JP2018206691A - Positive electrode active material, positive electrode and sodium ion battery, and manufacturing method of positive electrode active material - Google Patents

Abstract

To provide a positive electrode active material for sodium ion batteries in which the battery performance of a sodium ion battery is further improved.SOLUTION: A positive electrode active material for a sodium ion battery includes an iron phosphate compound expressed by NaFe(PO)(OH)(HO)(0≤a≤1.333, 1.000≤b≤1.333, 0≤c≤1, 0≤d≤1).SELECTED DRAWING: Figure 8

Description

  The present invention relates to a positive electrode active material, a positive electrode and a sodium ion battery, and a method for producing a positive electrode active material, and in particular, a positive electrode active material used in a sodium ion battery such as a sodium ion secondary battery, a positive electrode and a sodium ion battery, In addition, the present invention relates to a method for producing a positive electrode active material.

  Conventionally, in a sodium ion battery such as a sodium ion secondary battery, use of a layered metal oxide as a positive electrode active material has been studied (see Patent Document 1).

Japanese Patent No. 5870930

  By the way, when a layered metal oxide is used as the positive electrode active material for a sodium ion battery, the crystal structure is unstable because the bond between the metal atom and the oxygen atom is weak, and deterioration due to charge / discharge may be accelerated. There is sex.

  Accordingly, an object of the present invention is to provide a positive electrode active material for a sodium ion battery having a stable crystal structure capable of further improving the battery performance of the sodium ion battery, a positive electrode and a sodium ion battery, and a method for producing the positive electrode active material Is to provide.

The positive electrode active material according to the present invention is a positive electrode active material for a sodium ion battery, and Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 <= b <= 1.333, 0 <= c <= 1, 0 <= d <= 1) It is characterized by including the iron phosphate compound represented.

  In the positive electrode active material according to the present invention, the iron phosphate compound satisfies c + d = 1.

  In the positive electrode active material according to the present invention, the iron phosphate compound satisfies b-1 = c / 3 = (1-d) / 3.

The positive electrode active material according to the present invention is a positive electrode active material for a sodium ion battery, and includes an iron phosphate compound represented by Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d , In the XRD measurement using the CuKα ray as an X-ray source, the iron phosphate compound has a first peak with the highest intensity in a range of 2θ = 26.05 ° to 26.95 °, and 2θ = 27.15. There is a second peak having the second highest intensity in the range of from ° to 28.20 °.

In the positive electrode active material according to the present invention, the crystal structure of the iron phosphate compound belongs to the space group I4 ₁ / amd.

  The positive electrode according to the present invention includes the positive electrode active material described above.

  The sodium ion battery which concerns on this invention is equipped with said positive electrode, It is characterized by the above-mentioned.

  The method for producing a positive electrode active material according to the present invention is a method for producing a positive electrode active material for a sodium ion battery, and a raw material solution preparation step of preparing a raw material solution by dissolving an iron raw material and a phosphorus raw material in water And a hydrothermal synthesis step in which the raw material solution is heat-treated at 150 ° C. or higher and lower than 300 ° C. for hydrothermal synthesis.

In the method for producing a positive electrode active material according to the present invention, the iron raw materials are Fe (NO ₃ ) ₃ · 9H ₂ O, FeC ₂ O ₄ · 2H ₂ O, Fe (NH ₄ ) ₂ (SO ₄ ) ₂ · 4H. _It contains at least one of ₂ O, and the phosphorus raw material contains H ₃ PO ₄ .

  In the method for producing a positive electrode active material according to the present invention, the raw material solution preparation step includes a first sodium raw material in the raw material solution.

In the method for producing a positive electrode active material according to the present invention, the first sodium raw material is NaNO ₃ , Na ₂ CO ₃ , NaOH, CH ₃ COONa, NaH ₂ PO ₄ , Na ₃ PO ₄ .12H ₂ O and Na _2. It includes at least one of HPO ₄ .

  In the method for producing a positive electrode active material according to the present invention, a mixture obtained by hydrothermal synthesis in the hydrothermal synthesis step and a second sodium raw material are mixed to form a mixture, and the mixture is heated to 150 ° C. or higher. It is characterized in that it is subjected to a solid phase reaction by heat treatment at 500 ° C. or lower.

In the method for producing a positive electrode active material according to the present invention, the second sodium raw material is NaNO ₃ , Na ₂ CO ₃ , NaOH, CH ₃ COONa, NaH ₂ PO ₄ , Na ₃ PO ₄ .12H ₂ O and Na _2. It includes at least one of HPO ₄ .

  The method for producing a positive electrode active material according to the present invention is characterized in that the mixture is heat-treated in a reducing atmosphere to cause a solid phase reaction.

  In the method for producing a positive electrode active material according to the present invention, a mixture obtained by hydrothermal synthesis in the hydrothermal synthesis step and a third sodium raw material are dissolved in water to prepare a mixed solution. It is characterized by hydrothermal synthesis by heat treatment at a temperature not lower than 300 ° C and lower than 300 ° C.

In the method for producing a positive electrode active material according to the present invention, the third sodium raw material is NaNO ₃ , Na ₂ CO ₃ , NaOH, CH ₃ COONa, NaH ₂ PO ₄ , Na ₃ PO ₄ .12H ₂ O and Na _2. It includes at least one of HPO ₄ .

  The method for producing a positive electrode active material according to the present invention is characterized in that the mixed solution is heat-treated in a reducing atmosphere and hydrothermally synthesized.

  The method for producing a positive electrode active material according to the present invention includes assembling a battery using a compound hydrothermally synthesized in the hydrothermal synthesis step as a positive electrode active material and using a material containing sodium as a negative electrode active material to cause a discharge reaction. Thus, the composition is characterized in that sodium is contained in the composite.

  According to the above configuration, the movement of sodium ions in the crystal is easy and the crystal structure is stable, so that the battery performance of the sodium ion battery can be improved.

In the embodiment of the present invention, Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) It is a flowchart which shows the process of producing | generating IHP which is an iron phosphate compound represented. In embodiment of this invention, it is a graph which shows the X-ray-diffraction measurement result of IHP of Example 3. FIG. In embodiment of this invention, it is a figure which shows the crystal structure analysis result of IHP of Example 3. FIG. In embodiment of this invention, it is a figure which shows the crystal structure analysis result of IHP of Examples 1-7. In embodiment of this invention, it is a figure for demonstrating reaction which has arisen in the positive electrode and negative electrode of a sodium ion battery. In embodiment of this invention, it is a figure which shows Na amount of IHPNa of Examples 8-25. In embodiment of this invention, it is a figure which shows the amount of Na of IHPNa of Examples 27-34. In embodiment of this invention, it is a graph which shows the charging / discharging characteristic of the coin cell using the positive electrode active material which consists of IHP. In embodiment of this invention, it is a graph which shows the charging / discharging characteristic of the coin cell using the positive electrode active material which consists of IHPNa. In embodiment of this invention, it is a graph which shows charging / discharging cycling characteristics. In embodiment of this invention, it is a graph which shows the X-ray-diffraction measurement result of the positive electrode active material of each discharge state.

  Embodiments of the present invention will be described below in detail with reference to the drawings. Sodium ion batteries, such as a sodium ion secondary battery, are used for a notebook personal computer, a mobile phone, an electric vehicle, etc., for example.

The positive electrode active material for a sodium ion battery is Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1). This iron phosphate compound preferably satisfies c + d = 1. The iron phosphate compound more preferably satisfies b-1 = c / 3 = (1-d) / 3.

Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) The crystal structure of the iron phosphate compound represented by) is composed of tetragonal crystals. The crystal structure of the iron phosphate compound may be a crystal structure belonging to the space group I4 ₁ / amd represented by the Hermann-Morgan symbol (international symbol). The crystal structure of this iron phosphate compound allows sodium ions to easily move in the crystal, and can easily reversibly remove and insert sodium ions, and can function as a positive electrode active material for sodium ion batteries. . Moreover, since this iron phosphate compound contains an oxygen atom in the form of PO ₄ , the P—O bond is strengthened and the structural stability is enhanced. In addition, when not satisfy | filling the numerical range of a, b, c, d, crystal structure changes and it can transfer to a phase with low electrochemical activity.

Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) In the XRD measurement (X-ray diffraction measurement) using CuKα ray as an X-ray source, the iron phosphate compound represented by) has the highest intensity in the range of 2θ = 26.05 ° to 26.95 °. One peak and a second peak having the second highest intensity in the range of 2θ = 27.15 ° to 28.20 °.

Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) ) In the case where a is 0 (a = 0), Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 1.000 ≦ b ≦ 1. 333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1), and may be an iron phosphate compound (hereinafter sometimes referred to as IHP). The crystal structure of IHP is a tetragonal crystal, and may be a crystal structure belonging to the space group I4 ₁ / amd represented by the Hermann-Morgan symbol (international symbol).

Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) ) Is a Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 <a ≦ 1.333) when a is not 0 (a ≠ 0). , 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1), and may be an iron phosphate compound (hereinafter sometimes referred to as IHPNa). The crystal structure of IHPNa is a tetragonal crystal and may be a crystal structure belonging to the space group I4 ₁ / amd represented by the Hermann-Morgan symbol (international symbol).

Next, the manufacturing method of the positive electrode active material for sodium ion batteries is demonstrated. The manufacturing method of the positive electrode active material for sodium ion batteries is Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1. 333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1).

First, iron phosphate represented by Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) A method for producing the compound IHP will be described. FIG. 1 shows phosphorus represented by Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1). It is a flowchart which shows the process of producing | generating IHP which is an iron oxide compound. The step of generating IHP includes a raw material solution preparation step (S10) and a hydrothermal synthesis step (S12).

  The raw material solution preparation step (S10) is a step of preparing a raw material solution by dissolving an iron raw material and a phosphorus raw material in water.

The iron raw material includes, for example, an iron compound containing at least one of Fe (NO ₃ ) ₃ .9H ₂ O, FeC ₂ O ₄ .2H ₂ O and Fe (NH ₄ ) ₂ (SO ₄ ) ₂ .2H ₂ O However, the present invention is not particularly limited to these iron compounds. For the iron raw material, these iron compounds may be used alone or in combination. A general commercial item can be used for these iron compounds.

As the phosphorus raw material, for example, a phosphorus compound such as H ₃ PO ₄ can be used, but is not particularly limited to these phosphorus compounds. A general commercial item can be used for a phosphorus compound.

  An iron raw material and a phosphorus raw material are dissolved in water such as deionized water (ion exchange water) to prepare a raw material solution.

  The hydrothermal synthesis step (S12) is a step of hydrothermal synthesis by heat-treating the prepared raw material solution at 150 ° C. or higher and lower than 300 ° C.

  The prepared raw material solution is sealed in a pressure-resistant sealed container such as a carbon steel vessel, and is hydrothermally synthesized by heating and pressurizing with an induction heating furnace or the like. Since the raw material solution is sealed and sealed in a pressure-resistant sealed container, when heated in an induction heating furnace or the like, it is heated at high pressure and hydrothermally synthesized.

  When enclosing the raw material solution in the pressure-resistant sealed container, it is preferable to enclose the raw material solution while blowing air with an inert gas such as argon gas. The atmosphere in the pressure-resistant sealed container may be an air atmosphere, but is preferably an inert gas atmosphere such as argon gas. The heating furnace is not particularly limited as long as it can be heated at 150 ° C. or more and less than 300 ° C., but a swinging induction heating furnace or a general muffle furnace can be used.

  The heat treatment temperature is preferably 150 ° C. or higher and lower than 300 ° C. This is because it is difficult to generate IHP when the heat treatment temperature is lower than 150 ° C. or when the heat treatment temperature is higher than 300 ° C. The pressure in the pressure sealed container is preferably 2 MPa or more and 50 MPa or less.

  The heat treatment time is preferably 10 minutes to 4 hours. This is because when the heat treatment time is shorter than 10 minutes, the hydrothermal synthesis reaction is not completed, and the yield of IHP may be reduced. This is because when the heat treatment time is longer than 4 hours, the performance of IHP is lowered, and the performance of the sodium ion battery may be lowered.

  The product produced by hydrothermal synthesis is separated from the remaining solution by a centrifuge or the like. The product is washed with deionized water (ion exchange water) or the like, and then dried with a dryer or the like, for example, at 30 ° C. or higher and 150 ° C. or lower. Further, the product may be vacuum dried after being washed with deionized water (ion exchange water) or the like. In this way, IHP is produced in the form of powder or the like.

Next, Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 <a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ A method for producing IHPNa which is an iron phosphate compound represented by d ≦ 1) will be described. The generation method of IHPNa can be generated by four types of generation methods. More specifically, the method of producing IHPNa includes a method of directly synthesizing and producing IHPNa, a method of producing IHPNa by a solid phase reaction method, a method of producing IHPNa by a hydrothermal synthesis method using IHP, There exists the method of producing | generating with an electrochemical method using IHP.

  First, a method for directly synthesizing and generating IHPNa will be described. The method of directly synthesizing and producing IHPNa is different from the above-described method of producing IHP in the raw material solution preparation step. In the raw material solution preparation step, the first sodium raw material, the iron raw material, and the phosphorus raw material are dissolved in water to prepare the raw material solution.

Examples of the first sodium raw material include a sodium compound containing at least one of NaNO ₃ , Na ₂ CO ₃ , NaOH, CH ₃ COONa, NaH ₂ PO ₄ , Na ₃ PO ₄ .12H ₂ O, and Na ₂ HPO ₄ . However, it is not particularly limited to these sodium compounds. As the first sodium raw material, these sodium compounds may be used alone or in combination. A general commercial item can be used for the first sodium raw material. For the iron raw material and the phosphorus raw material, the same raw material as the iron raw material and the phosphorus raw material used in the raw material solution preparation step (S10) can be used.

  A raw material solution is prepared by dissolving a first sodium raw material, an iron raw material, and a phosphorus raw material in water such as deionized water (ion-exchanged water). The first sodium raw material, the iron raw material, and the phosphorus raw material may be mixed so that Na is 2 times or more and 30 times or less in a mass ratio with Fe. Moreover, it is good to mix so that P may become 1 time or more and 10 times or less in the substance amount ratio with Fe. For example, it is preferable that Na, Fe, and P are Na: Fe: P = 10: 1: 1 in terms of the amount of each element. Moreover, about pH of a raw material solution, it is good to set it as pH 4 or less.

  The prepared raw material solution is heat-treated at 150 ° C. or higher and lower than 300 ° C. for hydrothermal synthesis. The hydrothermal synthesis may be hydrothermally synthesized by the same method as in the hydrothermal synthesis step (S12), and detailed description thereof is omitted. In this way, IHPNa is produced in the form of powder or the like.

Next, a method of generating by a solid phase reaction method using IHP will be described. First, powdered IHP and a second sodium raw material are mixed to form a mixture. Powdered IHP can be produced by performing the raw material solution preparation step (S10) and the hydrothermal synthesis step (S12). Examples of the second sodium raw material include a sodium compound containing at least one of NaNO ₃ , Na ₂ CO ₃ , NaOH, CH ₃ COONa, NaH ₂ PO ₄ , Na ₃ PO ₄ .12H ₂ O, and Na ₂ HPO ₄ . However, it is not particularly limited to these sodium compounds. For the second sodium raw material, these sodium compounds may be used alone or in combination. Powdered IHP and the second sodium raw material are mixed to form a pellet-like mixture.

  Next, the mixture is heat-treated at 200 ° C. or higher and 500 ° C. or lower to cause a solid phase reaction. The heat treatment temperature is preferably 200 ° C. or higher and 500 ° C. or lower. This is because it becomes difficult to generate IHPNa when the heat treatment temperature is lower than 200 ° C. or when the heat treatment temperature is higher than 500 ° C.

  The heat treatment time is preferably 10 minutes to 4 hours. This is because when the heat treatment time is shorter than 10 minutes, the solid phase reaction is not completed, and the yield of IHPNa may be reduced. This is because when the heat treatment time is longer than 4 hours, the performance of IHPNa is lowered, and thus the performance of the sodium ion battery may be lowered.

  The mixture may be heat-treated in an inert atmosphere such as argon gas or a reducing atmosphere containing hydrogen gas or the like. The mixture is preferably heat treated in a reducing atmosphere in order to insert more Na into the IHP. The heating furnace is not particularly limited as long as it can be heated at 200 ° C. or more and 500 ° C. or less, but a swinging induction heating furnace or a general muffle furnace can be used. In this way, IHPNa is produced in the form of powder or the like.

Next, the method of producing | generating by a hydrothermal synthesis method using IHP is demonstrated. First, powdered IHP and a third sodium raw material are dissolved in ion-exchanged water to prepare a mixed solution. Powdered IHP can be produced by performing the above raw material solution preparation step (S10) and the hydrothermal synthesis step (S12). The third sodium raw material includes, for example, a sodium compound containing at least one of NaNO ₃ , Na ₂ CO ₃ , NaOH, CH ₃ COONa, NaH ₂ PO ₄ , Na ₃ PO ₄ .12H ₂ O, and Na ₂ HPO ₄ However, it is not particularly limited to these sodium compounds. As the third sodium raw material, these sodium compounds may be used alone or in combination. Powdered IHP and a third sodium raw material are dissolved in water such as deionized water (ion exchange water) to prepare a mixed solution.

  Next, the mixed solution is subjected to hydrothermal synthesis by heat treatment at 150 ° C. or higher and lower than 300 ° C. The prepared mixed solution is sealed together with a reducing agent, for example, in a pressure-tight sealed container such as a carbon steel vessel, and is hydrothermally synthesized by heating and pressurizing in a reducing atmosphere in an induction heating furnace or the like. By heating and pressurizing in a reducing atmosphere, Fe is reduced, Na is inserted, and IHPNa is generated. As the reducing agent, for example, ascorbic acid can be used, but other reducing agents may be used. In this way, IHPNa is produced in the form of powder or the like.

  Next, a method of generating by an electrochemical method using IHP will be described. IHP can be generated by performing the raw material solution preparation step (S10) and the hydrothermal synthesis step (S12). A battery is assembled using IHP as a positive electrode active material and a material containing sodium as a negative electrode active material, and a discharge reaction is caused to electrochemically contain sodium in IHP. Metallic sodium or the like can be used as the material containing sodium serving as the negative electrode active material, but is not particularly limited. Thereby, the insertion reaction of sodium to IHP occurs, and IHPNa is produced.

  Next, the positive electrode for sodium ion batteries will be described.

The positive electrode is Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ a positive electrode active material containing an iron phosphate compound represented by d ≦ 1), a conductive material, a binder, a thickener, and a current collector. For this iron phosphate compound, IHP or IHPNa may be used.

  As the conductive material, ketjen black, acetylene black, graphite, carbon fiber, or the like can be used. As the binder, styrene butadiene rubber, fluorine resin such as polytetrafluoroethylene, polyethylene resin, polypropylene resin, or the like can be used. As the thickener, carboxymethyl cellulose or the like can be used. For the current collector, a metal foil or a metal sheet made of aluminum, stainless steel, copper, nickel, or the like can be used.

Next, the manufacturing method of a positive electrode is demonstrated. First, Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d A positive electrode active material containing an iron phosphate compound represented by ≦ 1), a conductive material, a binder, and a thickener are mixed to produce a slurry. The mixing ratio of the positive electrode active material, the conductive material, the binder, and the thickener is, for example, in mass ratio, positive electrode active material: conductive material: binder: thickener = 80: 10: 6: 4 It is good to do. As a mixing method of these materials, a general mixing method using a ball mill or the like can be used.

  The produced slurry is applied to the surface of a current collector such as a metal foil. The slurry can be applied to the current collector by a general application method using a brush or a spatula. The current collector coated with the slurry is dried by a dryer or the like at 30 ° C. or higher and 150 ° C. or lower, for example. In this way, a positive electrode for a sodium ion battery is manufactured.

  Next, a sodium ion battery will be described. As an example, a sodium ion secondary battery will be described.

The sodium ion secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode is Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ A positive electrode active material containing an iron phosphate compound represented by d ≦ 1) is provided. For this iron phosphate compound, IHP or IHPNa may be used. Examples of the negative electrode include TiO ₂ , Nb-doped TiO ₂ , lithium titanate (Li ₄ Ti ₅ O ₁₂ ), Sn, Si, SiO, SnO, Sn—Cu alloy thin film, Sn—Fe alloy thin film, carbon nanotube, Si— It is composed of Sn—Ti alloy, carbon nanofiber, MoO ₃ , SnO ₂ , P simple substance, metallic sodium, hard carbon, graphite, and Pb. In the case where the positive electrode is made of a positive electrode active material containing IHP, the negative electrode is preferably made of a material containing sodium such as metallic sodium. The electrolyte is composed of, for example, a mixture of a solute 1M NaClO ₄ / solvent propylene carbonate and 2% by volume fluoroethylene carbonate, or the like. The separator is made of, for example, a nonwoven fabric made of glass fiber. The shape of the sodium ion secondary battery can be a coin shape, a cylindrical shape, a square shape, or the like.

As described above, according to the above configuration, the positive electrode active material for a sodium ion _{battery, Na a Fe b (PO 4} ) (OH) c (H 2 O) d ( where, 0 ≦ a ≦ 1.333,1.000 Since the iron phosphate compound represented by ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) is included, the battery performance of the sodium ion battery can be improved. This iron phosphate compound has a stable crystal structure belonging to the space group I4 ₁ / amd represented by the Hermann-Morgan symbol (international symbol), and sodium ions easily move within the crystal. Ions can be easily reversibly inserted and removed, and can function as a positive electrode active material of a sodium ion battery.

Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.) For use as a positive electrode active material for sodium ion batteries. 333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) was generated.

(Generation of INP)
First, iron phosphate represented by Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) The compound IHP was produced. A method for generating IHP according to the first to seventh embodiments will be described.

An iron raw material and a phosphorus raw material were dissolved in ion exchange water to prepare a raw material solution. Fe (NO ₃ ) · 9H ₂ O (purity 99%) was used as the iron raw material. H ₃ PO ₄ (purity 85%) was used as the phosphorus raw material. The concentration of the iron raw material was 0.1 mol / l. In the IHPs of Examples 1 to 7, the heat treatment conditions were changed.

  The raw material solution was put into a pressure-resistant sealed container, and sealed by replacing the air by blowing argon gas. A pressure-resistant sealed container in which the raw material solution was sealed was heated in a rocking induction heating furnace and hydrothermally synthesized. The heating temperature was 150 ° C. or higher and lower than 300 ° C. The heating time was 10 minutes to 4 hours. After the synthesis, only the synthesized product was extracted with a centrifuge. The extracted composition was vacuum dried to obtain powdered IHP.

  The produced IHPs of Examples 1 to 7 were subjected to crystal structure analysis by X-ray diffraction measurement. For X-ray diffraction measurement, a fully automatic horizontal multi-purpose X-ray diffractometer SmartLab (SmartLab) manufactured by Rigaku was used. The X-ray source was Cu, acceleration voltage 45 kV, acceleration current 200 mA, step angle 0.02 degrees. The bending angle 2θ was measured under the condition of 10 ° to 90 °. The crystal structure analysis was performed from the measurement data using the Rietveld method using the Rietan-FP program.

As a representative, a method for analyzing the crystal structure of IHP of Example 3 will be described. FIG. 2 is a graph showing the results of X-ray diffraction measurement of IHP of Example 3. In the graph of FIG. 2, the horizontal axis indicates the diffraction angle 2θ, the vertical axis indicates the intensity, and the measurement data and analysis results are indicated by solid lines. _{Incidentally, R wp} is 7.670, _{R e} is 4.787, S is 1.6023. From this measurement data, the crystal structure was analyzed using the Rietveld method. FIG. 3 is a view showing the crystal structure analysis result of IHP of Example 3. In FIG. 3, the kind of element which comprises IHP of Example 3, the occupation rate of each site | part of each element, and x, y, z coordinate of each element are shown. The chemical composition of the IHP of Example 3 is Fe _1.135 (PO ₄ ) (OH) _0.405 (H ₂ O) where b = 1.135, c = 0.405, and d = 0.595 _. It was found to be ₅₉₅ . The crystal structure of IHP of Example 3 was tetragonal and belonged to the space group I 4 ₁ / amd represented by the Hermann-Morgan symbol (international symbol).

For IHP of other examples, the crystal structure analysis was performed in the same manner as IHP of Example 3. FIG. 4 is a diagram showing the results of crystal structure analysis of IHP in Examples 1 to 7. In Fe _{1 + x} (PO ₄ ) (OH) _3x (H ₂ O) _1-3x , which is the theoretical formula of IHP, _{1 + x} of the Fe amount corresponds to b, 3x of the OH amount corresponds to c, and the amount of H ₂ O 1-3x corresponds to d. In addition, the values of Fe, PO ₄ , O, and H in the IHP of each example indicate the amount of each element. For example, in the case of the IHP of Example 1, the Fe amount is 1.110, the PO ₄ amount is 1.000, the O amount is 5.000, and the H amount is 1.671. In addition, the value of X in the IHP in each example corresponds to x in the IHP theoretical formula. The chemical compositions of the IHPs of Examples 1 to 7 were such that the Fe amount was 1.000 ≦ b ≦ 1.333, the OH amount was 0 ≦ c ≦ 1, and the H ₂ O amount was 0 ≦ d ≦ 1. It was. In addition, the crystal structures of the IHPs of Examples 1 to 7 were tetragonal and belonged to the space group I 4 ₁ / amd represented by the Hermann-Morgan symbol (international symbol).

(INPNa generation)
Next, Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 <a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ IHPNa which is an iron phosphate compound represented by d ≦ 1) was produced. About the production | generation method of IHPNa, it performed by three types of methods, a solid-phase reaction method, a hydrothermal synthesis method, and an electrochemical method.

First, the production method of the solid phase reaction method will be described. IHPNa of Examples 8 to 25 was produced by the solid phase reaction method. Powdered IHP and a sodium raw material were mixed to form a mixture. In Examples 8 to 25, the IHP of Example 3 was used as the powdery IHP. In Examples 8 to 23, NaOH was used as a sodium raw material. In Example 24, Na ₂ CO ₃ was used as the sodium raw material. In Example 25, NaNO ₃ was used as a sodium raw material.

  The mixture was subjected to a solid phase reaction by heating in an argon gas atmosphere or a mixed gas atmosphere of hydrogen gas and argon gas. The concentration of hydrogen gas in the mixed gas was 5% by mass. The synthesis temperature was 200 ° C to 500 ° C. The synthesis time was 2 to 108 hours. In this way, IHPNa of Examples 8 to 25 were obtained.

Next, for the IHPNa of Examples 8 to 25, the amount of Na was measured from the battery capacity. First, a method for obtaining the Na amount from the battery capacity will be described. FIG. 5 is a diagram for explaining a reaction occurring in the positive electrode and the negative electrode of the sodium ion battery. Sodium ions Na ⁺ move by diffusing inside the battery. The electron e ⁻ travels through a circuit outside the battery and becomes a current. From this, when the current value (charge amount) is measured, the amount of sodium ion Na ⁺ that has moved inside the battery can be determined.

More specifically, the number of moles of the transferred electron e ⁻ can be obtained by dividing the amount of charge (As) measured by charging and discharging the battery by the Faraday constant (As / mol). In the battery, since the sodium ion Na ⁺ moves in the same amount as the electron, the calculated number of moles of the electron e ⁻ becomes the number of moles of the sodium ion Na ⁺ moved as it is. Since the number of moles of the material used for the electrode is determined from the electrode weight, the amount of Na in IHPNa is determined.

FIG. 6 is a graph showing the amount of Na in IHPNa of Examples 8 to 25. In FIG. 6, the amount of Na is shown together with the synthesis conditions of Examples 8 to 25. In IHPNa of Examples 8 to 25, it was found that the amount of Na was in the range of 0 <a ≦ 1.333. Further, as a result of analyzing the crystal structure of the IHPNa of Examples 8 to 25 by the X-ray diffraction measurement method in the same manner as the crystal structure analysis of the IHP of Examples 1 to 7, the crystal of the IHPNa of Examples 8 to 25 was obtained. The structure was tetragonal and belonged to the space group I4 ₁ / amd represented by the Hermann-Morgan symbol (international symbol).

Next, a method for producing IHPNa by the hydrothermal synthesis method will be described. IHPNa of Example 26 was produced by a hydrothermal synthesis method. Powdered IHP and a sodium raw material were dissolved in ion-exchanged water to prepare a mixed solution. As powder IHP, IHP of Example 3 was used. NaNO ₃ was used as the sodium raw material.

  The mixed solution and the reducing agent were placed in a pressure-resistant sealed container, and sealed by replacing the air by blowing argon gas. Ascorbic acid was used as the reducing agent. A pressure-resistant sealed container enclosing the mixed solution and the reducing agent was heated in a swinging induction heating furnace and hydrothermally synthesized. The synthesis temperature was 150 ° C. The synthesis time was 20 minutes. After the synthesis, only the synthesized product was extracted with a centrifuge. The extracted composition was vacuum-dried to obtain powdery IHPNa.

Next, for the IHPNa of Example 26, the amount of Na was measured from the battery capacity in the same manner as the IHPNa of Examples 8 to 25. The Na amount of IHPNa of Example 26 was 0.090, and the battery capacity was 12 (mAh / g). From this measurement result, it was found that the amount of Na in the IHPNa of Example 26 was in the range of 0 <a ≦ 1.333. In addition, as a result of analyzing the crystal structure of IHPNa of Example 26 by the X-ray diffraction measurement method in the same manner as the crystal structure analysis of IHP of Examples 1 to 7, the crystal structure of IHPNa of Example 26 is square. And was assigned to the space group I 4 ₁ / amd represented by the Hermann-Morgan symbol (international symbol).

  Next, a generation method using an electrochemical method will be described. The electrochemical method produced IHPNa of Examples 27-34. A battery was assembled using powdered IHP as a positive electrode active material and sodium as a negative electrode active material to cause a discharge reaction. The battery type was a coin cell. Then, by causing a discharge reaction, sodium was inserted into IHP to obtain IHPNa.

  For Examples 27 to 30, the IHP of Example 3 was used. In Examples 31 to 34, IHP synthesized by heating to 150 ° C. in a muffle furnace was used instead of the swing induction heating furnace in contrast to the IHP synthesis method of Example 3. The holding time was 0 minutes to 4 hours.

Next, for the IHPNa of Examples 27 to 34, the amount of Na was measured from the battery capacity in the same manner as the IHPNa of Examples 8 to 25. FIG. 7 is a graph showing the amount of Na of IHPNa in Examples 27 to 34. In FIG. 7, the amount of Na is shown together with the synthesis conditions of IHP used in Examples 27 to 34. In IHPNa of Examples 27 to 34, it was found that the amount of Na was in the range of 0 <a ≦ 1.333. Further, as a result of analyzing the crystal structure of the IHPNa of Examples 27 to 34 by the X-ray diffraction measurement method in the same manner as the crystal structure analysis of the IHP of Examples 1 to 7, the crystal of the IHPNa of Examples 27 to 34 was obtained. The structure was tetragonal and was assigned to the space group I4 ₁ / amd represented by the Hermann-Morgan symbol (international symbol).

(Battery performance evaluation test)
A coin cell was produced as a sodium ion battery, and the charge / discharge characteristics of the coin cell were evaluated. First, a method for producing a coin cell will be described. Two types of coin cells were produced, one using a positive electrode active material made of IHP and one using a positive electrode active material made of IHPNa. As the IHP, the IHP of Example 3 was used. As the IHPNa, the IHPNa of Example 18 was used. The coin cell size was 20 mm in diameter and 3.2 mm in thickness.

  First, the positive electrode of the coin cell will be described. A slurry was prepared by mixing a powdered positive electrode active material, a conductive material, a binder, and a thickener. Ketjen black was used as the conductive material. Styrene butadiene rubber was used as the binder. Carboxymethylcellulose was used as the thickener. Regarding the mixing ratio of the positive electrode active material, the conductive material, the binder, and the thickener, the positive electrode active material: conductive material: binder: thickener = 80: 10: 6: 4 in mass ratio. . Next, slurry was applied to the surface of the aluminum foil and dried at 30 to 120 ° C. to form a positive electrode.

Metal sodium was used for the negative electrode of the coin cell. As the electrolytic solution, a mixed solution of a solute 1M NaClO ₄ / solvent propylene carbonate and 2% by volume of fluoroethylene carbonate was used. A nonwoven fabric made of glass fiber was used for the separator.

  Next, the charge / discharge characteristics of each coin cell were evaluated. The cut voltage was 1.5 V to 4.0 V, the test temperature was 25 ° C., the charge / discharge rate was 0.1 C, and the cycle number was 3 cycles. FIG. 8 is a graph showing charge / discharge characteristics of a coin cell using a positive electrode active material made of IHP. FIG. 9 is a graph showing charge / discharge characteristics of a coin cell using a positive electrode active material made of IHPNa. In the graphs of FIGS. 8 and 9, the horizontal axis represents the capacity during charging and discharging, the vertical axis represents the voltage, the charging case is indicated by a solid line, and the discharging case is indicated by a broken line. 8 and 9 both show the characteristics of the third cycle.

  As shown in FIG. 8, the coin cell using the positive electrode active material made of IHP was reversibly chargeable / dischargeable, and the capacity during charging / discharging was about 150 mAh / g. Further, no plateau (potential flat portion) was observed during charging and discharging. As shown in FIG. 9, the coin cell using the positive electrode active material made of IHPNa was reversibly chargeable / dischargeable, and the capacity during charging / discharging was about 75 mAh / g. Further, no plateau (potential flat portion) was observed during charging and discharging. From this result, it was found that IHP and IHPNa have high performance as a positive electrode active material of a sodium ion battery. In addition, when a positive electrode active material made of IHP or IHPNa is used, there is no plateau (potential flat portion) during charging / discharging, so the potential required for sodium insertion changes continuously, so the battery charge rate It was found that the estimation accuracy of was improved.

  Next, the charge / discharge cycle characteristics of the coin cell using the positive electrode active material made of IHP of Example 3 were evaluated. The cut voltage was 1.5 V to 4.0 V, the test temperature was 25 ° C., the charge / discharge rate was 0.1 C, and the cycle number was 100 times. FIG. 10 is a graph showing charge / discharge cycle characteristics. In the graph of FIG. 10, the horizontal axis represents the number of cycles, the vertical axis represents the discharge capacity, and the change in the discharge capacity is represented by a solid line. While the initial discharge capacity was 110.6 (mAh / g), the discharge capacity after 100 cycles was 71.8 (mAh / g), and the maintenance rate was 64.9%. It was. From this result, it was clarified that the coin cell using the positive electrode active material made of IHP of Example 3 can be repeatedly charged and discharged even when the number of cycles is 100.

  Regarding the coin cell using the positive electrode active material made of IHP of Example 3, the structural change of the positive electrode active material during charge and discharge was evaluated. First, the structural change evaluation method will be described. The charge / discharge states were four types: 0% discharge state (100% charge state), 50% discharge state, 70% discharge state, and 100% discharge state (0% charge state). The structural change of the positive electrode active material was evaluated by X-ray diffraction measurement. For X-ray diffraction measurement, a fully automatic horizontal multi-purpose X-ray diffractometer SmartLab (SmartLab) manufactured by Rigaku was used. The X-ray source was Cu, acceleration voltage 45 kV, acceleration current 200 mA, step angle 0.02 degrees. The bending angle 2θ was measured under the condition of 20 ° to 30 °.

FIG. 11 is a graph showing the X-ray diffraction measurement results of the positive electrode active material in each discharge state. The horizontal axis represents the diffraction angle 2θ, the vertical axis represents the intensity (arbitrary intensity), and the measurement data is indicated by a solid line. In each discharge state, two peaks, peak (a) and peak (b), were observed. Moreover, the intensity of the peak (a) was larger than the intensity of the peak (b). More specifically, in the 0% discharge state (100% charge state), the peak (a) is 2θ = 26.55 ° or more and 26.95 ° or less, and the peak (b) is 2θ = 27.80 °. It was above 28.20 °. In the 100% discharge state (0% charge state), the peak (a) is 2θ = 26.05 ° or more and 26.45 ° or less, and the peak (b) is 2θ = 27.15 ° or more and 27.55 °. It was the following. Moreover, the peak (a) and the peak (b) in the 50% discharge state and the 70% discharge state are a 0% discharge state (100% charge state), a 100% discharge state (0% charge state), and It was found that the peak (a) and the peak (b) were respectively in the middle. From this result, the iron phosphate compound represented by Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d is an XRD measurement (X-ray diffraction measurement) using CuKα rays as an X-ray source. The first highest intensity peak in the range of 2θ = 26.05 ° to 26.95 ° and the second highest intensity in the range of 2θ = 27.15 ° to 28.20 °. It became clear that there was a peak.

S10 Raw material solution preparation process S12 Hydrothermal synthesis process

Claims (18)

A positive electrode active material for a sodium ion battery,
Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) A positive electrode active material comprising an iron phosphate compound represented by: The positive electrode active material according to claim 1,
The positive electrode active material, wherein the iron phosphate compound satisfies c + d = 1. The positive electrode active material according to claim 2,
The positive electrode active material, wherein the iron phosphate compound satisfies b-1 = c / 3 = (1-d) / 3. A positive electrode active material for a sodium ion battery,
An iron phosphate compound represented by Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d ,
In the XRD measurement using the CuKα ray as an X-ray source, the iron phosphate compound has a first peak with the highest intensity in a range of 2θ = 26.05 ° to 26.95 °, and 2θ = 27.15. A positive electrode active material characterized in that there is a second peak having the second highest intensity in the range of from ° to 28.20 °. A positive electrode active material according to any one of claims 1 to 4,
The positive electrode active material, wherein the crystal structure of the iron phosphate compound belongs to a space group I4 ₁ / amd.   A positive electrode comprising the positive electrode active material according to claim 1.   A sodium ion battery comprising the positive electrode according to claim 6. A method for producing a positive electrode active material for a sodium ion battery,
A raw material solution preparation step of preparing a raw material solution by dissolving an iron raw material and a phosphorus raw material in water;
A hydrothermal synthesis step of hydrothermally synthesizing the raw material solution by heat treatment at 150 ° C. or higher and lower than 300 ° C .;
A method for producing a positive electrode active material comprising: A method for producing a positive electrode active material for a sodium ion battery according to claim 8,
The iron raw material includes at least one of Fe (NO ₃ ) ₃ · 9H ₂ O, FeC ₂ O ₄ · 2H ₂ O, Fe (NH ₄ ) ₂ (SO ₄ ) ₂ · 4H ₂ O,
The method for producing a positive electrode active material, wherein the phosphorus raw material contains H ₃ PO ₄ . It is a manufacturing method of the positive electrode active material of Claim 8 or 9, Comprising:
The said raw material solution preparation process contains the 1st sodium raw material in the said raw material solution, The manufacturing method of the positive electrode active material characterized by the above-mentioned. It is a manufacturing method of the positive electrode active material according to claim 10,
The first sodium raw material contains at least one of NaNO ₃ , Na ₂ CO ₃ , NaOH, CH ₃ COONa, NaH ₂ PO ₄ , Na ₃ PO ₄ .12H ₂ O, and Na ₂ HPO _4. A method for producing a positive electrode active material. A method for producing a positive electrode active material according to any one of claims 8 to 11,
The mixture hydrothermally synthesized in the hydrothermal synthesis step and the second sodium raw material are mixed to form a mixture, and the mixture is heat-treated at 150 ° C. or higher and 500 ° C. or lower to cause a solid phase reaction. A method for producing a positive electrode active material. It is a manufacturing method of the positive electrode active material of Claim 12, Comprising:
The second sodium raw material includes at least one of NaNO ₃ , Na ₂ CO ₃ , NaOH, CH ₃ COONa, NaH ₂ PO ₄ , Na ₃ PO ₄ .12H ₂ O, and Na ₂ HPO _4. A method for producing a positive electrode active material. It is a manufacturing method of the positive electrode active material of Claim 13, Comprising:
A method for producing a positive electrode active material, wherein the mixture is subjected to a solid phase reaction by heat treatment in a reducing atmosphere. A method for producing a positive electrode active material according to any one of claims 8 to 11,
Hydrolyzed synthesis is performed by dissolving the synthesized product hydrothermally synthesized in the hydrothermal synthesis step and the third sodium raw material in water to prepare a mixed solution, and heat-treating the mixed solution at 150 ° C. or higher and lower than 300 ° C. And a method for producing a positive electrode active material. It is a manufacturing method of the positive electrode active material of Claim 15, Comprising:
The third sodium raw material includes at least one of NaNO ₃ , Na ₂ CO ₃ , NaOH, CH ₃ COONa, NaH ₂ PO ₄ , Na ₃ PO ₄ .12H ₂ O, and Na ₂ HPO _4. A method for producing a positive electrode active material. It is a manufacturing method of the positive electrode active material of Claim 15 or 16,
A method for producing a positive electrode active material, wherein the mixed solution is heat-treated in a reducing atmosphere and hydrothermally synthesized. A method for producing a positive electrode active material according to any one of claims 8 to 11,
The composite synthesized hydrothermally in the hydrothermal synthesis step is used as a positive electrode active material, a material containing sodium is used as a negative electrode active material, a battery is assembled, and a discharge reaction is caused. A method for producing a positive electrode active material, comprising:

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