Negative electrode active material for secondary battery, negative electrode for secondary battery, and secondary battery

Abstract

This negative electrode active material for secondary battery contains a composite metal oxide represented by formula (1).　(1): NaaFebM1 cM2 dO2 (the valence of Fe in formula (1) is trivalent. M1 is any one or more selected from the group consisting of Ni, Co, V, Cr, Mg, and Al. M2 is any one or more selected from the group consisting of Ti, Nb, and Mo. a is greater than 0 and less than 1. b is 0.35-0.65. c is 0-0.15. d is 0.35-0.65.)

Description

Negative electrode active material for secondary batteries, negative electrode for secondary batteries, secondary batteries

[0001] The present invention relates to a negative electrode active material for a secondary battery, a negative electrode for a secondary battery, and a secondary battery. This application claims priority based on Japanese Patent Application No. 2024-123614, filed on July 30, 2024, the contents of which are incorporated herein by reference.

[0002] Lithium secondary batteries are widely used in many fields. However, lithium, an essential material for lithium secondary batteries, is not abundant in resources. For this reason, there is a demand for reducing the procurement risk of materials used in lithium secondary batteries. On the other hand, sodium, which belongs to the same alkali metal category as lithium, is present in the Earth's crust in an amount more than 1,000 times that of lithium, making it an abundant resource. Sodium also has a relatively high standard potential. For these reasons, sodium secondary batteries are attracting attention as next-generation batteries. Furthermore, proton batteries, which use hydrogen ions (protons), have fewer resource constraints and are attracting attention as next-generation batteries.

[0003] Patent Document 1 describes a sodium ion battery. ２ Ti ６ O １３ The document describes a negative electrode active material for a sodium ion battery in which part of Ti in the crystalline phase is substituted with M (M is at least one of Fe, V, Mn, Mo, Al, Cr, Mg, Nb, W, Zr, Ta, and Sn).

[0004] Japanese Patent Application Publication No. 2014-78353

[0005] Sodium has a standard electrode potential approximately 0.3 V higher than that of lithium, an ionic volume more than twice as large, and an atomic weight more than three times as large. Therefore, it is difficult to utilize negative electrode active materials commonly used in lithium secondary batteries as negative electrode active materials for sodium secondary batteries. Specifically, for example, graphite is a material commonly used as a negative electrode active material in lithium secondary batteries. However, graphite cannot absorb and release sodium ions. Therefore, graphite cannot be used as a negative electrode active material for sodium secondary batteries.

[0006] Conventionally, negative electrode active materials used in sodium secondary batteries include metallic sodium and hard carbon. However, sodium secondary batteries using conventional negative electrode active materials could not use water as a solvent for the electrolyte. By using water as the solvent for the electrolyte, sodium secondary batteries can be produced with improved safety compared to batteries using nonaqueous solvents such as organic solvents. For this reason, it is desirable to use water as a solvent for the electrolyte in sodium secondary batteries.

[0007] The present invention has been made in view of the above circumstances, and aims to provide a negative electrode active material for a secondary battery that can use water as a solvent for an electrolyte and that can form a secondary battery having high charge / discharge capacity and good cycle characteristics. Another aim of the present invention is to provide a negative electrode for a secondary battery and a secondary battery that include the negative electrode active material for a secondary battery of the present invention.

[0008] In order to solve the above problems, the present inventors have focused on the composition of a composite metal oxide that can be used as a negative electrode active material for a secondary battery and have conducted extensive research. ０．５ Ti ０．５ O ２ is oxidized using water as an oxidizing agent to convert divalent Fe to trivalent and to remove Na, resulting in the formation of Na defects. ０．５ Fe ０．５ Ti ０．５ O ２ ) can be used as a negative electrode active material for secondary batteries.

[0009] More specifically, Na ０．５ Fe ０．５ Ti ０．５ O ２ It was confirmed that in a sodium secondary battery having a negative electrode containing Na, not only an electrolyte solution using an organic solvent as a solvent but also an electrolyte solution using water as a solvent can be used. ０．５ Fe ０．５ Ti ０．５ O ２ This is because in a sodium secondary battery having a negative electrode containing

[0010] Moreover, Na ０．５ Fe ０．５ Ti ０．５ O ２ It has been confirmed that a sodium secondary battery having a negative electrode containing Na can obtain a sufficiently high charge / discharge capacity and good cycle characteristics, whether using an electrolyte solution containing an organic solvent or an electrolyte solution containing water as a solvent. ０．５ Fe ０．５ Ti ０．５ O ２ A sodium secondary battery having a negative electrode containing NaFe ０．５ Ti ０．５ O ２ It was confirmed that the charge-discharge capacity at the first cycle was higher than that of a sodium secondary battery having a negative electrode containing Na. ０．５ Fe ０．５ Ti ０．５ O ２ This is presumably because the content of 0.5-valent Na increases the number of sodium ions that can be inserted into the negative electrode.

[0011] Furthermore, the present inventors have found that the above Na ０．５ Fe ０．５ Ti ０．５ O ２ The function of the negative electrode containing Na ０．５ Fe ０．５ Ti ０．５ O ２ The proportions of Na, Fe, Ti, and O in the alloy were examined, and elements capable of substituting a portion of Na or Fe, and elements capable of substituting a portion or all of Ti were also examined.

[0012] As a result, the inventors have found that, in a composite metal oxide containing Na, Fe, Ti, and O in specific proportions, a portion of the Na and / or Fe may be substituted with one or more elements selected from the group consisting of Ni, Co, V, Cr, Mg, Al, Mg, and Al, and a portion or all of the Ti may be substituted with one or more elements selected from the group consisting of Ti, Nb, and Mo, and have arrived at the present invention. The present invention provides the following means.

[0013] [1] A negative electrode active material for a secondary battery, comprising a composite metal oxide represented by general formula (1): Na ａ Fe ｂ M １ ｃ M ２ ｄ O ２ (1) (The valence of Fe in formula (1) is trivalent. M １ is at least one selected from the group consisting of Ni, Co, V, Cr, Mg, and Al. ２ is one or more selected from the group consisting of Ti, Nb, and Mo. a is greater than 0 and less than 1. b is 0.35 to 0.65. c is 0 to 0.15. d is 0.35 to 0.65.

[0014] [2] The negative electrode active material for a secondary battery according to [1], wherein b + c + d in general formula (1) is 1. [3] The negative electrode active material for a secondary battery according to [1] or [2], wherein c is 0 and b = d in general formula (1). [4] The negative electrode active material for a secondary battery according to [2], wherein a, b, and d in general formula (1) are all 0.5.

[0015] [5] M in general formula (1) ２ [5] The negative electrode active material for a secondary battery according to any one of [1] to [4], wherein is Ti.

[0016] [6] A negative electrode for a secondary battery, comprising the negative electrode active material for a secondary battery according to any one of [1] to [5].

[0017] [7] A secondary battery having a positive electrode, a negative electrode, and an electrolyte solution, wherein the negative electrode contains the negative electrode active material for a secondary battery according to any one of [1] to [5].

[0018] [8] The secondary battery according to [7], wherein the electrolytic solution is an aqueous solution containing one or more electrolytes and one or more organic salts, the electrolyte is a sodium salt of a monocarboxylic acid having an alkyl group with 1 to 10 carbon atoms, the organic salt is selected from the group consisting of an ammonium salt, a lithium salt, a potassium salt, and a tetramethylammonium salt of a monocarboxylic acid having an alkyl group with 1 to 10 carbon atoms, and the secondary battery is a sodium secondary battery.

[0019] [9] The secondary battery according to [8], wherein the electrolytic solution is an aqueous solution containing sodium propionate and ammonium acetate.

[10] The secondary battery according to [8] or [9], wherein the ratio of the total number of moles of the electrolyte and the organic salt contained in the electrolytic solution to the number of moles of water is 1:1 to 1:2.

[11] The secondary battery according to [7], wherein the electrolytic solution is an aqueous solution containing one or more selected from an organic salt, an inorganic salt, an organic acid, and an inorganic salt, and wherein the secondary battery is a proton battery.

[12] The secondary battery according to

[11] , wherein the electrolytic solution is an aqueous solution containing ammonium acetate.

[13] The secondary battery according to

[11] or

[12] , wherein the ratio of the total number of moles of the organic salt, the inorganic salt, the organic acid, and the inorganic salt contained in the electrolytic solution to the number of moles of water is 1:2 to 1:50.

[0020] The negative electrode active material for a secondary battery according to an embodiment of the present invention includes a composite metal oxide represented by general formula (1). Therefore, by providing a secondary battery with a negative electrode including the negative electrode active material for a secondary battery according to an embodiment of the present invention, not only an electrolyte solution using an organic solvent as the solvent but also an electrolyte solution using water as the solvent can be used. Furthermore, by providing a negative electrode including the negative electrode active material for a secondary battery according to an embodiment of the present invention, a secondary battery having a high charge / discharge capacity and good cycle characteristics can be formed.

[0021] FIG. 1 is a chart showing the X-ray diffraction pattern of the composite metal oxide powder of Comparative Example 1. FIG. 2 is a chart showing the X-ray diffraction pattern of the composite metal oxide powder of Example 1. FIG. 3 is a graph showing the relationship between the voltage and charge / discharge capacity of the evaluation bipolar electrochemical cell of Example 1. FIG. 4 is a graph showing the relationship between the voltage and charge / discharge capacity of the evaluation bipolar electrochemical cell of Comparative Example 1. FIG. 5 is a graph showing the relationship between the voltage and charge / discharge capacity of the evaluation bipolar electrochemical cell of Example 2. FIG. 6 is a graph showing the relationship between the discharge capacity and the number of cycles of the evaluation bipolar electrochemical cells of Examples 2 and 3. FIG. 7A is a graph showing the relationship between the potential of the working electrode relative to the reference electrode (Ag / AgCl electrode) and the charge / discharge capacity when the current density of the three-electrode electrochemical cell of Example 4 is 50 mA / g. FIG. 7B is a graph showing the relationship between the voltage of the working electrode relative to the reference electrode (Ag / AgCl electrode) and the charge capacity when the current density of the three-electrode electrochemical cell of Example 4 is 100 mA / g, 200 mA / g, 500 mA / g, or 1000 mA / g. FIG. 8 is a graph showing the relationship between the voltage and charge / discharge capacity of the three-electrode electrochemical cell of Example 5. FIG. 9A is a graph showing the relationship between the voltage and charge / discharge capacity of the three-electrode electrochemical cell of Example 7-1 (electrolyte: aqueous acetic acid solution). FIG. 9B is a graph showing the relationship between the voltage and charge / discharge capacity of the three-electrode electrochemical cell of Example 7-2 (electrolyte: aqueous sodium acetate solution). FIG. 9C is a graph showing the relationship between the voltage and charge / discharge capacity of the three-electrode electrochemical cell of Example 7-3 (electrolyte: aqueous ammonium acetate solution). FIG. 10A is a graph showing the relationship between the voltage and charge / discharge capacity of the three-electrode electrochemical cell of Example 8-2 (electrolyte: molar ratio of NH ４ 10B is a graph showing the relationship between the voltage and the charge / discharge capacity of the three-electrode electrochemical cell of Example 8-1 (molar ratio in the electrolyte, NH ４FIG. 11 is a graph showing the relationship between discharge capacity and cycle number and the relationship between coulombic efficiency and cycle number for the three-electrode electrochemical cells of Examples 8-1 and 8-2. FIG. 12A is a graph showing the relationship between voltage and charge / discharge capacity for the three-electrode electrochemical cell of Example 9-1 (negative electrode current collector is titanium foil). FIG. 12B is a graph showing the relationship between voltage and charge / discharge capacity for the three-electrode electrochemical cell of Example 9-2 (negative electrode current collector is aluminum foil). FIG. 13 is a graph showing the relationship between discharge capacity and cycle number and the relationship between coulombic efficiency and cycle number for the three-electrode electrochemical cells of Examples 9-1 and 9-2.

[0022] Hereinafter, the negative electrode active material for a secondary battery, the negative electrode for a secondary battery, and the secondary battery according to this embodiment will be described in detail. Note that the secondary battery hereinafter refers to a sodium secondary battery and a proton battery. "Negative electrode active material for a secondary battery" The negative electrode active material for a secondary battery according to this embodiment contains a composite metal oxide represented by general formula (1). Na ａ Fe ｂ M １ ｃ M ２ ｄ O ２ (1) (The valence of Fe in formula (1) is trivalent. M １ is at least one selected from the group consisting of Ni, Co, V, Cr, Mg, and Al. ２ is one or more selected from the group consisting of Ti, Nb, and Mo. a is greater than 0 and less than 1. b is 0.35 to 0.65. c is 0 to 0.15. d is 0.35 to 0.65.

[0023] In the composite metal oxide represented by formula (1), Na is an essential element. a, which indicates the proportion of Na contained in the composite metal oxide represented by formula (1), is greater than 0 and less than 1, preferably 0.35 to 0.65, and most preferably 0.5. Because a is greater than 0 and less than 1, the first cycle charge / discharge capacity of a secondary battery equipped with a negative electrode containing the composite metal oxide represented by formula (1) is high. Furthermore, because a is greater than 0 and less than 1, a secondary battery equipped with a negative electrode containing the composite metal oxide represented by formula (1) can achieve high charge / discharge capacity and good cycle characteristics.

[0024] In the composite metal oxide represented by formula (1), trivalent Fe is an essential element. b, which indicates the proportion of trivalent Fe contained in the composite metal oxide represented by formula (1), is 0.35 to 0.65, preferably 0.45 to 0.55, and most preferably 0.5. Since b is 0.35 to 0.65, a secondary battery equipped with a negative electrode containing the composite metal oxide represented by formula (1) can achieve high charge / discharge capacity and good cycle characteristics.

[0025] In the composite metal oxide represented by formula (1), M １ is at least one selected from the group consisting of Ni, Co, V, Cr, Mg, and Al. １ is an element that may be contained as necessary. １ It is preferable that the metal oxide is not contained, since it is a composite metal oxide that is easier to procure as a material.

[0026] Therefore, M contained in the composite metal oxide represented by formula (1) １ The value c, which indicates the ratio of (a) to (b), is 0 to 0.15, preferably 0 to 0.05, and most preferably 0. Since c is 0 to 0.15, a secondary battery having a negative electrode containing the composite metal oxide represented by formula (1) can obtain high charge / discharge capacity and good cycle characteristics.

[0027] In the composite metal oxide represented by formula (1), M ２ is an essential element. ２ is at least one selected from the group consisting of Ti, Nb, and Mo.２ Ti is preferred because the material is easily available and a secondary battery having a high charge / discharge capacity and good cycle characteristics can be easily obtained.

[0028] M contained in the composite metal oxide represented by formula (1) ２ The value d, which indicates the ratio of , is 0.35 to 0.65, preferably 0.45 to 0.55, and most preferably 0.5. Since d is 0.35 to 0.65, a secondary battery having a negative electrode containing the composite metal oxide represented by formula (1) can obtain high charge / discharge capacity and good cycle characteristics.

[0029] In this embodiment, the composite metal oxide represented by formula (1) is M １ (in other words, c is 0), trivalent Fe and M ２ In this case, the composite metal oxide represented by formula (1) can be easily produced with good yield and excellent productivity.

[0030] In this embodiment, in the composite metal oxide represented by formula (1), a represents the proportion of Na, b represents the proportion of trivalent Fe, M ２ It is preferable that the value of d, which indicates the ratio of Na, b, which indicates the ratio of trivalent Fe, and M are all 0.5. This is because a secondary battery having a negative electrode containing the composite metal oxide represented by formula (1) can be easily manufactured with a high yield, and a higher charge / discharge capacity and better cycle characteristics can be obtained. The composite metal oxide represented by formula (1) has a value of a, which indicates the ratio of Na, b, which indicates the ratio of trivalent Fe, and M. ２ The ratio d is 0.5 in all cases, and M ２ Na where is Ti ０．５ Fe ０．５ Ti ０．５ O ２ It is most preferable that:

[0031] In the composite metal oxide represented by formula (1), b represents the proportion of trivalent Fe, and M １ c, which indicates the ratio of ２The closer to 1 the sum (b+c+d) of b and d, which indicates the ratio of b to c, is, the more preferable, and it is most preferable that b+c+d is 1 (b+c+d=1). This is because a secondary battery having a negative electrode containing the composite metal oxide represented by formula (1) can be easily produced, and a high charge / discharge capacity and good cycle characteristics can be obtained.

[0032] (Method for producing a composite metal oxide) Next, regarding the method for producing a composite metal oxide represented by formula (1), a composite metal oxide (Na ０．５ Fe ０．５ M ２ ０．５ O ２ (The valence of Fe in the formula is trivalent. M ２ is M in formula (1) ２ The following describes an example of the production of a fluororesin (fluororesin) containing fluororesin.

[0033] First, as a raw material, Na ２ CO ３ and FeC ２ O ４ ・2H ２ O and TiO ２ M such as ２ The above raw materials are then mixed with Na, Fe, and M. ２ The amount of Na:Fe:M is expressed as a molar ratio. ２ The components are weighed out so that the ratio is 2:1:1, and mixed using a known method such as a method using a ball mill to obtain a mixture.

[0034] Next, the resulting mixture is compressed into pellets, and then the resulting pellets are fired. The firing of the pellets is carried out, for example, in an inert gas atmosphere at a temperature of 600°C to 900°C for 5 hours to 48 hours. ０．５ M ２ ０．５ O ２ (Fe in the formula is divalent. M ２ is M in formula (1) ２ (which is the same as the formula (1).)

[0035] Next, the fired pellet is immersed in water for 30 minutes. ０．５ M ２ ０．５ O ２ (The valence of Fe in the formula is 2. M ２ is M in formula (1) ２ ) is oxidized by water as an oxidizing agent, and the divalent Fe becomes trivalent while Na is released, resulting in a complex metal oxide (Na ０．５ Fe ０．５ M ２ ０．５ O ２ (The valence of Fe in the formula is trivalent. M ２ is M in formula (1) ２ is generated.

[0036] Thereafter, the pellets are recovered from the water in which they have been immersed by suction filtration or the like, and then dried by a known method. ０．５ Fe ０．５ M ２ ０．５ O ２ (The valence of Fe in the formula is trivalent. M ２ is M in formula (1) ２ ) A complex metal oxide is obtained.

[0037] In this embodiment, as an example of a method for producing a composite metal oxide represented by formula (1), a case where a composite metal oxide in formula (1) is produced in which c is 0 and a, b, and d are all 0.5 has been described as an example. However, other composite metal oxides represented by formula (1) can be produced, for example, by the method shown below.

[0038] That is, in the above-mentioned manufacturing method, it can be manufactured by appropriately adjusting any one or more conditions selected from the following (a) to (c): (a) Na ２ CO ３ and FeC ２ O ４ ・2H ２ O and M ２ and, if necessary, M １The raw material is an oxide containing Na, Fe, and M. １ and M ２ (b) Adjusting the conditions for firing pellets made of a mixture of raw materials; (c) Adjusting the conditions for immersing the fired pellets in water to oxidize them.

[0039] The negative electrode active material for a secondary battery of this embodiment may contain a composite metal oxide represented by formula (1), and may contain a known negative electrode active material together with the composite metal oxide represented by formula (1) as needed, as long as the properties of the composite metal oxide represented by formula (1) are not impaired. Examples of known negative electrode active materials include TiS ２ , TiO ２ , Na ４ Ti ５ O １２ , V ２ O ５ , V.O. ２ , MoO ３ , MoO ２ The negative electrode active material for a secondary battery according to the present embodiment may contain inevitable impurities, such as raw materials remaining without reacting and components of the fired pellets containing divalent Fe remaining without oxidizing.

[0040] "Negative electrode for secondary battery" Next, an example of the negative electrode for secondary battery (hereinafter sometimes abbreviated as "negative electrode") of this embodiment will be described. The negative electrode of this embodiment is composed of a negative electrode current collector and a negative electrode active material layer.

[0041] The negative electrode current collector may be, for example, aluminum foil, nickel foil, stainless steel foil, or titanium foil. Aluminum foil is preferred as the negative electrode current collector for a proton battery. The negative electrode active material layer contains the negative electrode active material for a secondary battery of this embodiment, a conductive additive, and a binder.

[0042] Examples of the conductive additive include carbon materials, conductive oxides, and metal powders. Examples of the carbon material that can be used as the conductive additive include carbon black such as acetylene black, ketjen black, furnace black, and thermal black, graphite, hard carbon, carbon nanotubes, and carbon fibers. The conductive additive may also serve as a negative electrode active material. Only one type of conductive additive may be used, or two or more types may be mixed and used.

[0043] Examples of binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene butadiene rubber, polypropylene (PP), polyethylene (PE), carboxymethyl cellulose (CMC), polyacrylic acid (PAA) salt, polyvinyl alcohol (PVA), etc. Only one type of binder may be used, or two or more types may be mixed and used.

[0044] The ratio of the negative electrode active material for a secondary battery to the conductive additive contained in the negative electrode active material layer can be, for example, 80:20 to 99:1 in mass ratio (negative electrode active material:conductive additive). When the ratio of the negative electrode active material to the conductive additive is within the above range, a sufficient amount of the negative electrode active material is contained, thereby obtaining a sufficiently high charge / discharge capacity. Furthermore, when the ratio of the negative electrode active material to the conductive additive is within the above range, a sufficient amount of the conductive additive is contained, thereby obtaining sufficient electronic conductivity. Therefore, when the ratio of the negative electrode active material to the conductive additive is within the above range, a secondary battery including the negative electrode of this embodiment can obtain a higher charge / discharge capacity and better cycle characteristics.

[0045] The ratio of the total mass of the negative electrode active material for a secondary battery and the conductive additive contained in the negative electrode active material layer to the binder (negative electrode active material + conductive additive: binder) can be, for example, 90:10 to 99:1. When the ratio of the total mass of the negative electrode active material and the conductive additive to the mass of the binder is within the above range, the negative electrode active material and the conductive additive are stably fixed to the surface of the negative electrode current collector by the binder. Furthermore, when the ratio of the total mass of the negative electrode active material and the conductive additive to the mass of the binder is within the above range, the negative electrode active material and the conductive additive are sufficiently contained in the negative electrode active material layer. Therefore, when the ratio of the total mass of the negative electrode active material and the conductive additive to the mass of the binder is within the above range, a secondary battery can be obtained that has a higher charge / discharge capacity and better cycle characteristics.

[0046] (Method for Manufacturing Negative Electrode) The negative electrode of this embodiment can be manufactured, for example, by the method described below. That is, the negative electrode active material for a secondary battery of this embodiment and a conductive additive are mixed in a predetermined ratio to form a mixture. The obtained mixture is mixed with a predetermined amount of a binder dissolved in a solvent to form a negative electrode slurry. As the solvent, for example, one or more selected from N-methylpyrrolidone (NMP), water, ethanol, ethyl methyl ketone, etc. can be used. Thereafter, the negative electrode slurry is applied to the surface of a negative electrode current collector and dried. Through the above steps, the negative electrode of this embodiment is obtained.

[0047] "Secondary Battery" Next, the secondary battery of this embodiment will be described using an example. The secondary battery of this embodiment has a positive electrode, the negative electrode of this embodiment, an electrolyte, and a separator disposed between the positive electrode and the negative electrode. Examples of the secondary battery include a sodium secondary battery and a proton battery.

[0048] (Positive Electrode) The positive electrode may be, for example, a positive electrode current collector and a positive electrode active material layer. The positive electrode current collector may be, for example, aluminum foil, nickel foil, stainless steel foil, or titanium foil.

[0049] The positive electrode active material layer contains a positive electrode active material for secondary batteries (hereinafter sometimes abbreviated as "positive electrode active material"), a conductive additive, and a binder. As the positive electrode active material for sodium secondary batteries, one or more known materials that can release sodium ions during charging and occlude sodium ions during discharging can be used. Specifically, as the positive electrode active material for sodium secondary batteries, for example, Na ０．６７ MnO ２ , NaFeO ２ , NaNiO ２ , NaCoO ２ , NaMnO ２ , NaNi ０．５ Mn ０．５ O ２ , Na ０．６７ Ni ０．３３ Mn ０．６７ O ２ , Na ０．６７ Cu ０．３３ Mn ０．６７ O ２ As the positive electrode active material for a proton battery, one or more known materials that can release hydrogen ions (protons) during charging and absorb hydrogen ions (protons) during discharging can be used. Specifically, as the positive electrode active material for a proton battery, nickel hydroxide (Ni(OH) ２ ), manganese dioxide (MnO ２ ), tungsten oxide (WO ３ ), molybdenum oxide (MoO ３ ) etc. can be used.

[0050] Known conductive additives and binders can be used as materials for the positive electrode active material layer. Examples of conductive additives that can be used as materials for the positive electrode active material layer include the same conductive additives as those listed as materials for the negative electrode active material layer. Also, examples of binders that can be used as materials for the positive electrode active material layer include the same binders as those listed as materials for the negative electrode active material layer. The positive electrode can be manufactured using known methods.

[0051] (Separator) A known separator can be used. For example, a porous membrane made of polyethylene (PE), polypropylene (PP), cellulose, polyvinylidene fluoride, or the like can be used as the separator. When the electrolytic solution is an aqueous electrolyte using water as a solvent and the separator is made of polyethylene (PE) or polypropylene (PP), it is preferable to use a separator whose surface is coated with a polar polymer or the like to improve its affinity for water.

[0052] (Electrolyte for Sodium Secondary Battery) The electrolyte provides ionic conduction between the positive electrode active material and the negative electrode active material. The electrolyte contains an electrolyte and a solvent. In this embodiment, it is preferable to use an aqueous solution containing an electrolyte and an organic salt as the electrolyte for the sodium secondary battery. Since such an electrolyte uses water as the solvent, it has better safety than, for example, a solution using an organic solvent as the solvent. Furthermore, since the above-mentioned electrolyte contains an electrolyte and an organic salt, the concentration of salt (electrolyte (sodium salt) + organic salt) in the electrolyte can be made higher than when an organic salt is not contained. An aqueous solution in which salt is dissolved at a high concentration has high ionic conductivity. Therefore, by using the above-mentioned electrolyte, excellent rapid charging performance can be obtained.

[0053] Examples of the electrolyte include sodium salts of monocarboxylic acids having an alkyl group with 1 to 10 carbon atoms, sodium salts of aromatic monocarboxylic acids, sodium salts of aromatic dicarboxylic acids, and NaPF ６ , NaBF ４ , NaClO ４ , NaCF ３ SO ３ , NaN(CF ３ SO ２ ) ２ , NaN(C ２ F ５ SO ２ ) ２ , NaN(FSO ２ ) ２ The electrolyte may be used alone or in combination of two or more.

[0054] In this embodiment, it is preferable to use a sodium salt of a monocarboxylic acid having an alkyl group containing 1 to 10 carbon atoms as the electrolyte. This is because an electrolyte solution with good sodium ion conductivity can be obtained, and further, a coating can be formed on the electrode surface. Among sodium salts of monocarboxylic acids having an alkyl group containing 1 to 10 carbon atoms, it is preferable to use sodium propionate or sodium acetate as the electrolyte, and it is more preferable to use sodium propionate because the material is easily procurable and has high solubility.

[0055] The organic salt may be any salt that can be dissolved in water together with the electrolyte. Only one type of organic salt may be used, or two or more types may be mixed and used. By using a mixture of two or more types of organic salts, the concentration of salt (electrolyte (sodium salt) + organic salt) in the electrolytic solution may be increased in some cases.

[0056] The organic salt is preferably selected from the group consisting of ammonium salts, lithium salts, potassium salts, and tetramethylammonium salts of monocarboxylic acids having an alkyl group of 1 to 10 carbon atoms; ammonium salts, lithium salts, potassium salts, and tetramethylammonium salts of aromatic monocarboxylic acids; and ammonium salts, lithium salts, potassium salts, and tetramethylammonium salts of aromatic dicarboxylic acids, and more preferably selected from the group consisting of ammonium salts, lithium salts, potassium salts, and tetramethylammonium salts of monocarboxylic acids having an alkyl group of 1 to 10 carbon atoms. The reason for this is that, by dissolving these organic salts in water together with a sodium salt of a monocarboxylic acid having an alkyl group of 1 to 10 carbon atoms as an electrolyte, an electrolytic solution having high ionic conductivity in which the salts (electrolyte (sodium salt) + organic salt) are dissolved at a high concentration is easily obtained.

[0057] Specifically, it is preferable to use ammonium acetate, potassium acetate, ammonium propionate, or potassium propionate as the organic salt, and it is more preferable to use ammonium acetate because the material is easily available and an electrolyte solution with higher ionic conductivity can be obtained.

[0058] The ratio of the electrolyte (sodium salt) to the organic salt contained in the electrolyte solution for sodium secondary batteries is preferably, for example, 0.1:0.9 to 0.3:0.7 in molar ratio (electrolyte:organic salt). When the ratio of the electrolyte to the organic salt is within this range, the organic salt is contained in sufficient quantity, making it easier to obtain an aqueous solution with higher ionic conductivity, and a sodium secondary battery with better rapid charging performance is obtained. Furthermore, when the ratio of the electrolyte to the organic salt is within this range, the electrolyte is contained in sufficient quantity, resulting in a sodium secondary battery with high charge / discharge capacity and good cycle characteristics.

[0059] It is more preferable that the ratio of the total number of moles of the electrolyte (sodium salt) and organic salt contained in the electrolyte solution for sodium secondary batteries to the number of moles of water (electrolyte + organic salt:water) be 1:1 to 1:2. When the number of moles of water relative to the total number of moles of the electrolyte and organic salt is within the above range, the electrolyte solution contains a sufficient amount of (electrolyte + organic salt), and the activity of free water becomes extremely low. This makes it easier to obtain an electrolyte solution consisting of an aqueous solution with higher electrochemical stability, enabling the sodium secondary battery to operate at a higher voltage.

[0060] In this embodiment, a preferred example of an electrolyte solution for a sodium secondary battery has been described using an aqueous solution containing an electrolyte and an organic salt. However, the organic salt may be contained as needed, or may not be contained. Furthermore, the solvent contained in the electrolyte solution may be any solvent capable of dissolving the electrolyte. Therefore, the solvent contained in the electrolyte solution is not limited to water, but may be a non-aqueous solvent, and may be appropriately determined depending on the type of electrolyte, etc. Furthermore, the solvent contained in the electrolyte solution may be one type only, or two or more types may be mixed together.

[0061] Examples of non-aqueous solvents that can be used as the solvent for the electrolyte include monoglyme (G1), diglyme (G2), triglyme (G3), tetraglyme (G4), pentaglycine (G5), fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), γ-butyrolactone, N-methylpyrrolidone (NMP), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), tetrahydrofuran (THF), sulfolane (SL), acetonitrile (AN), succinonitrile (SN), methyl acetate (MA), ethyl acetate (EA), dimethyl sulfoxide (DMSO), and trimethyl phosphate (TMP).

[0062] Among these non-aqueous solvents, it is preferable to use one or more selected from diglyme (G2), fluoroethylene carbonate (FEC), and propylene carbonate (PC). Diglyme (G2) is particularly preferable as the non-aqueous solvent, since it can form a sodium secondary battery having a high charge / discharge capacity and good cycle characteristics.

[0063] Furthermore, the non-aqueous solvent may be, for example, a gel obtained by mixing a non-aqueous solvent with a polymer. Known polymers can be used as the polymer to be mixed with the non-aqueous solvent. Examples of polymers that can be mixed with the non-aqueous solvent include polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP).

[0064] (Electrolyte for Proton Batteries) The electrolyte used contains one or more selected from organic salts, inorganic salts, organic acids, and inorganic acids, and a solvent. It is preferable to use an aqueous solution containing one or more selected from organic salts, inorganic salts, organic acids, and inorganic acids as the electrolyte for proton batteries. The organic salt is preferably an ammonium salt of a monocarboxylic acid having an alkyl group with 1 to 10 carbon atoms, and more preferably ammonium acetate. Examples of inorganic salts include phosphates, nitrates, and sulfates. The organic acid is preferably a monocarboxylic acid having an alkyl group with 1 to 10 carbon atoms, and more preferably acetic acid. Examples of inorganic acids include phosphoric acid, nitric acid, and sulfuric acid.

[0065] The ratio of the total number of moles of organic salts, inorganic salts, organic acids, and inorganic salts contained in the proton battery electrolyte to the number of moles of water (organic salts + inorganic salts + organic acids + inorganic salts:water) is preferably 1:2 to 1:50. The solvent contained in the electrolyte may be any solvent capable of dissolving organic salts, inorganic salts, organic acids, and inorganic salts. Therefore, the solvent contained in the electrolyte is not limited to water and may be a non-aqueous solvent, which can be appropriately determined as needed. The solvent contained in the electrolyte may be a single solvent or a mixture of two or more solvents. Examples of non-aqueous solvents that can be used as solvents for proton battery electrolytes include the non-aqueous solvents that can be used as solvents for the electrolytes for sodium secondary batteries described above. As with the electrolytes for sodium secondary batteries, the non-aqueous solvent may be, for example, a mixture of a non-aqueous solvent and a polymer to form a gel.

[0066] (Method for manufacturing secondary battery) The secondary battery of this embodiment can be manufactured by a known method using the above-mentioned positive electrode, the negative electrode of this embodiment, an electrolyte solution, and a separator. The shape of the secondary battery of this embodiment can be, for example, a coin type, a laminate type, a cylindrical type, a square type, or the like, and is not particularly limited.

[0067] The secondary battery of this embodiment has a negative electrode of this embodiment containing the composite metal oxide represented by formula (1) as the negative electrode active material. Therefore, the secondary battery of this embodiment can use not only an electrolyte solution using a nonaqueous solvent as the solvent, but also an electrolyte solution using water. Moreover, by including the negative electrode of this embodiment containing the composite metal oxide represented by formula (1), the secondary battery of this embodiment has a high charge / discharge capacity and good cycle characteristics.

[0068] (Uses of Secondary Battery) The secondary battery of the present embodiment can be suitably used, for example, as a power source for small electronic devices such as mobile phones, portable audio devices, and notebook computers; a power source for transportation equipment such as automobiles, motorcycles, electric chairs, forklifts, trains, airplanes, and ships; a power source for outdoor / indoor installation in factories, houses, and the like; a load-leveling power source for various power generation such as a charging device for solar cells and a charging device for wind power generation, and as a replacement for existing lead-acid batteries and existing nickel-metal hydride batteries.

[0069] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the specific embodiments, and various modifications and changes are possible within the scope of the requirements of the present invention as set forth in the claims.

[0070] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0071] [Example 1, Comparative Example 1] Composite metal oxides of Example 1 and Comparative Example 1 were produced by the following method. ２ CO ３ (Wako Pure Chemical Industries, Ltd.) and FeC ２ O ４ ・2H ２ O (manufactured by Wako Pure Chemical Industries, Ltd.) and TiO ２ (manufactured by Wako Pure Chemical Industries, Ltd.) was prepared. The raw materials were weighed out so that the amounts of Na, Fe, and Ti were in a molar ratio of Na:Fe:Ti=2:1:1, and mixed using a ball mill to prepare a mixture.

[0072] The mixture was then compressed into pellets, which were then fired in an inert gas atmosphere at 700° C. for 12 hours to obtain pellets made of the composite metal oxide of Comparative Example 1.

[0073] The X-ray diffraction pattern of the composite metal oxide of Comparative Example 1 was measured using CuKα radiation with an X-ray diffraction (XRD) measurement device (D2 PHASER (manufactured by Bruker)). FIG. 1 is a chart showing the X-ray diffraction pattern of the composite metal oxide powder of Comparative Example 1. The numerical values ​​written at the peak positions in FIG. 1 are Miller indices.

[0074] As can be seen from FIG. 1, the composition of the composite metal oxide of Comparative Example 1 is NaFe ０．５ Ti ０．５ O ２ (The valence of Fe in the formula is 2.) More specifically, as shown in FIG. 1, NaFe was present at 2θ positions of 15.9° to 16.9°, 32.3° to 33.6°, 34.2° to 35.1°, 35.8° to 36.5°, 40.7° to 41.6°, 52.3° to 53.1°, 56.7° to 57.5°, and 60.8° to 62.1°. ０．５ Ti ０．５ O ２ (The valence of Fe in the formula is 2) was detected. The a-axis length and c-axis length in the R-3m space group of the crystal were calculated from the position, intensity, and shape of these peaks to be 3.000 Å and 16.63 Å, respectively.

[0075] Next, the pellets made of the composite metal oxide of Comparative Example 1 were immersed in water for 30 minutes, and then recovered from the water by suction filtration and vacuum-dried at a temperature of 80°C to obtain pellets made of the composite metal oxide of Example 1.

[0076] The X-ray diffraction (XRD) pattern of the composite metal oxide of Example 1 was measured in the same manner as for the composite metal oxide of Comparative Example 1. Figure 2 is a chart showing the X-ray diffraction pattern of the powder of the composite metal oxide of Example 1. The numerical values ​​written at the peak positions in Figure 2 are Miller indices.

[0077] As can be seen from FIG. 2, the composition of the composite metal oxide of Example 1 is０．５ Fe ０．５ Ti ０．５ O ２ (The valence of Fe in the formula is trivalent.) More specifically, as shown in FIG. 2, Na was present at positions where 2θ is 16.0° to 17.2°, 32.9° to 34.1°, 34.9° to 35.8°, 36.4° to 37.1°, 41.1° to 42.3°, 52.7° to 53.8°, 57.1° to 58.4°, and 62.0° to 63.1°. ０．５ Fe ０．５ Ti ０．５ O ２ (The valence of Fe in the formula is trivalent) crystals were detected. The a-axis length and c-axis length in the R-3m space group of the crystals calculated from the position, intensity, and shape of these peaks were 2.975 Å and 16.60 Å, respectively.

[0078] (Evaluation of Battery Characteristics) The composite metal oxides of Example 1 and Comparative Example 1 obtained in this manner were used as negative electrode active materials for sodium secondary batteries, and negative electrodes for sodium secondary batteries were obtained by the method described below.

[0079] A negative electrode active material for a sodium secondary battery and acetylene black (AB) (HS-100, manufactured by Denka Co., Ltd.) serving as a conductive additive were mixed at a mass ratio (negative electrode active material:conductive additive) of 90:10 to form a mixture. Polyvinylidene fluoride (PVDF) (manufactured by Kureha Corporation) serving as a binder was dissolved in N-methylpyrrolidone (NMP) as a solvent. The above mixture, additional acetylene black (AB), and a polyvinylidene fluoride (PVDF) solution were mixed to form a negative electrode slurry. The mass ratio (negative electrode active material:conductive additive:binder) of the negative electrode slurry was 76.5:13.5:10. Thereafter, the negative electrode slurry was applied to the entire surface of an aluminum foil having a length of 100 mm, a width of 200 mm, and a thickness of 0.02 mm as a negative electrode current collector in a manner of 100 cm. ２ / g and dried. By the above steps, negative electrodes for sodium secondary batteries of Example 1 and Comparative Example 1 were obtained.

[0080] The negative electrodes for sodium secondary batteries of Example 1 and Comparative Example 1 were used as the negative electrodes (working electrodes), sodium metal foil of 13 mm in length, 13 mm in width, and 0.1 mm in thickness was used as the counter electrode, and the electrolyte NaPF ６ The above was dissolved in propylene carbonate (PC) at a concentration of 1 mol / L, and the bipolar electrochemical cells for evaluation of Example 1 and Comparative Example 1 were fabricated using the solution.

[0081] For the bipolar electrochemical evaluation cells of Example 1 and Comparative Example 1, charge / discharge tests were performed at room temperature from the 1st cycle to the 50th cycle at a current density of 10 mA / g and a voltage range of 1.0 to 2.5 V, and the charge / discharge capacities were measured. The results are shown in FIGS. 3 and 4. FIG. 3 is a graph showing the relationship between the voltage and charge / discharge capacity of the bipolar electrochemical evaluation cell of Example 1. FIG. 4 is a graph showing the relationship between the voltage and charge / discharge capacity of the bipolar electrochemical evaluation cell of Comparative Example 1. In FIGS. 3 and 4, the downward-sloping curves correspond to the charge curves, and the upward-sloping curves correspond to the discharge curves.

[0082] As shown in Figure 3, the charge-discharge capacity of the evaluation bipolar electrochemical cell of Example 1 in the first cycle (initial) was 130 mAh / g. In contrast, as shown in Figure 4, the charge-discharge capacity of the evaluation bipolar electrochemical cell of Comparative Example 1 in the first cycle (initial) was 40 mAh / g, which was much lower than that of the evaluation bipolar electrochemical cell of Example 1. This is because the composite metal oxide (Na ０．５ Fe ０．５ Ti ０．５ O ２ ) contains 0.5-valent Na, so that the composite metal oxide (NaFe ０．５ Ti ０．５ O ２ This is because the number of sodium ions that can be inserted into the negative electrode is larger than that of Comparative Example 1. In addition, Example 1 shown in Figure 3 shows a smaller decrease in charge / discharge capacity from the 1st cycle to the 50th cycle than Comparative Example 1 shown in Figure 4, and it was confirmed that the capacity is less likely to deteriorate.

[0083] [Example 2] As the electrolytic solution, the electrolyte NaPF ６A bipolar electrochemical cell for evaluation of Example 2 was produced in the same manner as in Example 1, except that a solution of the above in diglyme (G2) at a concentration of 1 mol / L was used.

[0084] The bipolar electrochemical cell for evaluation of Example 2 was subjected to a charge-discharge test at room temperature from the first cycle to the 1000th cycle at a current density of 100 mA / g and a voltage range of 1.0 to 2.5 V, and the charge-discharge capacity was measured. The results are shown in Figure 5. Figure 5 is a graph showing the relationship between the voltage and charge-discharge capacity of the bipolar electrochemical cell for evaluation of Example 2. In Figure 5, the curve sloping downward to the right corresponds to the charge curve, and the curve sloping upward to the right corresponds to the discharge curve.

[0085] As shown in Fig. 5, the evaluation bipolar electrochemical cell of Example 2 had a first cycle (initial) charge / discharge capacity exceeding 100 mAh / g, similar to the evaluation bipolar electrochemical cell of Example 1 shown in Fig. 3, which was significantly larger than that of the evaluation bipolar electrochemical cell of Comparative Example 1. Furthermore, as shown in Fig. 5, Example 2 showed a small decrease in charge / discharge capacity from the first cycle to the 1000th cycle, and was confirmed to be resistant to capacity degradation and to have good cycle characteristics.

[0086] [Example 3] The electrolyte used in Example 1 (NaPF ６ A bipolar electrochemical cell for evaluation of Example 3 was prepared in the same manner as in Example 1, except that a solution prepared by adding fluoroethylene carbonate (FEC) to a concentration of 1% by volume of ethylene carbonate (propylene carbonate) dissolved in propylene carbonate (PC) at a concentration of 1 mol / L was used.

[0087] The bipolar electrochemical cell for evaluation of Example 3 was subjected to a charge-discharge test at room temperature from the first cycle to the 1000th cycle at a current density of 100 mA / g and a voltage range of 1.0 to 2.5 V, and the discharge capacity was measured. The results are shown in Figure 6. Figure 6 is a graph showing the relationship between the discharge capacity and the number of cycles for the bipolar electrochemical cells for evaluation of Examples 2 and 3.

[0088] 6, in the evaluation bipolar electrochemical cell of Example 2, the decrease in discharge capacity up to the 1000th cycle was small, and a discharge capacity of 90 mAh / g was maintained up to the 1000th cycle. Furthermore, in the evaluation bipolar electrochemical cell of Example 2, the ratio of the discharge capacity at the 1000th cycle to the maximum discharge capacity (discharge capacity retention rate) was 91%, confirming that the cell had good cycle characteristics.

[0089] 6, the bipolar electrochemical cell for evaluation in Example 3 showed little decrease in discharge capacity up to about the 400th cycle. However, the bipolar electrochemical cell for evaluation in Example 3 showed a decrease in discharge capacity after the 400th cycle. This indicates that the use of the electrolyte solution used in Example 2 provides better cycle characteristics than the use of the electrolyte solution used in Example 3.

[0090] [Example 4] The negative electrode for a sodium secondary battery of Example 1 was used as the working electrode. ０．６７ MnO ２ An electrode was prepared in the same manner as the working electrode, except that a silver / silver chloride (Ag / AgCl) electrode was used as the counter electrode. A silver electrode having a silver chloride film on its surface was immersed in a saturated KCl aqueous solution to form a reference electrode. The working electrode was evaluated by the following method using the three-electrode electrochemical cell of Example 4 having these working electrode, counter electrode, and reference electrode.

[0091] First, an aqueous solution was prepared by mixing sodium propionate (electrolyte), ammonium acetate (organic salt), and water (solvent) in a molar ratio (electrolyte:organic salt:water) of 0.1:0.9:1.4. Then, using this electrolyte in the three-electrode electrochemical cell of Example 4, a charge-discharge test was performed from the first cycle to the fifth cycle at room temperature, with a current density of 50 mA / g and a voltage range of −1.0 to 0.2 V, and the charge-discharge capacity was measured. The results are shown in FIG. 7A.

[0092] 7A is a graph showing the relationship between the potential of the working electrode relative to the reference electrode (Ag / AgCl electrode) and the charge / discharge capacity when the current density of the three-electrode electrochemical cell of Example 4 is 50 mA / g. In FIG. 7A, the downward-sloping curve corresponds to the charge curve, and the upward-sloping curve corresponds to the discharge curve. As shown in FIG. 7A, the three-electrode electrochemical cell of Example 4 had a large charge / discharge capacity of about 120 mAh / g at the first cycle (initial), and the decrease in charge / discharge capacity from the first cycle to the fifth cycle was small, confirming stable charge / discharge characteristics.

[0093] Next, the first cycle charge capacity was measured in the three-electrode electrochemical cell of Example 4 using the above electrolyte at room temperature and at a current density of 100 mA / g, 200 mA / g, 500 mA / g, or 1000 mA / g in the voltage range of −1.0 to 0.2 V. The results are shown in FIG.

[0094] 7B is a graph showing the relationship between the voltage of the working electrode relative to the reference electrode (Ag / AgCl electrode) and the charge capacity when the current density of the three-electrode electrochemical cell of Example 4 was 100 mA / g, 200 mA / g, 500 mA / g, or 1000 mA / g. As shown in FIG. 7B, the three-electrode electrochemical cell of Example 4 demonstrated a high first-cycle (initial) charge capacity even at a high current density of 1000 mA / g, demonstrating excellent rapid charge performance.

[0095] [Example 5] As the electrolyte, an aqueous solution prepared by mixing sodium propionate as an electrolyte, ammonium acetate as an organic salt, and water as a solvent at a molar ratio (electrolyte:organic salt:water) of 0.1:0.9:1.4 was used, as in Example 4. A three-electrode electrochemical cell of Example 5 was fabricated in the same manner as in Example 4, except that the working electrode used in Example 4 was used as the counter electrode, and the counter electrode used in Example 4 was used as the working electrode.

[0096] A charge-discharge test was performed on the three-electrode electrochemical cell of Example 5 from the first cycle to the tenth cycle at room temperature, with a current density of 100 mA / g and a voltage ranging from 0 to 2.0 V, and the charge-discharge capacity was measured. The results are shown in Figure 8. Figure 8 is a graph showing the relationship between the voltage and charge-discharge capacity of the three-electrode electrochemical cell of Example 5. In Figure 8, the downward-sloping curve corresponds to the charge curve, and the upward-sloping curve corresponds to the discharge curve.

[0097] As shown in FIG. 8, in the three-electrode electrochemical cell of Example 5, the charge / discharge capacity in the first cycle (initial) exceeded 100 mAh / g, and the decrease in charge / discharge capacity from the first cycle to the tenth cycle was small, confirming that the cell exhibited stable charge / discharge characteristics.

[0098] Example 6 Example 6-1 In the electrochemical cell for evaluation of Example 4, a charge-discharge test was carried out at room temperature with a current density of 50 mA / g and a voltage in the range of −1.0 to 0.2 V from the 1st cycle to the 1000th cycle, and the charge-discharge capacity was measured.

[0099] The charge / discharge capacity at the first cycle (initial) is shown in Table 1. In addition, the ratio of the discharge capacity at the 1000th cycle to the maximum discharge capacity from the first cycle to the 1000th cycle (discharge capacity retention rate (%)) was calculated, and the cycle characteristics were evaluated according to the following criteria. The results are shown in Table 1. [Criteria] A: Discharge capacity retention rate is 90% or more. B: Discharge capacity retention rate is 70% or more and less than 90%. C: Discharge capacity retention rate is 50% or more and less than 70%.

[0100] (Examples 6-2 to 6-6) In the evaluation electrochemical cell of Example 4, except that the composite metal oxides of Examples 6-2 to 6-6 were used instead of the composite metal oxide contained in the negative electrode, charge-discharge tests were performed from the 1st cycle to the 1000th cycle in the same manner as in Example 6-1, and the charge-discharge capacity was measured.

[0101] The charge / discharge capacity at the first cycle (initial) is shown in Table 1. In addition, the ratio of the discharge capacity at the 1000th cycle to the maximum discharge capacity from the 1st cycle to the 1000th cycle (discharge capacity retention rate) was calculated, and the cycle characteristics were evaluated in the same manner as in Example 6-1. The results are shown in Table 1.

[0102] The composite metal oxides of Examples 6-2 to 6-5 were prepared by the same method as in Example 1 except that Na ２ CO ３ and FeC ２ O ４ ・2H ２ O and TiO ２ The amount of each of the above components was adjusted to change the number of moles of Na, Fe, and Ti in the raw materials, and the firing conditions for pellets made from the mixture of the raw materials and the conditions for immersing the fired pellets in water to oxidize them were also adjusted to produce the powder.

[0103] Thereafter, the compositions of the composite metal oxides of Examples 6-2 to 6-5 were confirmed in the same manner as in Example 1. As a result, all of them contained Na ａ Fe ｂ Ti ｄ O ２ (The valence of Fe in the formula is trivalent.) Table 1 shows the molar ratios of Na, Fe, and Ti in the composite metal oxides of Examples 6-1 to 6-5.

[0104] The composite metal oxide of Example 6-6 was prepared by the same method as in Example 1 except that Na ２ CO ３ and FeC ２ O ４ ・2H ２ O and TiO ２ and Al(OH) ３ (manufactured by Wako Pure Chemical Industries, Ltd.) was used, and the amounts of these oxides were adjusted to change the number of moles of Na, Fe, Ti, and Al in the raw materials, while adjusting the conditions for firing pellets made from the mixture of raw materials and the conditions for immersing the fired pellets in water to oxidize them.

[0105] Thereafter, the composition of the composite metal oxide of Example 6-6 was confirmed in the same manner as in Example 1. As a result, Na ａ Fe ｂ Al ｃ Ti ｄ O ２(The valence of Fe in the formula is trivalent.) The molar ratios of Na, Fe, Ti and Al in the composite metal oxide of Example 6-6 are shown in Table 1.

[0106]

[0107] As shown in Table 1, among Examples 6-1 to 6-6, the evaluation electrochemical cell of Example 6-1, in which the molar ratio of Na:Fe:Ti in the composite metal oxide was 0.5:0.5:0.5, had the best capacity and cycle characteristics. Also, as shown in Table 1, it was confirmed that the evaluation electrochemical cell in which the molar ratio of Na:Fe:Ti in the composite metal oxide was closer to 0.5:0.5:0.5 exhibited better results in terms of capacity and cycle characteristics.

[0108] Furthermore, it was confirmed from the results of Examples 1, 5, and 6 that better cycle characteristics can be obtained when b + c + d in the composite metal oxide represented by formula (1) is 1. It was also confirmed from the results of Examples 1 and 6 that better capacity can be obtained when c is 0 and b = d in the composite metal oxide represented by formula (1).

[0109] [Example 7] (Example 7-1) The composite metal oxide Na of Example 1 ０．５ Fe ０．５ Ti ０．５ O ２ (where the valence of Fe is trivalent) was used as the negative electrode active material for a secondary battery, and a negative electrode for a secondary battery was obtained by the method described below. The negative electrode active material for a secondary battery and acetylene black (AB) (HS-100 manufactured by Denka Co., Ltd.) as a conductive additive were mixed in a mass ratio (negative electrode active material:conductive additive) of 60:20 to obtain a mixture. Polyvinyl alcohol (PVA) and aramid fiber (aramid) as binders were dissolved in N-methylpyrrolidone (NMP) as a solvent, and mixed with the above mixture to obtain a negative electrode slurry. The mass ratio of the negative electrode active material, conductive additive, and binder contained in the negative electrode slurry was 60:20:10:10 (negative electrode active material:conductive additive:PVA:aramid). Thereafter, the negative electrode slurry was applied to the entire surface of a titanium foil having a length of 100 mm, a width of 200 mm, and a thickness of 0.02 mm as a negative electrode current collector in a thickness of 100 cm.２ The negative electrode for a secondary battery of Example 7 was obtained by the above steps.

[0110] Nickel hydroxide (Ni(OH)) as a positive electrode active material ２ ) and acetylene black (AB) (HS-100 manufactured by Denka Co., Ltd.) as a conductive additive were mixed in a mass ratio (positive electrode active material: conductive additive) of 80:10 to obtain a mixture. Polyvinylidene fluoride (PVDF) (manufactured by Kureha Corporation) as a binder was dissolved in N-methylpyrrolidone (NMP) as a solvent and mixed with the above mixture to obtain a positive electrode slurry. The mass ratio (positive electrode active material: conductive additive: binder) of the positive electrode slurry was 80:10:10. Thereafter, the positive electrode slurry was applied to the entire surface of a titanium foil having a length of 100 mm, a width of 200 mm, and a thickness of 0.02 mm as a positive electrode current collector, in a thickness of 100 cm. ２ The positive electrode for a secondary battery of Example 7 was obtained by the above steps.

[0111] Nickel oxyhydroxide (NiOOH) as a reference electrode material and acetylene black (AB) (HS-100, manufactured by Denka Co., Ltd.) as a conductive additive were mixed in a mass ratio (reference electrode material:conductive additive) of 87:5 to prepare a mixture. Polyvinylidene fluoride (PVDF) (manufactured by Kureha Corporation) as a binder was dissolved in N-methylpyrrolidone (NMP) as a solvent and mixed with the above mixture to prepare a reference electrode slurry. The mass ratio (reference electrode material:conductive additive:binder) of the reference electrode material, conductive additive, and binder contained in the reference electrode slurry was 87:5:8. Thereafter, the reference electrode slurry was applied in a 100 cm area over the entire surface of a titanium foil measuring 100 mm in length, 200 mm in width, and 0.02 mm in thickness as a reference electrode current collector. ２ The coating was applied in an amount of 1000 μg / g and then dried. A reference electrode was obtained by the above steps.

[0112] The negative electrode for the secondary battery of Example 7 was used as the working electrode, the positive electrode for the secondary battery of Example 7 was used as the counter electrode, and the reference electrode of Example 7 was used as the reference electrode. A three-electrode electrochemical cell of Example 7-1 having these working electrode, counter electrode, and reference electrode was prepared, and the working electrode was evaluated by the method described below. As the electrolyte, an aqueous solution was prepared by mixing acetic acid (HOAc), an organic acid, and water, a solvent, in a molar ratio (organic acid:water) of 1:10. The pH of the electrolyte was 1.1. Then, in the three-electrode electrochemical cell of Example 7-1, a charge-discharge test was performed from the first cycle to the tenth cycle at room temperature, with a current density of 50 mA / g and a voltage range of -1.5 to -0.2 V, and the charge-discharge capacity was measured. The results are shown in FIG. 9A.

[0113] Example 7-2 An aqueous solution was prepared as an electrolyte by mixing an organic salt, sodium acetate (NaOAc), and a solvent, water, at a molar ratio (organic salt:water) of 1:10. The pH of the electrolyte was 8 to 9. A three-electrode electrochemical cell of Example 7-2 was prepared in the same manner as in Example 7-1, except that the above electrolyte was used. A charge-discharge test was performed at room temperature from the first cycle to the tenth cycle at a current density of 50 mA / g and a voltage range of −1.5 to −0.2 V, and the charge-discharge capacity was measured. The results are shown in FIG. 9B.

[0114] (Example 7-3) As an electrolyte, an organic salt, ammonium acetate (NH ４ An aqueous solution was prepared by mixing organic salt (OAc) and water as a solvent at a molar ratio (organic salt:water) of 1:10. The pH of the electrolyte solution was 7. A three-electrode electrochemical cell of Example 7-3 was prepared in the same manner as in Example 7-1, except that the above electrolyte solution was used. A charge-discharge test was performed from the first cycle to the tenth cycle at room temperature with a current density of 50 mA / g and a voltage range of -1.5 to -0.2 V, and the charge-discharge capacity was measured. The results are shown in FIG. 9C.

[0115] 9A to 9C, it was confirmed that the largest charge / discharge capacity and better cycle characteristics were obtained when ammonium acetate was used as the organic salt.

[0116] [Example 8] (Example 8-1) Using the electrolyte solution of Example 7-3, a charge-discharge test was performed in the three-electrode electrochemical cell of Example 7-3 at room temperature, with a current density of 50 mA / g and a voltage range of −1.5 to −0.4 V, from the 1st cycle to the 100th cycle, and the charge-discharge capacity was measured. The results are shown in FIG. 10B.

[0117] (Example 8-2) As an electrolyte, an organic salt, ammonium acetate (NH ４ An aqueous solution was prepared by mixing organic salt (OAc) and water as a solvent at a molar ratio (organic salt:water) of 1:3. A three-electrode electrochemical cell of Example 8-2 was fabricated in the same manner as in Example 7-3, except that the above-described electrolyte solution was used. Then, in the same manner as in Example 8-1, a charge-discharge test was performed at room temperature, with a current density of 50 mA / g and a voltage range of -1.5 to -0.4 V, from the 1st cycle to the 70th cycle, and the charge-discharge capacity was measured. The results are shown in FIG. 10A.

[0118] FIG. 11 is a graph showing the relationship between discharge capacity and cycle number, and the relationship between coulombic efficiency and cycle number for the three-electrode electrochemical cells of Examples 8-1 and 8-2. As shown in FIGS. 10A, 10B, and 11, the cells using an electrolyte solution with a molar ratio (organic salt:water) of 1:3 and the cells using an electrolyte solution with a molar ratio (organic salt:water) of 1:10 exhibited comparable charge / discharge capacities and cycle characteristics. The changes in discharge capacity and coulombic efficiency when using an electrolyte solution with a molar ratio (organic salt:water) of 1:10 were almost identical to the changes in discharge capacity and coulombic efficiency when using an electrolyte solution with a molar ratio (organic salt:water) of 1:3, demonstrating that good cycle characteristics were obtained.

[0119] [Example 9] (Example 9-1) The negative electrode slurry of Example 7 was applied to a titanium foil as a negative electrode current collector to a coating thickness of 10 mil (0.254 mm) and dried to obtain a negative electrode for a secondary battery. In the negative electrode layer after drying, the mass loading of the negative electrode active material per unit area (mass loading) was 4.973 mg / cm. ２The negative electrode for secondary batteries was then punched out with a 10 mm diameter punch to obtain a 10 mm diameter disk-shaped negative electrode for secondary batteries of Example 9-1. The positive electrode slurry of Example 7 was applied to a titanium foil serving as a positive electrode current collector to a coating thickness of 10 mil (0.254 mm) and dried to obtain a positive electrode for secondary batteries. The positive electrode for secondary batteries was then punched out with a 15 mm diameter punch to obtain a 15 mm diameter disk-shaped positive electrode for secondary batteries of Example 9. The reference electrode slurry of Example 7 was applied to a 5 μm thick titanium foil serving as a reference electrode current collector and dried to obtain a reference electrode of Example 9.

[0120] The negative electrode for the secondary battery of Example 9-1 was used as the working electrode, the positive electrode for the secondary battery of Example 9 was used as the counter electrode, and the reference electrode of Example 9 was used as the reference electrode. A polyolefin nonwoven fabric was used as the separator. A three-electrode electrochemical cell of Example 9-1 having these working electrode, counter electrode, reference electrode, and separator was fabricated, and the working electrode was evaluated by the method described below. Ammonium acetate (NH), an organic salt, was used as the electrolyte. ４ An aqueous solution was prepared by mixing organic salt (OAc) and water as a solvent at a molar ratio (organic salt:water) of 1:3. Then, using the above electrolyte in the three-electrode electrochemical cell of Example 9-1, a charge-discharge test was performed from the 1st cycle to the 100th cycle at room temperature with a current density of 50 mA / g and a voltage range of −1.5 to −0.4 V, and the charge-discharge capacity was measured. The results are shown in FIG. 12A.

[0121] (Example 9-2) The negative electrode slurry of Example 7 was applied to an aluminum foil as a negative electrode current collector to a coating thickness of 10 mil (0.254 mm) and dried to obtain a negative electrode for a secondary battery. In the negative electrode layer after drying, the mass loading of the negative electrode active material per unit area (mass loading) was 5.837 mg / cm. ２The negative electrode for secondary batteries was then punched out with a punch having a diameter of 10 mm to obtain a disk-shaped negative electrode for secondary batteries of Example 9-2 having a diameter of 10 mm. A three-electrode electrochemical cell of Example 9-2 was fabricated in the same manner as in Example 9-1, except that the negative electrode for secondary batteries of Example 9-2 was used as the working electrode. Then, the working electrode was evaluated using the three-electrode electrochemical cell of Example 9-2 in the same manner as in Example 9-1. The obtained results are shown in FIG. 12B.

[0122] 13 is a graph showing the relationship between discharge capacity and cycle number, and the relationship between coulombic efficiency and cycle number for the three-electrode electrochemical cells of Examples 9-1 and 9-2. It was found that when aluminum foil was used as the negative electrode current collector, excellent cycle characteristics and high coulombic efficiency were obtained. This is thought to be because the hydrogen overvoltage of aluminum foil is higher than that of titanium foil, suppressing the reductive decomposition of water during charging.

[0123] The negative electrode active material for secondary batteries of this embodiment is suitably applied to sodium secondary batteries and proton batteries.

Claims

1. A negative electrode active material for a secondary battery, comprising a composite metal oxide represented by general formula (1): Na ａ Fe ｂ M １ ｃ M ２ ｄ O ２ (1) (The valence of Fe in formula (1) is trivalent. M １ is at least one selected from the group consisting of Ni, Co, V, Cr, Mg, and Al. ２ is one or more selected from the group consisting of Ti, Nb, and Mo. a is greater than 0 and less than 1. b is 0.35 to 0.

65. c is 0 to 0.

15. d is 0.35 to 0.

65.

2. The negative electrode active material for a secondary battery according to claim 1, wherein b+c+d in general formula (1) is 1.

3. The negative electrode active material for a secondary battery according to claim 1, wherein c in general formula (1) is 0 and b=d.

4. The negative electrode active material for a secondary battery according to claim 2, wherein a, b, and d in general formula (1) are all 0.

5.

5. M in general formula (1) ２ The negative electrode active material for a secondary battery according to claim 1 , wherein is Ti.

6. A negative electrode for a secondary battery, comprising the negative electrode active material for a secondary battery according to any one of claims 1 to 5.

7. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode contains the negative electrode active material for secondary batteries according to any one of claims 1 to 5.

8. The secondary battery according to claim 7, wherein the electrolytic solution is an aqueous solution containing one or more electrolytes and one or more organic salts, the electrolyte is a sodium salt of a monocarboxylic acid having an alkyl group with 1 to 10 carbon atoms, and the organic salt is selected from the group consisting of an ammonium salt, lithium salt, potassium salt, and tetramethylammonium salt of a monocarboxylic acid having an alkyl group with 1 to 10 carbon atoms, and the secondary battery is a sodium secondary battery.

9. The secondary battery according to claim 8, wherein the electrolyte is an aqueous solution containing sodium propionate and ammonium acetate.

10. The secondary battery according to claim 8, wherein the ratio of the total number of moles of the electrolyte and the organic salt contained in the electrolytic solution to the number of moles of water is 1:1 to 1:

2.

11. The secondary battery according to claim 7, wherein the electrolyte is an aqueous solution containing at least one selected from the group consisting of an organic salt, an inorganic salt, an organic acid, and an inorganic salt, and the secondary battery is a proton battery.

12. The secondary battery according to claim 11, wherein the electrolyte is an aqueous solution containing ammonium acetate.

13. The secondary battery according to claim 11, wherein the ratio of the total number of moles of the organic salt, the inorganic salt, the organic acid, and the inorganic salt contained in the electrolyte solution to the number of moles of water is 1:2 to 1:50.

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