JP5460979B2 - Transition metal phosphate, positive electrode for sodium secondary battery using the same, and secondary battery using the positive electrode - Google Patents

Description

  The present invention relates to a transition metal phosphate and a sodium secondary battery using the same. Specifically, the present invention relates to a transition metal phosphate used for a positive electrode active material for a sodium secondary battery, a positive electrode for a sodium secondary battery using the same, and a sodium secondary battery using the positive electrode.

  Non-aqueous electrolyte secondary batteries, especially lithium secondary batteries, have been put into practical use and widely used as small power sources for mobile phones and notebook computers. In addition, there is an increasing demand for non-aqueous electrolyte secondary batteries for large power sources such as electric vehicles and distributed power storage.

  However, the lithium used in the lithium secondary battery cannot be said to be abundant in terms of resources, and there is a concern about the depletion of lithium resources in the future. On the other hand, sodium belonging to the same alkali metal is present in abundant resources as compared with lithium, and is one digit cheaper than lithium. Moreover, since sodium has a relatively high standard potential, it is considered that a sodium secondary battery can be a high-capacity secondary battery. Here, as a sodium secondary battery, a positive electrode active material containing sodium is used for the positive electrode, and a secondary battery using metal sodium or a sodium alloy is used for the negative electrode, or a positive electrode active material containing sodium is used for the positive electrode, In addition, a secondary battery using a carbon material or the like for the negative electrode can be used. If a sodium secondary battery can be used instead of the current lithium secondary battery, there is no need to worry about resource depletion. For example, a large battery such as an in-vehicle secondary battery or a distributed power storage secondary battery can be used. Secondary batteries can be produced in large quantities.

By the way, as a positive electrode active material used for the positive electrode of a sodium secondary battery, for example, in Patent Document 1, raw materials are mixed and fired at 750 ° C. for 8 hours to obtain sodium iron phosphate (NaFePO ₄ ). The use of this as a positive electrode active material is disclosed.

Special table 2004-533706 gazette

  However, even when the transition metal phosphate according to the conventional technique as disclosed in Patent Document 1 is used as a positive electrode active material for a secondary battery, it cannot be said that the discharge capacity of the battery is sufficient.

  Under such circumstances, an object of the present invention is to provide a transition metal phosphate that can be suitably used as a positive electrode active material for an inexpensive and high-capacity sodium secondary battery. Another object of the present invention is to provide a positive electrode for a sodium secondary battery using a transition metal phosphate and a sodium secondary battery using the positive electrode.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that the following inventions meet the above object, and have reached the present invention.

That is, the present invention relates to the following invention.
<1> Transition metal phosphate containing Na, P and a transition metal element, wherein the transition metal phosphate has a BET specific surface area of 1 m ² / g or more and 50 m ² / g or less .
<2> The transition metal phosphate according to <1>, wherein the transition metal phosphate has an orthorhombic crystal structure.
<3> The transition metal phosphate according to <1> or <2>, which is represented by the formula (I).
_{Na x M y PO 4 (I} )
(However, in the formula (I), x is in the range of more than 0 and 1.5 or less, y is in the range of 0.8 to 1.2, and M is one or more transition metal elements. )
<4> The transition metal phosphate according to <3>, wherein the M contains at least Fe or Mn.
<5> The transition metal phosphate is composed of particles, and the D50 value thereof (where D50 is a volume-based cumulative particle size distribution, in which 50% cumulative volume frequency is calculated from the smaller particle size. The transition metal phosphate according to any one of <1> to <4>, wherein the particle size is 50% or less.
<6> A positive electrode active material for a sodium secondary battery, comprising the transition metal phosphate according to any one of <1> to <5>.
<7> A positive electrode for a sodium secondary battery, using the positive electrode active material according to <6>.
<8> A sodium secondary battery using the positive electrode according to <7>.
<9> The sodium secondary battery according to <8>, further including a separator.
<10> The sodium secondary battery according to <9>, 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.

  The transition metal phosphate of the present invention is useful as a positive electrode active material, has excellent characteristics as a secondary battery such as a higher capacity than conventional, and can realize an inexpensive sodium secondary battery, It is extremely useful industrially.

Hereinafter, the present invention will be described in detail.
The transition metal phosphate of the present invention is a transition metal phosphate containing Na, P and a transition metal element, and has a BET specific surface area of 1 m ² / g or more and 50 m ² / g or less. The present inventors have found that the transition metal phosphate obtained by firing according to the prior art has a small BET specific surface area, and this is a problem in application as a positive electrode active material of a sodium secondary battery.

The BET specific surface area of the transition metal phosphate of the present invention is 1 m ² / g or more and 50 m ² / g or less. Low discharge capacity in the case of a sodium secondary battery when the BET specific surface area is below 1 m ² / g, when more than 50 m ² / g reduced packing property at the time of the transition metal phosphate in the electrode sheet The discharge capacity in the case of a sodium secondary battery is not sufficient. From the viewpoint of increasing the discharge capacity, BET specific surface area of the transition metal phosphate is preferably at least 3m ² / g, more preferably at least ^{5m 2 / g, 10m 2 /} g or more is more preferred. Moreover, 45 m < ² > / g or less is preferable and 40 m < ² > / g or less is more preferable from a viewpoint that the filling property at the time of making the said transition metal phosphate into an electrode sheet increases. By setting it as said BET specific surface area value, the sodium secondary battery which can ensure the electroconductive path between particle | grains and can show high discharge capacity can be obtained.

  The transition metal phosphate of the present invention preferably has an orthorhombic crystal structure in order to obtain a transition metal phosphate with few impurity crystal phases.

The orthorhombic crystal structure includes P222, P222 ₁ , P2 ₁ 2 ₁ 2, P2 ₁ 2 ₁ 2 ₁ , C222 ₁ , C222, F222, I222, I2 ₁ 2 ₁ 2 ₁ , Pmm2, Pmc2 ₁ , Pcc2, Pma2, Pca2 ₁ , Pnc2, Pmn2 ₁ , Pba2, Pna2 ₁ , Pnn2, Cmm2, Cmc2 ₁ , Ccc2, Amm2, Abm2, Ama2, Aba2, Fmm2, Fdd2, Imm2, Iba2, Pm2, P Pban, Pmma, Pnna, Pmna, Pcca, Pbam, Pccn, Pbcm, Pnnm, Pmmn, Pbcn, Pbca, Pnma, Cmcm, Cmca, Cmmm, Cccm, Cmma, Ccca, Fmmm, Fdd, Immm, b, Immm, b Selected space group The orthorhombic crystal structure is preferably a crystal structure belonging to the space group Pnma in that the sodium secondary battery obtained has a higher capacity.

The transition metal phosphate of the present invention is preferably represented by the following formula (I) in order to obtain a sodium secondary battery with higher capacity.
_{Na x M y PO 4 (I} )
(However, in the formula (I), x is in the range of more than 0 and 1.5 or less, y is in the range of 0.8 to 1.2, and M is one or more transition metal elements.)

  Here, in the formula (I), M can be arbitrarily selected from one or more transition metal elements. Examples of the transition metal element include Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. When the transition metal phosphate of the present invention is used as a positive electrode active material, M is preferably a divalent transition metal element in that a high-capacity sodium secondary battery can be obtained.

  In addition, in the formula (I), M preferably contains at least Fe or Mn, and particularly preferably M is Fe and / or Mn in that a high-capacity and inexpensive secondary battery can be obtained.

Furthermore, in the formula (I), the value of x can be selected in the range of more than 0 and not more than 1.5, preferably in the range of not less than 0.8 and not more than 1.2, particularly preferably 1.
The value of y can be selected in the range of 0.8 to 1.2, preferably in the range of 0.9 to 1.1, and more preferably 1.

The transition metal phosphate of the present invention is usually a particulate material composed of primary particles and agglomerated particles formed by agglomeration of primary particles, and the D50 value thereof can be obtained by laser diffraction scattering method particle size distribution measurement. it can. The D50 value means a particle size in which the cumulative volume frequency calculated by the volume fraction is 50% calculated from the smaller particle size in the volume-based cumulative particle size distribution of the particles.
The D50 value of the transition metal phosphate particulate material of the present invention is preferably 0.01 μm or more and 50 μm or less. When the D50 value is less than 0.01 μm or exceeds 50 μm, there is a case where an output at a high current rate cannot be sufficiently obtained when used for a sodium secondary battery. From the viewpoint of increasing the discharge capacity, the D50 value of the transition metal phosphate particulate material is preferably 0.03 μm or more, more preferably 0.05 μm or more, and particularly preferably 0.1 μm or more. Further, when used as a positive electrode active material for a sodium secondary battery, the conductive path tends to increase. By setting it as said D50 value, the secondary battery which can show high discharge capacity can be obtained.

  The transition metal phosphate particles of the present invention are usually spherical or rod-like particles, and the aspect ratio represented by a / b when the major axis of the particles is a and the minor axis is b is usually 1 to 100. The shape of the transition metal phosphate particles can be confirmed by SEM observation.

  In addition, in the range which does not impair the objective of this invention, you may substitute a part of Na, P, and transition metal element in the transition metal phosphate of this invention with another element. Here, as other elements, Li, B, C, N, F, Mg, Al, Si, S, Cl, K, Ca, Sc, Zn, Ga, Ge, Rb, Sr, Y, Zr, Nb, Examples of the element include Mo, Tc, Ru, Pd, Rh, Ag, In, Sn, I, Ba, Hf, Ta, W, Ir, and Ln (lanthanoid).

Further, using the transition metal phosphate of the present invention as a core material, the surface of the particle (core material) is further B, Al, Mg, Ga, In, Si, Ge, Sn, Nb, Ta, W, Mo and A surface treatment such as deposition of a compound containing one or more elements selected from transition metal elements may be performed. 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.
In the case of heat treatment after the above-described deposition treatment, the BET specific surface area of the powder after the deposition heat treatment is the BET specific surface area in the transition metal phosphate of the present invention, although it depends on the temperature of the heat treatment. In this case, the range of the BET specific surface area of the transition metal phosphate in the present invention is that before deposition.

Next, the method for producing the transition metal phosphate of the present invention will be described.
The transition metal phosphate of the present invention can be preferably produced by the following precipitation reaction. That is, a compound containing an element that can become a corresponding transition metal phosphate is weighed so as to have a predetermined composition, and an aqueous solution in which each weighed compound is dissolved is prepared, and precipitation is generated by bringing each aqueous solution into contact with each other. The desired transition metal phosphate can be produced from the product. For example, when M is Fe, sodium iron phosphate represented by NaFePO ₄ which is one of the preferred compositions is sodium hydroxide, iron (II) chloride tetrahydrate, diammonium hydrogen phosphate Na. : Fe: P molar ratio is adjusted to a predetermined ratio, then each compound weighed is completely dissolved in ion-exchanged water to prepare an aqueous solution, and a precipitate obtained by contacting each aqueous solution Can be obtained by solid-liquid separation.

As a raw material for the transition metal phosphate, Na, M, and P can be used alone or as a compound. In particular, compounds containing Na, M, and P, such as oxides, hydroxides, oxyhydroxides, carbonates, sulfates, nitrates, acetates, halides, ammonium salts, oxalates, alkoxides, etc. Of these, those that can be dissolved in water to form an aqueous solution can be suitably used. When the simple substance or the compound is difficult to dissolve in water, an aqueous solution containing each of the elements dissolved in an aqueous solution containing hydrochloric acid, sulfuric acid, nitric acid, acetic acid or the like can be used. Among these, as the compound containing Na, hydroxides and carbonates are preferable, as the compound containing M, chlorides and nitrates are preferable, and as the compound containing P, phosphoric acid (H ₃ PO ₄ ), Its ammonium salt is preferred. Moreover, you may use the composite compound containing 2 or more types of Na, M, and P as a raw material. For example, NaH ₂ PO ₄ , Na ₂ HPO ₄ , Na ₃ PO _{4 and the} like can be mentioned as complex oxides containing Na and P, and as a compound containing M and P, a phosphate of M (for example, And iron phosphate, manganese phosphate, etc.). Among these complex oxides, NaH ₂ PO ₄ is particularly useful.

  Moreover, the solid-liquid separation of the precipitate obtained by bringing the metal compound aqueous solution into contact can be performed, for example, by removing water by a method such as filtration, centrifugation, or heating. For example, when performing solid-liquid separation by heating, the heating temperature range is preferably 50 ° C. or higher and 250 ° C. or lower, more preferably 80 ° C. or higher and 200 ° C. or lower, and further preferably 90 ° C. or higher and 180 ° C. or lower. It is as follows. Moreover, the solid substance obtained by solid-liquid separation can also be washed. Although the solvent used for the washing is not particularly limited, it is preferably water, more preferably pure water and / or ion exchange water. A transition metal phosphate can be obtained by washing with pure water and / or ion-exchanged water and then drying. A preferable temperature range for the drying is the same as the temperature range for the heating. The atmosphere during heating and drying is not particularly limited, and air, oxygen, nitrogen, argon, or a mixed gas thereof can be used, but an inert atmosphere or reduction that does not contain oxygen. Sexual atmosphere is preferred. Moreover, drying can also be performed in a reduced pressure atmosphere. Further, washing and drying may be repeated twice or more, and baking may be performed after drying.

  The transition metal phosphate obtained by the above precipitation reaction can be pulverized and classified using a ball mill, a vibration mill, a jet mill or the like to adjust the particle size. The pulverization and firing may be repeated twice or more, and the resulting transition metal phosphate can be washed or classified as necessary.

  The above-mentioned transition metal phosphate can be used as a positive electrode active material without being treated or after being subjected to a surface treatment such as deposition.

  Next, the positive electrode for sodium secondary batteries containing the transition metal phosphate of the present invention as a positive electrode active material will be described.

  The positive electrode can be produced by supporting a positive electrode mixture containing a positive electrode active material, a conductive material and a binder in the present invention on a positive electrode current collector. In this case, the positive electrode for a sodium secondary battery has a conductive material. As the conductive material, a carbon material can be used. As the carbon material, graphite powder, carbon black (for example, Ketjen Black (trade name, manufactured by Ketjen Black International Co., Ltd.), acetylene black, etc.), fibrous carbon material And so on. Since carbon black is fine and has a large surface area, adding a small amount to the positive electrode mixture can improve the conductivity inside the positive electrode and improve the charge / discharge efficiency and output characteristics. In some cases, the binding between the mixture and the positive electrode current collector is lowered, and the internal resistance is increased. Usually, the ratio of the conductive material in the positive electrode mixture is 5 to 30 parts by weight with respect to 100 parts by weight of the positive electrode active material powder. When a fibrous carbon material is used as the conductive material, this ratio can be lowered.

In order to further increase the conductivity of the positive electrode for a sodium secondary battery, the conductive material may preferably contain a fibrous carbon material. When the length of the fibrous carbon material is l and the diameter of the cross section perpendicular to the length direction of the material is m, l / m is usually 20 to 1000. Further, when the length of the fibrous carbon material is l and the volume-based average particle diameter (D50) of the primary particles and the aggregated particles of the primary particles in the positive electrode active material in the present invention is n, the value of l / n is Usually, it is 2-100, Preferably it is 2-50. When l / n is less than 2, the conductivity between particles in the positive electrode active material may not be sufficient, and when it exceeds 100, the binding property between the positive electrode mixture and the positive electrode current collector decreases. There is a case. Moreover, in the fibrous carbon material, it is better that the electric conductivity is high. The electrical conductivity of the fibrous carbon material is measured for a sample molded so that the density of the fibrous carbon material is 1.0 to 1.5 g / cm ^3, and the electrical conductivity in that case is usually 1 S / cm. It is above, Preferably it is 2 S / cm or more.

  Specific examples of the fibrous carbon material include graphitized carbon fiber and carbon nanotube. The carbon nanotube may be either a single wall or a multiwall. As the fibrous carbon material, a commercially available material may be pulverized and prepared so as to be in the above ranges of l / m and l / n. Here, the pulverization may be either dry or wet. Examples of the dry pulverization include pulverization using a ball mill, a rocking mill, and a planetary ball mill. Examples of the wet pulverization include pulverization using a ball mill and a disperser. Examples of the disperser include Dispermat (product name, manufactured by Eiko Seiki Co., Ltd.).

  In the positive electrode for a sodium secondary battery of the present invention, when a fibrous carbon material is used, the proportion of the fibrous carbon material is set to 0. It is preferably 1 to 30 parts by weight. Further, as the conductive material, a fibrous carbon material and other carbon materials (graphite powder, carbon black, etc.) may be used in combination. In this case, the other carbon material is preferably spherical and fine. When other carbon materials are used in combination, the proportion of the material is 0.1 to 30 parts by weight with respect to 100 parts by weight of the positive electrode active material powder.

  As the binder, a thermoplastic resin can be used. Specifically, polyvinylidene fluoride (hereinafter sometimes referred to as PVdF), polytetrafluoroethylene (hereinafter sometimes referred to as PTFE), and four fluorine. Fluoropolymers such as fluorinated ethylene / hexafluoropropylene / vinylidene fluoride copolymers, propylene hexafluoride / vinylidene fluoride copolymers, tetrafluoroethylene / perfluorovinyl ether copolymers, polyethylene, polypropylene, etc. Polyolefin resin and the like. Moreover, you may use these 2 types or more in mixture. Further, by using a fluororesin and a polyolefin resin as a binder, the ratio of the fluororesin to the positive electrode mixture is 1 to 10% by weight, and the ratio of the polyolefin resin is 0.1 to 2% by weight, A positive electrode mixture excellent in binding property with the positive electrode current collector can be obtained.

  As the positive electrode current collector, Al, Ni, stainless steel or the like can be used, but Al is preferable in that it is easily processed into a thin film and is inexpensive. As a method of supporting the positive electrode mixture on the positive electrode current collector, there is a method of pressure molding, or a method of pasting using an organic solvent or the like, applying onto the positive electrode current collector, drying and pressing to fix the positive electrode current collector. Can be mentioned. When making a paste, a slurry composed of a positive electrode active material, a conductive material, a binder, and an organic solvent is prepared. Examples of the organic solvent include amine solvents such as N, N-dimethylaminopropylamine and diethylenetriamine, ether solvents such as tetrahydrofuran, ketone solvents such as methyl ethyl ketone, ester solvents such as methyl acetate, dimethylacetamide, N-methyl- Examples include amide solvents such as 2-pyrrolidone.

  Examples of the method for applying the positive electrode mixture to 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. By the method mentioned above, the sodium secondary battery positive electrode in the present invention can be produced.

Next, a sodium secondary battery having the positive electrode for sodium secondary battery of the present invention will be described.
A sodium secondary battery has an electrode group obtained by laminating and winding a separator, a negative electrode, and the above-described positive electrode in a container such as a battery can, and then an electrolytic solution comprising an organic solvent containing an electrolyte. Can be impregnated.

  As the shape of the electrode group, for example, a shape in which the 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, etc. Can be mentioned. In addition, examples of the shape of the battery include a paper shape, a coin shape, a cylindrical shape, and a square shape.

  The negative electrode only needs to have a negative electrode material that can be doped / undoped with sodium ions at a lower potential than the positive electrode, or an electrode in which a negative electrode mixture containing the negative electrode material is supported on a negative electrode current collector, or a negative electrode The electrode which consists of material single can be mentioned. Examples of the negative electrode material include carbon materials, chalcogen compounds (oxides, sulfides, and the like), nitrides, metals, and alloys that can be doped / undoped with sodium ions at a lower potential than the positive electrode. Moreover, you may use these negative electrode materials in mixture.

  The negative electrode material is exemplified below. Specific examples of the carbon material include graphite such as natural graphite and artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers, and fired organic polymer compounds. The material which can dope and dedope of sodium ion can be mentioned. These carbon materials, oxides, sulfides, and nitrides may be used in combination, and may be crystalline or amorphous. Further, these carbon materials, oxides, sulfides and nitrides are mainly carried on a negative electrode current collector and used as a negative electrode.

  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.

Specific examples of the metal that can be doped / undoped with sodium ions at a potential lower than that of the positive electrode include sodium metal, silicon metal, and tin metal. Examples of the alloy that can be doped / undoped with sodium ions at a lower potential than the positive electrode include sodium alloys such as Na—Al, Na—Ni, and Na—Si, silicon alloys such as Si—Zn, and Sn—Mn. In addition to tin alloys such as Sn—Co, Sn—Ni, Sn—Cu, and Sn—La, alloys such as Cu ₂ Sb and La ₃ Ni ₂ Sn ₇ can also be cited. These metals and alloys are mainly used alone as a negative electrode (for example, used in a foil shape).

  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. In the case where the electrolytic solution does not contain ethylene carbonate described later, when a negative electrode mixture containing polyethylene carbonate is used, the cycle characteristics and large current discharge characteristics of the obtained battery may be improved.

  Examples of the negative electrode current collector include Cu, Ni, and stainless steel. Cu may be used because it is difficult to make an alloy with sodium and it is easy to process into a thin film. The method of supporting the negative electrode mixture on the negative electrode current collector is the same as in the case of the positive electrode described above. The method is a method of pressure molding, pasted using a solvent, applied onto the negative electrode current collector, dried 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 separator using 2 or more types of the said material, and the said material may be laminated | stacked. 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. The thickness of a separator is about 5-200 micrometers normally, Preferably it is about 5-40 micrometers.

  The separator preferably has a porous film containing a thermoplastic resin. In a sodium secondary battery, normally, when an abnormal current flows in the battery due to a short circuit between the positive electrode and the negative electrode, the current is interrupted to prevent an excessive current from flowing (shut down). is important. 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 the separator, thermal breakage can be further prevented. 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 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 of the heat resistant porous layer 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, and the like. Is mentioned. 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. Similar to the heat resistant porous layer, the porous film has fine pores, and the pore size 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 secondary battery, when the normal use temperature is exceeded, the porous film plays a role of closing the micropores of the heat-resistant porous layer 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 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 polyethylene include polyethylene such as low density polyethylene, high density polyethylene, and linear polyethylene, and ultrahigh molecular weight polyethylene (weight average molecular weight of 1 million or more). 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, the electrolytes include NaClO ₄ , NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) ₂ , NaN (SO ₂ C ₂ F ₅ ) ₂ , NaN. (SO ₂ CF ₃ ) (COCF ₃ ), Na (C ₄ F ₉ SO ₃ ), NaC (SO ₂ CF ₃ ) ₃ , NaBPh ₄ , Na ₂ B ₁₀ Cl ₁₀ , NaBOB (where BOB is bis ( oxalato) borate)), sodium salts such as lower aliphatic carboxylic acid sodium salt and NaAlCl _4, and two or more of these may be used in combination. The sodium salt is selected from the group consisting of NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) ₂ and NaC (SO ₂ CF ₃ ) ₃ containing fluorine. It is preferable to use those containing at least one kind.

  In the electrolytic solution, examples of the organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, vinylene carbonate, isopropyl methyl carbonate, propyl methyl carbonate, ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolane. Carbonates such as 2-one and 1,2-di (methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetra Ethers such as fluoropropyl difluoromethyl ether, tetrahydrofuran and 2-methyltetrahydrofuran; esters such as methyl formate, methyl acetate and γ-butyrolactone; acetonitrile, Nitriles such as tyronitrile; Amides such as N, N-dimethylformamide and N, N-dimethylacetamide; Carbamates such as 3-methyl-2-oxazolidone; Sulfolane, dimethyl sulfoxide, 1,3-propane sultone and the like A sulfur-containing compound or one obtained by further introducing a fluorine substituent into the above organic solvent can be used, but usually two or more of these are used in combination. Among these, a mixed solvent containing carbonates is preferable, and a mixed solvent of cyclic carbonate and acyclic carbonate or cyclic carbonate and ether is more preferable.

A solid electrolyte may be used instead of the above electrolytic 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. Also, sulfide electrolytes such as Na ₂ S—SiS ₂ , Na ₂ S—GeS ₂ , Na ₂ S—P ₂ S ₂ , Na ₂ S—B ₂ S ₃ , or Na ₂ S—SiS ₂ —Na ₃ PO ₄ When using an inorganic compound electrolyte containing sulfide such as Na ₂ S—SiS ₂ —Na ₂ SO ₄ and a NASICON type electrolyte such as NaZr ₂ (PO ₄ ) ₃ , safety may be further improved. In the sodium 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.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these. Unless otherwise specified, powder X-ray diffraction measurement, particle size distribution measurement, BET specific surface area measurement and SEM observation of transition metal phosphates were performed by the following methods. The coin-type battery for charge / discharge test was produced by the following method.

(1) Powder X-ray diffraction measurement of transition metal phosphate As a powder X-ray diffraction apparatus, a powder X-ray diffraction measurement apparatus RINT2500TTR manufactured by Rigaku Corporation was used under the following conditions.
X-ray: CuKα
Voltage-current: 40 kV-140 mA
Measurement angle range: 2θ = 10-80 °
Step: 0.02 °
Scanning speed: 4 ° / min Divergent slit width: (DS) 1 °
Scattering slit width: (SS) 1 °
Light receiving slit width: (RS) 0.3 mm

(2) Particle size distribution measurement of transition metal phosphate It measured using the master sizer 2000 by Malvern as a laser diffraction scattering method particle size distribution measuring apparatus. As the dispersion medium, an aqueous 0.2 wt% sodium hexametaphosphate solution was used. As the measured value D50, the value of the particle size viewed from the fine particle side when 50% accumulated in the volume-based cumulative particle size distribution was used.

(3) Measurement of BET specific surface area of transition metal phosphate 1 g of transition metal phosphate powder was dried at 150 ° C. for 15 minutes in a nitrogen stream, and then measured using a Micrometrix Flowsorb II2300.

(4) SEM observation of transition metal phosphate As a scanning electron microscope observation apparatus, JSM-5500 made by JEOL Datum Co., Ltd. was used, and observation was performed under the condition of an acceleration voltage of 20 kV. For the aspect ratio (a / b) of the particles, the major axis a and minor axis b of 50 particles arbitrarily extracted from the obtained SEM observation photograph were measured, and the average value thereof was adopted.

(5) Production of coin-type battery for charge / discharge test Positive electrode active material powder, which will be described later as an example, acetylene black (hereinafter, also referred to as AB, manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive material, and a binder SUS that becomes a positive electrode mixture by mixing and kneading PTFE (manufactured by Daikin Industries, Ltd.) so that the positive electrode active material: AB: PTFE is 75: 20: 5 in a weight ratio. The positive electrode mixture was applied to a mesh (# 100, 10 mmφ) and vacuum-dried at 150 ° C. for 8 hours to obtain a positive electrode. The weight of the obtained positive electrode was measured, the weight of the SUS mesh was subtracted from the weight of the positive electrode, the weight of the positive electrode mixture was calculated, and the weight of the positive electrode active material powder was calculated from the weight ratio of the positive electrode mixture. A solution obtained by dissolving NaClO ₄ in propylene carbonate (hereinafter sometimes referred to as PC) as an electrolyte so as to be 1 mol / liter (hereinafter sometimes referred to as NaClO ₄ / PC). Using a polyethylene porous membrane as a separator and metallic sodium as a negative electrode, a coin-type battery (R2032) was produced by combining these.

Using the above coin-type battery, a charge / discharge test was performed under the conditions shown below while maintaining at 25 ° C.
(Cell structure) Bipolar type Positive electrode: Electrode containing positive electrode active material Negative electrode: Electrode made of metallic sodium
Electrolyte: 1M NaClO ₄ / PC
(Charge / discharge conditions)
Voltage range: 1.5-4.2V
Charging rate: 0.05C rate (speed to fully charge in 20 hours)
Discharge rate: 0.05C rate (rate of complete discharge in 20 hours)

Example 1
(A) Synthesis of transition metal phosphate powder S ₁ Sodium hydroxide (NaOH); 1.8 g, diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ); 2.7 g, iron (II) chloride tetrahydrate Japanese (FeCl ₂ .4H ₂ O); 2.0 g was weighed and each weighed compound was placed in a glass 100 ml beaker. Next, 33 g of ion-exchanged water was added to the beaker and dissolved completely with stirring to prepare each compound aqueous solution. Next, while adding an aqueous sodium hydroxide solution and an aqueous diammonium hydrogen phosphate solution and stirring well, the iron (II) chloride tetrahydrate aqueous solution is further added thereto to obtain a solid-liquid mixture containing solids. It was. The obtained solid-liquid mixture was put into an eggplant-shaped flask, and then the eggplant-shaped flask was heated in an oil bath set at 170 ° C. to obtain a dry solid product in which water was evaporated. Next, the dryness article was collected, washed with water, filtered to give a transition metal phosphate powder S ₁ and then dried.

(B) Various Evaluations of Transition Metal Phosphate Powder S _{1 When} X-ray diffraction measurement was performed on the powder S ₁ , it was found to be single-phase orthorhombic NaFePO ₄ (malisite) (FIG. 1). ). The measured particle size distribution and the BET specific surface area of the powder S _1, D50 is 1.3 .mu.m, the BET specific surface area was 20 m ² / g. Further, SEM observation of the powder S ₁ revealed that the average value of the aspect ratio a / b was 9 when the major axis of the particles was a and the minor axis was b, including rod-shaped particles (FIG. 2). . Next, to prepare a coin battery using Powder S ₁ as the positive electrode active material was subjected to a charge and discharge test, it is confirmed that can be charged and discharged, the discharge capacity of 5th cycle was 78 mAh / g.

Example 2
(A) Synthesis of transition metal phosphate powder S ₂ A solid product is separated by filtration instead of a dry product obtained by evaporating water to obtain a separated product, except that the separated product is washed, filtered and dried. to obtain a transition metal phosphate powder S ₂ in the same manner as in example 1.

(B) Various Evaluations of Transition Metal Phosphate Powder S _{2 When} X-ray diffraction measurement was performed on the powder S ₂ , it was found to be a single-phase orthorhombic NaFePO ₄ (FIG. 1). The measured particle size distribution and the BET specific surface area of the powder S _2, D50 is 1.8 .mu.m, the BET specific surface area was 36m ² / g. Further, when SEM observation of the powder S ₂ was performed, the average value of the aspect ratio a / b was 5 when the major axis of the particle was a and the minor axis was b, including rod-shaped particles (FIG. 3). . Next, to prepare a coin battery using Powder S ₂ as the positive electrode active material was subjected to a charge and discharge test, it is confirmed that can be charged and discharged, the discharge capacity at the fifth cycle was 80mAh / g.

Example 3
(A) Synthesis of transition metal phosphate powder S ₃ Except that acetylene black as a conductive material was added to the solid-liquid mixture by 10% by weight with respect to the obtained transition metal phosphate, and the mixture was stirred and mixed. example obtain a transition metal phosphate powder S ₃ in the same manner as 1.

(B) Various Evaluations of Transition Metal Phosphate Powder S _{3 When} X-ray diffraction measurement was performed on the powder S ₃ , it was found to be single-phase orthorhombic NaFePO ₄ (FIG. 1). The measured particle size distribution and the BET specific surface area of the powder S _3, D50 is 2.6 [mu] m, BET specific surface area was 32m ² / g. Furthermore, it was subjected to SEM observation of the powder S _3, comprises a rod-like particles, that acetylene black was uniformly attached was verified on each particle (Fig. 4). The average aspect ratio a / b was 7 when the major axis of the particles was a and the minor axis was b. Next, to prepare a coin battery using Powder S ₃ as the positive electrode active material was subjected to a charge and discharge test, it is confirmed that can be charged and discharged, the discharge capacity at the fifth cycle was 85mAh / g.

Example 4
(A) Synthesis of transition metal phosphate powder S ₄ Diammonium hydrogen phosphate; phosphoric acid (H ₃ PO ₄ ) aqueous solution (phosphoric acid concentration 85 wt%, specific gravity 1.69) instead of 2.7 g; 2 mL except for using, to obtain a transition metal phosphate powder S ₄ in the same manner as in example 3.

(B) Various Evaluations of Transition Metal Phosphate Powder S _{4 When} X-ray diffraction measurement was performed on the powder S ₄ , it was found to be a single-phase orthorhombic NaFePO ₄ (FIG. 1). The measured particle size distribution and the BET specific surface area of the powder S _4, D50 is 0.35 .mu.m, BET specific surface area was 18m ² / g. Furthermore, was subjected to SEM observation of the powder S _4, comprises a rod-like particles, it was confirmed that the acetylene black was uniformly deposited on the particles (FIG. 5). The average value of the aspect ratio a / b was 6 when the major axis of the particles was a and the minor axis was b. Next, to prepare a coin battery using Powder S ₄ as the positive electrode active material was subjected to a charge and discharge test, it is confirmed that can be charged and discharged, the discharge capacity of 5th cycle was 75 mAh / g.

Example 5
(A) Synthesis of transition metal phosphate powder S ₅ Sodium hydroxide (NaOH); 3.5 g, manganese (II) chloride tetrahydrate (MnCl ₂ .4H ₂ O); 3.1 g, phosphoric acid (H ₃ PO ₄ ) aqueous solution (phosphoric acid concentration 85% by weight, specific gravity 1.69); 2 mL was weighed, and each weighed compound was placed in a glass 100 mL beaker. Next, 33 g of ion-exchanged water was added to the beaker and dissolved completely with stirring to prepare each compound aqueous solution. Next, an aqueous sodium hydroxide solution and an aqueous manganese (II) chloride tetrahydrate solution were added and stirred well, and the phosphoric acid aqueous solution was further added thereto to obtain a solid-liquid mixture containing solids. The obtained solid-liquid mixture was put into an eggplant-shaped flask, and then the eggplant-shaped flask was heated in an oil bath set at 170 ° C. and evaporated to dryness until water evaporated to obtain a dried product. Next, the dryness products were collected, washed with water, filtered to give a transition metal phosphate powder S ₅ and then dried.

(B) Various Evaluations of Transition Metal Phosphate Powder S _{5 When} X-ray diffraction measurement was performed on the powder S ₅ , it was found to be single-phase orthorhombic NaMnPO ₄ (FIG. 6). The measured particle size distribution and the BET specific surface area of the powder S _5, D50 is 1.67 .mu.m, BET specific surface area was 4.0 m ² / g. Furthermore, was subjected to SEM observation of the powder S _5, spherical particles were confirmed (Fig. 7). Next, to prepare a coin battery using Powder S ₅ as the positive electrode active material was subjected to a charge and discharge test, it was confirmed that the charge and discharge.

Comparative Example 1
(A) Synthesis of comparative powder R _{1 As} raw materials, ferric trioxide (Fe ₂ O ₃ ); 3.2 g, sodium carbonate (Na ₂ CO ₃ ); 2.1 g, diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ); 5.1 g was weighed, and each raw material was sufficiently pulverized and mixed with a ball mill to obtain a raw material mixture. Next, the raw material mixture was filled in an alumina boat, and in an electric furnace, a comparative powder R ₁ was obtained by holding and firing at a temperature of 750 ° C. for 8 hours while supplying nitrogen gas at 5 liters / minute.

(B) Various evaluations of the comparative powder R _{1 When} X-ray diffraction measurement was performed on the powder R ₁ , the main phase was monoclinic Na ₃ Fe ₂ (PO ₄ ) ₃ , and the single phase NaFePO ₄ was It was not obtained (FIG. 8). The measured particle size distribution and the BET specific surface area of the powder R _1, D50 is 14 [mu] m, BET specific surface area was 0.10 m ² / g. Furthermore, it was subjected to SEM observation powder R _1, particle shape was irregular shape (FIG. 9). Next, a coin-type battery was prepared using the powder R ₁ as a positive electrode active material and a charge / discharge test was performed. As a result, it was confirmed that charge / discharge was possible, but the discharge capacity at the fifth cycle was as low as 1 mAh / g. .

Comparative Example 2
(A) Synthesis of Comparative Powder R ₂ Iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O): 5.1 g, sodium carbonate (Na ₂ CO ₃ ); 1.5 g, diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ); 3.8 g was weighed, and each raw material was sufficiently pulverized and mixed with a ball mill to obtain a raw material mixture. Next, the raw material mixture was filled in an alumina boat, in an electric furnace, a nitrogen gas with 5 liters / min aeration maintained at a temperature of 750 ° C. 24 hours to obtain Comparative Powder R ₂ by firing.

(B) Various evaluations of the comparative powder R _{2 When} X-ray diffraction measurement was performed on the powder R ₂ , the main phase was monoclinic Fe ₂ O ₃ and no single-phase NaFePO ₄ was obtained ( FIG. 8). The measured particle size distribution and the BET specific surface area of the powder R _2, D50 is 30 [mu] m, BET specific surface area was 0.26 m ² / g. Further, SEM observation of the powder R ₂ revealed that the particle shape was indefinite (FIG. 10). Next, a coin-type battery was prepared using the powder R ₂ as a positive electrode active material, and a charge / discharge test was performed. I could not.

Comparative Example 3
(A) except that the temperature during the synthesis calcination of Comparative Powder R ₃ to 800 ° C., in the same manner as in Comparative Example 2 to obtain a comparative powder R _3.

(B) Various evaluations of comparative powder R _{3 When} X-ray diffraction measurement was performed on powder R ₃ , the main phase was rhombohedral Fe ₂ O ₃ , and single-phase NaFePO ₄ was not obtained (see FIG. 8). The measured particle size distribution and the BET specific surface area of the powder R _3, D50 is 17 .mu.m, BET specific surface area was 0.47 m ² / g. Furthermore, it was subjected to SEM observation of the powder R _3, particle shape was irregular shape (FIG. 11). Next, to prepare a coin battery using Powder R ₃ as the positive electrode active material was subjected to a charge and discharge test, 1 discharge capacity cycle is as low as 1 mAh / g, it is charged and discharged up to the fifth cycle I could not.

Production Example 1 (Production of laminated porous film)
(1) Production of coating liquid After 272.7 g of calcium chloride was dissolved in 4200 g of N-methylpyrrolidone (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. Obtain the weight (Wi (g)) of each layer in the sample, and obtain the volume of each layer from Wi and the true specific gravity (true specific gravity i (g / cm ³ )) of the material of each layer, The porosity (volume%) was obtained from the following formula.
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 secondary battery capable of further increasing the thermal membrane breaking temperature can be obtained.

  Since the sodium secondary battery using the positive electrode containing the transition metal phosphate of the present invention has a high capacity, not only small applications such as portable electronic devices, but also medium-sized and It can be used in various applications such as large applications. 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.

It is a figure which shows the powder X-ray-diffraction analysis result in Examples 1-4. 2 is a view showing an SEM observation photograph in Example 1. FIG. 6 is a view showing an SEM observation photograph in Example 2. FIG. 6 is a view showing an SEM observation photograph in Example 3. FIG. 6 is a view showing an SEM observation photograph in Example 4. FIG. FIG. 6 is a graph showing the result of powder X-ray diffraction analysis in Example 5. 6 is a view showing an SEM observation photograph in Example 5. FIG. It is a figure which shows the powder X-ray-diffraction analysis result in Comparative Examples 1-3. 6 is a view showing an SEM observation photograph in Comparative Example 1. FIG. 6 is a view showing an SEM observation photograph in Comparative Example 2. FIG. 10 is a view showing an SEM observation photograph in Comparative Example 3. FIG.

Claims (9)

A transition metal phosphate of the formula (I), the transition BET specific surface area of the metal phosphate Ri der 1 m ² / g or more 50 m ² / g or less, have a crystal structure of orthorhombic transition metal phosphate, wherein to Rukoto.
_{Na x M y PO 4 (I} )
(However, in the formula (I), x is in the range of more than 0 and 1.5 or less, y is in the range of 0.8 to 1.2, and M is Fe and / or Mn.)
The transition metal phosphate according to claim 1, wherein the values of x and y in the transition metal phosphate represented by the formula (I) are each 1. The transition metal phosphate according to claim 1 or 2, wherein M is Fe or Mn. The transition metal phosphate is composed of particles, and the D50 value thereof (where D50 is 50% of the cumulative volume frequency distribution on a volume basis, the cumulative volume frequency at the time of 50% accumulation calculated from the smaller particle diameter) represents the value of the particle diameter at a.) is a transition metal phosphate according to any one of claims 1 3 is 0.01μm or more 50μm or less. A positive electrode active material for a sodium secondary battery, comprising the transition metal phosphate according to any one of claims 1 to 4 . A positive electrode for a sodium secondary battery, wherein the positive electrode active material according to claim 5 is used. A sodium secondary battery comprising the positive electrode according to claim 6 . The sodium secondary battery according to claim 7, further comprising a separator. The sodium secondary battery according to claim 8 , 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.