JP7676820B2 - Sodium Transition Metal Polyanions - Google Patents

Description

This disclosure relates to novel sodium transition metal polyanions and methods for producing them.

Sodium secondary batteries are attracting attention as post-lithium secondary batteries because they do not use lithium, a rare metal. Sodium transition metal-containing materials that contain sodium and a counter anion such as a transition metal oxide, polyanion, or fluoride are being considered as positive electrode active materials for sodium secondary batteries. Among sodium transition metal-containing materials, sodium-containing transition metal polyanions are expected to exhibit high charge/discharge capacity.

For example, it has been reported that sodium iron phosphate (NaFePO ₄ ), a sodium transition metal phosphate compound having an olivine structure, undergoes a reversible insertion/desorption reaction of Na cations (see, for example, Patent Document 1). However, the discharge capacity obtained is only low, at about 60 mAh/g.

JP 2009-206085 A

The present disclosure aims to provide at least one of a novel sodium transition metal polyanion, a positive electrode active material and a sodium secondary battery containing the same, and a method for producing the same.

In this disclosure, we have discovered a new sodium transition metal polyanion that can be used as a positive electrode active material in sodium secondary batteries, and further discovered that a sodium secondary battery using this sodium transition metal polyanion as a positive electrode material exhibits a higher discharge capacity than conventional sodium secondary batteries.

That is, the present invention is as described in the claims, and the gist of the present disclosure is as follows.
[1] A sodium transition metal polyanion having a powder X-ray diffraction pattern represented by the general formula _Na3M (OH)( _HPO4 )( _PO4 ) (wherein M is one or more selected from the group consisting of Fe, Mn, Ni, and Co) and having a monoclinic crystal structure.
[2] The sodium transition metal polyanion according to the above [1], wherein the monoclinic crystal is a monoclinic crystal belonging to the space group C2/m.
[3] The sodium transition metal polyanion according to the above [ ₁ ] or [2], in which the proportion of the powder X-ray diffraction pattern represented by the general formula Na3M(OH)( _HPO4 )( _PO4 ) (wherein M is one or more selected from the group consisting of Fe, Mn, Ni and Co) in the powder X-ray diffraction pattern of the sodium transition metal polyanion is 80% or more.
[4] The sodium transition metal polyanion according to any one of [1] to [3] above, wherein the lattice constants are a, 15.35 Å to 15.55 Å, b, 7.19 Å to 7.35 Å, c, 6.95 Å to 7.12 Å, and β, 95.8° to 97.5°.
[5] The sodium transition metal polyanion according to any one of [1] to [4] above, which comprises a coating layer.
[6] A method for producing a sodium transition metal polyanion according to any one of [1] to [5] above, comprising a step of subjecting a composition containing a sodium source, a transition metal source, a phosphate source, and water and having a pH of 4.0 or more and 11.0 or less to hydrothermal treatment at 80° C. or more and 180° C. or less.
[7] The method according to the above [6], wherein the pH is 6.0 or more and 9.0 or less.
[8] The method according to the above [6] or [7], wherein the transition metal source is one or more selected from the group consisting of oxides, hydroxides, oxyhydroxides, phosphates, sulfites, sulfates, nitrates, chlorides, acetates, bromides and fluorides containing one or more selected from the group consisting of Fe, Mn, Ni and Co.
[9] A positive electrode active material comprising the sodium transition metal polyanion according to any one of [1] to [5] above.
[10] A sodium secondary battery comprising a positive electrode containing the sodium transition metal polyanion according to any one of [1] to [5] above, a negative electrode, and an electrolyte.
[11] The sodium secondary battery according to the above [10], wherein the negative electrode contains at least NaTi ₂ (PO ₄ ) _3. [12] The sodium secondary battery according to the above [10] or [11], wherein the electrolyte contains at least NaClO ₄ .
[13] The sodium secondary battery according to any one of the above [10] to [12], wherein the electrolytic solution is an aqueous electrolytic solution.
[14] The sodium secondary battery according to any one of the above [10] to [12], wherein the electrolytic solution is a non-aqueous electrolytic solution.

The present disclosure provides at least one of a novel sodium transition metal polyanion, a positive electrode active material containing the same, a sodium secondary battery, and a method for producing the same.

Powder X-ray diffraction pattern of the sodium iron polyanion of Example 1. Powder X-ray diffraction pattern of the sodium iron polyanion of Example 2. Powder X-ray diffraction pattern of the sodium iron polyanion of Example 3. FIG. 1 shows the discharge capacity profile of a nonaqueous sodium secondary battery having the sodium iron polyanion of Example 1 as a positive electrode active material in Measurement Example 1 (in the figure, the solid line shows the discharge capacity at the first cycle, and the dashed line shows the discharge capacity at the tenth cycle). Discharge curve (first cycle) of an aqueous sodium secondary battery including NaTP of Example 1 in Measurement Example 2 Discharge curve (first cycle) of an aqueous sodium secondary battery including NaTP of Example 3 in Measurement Example 3

Sodium Transition Metal Polyanions
The sodium transition metal polyanion of the present disclosure will be described below with reference to an example of an embodiment.

The sodium transition metal polyanion of this embodiment is a sodium transition metal polyanion characterized by having a powder X-ray diffraction pattern represented by the general formula _Na3M (OH)( _HPO4 )( _PO4 ) (M is one or more selected from the group consisting of Fe, Mn, Ni, and Co) and having a monoclinic crystal structure.

In this embodiment, the "sodium transition metal polyanion (hereinafter also referred to as "NaTP" and, when the transition metal is iron, also referred to as "sodium iron polyanion" or "NaFeP")" is a compound consisting of sodium (Na), a transition metal, and two or more anions, and is preferably an artificially synthesized compound, i.e., a synthetic sodium transition metal polyanion.

The transition metal contained in NaTP of this embodiment is one or more selected from the group of iron (Fe), manganese (Mn), nickel (Ni) and cobalt (Co), and is preferably one or more selected from the group of iron, manganese and nickel, further at least one of iron and manganese, or further iron. NaTP of this embodiment preferably contains iron, and may contain one or more selected from the group of manganese, nickel and cobalt and iron, or may contain at least one of manganese and cobalt and iron. Furthermore, NaTP of this embodiment may contain at least one or more selected from the group of manganese, nickel and cobalt. The anion contains at least phosphate ion (PO ₄ ³⁻ ) and hydroxide ion (OH ⁻ ), and is preferably phosphate ion, hydrogen phosphate ion (HPO ₄ ²⁻ ) and hydroxide ion (OH ⁻ ).

The NaTP of this embodiment preferably does not contain one or more anions selected from the group consisting of chloride ions (Cl ⁻ ), bromide ions (Br ⁻ ), sulfate ions (SO ₄ ²⁻ ), and nitrate ions (NO ₃ ⁻ ), and more preferably does not contain chloride ions.

The NaTP of this embodiment has a powder X-ray diffraction pattern (hereinafter also referred to as "XRD pattern") represented by the general formula _Na3M (OH)( _HPO4 )( _PO4 ) (M is one or more selected from the group consisting of Fe, Mn, Ni, and Co). The XRD pattern may be measured using a general powder X-ray diffractometer. The following conditions can be exemplified as preferred measurement conditions for the XRD pattern.
Radiation source: CuKα radiation (λ=1.5405Å)
Measurement mode: Step scan
Scan condition: 20°/min
Measurement time: 3 seconds
2θ: 5° to 90°

The presence and the proportion of the XRD pattern (hereinafter also referred to as the "main XRD pattern") represented by the general formula _Na3M (OH)( _HPO4 )( _PO4 ) (M is one or more selected from the group consisting of Fe, Mn, Ni, and Co) can be confirmed by comparing (fitting) the main XRD pattern (hereinafter also referred to as the "simulation pattern") obtained by simulation with the measured XRD pattern of NaTP of this embodiment by Rietveld analysis. The simulation pattern may be an XRD pattern obtained by calculation of a compound in which V in _Na3V (OH)( _HPO4 )( _PO4 ) is replaced with any transition metal (M, i.e., one or more selected from the group consisting of Fe, Mn, Ni, and Co). As a preferred simulation pattern to be used in fitting NaTP of this embodiment, the simulation pattern described in Inorg. Chem. , 2008, 47, 10062-10066, the crystal data (Table ₁ ) and the coordination numbers and atomic arrangements of the constituent elements (Table 2) (hereinafter also referred to as "reference documents") of Na ₃ V(OH)(HPO ₄ )(PO 4 ) are exemplified by an XRD pattern obtained by substituting Fe for V in the crystal data (Table 1) and the coordination numbers and atomic arrangements of the constituent elements (Table 2) (hereinafter also referred to as "reference documents"). In this embodiment, the XRD pattern can be used as a simulation pattern in fitting of NaTP containing at least Fe.

In the case of NaTP of this embodiment, "having a main XRD pattern" means that the main XRD pattern can be confirmed in the XRD pattern obtained by XRD measurement of NaTP of this embodiment, i.e., the main XRD pattern is included.

In the present embodiment, the proportion of the main XRD pattern in the XRD pattern of NaTP (hereinafter also referred to as "purity") is 80% or more, preferably 85% or more, more preferably 90% or more, and even more preferably 99% or more. When the XRD pattern of NaTP of the present embodiment is only the main XRD pattern, the purity is 100%. It is permissible for NaTP of the present embodiment to contain a small amount of by-product phase, that is, a by-product phase to an extent that does not inhibit the desorption and insertion of sodium. Therefore, the purity may be 100% or less, or even less than 100%. The purity in the present embodiment can be determined by the reference intensity ratio (hereinafter also referred to as "RIR") method. Specifically, the purity can be determined by the ratio of area intensities in the comparison between the XRD pattern and the simulation pattern by Rietveld analysis.

The NaTP of this embodiment has a monoclinic crystal structure, and is preferably a monoclinic crystal belonging to the space group C2/m. A monoclinic crystal is a crystal structure in which the lattice constants a, b, and c are all different values, α and γ are all 90°, and β is a different value (i.e., other than 90°).

The lattice constants of NaTP in this embodiment are a 15.35 Å to 15.55 Å, preferably 15.40 Å to 15.50 Å, b 7.19 Å to 7.35 Å, preferably 7.20 Å to 7.30 Å, and c 6.95 Å to 7.12 Å, preferably 7.00 Å to 7.10 Å, and β 95.8° to 97.5°, preferably 96.3° to 97.0°. For example, the lattice constants of NaFeP (sodium iron polyanion) in this embodiment are a 15.45 Å to 15.55 Å, b 7.27 Å to 7.35 Å, c 7.04 Å to 7.12 Å, and β 95.9° to 96.7°. In addition, examples of the lattice constants of NaFeMnP (sodium iron manganese polyanion) of this embodiment include a of 15.50 Å to 15.55 Å, b of 7.30 Å to 7.35 Å, c of 7.05 Å to 7.12 Å, and β of 95.8° to 96.5°. Examples of the lattice constants of NaFeCoP (sodium iron cobalt polyanion) of this embodiment include a of 15.50 Å to 15.55 Å, b of 7.30 Å to 7.40 Å, c of 7.04 Å to 7.12 Å, and β of 95.9° to 96.7°.

The NaTP of this embodiment has both this purity and crystal structure, which makes it easier for sodium ions to be inserted and removed compared to conventional NaTP, and is therefore expected to exhibit high charge/discharge characteristics.

The crystallite diameter of the NaTP of this embodiment is preferably 20 Å or more and 1500 Å or less, and for example, the crystallite diameter of NaTP containing Fe is 20 Å or more or 30 Å or more, and 200 Å or less, 150 Å or less, or 100 Å or less. With a purity of 80% or more and a crystallite diameter in this range, the NaTP of this embodiment is more likely to facilitate efficient insertion and desorption of sodium, and is expected to exhibit high charge and discharge capacity as a positive electrode active material for sodium secondary batteries.

In this embodiment, the "crystallite size" is the size (hereinafter also referred to as " _WH size") determined by the Williamson-Hall method from two or more XRD peaks belonging to the XRD pattern of the general formula Na ₃ M (OH) (HPO 4 ) (PO ₄ ) (M is one or more selected from the group consisting of Fe, Mn, Ni, and Co). Specifically, the following plots are made for two or more XRD peaks that can be assigned to the general formula Na ₃ M (OH) (HPO ₄ ) (PO ₄ ) (M is one or more selected from the group consisting of Fe, Mn, Ni, and Co), preferably all XRD peaks that can be assigned to the general formula Na ₃ M (OH) (HPO ₄ ) (PO ₄ ) (M is one or more selected from the group consisting of Fe, Mn, Ni, and Co). The following linear approximation equation is determined by the least squares method of the obtained plot of multiple points, and the reciprocal of the y-intercept of the linear approximation equation is the crystallite size.

＜Plot＞
Y=(β・sinθ)/λ
X = sin θ / λ
<First-order approximation formula>
Y=2η・X+(1/ε) ...(1 formula)
In these formulas, β is half-width (°), θ is diffraction angle (°), λ is the wavelength of radiation source (nm), η is non-uniform distortion, and ε is crystallite diameter (Å), and 1/ε in the first-order approximation formula is y-intercept. In addition, in the present embodiment, there is no particular limitation on the XRD peak used for calculating WH diameter, and for example, all XRD peaks that belong to NaTP may be used.

The NaTP of this embodiment may have one or more layers (hereinafter also referred to as "coating layers") selected from the group consisting of metals, fluorine, fluoride, phosphorus, phosphorus compounds, metal oxides, carbon, compounds containing carbon, and conductive polymers on a part or all of the surface. The coating layer tends to increase the conductivity of NaTP. The coating layer is preferably one or more layers selected from the group consisting of compounds containing carbon, carbon, and conductive polymers, and is more preferably a carbon layer. The coating layer does not have to cover the entire surface of NaTP.

The shape of the NaTP in this embodiment may be any shape, and may be one or more selected from the group consisting of powder, granules, and molded body, with powder being preferred.

Since the NaTP of this embodiment easily undergoes insertion and desorption of sodium ions (Na ⁺ ), it can be used as an adsorbent, ion exchanger, hydrogen ion conductor, solid electrolyte, and battery material, and further can be used as an active material for electrochemical devices, in particular as a positive electrode active material for sodium secondary batteries, and further as a positive electrode active material for aqueous sodium secondary batteries.
Method for Producing Sodium Transition Metal Polyanions
The method for producing the sodium transition metal polyanion of this embodiment will be described below.

The NaTP of this embodiment can be obtained by a production method characterized by comprising a step of subjecting a composition containing a sodium source, a transition metal source, a phosphate source, and water and having a pH of 4.0 to 11.0 to hydrothermal treatment at 80°C to 180°C.

By subjecting a composition (hereinafter also referred to as the "raw material composition") that contains a sodium source, a transition metal source, a phosphate source, and water and has a pH of 4.0 or more and 11.0 or less to a hydrothermal treatment process (hereinafter also referred to as the "hydrothermal treatment process"), it is possible to significantly suppress the generation of by-product phases and obtain the NaTP of this embodiment.

The sodium source is at least one of a compound containing sodium (Na) or sodium, and is preferably a water-soluble sodium salt. Preferred sodium sources include one or more selected from the group consisting of sodium fluoride, sodium chloride, sodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium sulfite, sodium sulfate, sodium bicarbonate, and sodium carbonate, or one or more selected from the group consisting of sodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium sulfite, sodium sulfate, sodium bicarbonate, and sodium carbonate, and further one or more selected from the group consisting of sodium phosphate, sodium hydrogen phosphate, and disodium hydrogen phosphate.

The phosphate source is a compound containing phosphate ions, and is preferably at least one of water-soluble phosphoric acid and phosphate salts. Preferred phosphate sources include one or more selected from the group consisting of orthophosphoric acid, sodium phosphate, disodium hydrogen phosphate, ammonium phosphate, lithium phosphate, and potassium phosphate, and further, one or more selected from the group consisting of orthophosphoric acid, sodium phosphate, sodium hydrogen phosphate, and disodium hydrogen phosphate.

The raw material composition preferably contains sodium phosphate as a phosphoric acid and sodium source.

The transition metal source is a compound containing one or more selected from the group consisting of iron (Fe), manganese (Mn), nickel (Ni) and cobalt (Co), a compound containing at least one of iron, manganese and nickel, a compound containing at least one of iron and manganese, or a compound containing iron. The transition metal source is preferably one or more selected from the group consisting of oxides, hydroxides, oxyhydroxides, sulfites, sulfates, phosphates, carbonates, nitrates, chlorides and acetates containing one or more selected from the group consisting of iron, manganese, nickel and cobalt, more preferably at least one of sulfates, phosphates and carbonates, and more preferably phosphates.

The transition metal contained in the transition metal source is one or more selected from the group consisting of iron, manganese, nickel and cobalt, preferably one or more selected from the group consisting of iron, manganese and nickel, preferably at least one of iron or manganese, more preferably iron.

The raw material composition preferably contains one or more transition metal phosphates as a phosphate source and a transition metal source.

Examples of water include pure water, ion-exchanged water, and water derived from other raw materials (e.g., hydrates, structural water, or solvents in aqueous solutions).

The raw material composition may contain additives in addition to the sodium source, transition metal source, and phosphate source raw materials, and water. The additives may be one or more selected from the group consisting of oxalic acid, ascorbic acid, sulfurous acid, sodium sulfite, thiosulfate, sodium thiosulfate, sodium hydrogen monosulfide, hydrazine, and sodium hypochlorite, or one or more selected from the group consisting of ascorbic acid, sulfurous acid, sodium sulfite, thiosulfate, sodium thiosulfate, sodium hydrogen monosulfide, and sodium hypochlorite.

The pH of the raw material composition is 4.0 or more and 11.0 or less, preferably 5.0 or more and 10.0 or less, more preferably 6.0 or more and 9.0 or less, and even more preferably 6.5 or more and 8.5 or less.

The raw material composition is obtained by mixing the raw materials so that the composition is uniform, and is obtained by mixing these raw materials, water, and additives as necessary, by any method.

The hydrothermal treatment is preferably carried out by filling the raw material composition into a reaction vessel. There are no particular limitations on the reaction vessel as long as it can be used in the raw material composition and hydrothermal treatment process used in this embodiment, and an example of such a reaction vessel is a sealed pressure-resistant vessel whose inner surface is coated with Teflon resin or fluororesin.

The hydrothermal treatment temperature is 80°C or higher and 180°C or lower, preferably 90°C or higher and 170°C or lower, and more preferably 100°C or higher and 160°C or lower. Furthermore, when the pH of the raw material composition is 6.0 or higher and 9.0 or lower, the hydrothermal treatment temperature is preferably 100°C or higher and 180°C or lower, and even more preferably 100°C or higher and 150°C or lower.

Preferred hydrothermal treatment conditions, particularly for raw material compositions containing iron, include a hydrothermal treatment temperature of 100°C or higher and 180°C or lower, and a pH of greater than 7.5 and less than 8.5.

The duration of the hydrothermal treatment depends on the hydrothermal treatment temperature. For example, when the hydrothermal treatment temperature is 100°C or lower, the duration can be from 10 hours to 200 hours. When the hydrothermal treatment temperature exceeds 100°C, the duration can be from 3 hours to 100 hours. When the hydrothermal treatment temperature is 120°C or higher or 130°C or higher, crystallization is further promoted, and crystallization of NaTP of this embodiment can occur in 20 hours or less.

The manufacturing method of this embodiment may include at least one step selected from the group consisting of a washing step, a separation step, and a drying step.

The washing process is a process for removing impurities from NaTP. Any washing method can be used, but an example is a process in which NaTP is dispersed in a sufficient amount of pure water. This process may be carried out multiple times.

The separation process is a process for separating NaTP from the solvent. Any separation method may be used, and examples of the method include one or more methods selected from the group consisting of centrifugal sedimentation, filtration, and filter press.

The drying process is a process for removing moisture adsorbed to NaTP. Any drying method can be used, including treatment in air or in vacuum at 80°C to 120°C for 1 hour to 24 hours, and treatment in vacuum is preferred.

The manufacturing method of this embodiment may have a coating step of forming a coating layer on NaTP. The method of forming a coating layer on NaTP is not particularly limited, but for example, the method of mechanically milling the precursor of the coating layer and NaTP, or the method of pyrolysis can be exemplified. In addition, when forming a carbon layer as a coating layer on the surface of NaTP, the method of mechanically milling acetylene black and NaTP under an inert atmosphere, or the method of mixing NaTP and sucrose (C ₁₂ H ₂₂ O ₁₁ ) and then performing heat treatment under an inert atmosphere can be exemplified.

<Cathode active material>
Next, a positive electrode active material containing the sodium transition metal polyanion of the present disclosure will be described with reference to an example of an embodiment.
In this embodiment, the "positive electrode active material" refers to an electrode active material of an electrode having a higher potential among the electrodes constituting an electrochemical device, and in particular, an electrode active material of a positive electrode of a sodium secondary battery.

The positive electrode active material of this embodiment may contain only the NaTP of this embodiment, or may contain a sodium transition metal compound other than the NaTP of this embodiment. The mass ratio of NaTP to the mass of the positive electrode active material of this embodiment is 80 mass% or more and 100 mass% or less, preferably 90 mass% or more and 100 mass% or less, and more preferably 100 mass% (the positive electrode active material is only NaTP).

The positive electrode active material of this embodiment may have a coating layer, such as a carbon layer, on a part or the whole of the surface of the positive electrode active material, preferably a coating layer having electrical conductivity.
<Sodium secondary battery>
Next, a sodium secondary battery comprising a positive electrode containing the sodium transition metal polyanion of the present disclosure, a negative electrode, and an electrolyte solution will be described with reference to an example of an embodiment.

In this embodiment, a "sodium secondary battery" is an electrochemical device in which charging and discharging occurs by the insertion and removal of sodium ions (Na ⁺ ), and is synonymous with a sodium secondary battery, a sodium ion secondary battery, a sodium ion battery, a sodium storage battery, a Na secondary battery, a Na ion battery, or a Na storage battery.

A "nonaqueous electrolyte" is an electrolyte that contains a nonaqueous solvent as a solvent, and an "aqueous electrolyte" is an electrolyte that contains an aqueous solvent as a solvent.

A "non-aqueous sodium secondary battery" is a sodium secondary battery that uses a non-aqueous electrolyte as the electrolyte, and an "aqueous sodium secondary battery" is a sodium secondary battery that uses a non-aqueous electrolyte as the electrolyte.

<Positive electrode>
The positive electrode may include a positive electrode mixture containing the positive electrode active material including NaTP of the present disclosure, and a current collector.
The positive electrode mixture contains a positive electrode active material, a binder, a conductive material, and, if necessary, an additive. The binder, the conductive material, and the additive may each be a known material.

The binder may be one or more selected from the group consisting of fluororesin, polyethylene, polypropylene, SBR material, and imide material, as well as one or more selected from the group consisting of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and ethylene tetrafluoroethylene (ETFE).

The conductive material may be one or more selected from the group consisting of carbon materials, conductive fibers such as metal fibers, metal powders such as copper, silver, nickel, and aluminum, and organic conductive materials such as porphenylene derivatives. Preferred carbon materials include graphite, soft carbon, hard carbon, carbon black, ketjen black, acetylene black, graphite, activated carbon, carbon nanotubes, carbon fibers, and mesoporous carbon.

The positive electrode mixture may be produced by a known method, by mixing the positive electrode active material, binder, and conductive material in the desired ratio.

<Negative electrode>
The negative electrode may include a negative electrode mixture containing a negative electrode active material, a current collector, and, if necessary, an additive.
The negative electrode mixture contains a negative electrode active material, a binder, a conductive material, and, if necessary, an additive.

The negative electrode active material may contain a material that does not prevent the insertion and removal of sodium ions from the positive electrode active material, and may be one or more selected from the group consisting of platinum, zinc, carbon materials, materials that form an alloy with sodium, sodium-containing transition metal oxides, and sodium-containing polyanion materials. Preferred negative electrode active materials include one or more selected from the group consisting of carbon materials, polyimides, transition metal-containing cyano compounds, and transition metal-containing polyanion compounds, and are preferably one or more selected from the group consisting of activated carbon, Na ₂ Mn [Mn (CN) ₆ ], and NaTi ₂ (PO ₄ ) ₃ , and more preferably NaTi ₂ (PO ₄ ) ₃ .

The binder and conductive material may be any known material, and are the same as the binder and conductive material that can be used in the positive electrode mixture described above.

The negative electrode mixture may be produced by a known method, by mixing the negative electrode active material, binder, and conductive material in the desired ratio.

<Electrolyte>
The electrolyte is either a non-aqueous electrolyte or an aqueous electrolyte, and is preferably an aqueous electrolyte.
The electrolyte is a sodium salt, and _a soluble sodium salt is preferable. Preferred electrolytes include one or more selected from the group consisting of NaCl, _Na2SO4 , _NaNO3 , _NaClO4 , NaOH, and _Na2S . From the viewpoint of ease of handling, the electrolyte is preferably one or more selected from the group consisting of NaCl, _Na2SO4 , _{NaNO3 ,} and _NaClO4 , and preferably contains _NaClO4 .

There are no particular limitations on the electrolyte concentration in the electrolyte solution, but from the perspective of increasing the energy density of the sodium secondary battery, it is preferable that the electrolyte concentration (sodium salt concentration) in the electrolyte solution is high, and an example of a sodium salt concentration is a concentration of 1 mol/kg (1 m) or more and less than the saturated solubility.

The electrolyte may contain an additive. The additive is not particularly limited, but may be one or more selected from the group consisting of succinic acid, glutamic acid, maleic acid, citraconic acid, gluconic acid, itaconic acid, diglycol, cyclohexanedicarboxylic acid, cyclopentanetetracarboxylic acid, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, sulfolane, dimethyl sulfone, and N,N-dimethylmethanesulfonamide. The content of the additive may be, for example, 0.01% by mass or more and 10% by mass or less as a mass ratio of the additive to the mass of the electrolyte.
<Other configurations>
Other components such as the current collectors of the positive and negative electrodes and the separator may be any known components used in sodium secondary batteries or lithium secondary batteries.

Another embodiment of the sodium secondary battery of the present disclosure is a battery comprising a positive electrode containing a sodium transition metal polyanion represented by the general formula Na ₃ M(OH)(HPO ₄ )(PO ₄ ) (M is one or more selected from the group consisting of Fe, Mn, Ni, and Co), a negative electrode, and an electrolyte solution;
The negative electrode comprises at least _NaTi2 ( _PO4 ) ₃ ;
A sodium secondary battery in which the electrolyte contains at least _NaClO4 .

Another embodiment of the sodium secondary battery of the present disclosure is a sodium secondary battery comprising a positive electrode containing a sodium transition metal polyanion represented by the general formula _Na3M (OH)( _HPO4 )( _PO4 ) (M is one or more selected from the group consisting of Fe, Mn, Ni, and Co), a negative electrode, and an electrolyte, wherein the negative electrode contains at least _NaTi2 ( _PO4 ) ₃ , and the electrolyte is an aqueous electrolyte containing at least _NaClO4 .

A sodium secondary battery having a positive electrode containing NaTP of this embodiment exhibits a higher charge/discharge capacity than conventional sodium secondary batteries, particularly conventional aqueous sodium secondary batteries.

The present disclosure will be specifically described below with reference to examples. However, the present disclosure is not limited to these examples.
<Crystal phase identification and composition analysis>
The crystal phase of NaTP obtained in the examples was measured by powder X-ray diffraction. Powder X-ray diffraction was measured using a general X-ray diffraction device (device name: SmartLab, manufactured by Rigaku Corporation) under the following conditions.

Radiation source: CuKα radiation (λ=1.5405Å)
Measurement mode: Step scan
Scan condition: 20°/min
Measurement time: 3 seconds
Measurement range: 2θ=5° to 90°
The composition ratio of transition metals and sodium metal was determined by Rietvelt analysis using data processing software (product name: PDXL-2) attached to the X-ray diffractometer, and the crystal phase of the product was identified. The simulation pattern in the Rietvelt analysis was identified by calculating an XRD pattern in which V in Na ₃ V(OH)(HPO ₄ )(PO ₄ ) in the reference literature was replaced with an arbitrary M (i.e., Fe) using the data processing software, and comparing the simulation pattern with the measured XRD pattern. The purity of NaTP was determined from the abundance ratio (area intensity ratio) of the main XRD pattern by the RIR method.

The WH diameter was also calculated using all XRD peaks attributed to NaTP, and this was taken as the crystallite diameter.

Example 1
Iron phosphate ( _FePO4.n -hydrate, n=7) and sodium phosphate dodecahydrate ( _Na3PO4.12H2O ) were mixed, and then 85% _{orthophosphoric} acid ( _H3PO4 ) and pure water ( _H2O ) were added to obtain a composition _{having the} following composition.

Iron phosphate: 2.36g
Sodium phosphate dodecahydrate: 4.80 g
85% orthophosphoric acid: 0.36g
Pure water: 15g
The pH of the composition was 8.0.

The composition was filled into a Teflon (registered trademark) resin container with a lid and sealed, then placed in a thermostatic chamber and subjected to hydrothermal treatment under the following conditions.

Hydrothermal treatment temperature: 100°C
Hydrothermal treatment time: 120 hours Hydrothermal treatment pressure: autogenous pressure After the hydrothermal treatment, the product was washed with pure water, filtered, and then dried at 120°C for 4 hours under a vacuum atmosphere, and then pulverized to obtain a powdered sodium iron polyanion.

The resulting sodium iron polyanion had a purity of 100% (i.e., the XRD pattern of the sodium iron polyanion was only that represented by _Na3Fe (OH)( _HPO4 )( _PO4 )), and the crystal structure was a monoclinic crystal assigned to the space group C2/m. The lattice constants were a = 15.464(9) Å, b = 7.273(6) Å, and c = 7.049(1) Å, β = 96.692(5)°, and the crystallite size was 42 Å.

Example 2
The sodium iron polyanion of this example was obtained in the same manner as in Example 1, except that the raw material composition was changed to the following composition.

Iron phosphate: 2.36g
Sodium phosphate dodecahydrate: 4.80 g
85% orthophosphoric acid: 0.72g
Pure water: 15g
The pH of the composition was 7.0.

Figure 2 shows the XRD pattern of the obtained sodium iron polyanion. The obtained sodium iron polyanion had a purity of 99.9% and a monoclinic crystal structure assigned to the space group C2/m. The lattice constants were a 15.476(6) Å, b 7.280(1) Å, and c 7.048(3) Å, β 96.692(9)°, and the crystallite diameter was 37 Å.

Example 3
A sodium iron polyanion was obtained in the same manner as in Example 1, except that the hydrothermal treatment temperature was 150° C. and the hydrothermal treatment time was 16 hours.

Figure 3 shows the XRD pattern of the obtained sodium iron polyanion. The obtained sodium iron polyanion had a purity of 86.4% and a monoclinic crystal structure assigned to the space group C2/m. The lattice constants were a 15.471(7) Å, b 7.273(9) Å, and c 7.044(2) Å, β 96.688(1)°, and the crystallite diameter was 147 Å.

Example 4
A sodium iron manganese polyanion of this example containing iron and manganese in a ratio of Fe:Mn=3:1 was obtained in the same manner as in Example 1, except that the raw material composition was changed to the following composition, the hydrothermal treatment temperature was changed to 150°C, and the hydrothermal treatment time was changed to 16 hours.

Iron phosphate: 1.79g
Manganese phosphate monohydrate: 0.72g
Sodium phosphate dodecahydrate: 4.56 g
Pure water: 15g
The pH of the composition was 8.0.

The obtained sodium iron manganese polyanion had a purity of 99.9% and a monoclinic crystal structure assigned to the space group C2/m. The lattice constants were a 15.502(4) Å, b 7.331(2) Å, and c 7.118(2) Å, β 95.932(9)°, and the crystallite diameter was 97 Å.

Example 5
A sodium iron manganese polyanion of this example containing iron and cobalt in a ratio of Fe:Co=3:1 was obtained in the same manner as in Example 4, except that the raw material composition was changed to the following composition.

Iron phosphate: 1.79g
Cobalt phosphate octahydrate: 0.57g
Sodium phosphate dodecahydrate: 4.56 g
Pure water: 15g
The pH of the composition was 7.9.

The obtained sodium iron cobalt polyanion had a purity of 99.9% and a monoclinic crystal structure assigned to the space group C2/m. The lattice constants were a 15.513(6) Å, b 7.340(0) Å, and c 7.113(3) Å, β 96.324(1)°, and the crystallite diameter was 248 Å.

Measurement Example 1 (Evaluation of Non-Aqueous Sodium Secondary Battery)
(Formation of coating layer)
The NaTP obtained in Examples 1 to 3 was used, and sucrose was added so that the molar ratio of cellulose to NaTP was 0.05 to 0.07, and mixed in an agate mortar. After mixing, the mixture was heated at 250° C. for 3 hours under a nitrogen stream to form a coating layer (carbon layer) made of carbon on the surface of the NaTP, and NaTP having a carbon layer was obtained.
(Evaluation of non-aqueous sodium secondary battery characteristics)
NaTP having a carbon layer and a conductive binder (product name: TAB-2, manufactured by Hosen Co., Ltd.) were weighed out to a weight ratio of 2:1, and mixed in an agate mortar to prepare a positive electrode mixture. The obtained positive electrode mixture was placed on a SUS mesh (SUS316) having a diameter of 8 mm, and uniaxially pressed at 1 ton/ ^cm2 to prepare a positive electrode mixture pellet. The positive electrode mixture pellet was dried under reduced pressure at 150°C for 2 hours to prepare a positive electrode.

Using the obtained positive electrode, a nonaqueous sodium secondary battery with the following configuration was fabricated.

Test electrode: Positive electrode Counter electrode: Platinum (Pt) plate Reference electrode: Saturated calomel electrode Electrolyte: (Electrolyte) NaPF6 ₁ mol/ ^dm3
(Solvent) Ethylene carbonate (EC): Dimethyl carbonate (DMC)
= 1:1 (volume ratio)

A constant current charge/discharge test was performed using the nonaqueous sodium secondary battery under the following evaluation conditions:
Temperature: Room temperature (24.5±2.5℃)
Electrode potential: -2.18 to 2.02V (based on saturated calomel electrode)
Current value: constant current value of 1 mA/ ^cm2 Number of charge/discharge cycles: 10 cycles The potential based on the saturated calomel electrode corresponds to 0.76 to 4.98 V based on the sodium electrode.

As a result of measuring the discharge capacity, the discharge capacity at the 10th cycle was 153 mAh/g for the nonaqueous sodium secondary battery having NaTP of Example 1 as the positive electrode active material, 148 mAh/g for the nonaqueous sodium secondary battery having NaTP of Example 2 as the positive electrode active material, and 121 mAh/g for the nonaqueous sodium secondary battery having NaTP of Example 3 as the positive electrode active material.

Figure 4 shows the discharge capacity at the first and tenth cycles of a nonaqueous sodium secondary battery using NaTP of Example 1 as the positive electrode active material. The discharge capacity at the first cycle was 161 mAh/g.

Measurement Example 2 (Evaluation of Water-Based Sodium Secondary Battery)
(Formation of coating layer)
The NaTP obtained in Examples 1 and 2 was used, and a carbon layer (coating layer) was formed on the surface of the NaTP in the same manner as in Measurement Example 1, to obtain NaTP having a carbon layer.
(Evaluation of aqueous sodium battery characteristics)
NaTP with a carbon layer was used as the positive electrode active material, and this was mixed with AB and PTFE in a weight ratio of 60:30:10 to obtain a pellet-shaped positive electrode mixture with a diameter of 4 mm. An aqueous sodium secondary battery was produced, which was equipped with a positive electrode mixture as the working electrode (positive electrode), a plate-shaped zinc metal (Zn) as the counter electrode (negative electrode), a silver chloride electrode (Ag/AgCl) as the reference electrode, and an aqueous _NaClO4 solution with an electrolyte concentration ( _NaClO4 concentration) of 15.5 m as the electrolyte. The aqueous sodium secondary battery was charged and discharged under the following conditions, and its discharge capacity was measured.

Current density: 2mA/ ^cm2
Voltage: -1.2V to 1.3V (vs Ag/AgCl reference electrode)
Charge/discharge temperature: room temperature The discharge capacity at the first cycle was 79 mAh/g for the aqueous sodium secondary battery including NaTP of Example 1, and 75 mAh/g for the aqueous sodium secondary battery including NaTP of Example 2. Fig. 5 shows the discharge curve (first cycle) of the aqueous sodium secondary battery including NaTP of Example 1.

Measurement Example 3 (Evaluation of Water-Based Sodium Secondary Battery)
The aqueous sodium battery characteristics were evaluated in the same manner as in Measurement Example 2, except that NaTP of Examples 1 and 3 was used, and that a negative electrode having _NaTi2 ( _PO4 ) ₃ synthesized by the Pechini method as a negative electrode active material was used. The method for producing the negative electrode is shown below.
(Preparation of negative electrode)
A mixed solution obtained by mixing 40 ml of a solution in which Ti(OCH ₂ CH ₂ CH ₂ CH ₃ ) ₄ was dissolved in a 30% hydrogen peroxide solution, 15 ml of 28% ammonia water, 10 ml of a nitric acid solution of Na ₂ CO ₃ and citric acid in a molar amount twice that of Ti, 10 ml of an aqueous NH ₄ H ₂ PO ₄ solution, and ethylene glycol was evaporated to dryness at 80°C for 1 to 2 hours. The mixture was then heated in air at 140°C to obtain a brown gel composition. This was then baked in air at 350°C, and then baked in air at 800°C to obtain NaTi ₂ (PO ₄ ) ₃ .

The obtained NaTi ₂ (PO ₄ ) ₃ and acetylene black (AB) were mixed in a weight ratio of 70:30, and then treated in a planetary ball mill at 400 rpm for 1 hour under an Ar atmosphere to carbon-coat the NaTi ₂ (PO ₄ ) ₃ , which was used as the negative electrode active material. The obtained negative electrode active material and PTFE were mixed in a weight ratio of 90:10, and molded into pellets with a diameter of 4 mm to use as the negative electrode mixture.

Measurement Example 4 (Evaluation of Water-Based Sodium Secondary Battery)
(Formation of coating layer)
0.70 g of NaTP obtained in Examples 4 and 5 and 0.05 g of acetylene black (Li-435, manufactured by Denka Co., Ltd.) were mixed in an argon atmosphere for 1 hour at 600 rpm using a planetary ball mill to obtain a mixed powder. 0.25 g of acetylene black was added to the obtained mixed powder, and the mixture was mixed in an argon atmosphere for 0.5 hours at 200 rpm using a planetary ball mill to obtain a positive electrode active material having a weight ratio of NaTP: acetylene black of 70:30. The obtained positive electrode active material and PTFE were mixed in a weight ratio of 90:10 and rolled, and then a pellet having a diameter of 4 mm and a mass of 4 mg was cut out and used as a positive electrode.
(Aqueous sodium secondary battery evaluation)
The aqueous sodium battery characteristics were evaluated in the same manner as in Measurement Example 3, except that NaTP of Examples 4 and 5 was used, and that a negative electrode using NaTi ₂ (PO ₄ ) ₃ synthesized by the Pechini method as the negative electrode active material was used.

As a result of evaluating the aqueous sodium secondary battery, the first cycle discharge capacity of the cells using NaTP of Examples 4 and 5 was 76 mAh/g for Example 5 and 78 mAh/g for Example 6, respectively.

Comparative measurement example (non-aqueous sodium ion secondary battery)
A sodium-containing transition metal phosphate compound (NaFePO ₄ ) having an olivine structure was synthesized according to a method similar to that of Example 1 of JP 2009-206085 A. A pellet-shaped positive electrode mixture was obtained in the same manner as in Measurement Example 2, except that the obtained NaFePO ₄ was used as the positive electrode active material.

A non-aqueous sodium secondary battery was fabricated and subjected to a charge-discharge test in the same manner as in Measurement Example 2, except that the obtained positive electrode mixture was used. As a result, the discharge capacity in the first cycle was 60 mAh/g.

From these results, it can be confirmed that the NaTP of this embodiment exhibits more than twice the discharge capacity as a positive electrode active material for a non-aqueous sodium secondary battery compared to an olivine-based sodium-containing transition metal phosphate compound. Furthermore, from Measurement Examples 1 and 2, it can be confirmed that the discharge capacity of the positive electrode active material is lower in an aqueous sodium secondary battery compared to a non-aqueous sodium secondary battery. Nevertheless, it can be confirmed that the NaTP of this embodiment, also as a positive electrode active material for an aqueous sodium secondary battery, exhibits a discharge capacity equal to or greater than that of a non-aqueous sodium ion secondary battery that uses a conventional olivine-based sodium-containing transition metal phosphate compound as the positive electrode active material.

The sodium transition metal polyanion disclosed herein can be suitably used as an adsorbent, an ion exchanger, a hydrogen ion conductor, a solid electrolyte, a battery material, in particular an active material for electrochemical devices, and further as a positive electrode active material for sodium secondary batteries.

Claims (14)

A sodium transition metal polyanion having a powder X-ray diffraction pattern represented by the general formula _Na3M (OH)( _HPO4 )( _PO4 ) (wherein M is Fe alone or a combination of Fe with either Mn or Co) and having a monoclinic crystal structure. The sodium transition metal polyanion according to claim 1, wherein the monoclinic crystal belongs to the space group C2/m. 3. The sodium transition metal polyanion according to claim 1, wherein the proportion of a powder X-ray diffraction pattern represented by the general formula _Na3M (OH)( _HPO4 )( _PO4 ) (wherein M is Fe only , or a combination of Fe with Mn or Co) in the powder X-ray diffraction pattern of the sodium transition metal polyanion is 80% or more. The sodium transition metal polyanion according to any one of claims 1 to 3, wherein the lattice constants are a: 15.35 Å to 15.55 Å, b: 7.19 Å to 7.35 Å, c: 6.95 Å to 7.12 Å, and β: 95.8° to 97.5°. The sodium transition metal polyanion according to any one of claims 1 to 4, comprising a coating layer. The method for producing the sodium transition metal polyanion according to any one of claims 1 to 5, characterized in that it comprises a step of subjecting a composition containing a sodium source, a transition metal source, a phosphate source, and water and having a pH of 4.0 to 11.0 to hydrothermal treatment at 80°C to 180°C. The method according to claim 6, wherein the pH is 6.0 or more and 9.0 or less. 8. The method according to claim 6 or 7, wherein the transition metal source is at least one selected from the group consisting of oxides, hydroxides, oxyhydroxides, phosphates, sulfites, sulfates, nitrates, chlorides, acetates, bromides, and fluorides containing only Fe, or Fe and Mn or Co. A positive electrode active material containing the sodium transition metal polyanion according to any one of claims 1 to 5. A sodium secondary battery comprising a positive electrode containing the sodium transition metal polyanion according to any one of claims 1 to 5, a negative electrode and an electrolyte. The sodium secondary battery according to claim 10, wherein the negative electrode contains at least _NaTi2 ( _PO4 ) ₃ . The sodium secondary battery according to claim 10 or 11, wherein the electrolyte contains at least _NaClO4 . The sodium secondary battery according to any one of claims 10 to 12, wherein the electrolyte is an aqueous electrolyte. The sodium secondary battery according to any one of claims 10 to 12, wherein the electrolyte is a non-aqueous electrolyte.