate shape, a columnar shape, a needle shape, a whisker shape, and a fiber shape, and any particle can be used. It is preferable that

  Examples of the organic powder as the filler include, for example, styrene, vinyl ketone, acrylonitrile, methyl methacrylate, ethyl methacrylate, glycidyl methacrylate, glycidyl acrylate, methyl acrylate, and the like, or two or more kinds of copolymers; polytetrafluoroethylene, Fluororesin such as tetrafluoroethylene-6 fluorinated propylene copolymer, tetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride, etc .; melamine resin; urea resin; polyolefin; powder made of organic matter such as polymethacrylate Can be mentioned. An organic powder may be used independently and can also be used in mixture of 2 or more types. Among these organic powders, polytetrafluoroethylene powder is preferable from the viewpoint of chemical stability.

  Examples of inorganic powders as fillers include powders made of inorganic substances such as metal oxides, metal nitrides, metal carbides, metal hydroxides, carbonates, sulfates, and specific examples include alumina and silica. , Titanium dioxide, or calcium carbonate powder. An inorganic powder may be used independently and can also be used in mixture of 2 or more types. Among these inorganic powders, alumina powder is preferable from the viewpoint of chemical stability. It is more preferable that all of the particles constituting the filler are alumina particles, and it is even more preferable that all of the particles constituting the filler are alumina particles, and part or all of them are substantially spherical alumina particles.

  The filler content in the heat-resistant porous layer depends on the specific gravity of the filler material. For example, when all of the particles constituting the filler are alumina particles, the total weight of the heat-resistant porous layer is 100, The weight of the filler is usually 20 to 95 parts by weight, preferably 30 to 90 parts by weight. These ranges can be appropriately set depending on the specific gravity of the filler material.

  In the laminated porous film, the porous film contains a thermoplastic resin. The thickness of this porous film is usually 3 to 30 μm, more preferably 3 to 20 μm. Like the porous film, the porous film has micropores, and the size of the pores is usually 3 μm or less, preferably 1 μm or less. The porosity of the porous film is usually 30 to 80% by volume, preferably 40 to 70% by volume. In a sodium ion secondary battery, when the normal use temperature is exceeded, the porous film plays a role of closing the micropores by softening the thermoplastic resin constituting the porous film.

Examples of the thermoplastic resin contained in the porous film include those that soften at 80 to 180 ° C., and those that do not dissolve in the electrolyte solution in the sodium ion secondary battery may be selected. Specifically, examples of the thermoplastic resin include polyolefins such as polyethylene and polypropylene, and thermoplastic polyurethane, and a mixture of two or more of these may be used. In order to soften and shut down at a lower temperature, polyethylene is preferable as the thermoplastic resin. Specific examples of the polyethylene include polyethylene such as low density polyethylene, high density polyethylene, and linear polyethylene, and also include ultrahigh molecular weight polyethylene. In order to further increase the puncture strength of the porous film, the thermoplastic resin preferably contains at least ultra high molecular weight polyethylene. In addition, in terms of production of the porous film, the thermoplastic resin may preferably contain a wax made of polyolefin having a low molecular weight (weight average molecular weight of 10,000 or less).

In the electrolytic solution, examples of the electrolyte include NaClO ₄ , NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) ₂ , lower aliphatic carboxylic acid sodium salt, NaAlCl _{4 and the} like. These may be used alone or in combination of two or more thereof. Among these, it is preferable to use those containing at least one selected from the group consisting of NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ and NaN (SO ₂ CF ₃ ) ₂ containing fluorine.

  In the electrolytic solution, examples of the organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, and 1,2-di (methoxy). Carbonates such as carbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran Ethers such as methyl formate, methyl acetate, and γ-butyrolactone; Nitriles such as acetonitrile and butyronitrile; N, N-dimethylformamide, N, N-dimethylacetoa Amides such as carbon; Carbamates such as 3-methyl-2-oxazolidone; Sulfur-containing compounds such as sulfolane, dimethyl sulfoxide and 1,3-propane sultone, or those obtained by further introducing a fluorine substituent into the above organic solvent Usually, two or more of these are used in combination.

Moreover, you may use a solid electrolyte instead of the said electrolyte solution. As the solid electrolyte, for example, a polymer electrolyte such as a polyethylene oxide polymer compound, a polymer compound including at least one of a polyorganosiloxane chain or a polyoxyalkylene chain can be used. Moreover, what is called a gel type which hold | maintained the nonaqueous electrolyte solution in the polymer | macromolecule can also be used. Further, sulfide electrolytes such as Na ₂ S—SiS ₂ and Na ₂ S—GeS ₂ , NaTi ₂ (PO ₄ ) ₃ , NaFe ₂ (PO ₄ ) ₃ , Na ₂ (SO ₄ ) ₃ , Fe ₂ (SO ₄ ) _When NASCON type electrolytes such as ₂ (PO ₄ ) and Fe ₂ (MoO ₄ ) ₃ are used, safety may be further improved. In the sodium ion secondary battery of the present invention, when a solid electrolyte is used, the solid electrolyte may serve as a separator, and in that case, the separator may not be required.

  Thus, the sodium ion secondary battery manufactured using the electrode active material of this invention turns into a nonaqueous electrolyte secondary battery excellent in the cycling characteristics of the discharge at the time of charging / discharging repeatedly.

  Hereinafter, the present invention will be described more specifically with reference to examples.

1. Powder X-ray Diffraction Measurement Powder X-ray diffraction measurement for the electrode active material was performed using a powder X-ray diffractometer (RINT2500TTR manufactured by Rigaku Corporation) under the following conditions.
X-ray: CuKα
Voltage-current: 40 kV-140 mA
Measurement angle range: 2θ = 10 to 90 °
Step: 0.02 °
Scan speed: 4 ° / min

2. Evaluation of charge / discharge performance of positive electrode active material for sodium ion secondary battery A test battery including a positive electrode containing the electrode active material of the present invention was prepared, a constant current charge / discharge test was conducted, and cycle efficiency of discharge capacity (hereinafter simply referred to as “ It was sometimes called “cycle efficiency”. In addition, the cycle efficiency calculated the cycle efficiency of the discharge capacity of the nth cycle (n is an integer of 1-10) with respect to the discharge capacity of the first cycle by the following formula.
Cycle efficiency (%) = n-th discharge capacity / first-cycle discharge capacity × 100

The cell configuration and charge / discharge conditions of the test battery are shown below.
(Cell configuration) Bipolar positive electrode: electrode containing the electrode active material of the present invention Negative electrode: metallic sodium
Electrolyte: 1M NaClO ₄ / PC
(Charge / discharge conditions)
Voltage range: 1.5-4.0V
Charging rate: 0.1C rate (speed to fully charge in 10 hours)
Discharge rate: 0.1C rate (speed to fully discharge in 10 hours)
Number of cycles: 10 times

Hereinafter, a specific method for producing a test battery for a charge / discharge test will be described.
An electrode active material, which will be described later as an example, and acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive material and a PVdF binder (manufactured by Kureha Co., Ltd., PolyVinylidene DiFluoride) are used. : Weighed so as to have a composition of 5 (weight ratio), dissolved the binder in N-methylpyrrolidone (NMP), and then added the electrode active material and acetylene black to form a slurry, which is the current collector Coating was performed with a doctor blade on an aluminum foil having a thickness of 40 μm, and this was dried with a dryer to obtain an electrode sheet. This electrode sheet was punched to a diameter of 1.45 cm with an electrode punching machine to obtain a circular positive electrode.

A positive electrode is placed on the bottom lid of a coin cell (made by Hosen Co., Ltd.) with the aluminum foil side facing down, 1M NaClO ₄ / PC as the electrolyte, a polypropylene porous film as the separator, and metallic sodium (made by Aldrich) ) Was used as a negative electrode, and these were combined to produce a test battery. The test battery was assembled in a glove box in an argon atmosphere.

Example 1
(1) Synthesis of NaFeTiO ₄ Na ₂ CO ₃ , Fe ₃ O ₄ and TiO ₂ were weighed in a molar ratio of Na: Fe: Ti = 1: 1: 1, and then mixed in an agate mortar. . The obtained mixture was baked by holding at 1000 ° C. for 6 hours in an air atmosphere, and then pulverized in an agate mortar to obtain an electrode active material 1 for a sodium ion secondary battery.

(2) Powder X-ray diffraction analysis As a result of performing a powder X-ray diffraction analysis on the electrode active material 1 under the measurement conditions described above, the electrode active material 1 had a sodium complex oxide having a calcium ferrite type crystal structure ( FIG. 1).

(3) Evaluation of charge / discharge performance of electrode active material for sodium ion secondary battery A test battery was prepared using the electrode active material 1, and a constant current charge / discharge test was performed under the above-described conditions.
The cycle efficiency of the discharge capacity at the 10th cycle relative to the discharge capacity at the 1st cycle was 43% (FIG. 2).

Example 2
(1) Synthesis of NaFeTi _0.9 Mo _0.1 O ₄ Na ₂ CO ₃ , Fe ₃ O ₄ , TiO ₂ and MoO ₃ in a molar ratio of Na: Fe: Ti: Mo = 1: 1: 0.9: 0.1 After weighing in this manner, the mixture was mixed in an agate mortar. The obtained mixture was baked by holding at 1000 ° C. for 6 hours in an air atmosphere, and then pulverized in an agate mortar to obtain an electrode active material 2 for a sodium ion secondary battery.

(2) Powder X-ray diffraction analysis As a result of performing powder X-ray diffraction analysis using the electrode active material 2 under the measurement conditions described above, the electrode active material 2 has a sodium complex oxide having a calcium ferrite type crystal structure. (FIG. 3).

(3) Evaluation of charge / discharge performance of electrode active material for sodium ion secondary battery A test battery was prepared using the electrode active material 2, and a constant current charge / discharge test was performed under the above-described conditions.
The cycle efficiency of the discharge capacity at the 10th cycle relative to the discharge capacity at the 1st cycle was 93% (FIG. 4).

Example 3
(1) Synthesis of NaFeSnO ₄ Na ₂ CO ₃ , Fe ₃ O ₄ , and SnO ₂ were weighed in a molar ratio of Na: Fe: Sn = 1: 1: 1, and then mixed in an agate mortar. . After baking the obtained mixture by hold | maintaining at 1000 degreeC and 6 hours in an air atmosphere, it grind | pulverized with the agate mortar, and the electrode active material 3 for sodium ion secondary batteries was obtained.

(2) Powder X-ray diffraction analysis As a result of performing a powder X-ray diffraction analysis using the electrode active material 3 under the measurement conditions described above, the electrode active material 3 has a sodium complex oxide having a calcium ferrite type crystal structure. (FIG. 5).

(3) Evaluation of charge / discharge performance of electrode active material for sodium ion secondary battery A test battery was prepared using the electrode active material 3, and a constant current charge / discharge test was performed under the above-described conditions.
The cycle efficiency of the discharge capacity at the 10th cycle relative to the discharge capacity at the 1st cycle was 74% (FIG. 6).

Example 4
(1) Synthesis of NaFeSnO ₄ An electrode active material 4 for a sodium ion secondary battery was obtained in the same manner as in Example 3 except that it was calcined at 1100 ° C. for 6 hours in an air atmosphere.

(2) Powder X-ray diffraction analysis As a result of performing a powder X-ray diffraction analysis using the electrode active material 4 under the measurement conditions described above, the electrode active material 4 has a sodium composite oxide having a calcium ferrite type crystal structure. (FIG. 7).

(3) Evaluation of charge / discharge performance of electrode active material for sodium ion secondary battery A test battery was prepared using the electrode active material 4, and a constant current charge / discharge test was performed under the above-described conditions.
The cycle efficiency of the discharge capacity at the 10th cycle relative to the discharge capacity at the 1st cycle was 72% (FIG. 8).

Comparative Example (1) Synthesis of NaFeO ₂ Na ₂ CO ₃ and Fe ₃ O ₄ were weighed in a molar ratio of Na: Fe = 1: 1, and then mixed in an agate mortar. The obtained mixture was calcined in an air atmosphere at 750 ° C. for 6 hours and then pulverized in an agate mortar to obtain an electrode active material 5 for a sodium ion secondary battery.

(2) Powder X-ray diffraction analysis As a result of performing a powder X-ray diffraction analysis using the electrode active material 5 under the measurement conditions described above, the electrode active material 5 had a NaFeO ₂ type crystal structure (FIG. 9).

(3) Evaluation of charge / discharge performance of electrode active material for sodium ion secondary battery Using electrode active material 5, a test battery was prepared, and a constant current charge / discharge test was performed under the above-described conditions. The cycle efficiency of the discharge capacity at the 10th cycle relative to the discharge capacity at the 1st cycle was 37%, which was low (FIG. 10). The ratio of the discharge capacity at the 10th cycle to the discharge capacity at the 5th cycle was also low.

Production Example 1 (Production of laminated porous film)
(1) Production of coating solution After 272.7 g of calcium chloride was dissolved in 4200 g of NMP, 132.9 g of paraphenylenediamine was added and completely dissolved. To the obtained solution, 243.3 g of terephthalic acid dichloride was gradually added and polymerized to obtain para-aramid, which was further diluted with NMP to obtain a para-aramid solution (A) having a concentration of 2.0% by weight. To 100 g of the obtained para-aramid solution, 2 g of alumina powder (a) (Nippon Aerosil Co., Ltd., Alumina C, average particle size 0.02 μm) and 2 g of alumina powder (b) (Sumitomo Chemical Co., Ltd. Sumiko Random, AA03, average particles) 4 g in total as a filler is added and mixed, treated three times with a nanomizer, filtered through a 1000 mesh wire net, and degassed under reduced pressure to produce a slurry coating solution (B) did. The weight of alumina powder (filler) with respect to the total weight of para-aramid and alumina powder is 67% by weight.

(2) Production and Evaluation of Laminated Porous Film As a porous film that can be shut down, a polyethylene porous film (film thickness 12 μm, air permeability 140 seconds / 100 cc, average pore diameter 0.1 μm, porosity 50%) Was used. The polyethylene porous film was fixed on a PET film having a thickness of 100 μm, and the slurry-like coating liquid (B) was applied onto the porous film by a bar coater manufactured by Tester Sangyo Co., Ltd. While the coated porous film on the PET film is integrated, it is immersed in water which is a poor solvent to deposit a para-aramid porous layer (heat resistant porous layer), and then the solvent is dried to obtain a heat resistant porous layer. And a porous film 1 made of polyethylene were laminated. The thickness of the laminated porous film 1 was 16 μm, and the thickness of the para-aramid porous layer (heat resistant porous layer) was 4 μm. The laminated porous film 1 had an air permeability of 180 seconds / 100 cc and a porosity of 50%. When the cross section of the heat-resistant layer in the laminated porous film 1 was observed with a scanning electron microscope (SEM), relatively small pores of about 0.03 μm to 0.06 μm and relatively large fines of about 0.1 μm to 1 μm. It was found to have pores. The laminated porous film was evaluated by the following method.

Evaluation of laminated porous film (A) Thickness measurement The thickness of the laminated porous film and the thickness of the polyethylene porous film were measured in accordance with JIS standards (K7130-1992). Moreover, as the thickness of the heat-resistant layer, a value obtained by subtracting the thickness of the polyethylene porous film from the thickness of the laminated porous film was used.
(B) Measurement of air permeability by Gurley method The air permeability of the laminated porous film was measured with a digital timer type Gurley type densometer manufactured by Yasuda Seiki Seisakusho, based on JIS P8117.
(C) Porosity A sample of the obtained laminated porous film was cut into a square having a side length of 10 cm, and the weight W (g) and the thickness D (cm) were measured. The weight (Wi) of each layer in the sample is obtained, and the volume of each layer is obtained from Wi and the true specific gravity (g / cm ³ ) of the material of each layer. %).
Porosity (volume%) = 100 × {1− (W1 / true specific gravity 1 + W2 / true specific gravity 2 + ·· + Wn / true specific gravity n) / (10 × 10 × D)}

  In each of the above examples, when the laminated porous film obtained in Production Example 1 is used as a separator, a sodium ion secondary battery capable of further increasing the thermal membrane breaking temperature can be obtained.

  The sodium ion secondary battery using the electrode comprising the electrode active material of the present invention is excellent in cycle efficiency when repeated charging and discharging, so that not only small applications such as portable electronic devices, but also hybrid vehicles, It can be used in various applications such as medium and large applications for power storage. Furthermore, since abundant sodium as a resource is contained in the electrode active material and a cheaper non-aqueous electrolyte secondary battery can be manufactured, the present invention is extremely useful industrially.

1 is a diagram showing a powder X-ray diffraction analysis in Example 1. FIG. It is a figure which shows the charging / discharging performance evaluation in Example 1. FIG. FIG. 3 is a diagram illustrating a powder X-ray diffraction analysis in Example 2. It is a figure which shows the charging / discharging performance evaluation in Example 2. FIG. 6 is a diagram showing a powder X-ray diffraction analysis in Example 3. FIG. It is a figure which shows the charging / discharging performance evaluation in Example 3. FIG. FIG. 4 is a diagram showing a powder X-ray diffraction analysis in Example 4. It is a figure which shows the charging / discharging performance evaluation in Example 4. FIG. FIG. 3 is a diagram showing a powder X-ray diffraction analysis in Comparative Example 1. It is a figure which shows the charging / discharging performance evaluation in the comparative example 1.

Claims (12)

  An electrode active material for a sodium ion secondary battery, characterized by having a sodium composite oxide having a crystal structure of a calcium ferrite type crystal structure. The electrode active material according to claim 1, wherein the sodium composite oxide is represented by the following formula (I).
_{Na x Fe (2-y)} M y O 4 (I)
(In the formula, M represents one or more metal elements excluding Na and Fe, x is in the range of more than 0 and less than 2, and y is in the range of more than 0 and 2 or less.)   The electrode active material according to claim 2, wherein the M is one or more metal elements selected from Group 4, 5, 6 and 14 elements.   The electrode active material according to claim 2 or 3, wherein the M is one or more metal elements selected from Ti, Sn, Mo, and Nb.   The electrode active material according to any one of claims 2 to 4, wherein the M is composed of Ti and Mo. Firing a mixture comprising a compound containing an Na compound, an Fe compound, and M (where M is one or more metal elements excluding Na and Fe) at 650 ° C. to 1300 ° C. in an oxidizing atmosphere. A method for producing an electrode active material comprising a composite oxide represented by the following formula (I):
_{Na x Fe (2-y)} M y O 4 (I)
(In the formula, M represents one or more metal elements excluding Na and Fe, x is in the range of more than 0 and less than 2, and y is in the range of more than 0 and 2 or less.)   The method for producing an electrode active material according to claim 6, wherein the M is one or more metal elements selected from Ti, Sn, Mo, and Nb.   The method for producing an electrode active material according to claim 7, wherein the M is made of Ti and Mo.   A sodium ion secondary battery electrode comprising the electrode active material according to claim 1, a conductive material, and a binder.   A sodium ion secondary battery comprising the electrode according to claim 9.   The sodium ion secondary battery according to claim 10, further comprising a separator.   The sodium ion secondary battery according to claim 11, wherein the separator is a separator made of a laminated porous film in which a heat resistant porous layer containing a heat resistant resin and a porous film containing a thermoplastic resin are laminated.

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