Aqueous Sodium Ion Secondary Battery

Abstract

Provided is an aqueous sodium ion secondary battery that can exhibit a high electrochemical capacity compared to conventionally known sodium ion secondary batteries provided with an aqueous electrolytic solution. This aqueous sodium ion secondary battery is characterized by being provided with a positive electrode, a negative electrode, an electrolytic solution, and a separator, wherein the positive electrode comprises a positive electrode active material containing at least a sodium transition metal polyanion represented by general formula Na3-xMPO4CO3 (M represents at least one selected from the group consisting of Fe, Mn, Ni, and Co, and x is 0-2), and an aqueous electrolytic solution is provided as the electrolytic solution.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and particularly relates to an aqueous sodium-ion secondary battery.

Background Art

[0002] Sodium-ion secondary batteries do not use lithium, a rare metal, and thus have attracted attention as post-lithium-ion secondary batteries. As sodium-ion secondary batteries, those equipped with an electrolyte solution similar to that of current lithium-ion secondary batteries, mainly a non-aqueous electrolyte solution, have been mainly studied. On the other hand, sodium-ion secondary batteries using an aqueous electrolyte solution that uses inexpensive water as the electrolyte solution have been studied.

[0003] In Patent Document 1 and Non-Patent Document 1, it has been confirmed that charge and discharge occur due to the reversible insertion and desorption reaction of Na cations in a sodium-ion secondary battery equipped with an aqueous electrolyte solution. The electrochemical capacity in the first charge and discharge of these sodium-ion secondary batteries was about 120 mAh / g.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Non-Patent Documents

[0005]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] An object of the present invention is to provide an aqueous sodium-ion secondary battery that can exhibit a high electrochemical capacity when compared with a conventionally known sodium-ion secondary battery including an aqueous electrolyte solution. **Means for Solving the Problems**

[0007] The inventors have intensively studied a sodium-ion secondary battery including an aqueous electrolyte solution (hereinafter referred to as "aqueous sodium-ion secondary battery"). As a result, it has been found that an aqueous sodium-ion secondary battery capable of exhibiting a higher electrochemical capacity than a conventionally known aqueous sodium-ion secondary battery can be constructed by using a sodium-containing transition metal polyanion containing three Na cations as a positive electrode active material.

[0008] That is, the gist of the present invention is as follows. [1] An aqueous sodium-ion secondary battery comprising a positive electrode, a negative electrode, an electrolyte solution, and a separator, wherein the positive electrode includes a positive electrode active material containing at least a sodium transition metal polyanion represented by the general formula Na 3-x MPO4CO3 (M is at least any one selected from the group consisting of Fe, Mn, Ni, and Co, and x is 0 or more and 2 or less), and the electrolyte solution is an aqueous electrolyte solution. [2] The aqueous sodium-ion secondary battery according to [1] above, characterized in that a part or all of the surface of the sodium transition metal polyanion has carbon. [3] The aqueous sodium-ion secondary battery according to [1] or [2] above, wherein the crystallite diameter of the sodium transition metal polyanion is 90 Å or more and 400 Å or less. [4] The aqueous sodium-ion secondary battery according to any one of [1] to [3] above, wherein the aqueous electrolyte solution contains one or more selected from the group consisting of Na2SO4, NaNO3, and NaClO4. [5] The aqueous sodium-ion secondary battery according to any one of [1] to [4] above, wherein the aqueous electrolyte solution contains NaClO4. [6] The aqueous sodium secondary battery according to [5] above, wherein the electrolyte concentration in the electrolyte solution is 5 mol / kg or more. [7] The aqueous sodium-ion secondary battery according to any one of [1] to [6] above, wherein the negative electrode contains at least NaTi2(PO4)3. [8] The positive electrode contains at least the general formula Na 3-x A positive electrode active material containing a sodium transition metal polyanion represented by FePO4CO3 (x is 0 or more and 2 or less), the negative electrode contains at least NaTi2(PO4)3, and the aqueous electrolyte contains at least NaClO4. The aqueous sodium-ion secondary battery according to any one of [1] to [7] above. [9] At least the general formula Na 3-x A positive electrode active material for an aqueous sodium-ion secondary battery containing a sodium transition metal polyanion represented by MPO4CO3 (M is at least any one selected from the group of Fe, Mn, Ni, and Co, and x is 0 or more and 2 or less).

[10] The positive electrode active material for an aqueous sodium-ion battery according to [9] above, wherein M contains at least Fe. [Advantages of the Invention]

[0009] According to the present invention, an aqueous sodium-ion secondary battery capable of exhibiting a high electrochemical capacity can be provided when compared with a conventionally known sodium-ion secondary battery equipped with an aqueous electrolyte. [Brief Description of the Drawings]

[0010]

Figure 1

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Mode for Carrying Out the Invention

[0011] Hereinafter, preferred embodiments of the aqueous sodium-ion secondary battery of the present invention will be described. In the following, the terms sodium-ion secondary battery, sodium secondary battery, and sodium-ion battery are used with the same meaning respectively. The positive electrode of the aqueous sodium-ion secondary battery of the present embodiment contains at least a sodium transition metal polyanion represented by the general formula Na 3-x MPO4CO3 (M is at least any one selected from the group of Fe, Mn, Ni, and Co, and x is 0 or more and 2 or less) (hereinafter referred to as "Na polyanion"). The positive electrode active material contains a sodium transition metal polyanion, which is a sodium-containing transition metal phosphate carbonate, and is preferably an artificially synthesized synthetic sodium-containing transition metal phosphate carbonate. This functions as a positive electrode active material for an aqueous sodium-ion secondary battery. Further, the Na polyanion of the present embodiment is a positive electrode active material for an aqueous sodium-ion secondary battery containing at least a sodium transition metal polyanion represented by the general formula Na 3-x MPO4CO3 (M is at least any one selected from the group of Fe, Mn, Ni, and Co, and x is 0 or more and 2 or less). Here, M preferably contains at least Fe, more preferably at least any one of Fe and Mn, and particularly preferably Fe. Further, x may be 0 or more and 1 or less. Specific Na polyanions include one or more selected from the group consisting of sodium iron phosphate carbonate, sodium manganese phosphate carbonate, sodium nickel phosphate carbonate, and sodium cobalt phosphate carbonate, further one or more selected from the group consisting of sodium iron phosphate carbonate, sodium manganese phosphate carbonate, and sodium cobalt phosphate carbonate, still further at least one of sodium iron phosphate carbonate and sodium manganese phosphate carbonate, and still further sodium iron phosphate carbonate can be exemplified.

[0012] The physical properties of the Na polyanion are not particularly limited and may be appropriately adjusted according to the intended battery configuration. Examples of the physical properties include, but are not limited to, crystal system, purity, lattice constant, crystallinity, crystallite size, crystal orientation, pore diameter, pore distribution, pore volume, BET specific surface area, primary particle size, secondary particle size, powder particle size, particle size distribution, particle morphology, and particle size composition. The crystallite size of the Na polyanion is preferably 90 Å or more and 400 Å or less, 95 Å or more and 360 Å or less, 100 Å or more and 300 Å or less, or 150 Å or more and 280 Å or less. The crystallite size in this embodiment is the diameter obtained by the Williamson-Hall method from the XRD pattern (hereinafter also referred to as the "WH diameter"). The WH diameter is the crystallite size obtained from two or more XRD peaks attributable to the Na polyanion in the XRD pattern. Specifically, for two or more XRD peaks attributable to the Na polyanion, the following plots are performed respectively. The following linear approximation formula is obtained by the least squares method of the plurality of obtained plotted points, and the reciprocal of the y-intercept of the linear approximation formula is the crystallite size.

[0013] <Plot> Y = (β·sinθ) / λ X = sinθ / λ <Linear approximation formula> Y = 2η·X + (1 / ε) ···(Equation 1) In these formulas, β is the full width at half maximum (°), θ is the diffraction angle (°), λ is the wavelength of the radiation source (nm), η is the inhomogeneous strain, and ε is the crystallite size (Å). Also, 1 / ε in the first-order approximation formula is the y-intercept. 2η corresponds to the slope of the first-order approximation formula obtained by plotting the XRD peaks. Preferably, the WH diameter is the crystallite size determined from the XRD peaks corresponding to the (020), (220), and (002) planes in the XRD pattern. Specifically, for the XRD peaks corresponding to the (020), (220), and (002) planes, the above plots are performed respectively. The first-order approximation formula is obtained by the least squares method for the three resulting plots, and the reciprocal of the y-intercept of the first-order approximation formula is the crystallite size.

[0014] The XRD peaks corresponding to the (020), (121), and (202) planes of the Na polyanion in this embodiment can be confirmed as XRD peaks having peak tops at the following 2θ (hereinafter also referred to as the "main peak group"). (020) plane: 2θ = 26.39° to 27.06° (220) plane: 2θ = 33.19° to 33.88° (002) plane: 2θ = 34.59° to 34.99°

[0015] When the Na polyanion is represented by the general formula Na 3-x FePO4CO3 (when it is sodium iron phosphate carbonate), the 2θ of the main peak group is preferably the following values, and the crystallite size is preferably 150 Å or more and 400 Å or less, and particularly preferably 170 Å or more and 360 Å or less. (020) plane: 2θ = 26.86° to 26.94° (220) plane: 2θ = 33.56° to 33.69° (002) plane: 2θ = 34.76° to 34.86°

[0016] When the Na polyanion is represented by the general formula Na 3-xWhen represented by MnPO4CO3 (when it is sodium manganese phosphate carbonate), the 2θ of the main peak group is preferably the following value, and the crystallite size is preferably 90 Å or more and 200 Å or less, and particularly preferably 90 Å or more and 180 Å or less. (020) plane: 2θ = 26.39° - 26.47° (220) plane: 2θ = 33.19° - 33.29° (002) plane: 2θ = 34.59° - 34.77°

[0017] Na polyanion has the general formula Na 3-x When represented by NiPO4CO3 (when it is sodium cobalt phosphate carbonate), the 2θ of the main peak group is preferably the following value, and the crystallite size is preferably 95 Å or more and 300 Å or less, and particularly preferably 100 Å or more and 250 Å or less. (020) plane: 2θ = 26.91° - 27.06° (220) plane: 2θ = 33.68° - 33.88° (002) plane: 2θ = 34.84° - 34.99°

[0018] Na polyanion has the general formula Na 3-x When represented by CoPO4CO3 (when it is sodium cobalt phosphate carbonate), the 2θ of the main peak group is preferably the following value, and the crystallite size is preferably 150 Å or more and 400 Å or less, and particularly preferably 180 Å or more and 370 Å or less. (020) plane: 2θ = 26.88° - 26.94° (221) plane: 2θ = 33.63° - 33.69° (002) plane: 2θ = 34.84° - 34.88° The XRD pattern in this embodiment can be measured under the following conditions. Radiation source: CuKα ray (λ = 1.5405 Å) Measurement mode: Step scan Scan condition: 20° / min Measurement time: 3 s 2θ: 5° to 90° The obtained XRD pattern may be subjected to Rietveld refinement analysis using commercially available analysis software (for example, data processing software PDXL-2 attached to an X-ray diffractometer (for example, SmartLab, manufactured by Rigaku Corporation), etc.) to identify and analyze the crystal phase of the product.

[0019] The Na polyanion preferably belongs to the monoclinic crystal system. Further, it is more preferable that the monoclinic crystal system belongs to at least one of the space groups P21 / m and P21. The monoclinic crystal system to which the Na polyanion belongs is a crystal system in which the lattice constants a, b, and c are different values, α and γ are both 90°, and β is an angle other than 90°.

[0020] In this embodiment, the lattice constants of the Na polyanion are preferably a is 8.800 Å or more and 9.050 Å or less, b is 6.570 Å or more and 6.780 Å or less, c is 5.100 Å or more and 5.190 Å or less, α is 90.00°, γ is 90.00°, and β is 89.00° or more and 90.50° or less, respectively. More preferable lattice constants include a is 8.820 Å or more and 9.030 Å or less, b is 6.580 Å or more and 6.770 Å or less, α is 90.00°, γ is 90.00°, c is 5.110 Å or more and 5.180 Å or less, and β is 89.10° or more and 90.20° or less, respectively.

[0021] In this embodiment, the general formula Na 3-xThe lattice constants of the Na polyanion represented by FePO4CO3 are as follows: a is 8.930 Å or more and 8.985 Å or less, preferably 8.940 Å or more and 8.975 Å or less, more preferably 8.959 Å or more and 8.970 Å or less; b is 6.600 Å or more and 6.655 Å or less, preferably 6.610 Å or more and 6.645 Å or less, more preferably 6.615 Å or more and 6.635 Å or less; c is 5.125 Å or more and 5.175 Å or less, preferably 5.135 Å or more and 5.165 Å or less, more preferably 5.145 Å or more and 5.159 Å or less; α is 90.00°; γ is 90.00°; and β is 89.30° or more and 89.80° or less, preferably 89.50° or more and 89.70° or less.

[0022] In this embodiment, the general formula Na 3-x The lattice constants of the Na polyanion represented by MnPO4CO3 are as follows: a is 8.960 Å or more and 9.050 Å or less, preferably 8.970 Å or more and 9.030 Å or less, more preferably 8.981 Å or more and 9.030 Å or less, still more preferably 9.000 Å or more and 9.025 Å or less; b is 6.705 Å or more and 6.780 Å or less, preferably 6.715 Å or more and 6.775 Å or less, more preferably 6.729 Å or more and 6.770 Å or less, still more preferably 6.745 Å or more and 6.765 Å or less; c is 5.135 Å or more and 5.190 Å or less, preferably 5.145 Å or more and 5.180 Å or less, more preferably 5.150 Å or more and 5.180 Å or less; α is 90.00°; γ is 90.00°; and β is 89.90° or more and 90.50° or less, preferably 90.10° or more and 90.20°, more preferably 90.14° or more and 90.20° or less.

[0023] In this embodiment, the general formula Na 3-xThe lattice constants of the Na polyanion represented by NiPO4CO3 are as follows: a is 8.800 Å or more and 8.940 Å or less, preferably 8.815 Å or more and 8.930 Å or less, more preferably 8.885 Å or more and 8.930 Å or less, still more preferably 8.900 Å or more and 8.925 Å or less; b is 6.570 Å or more and 6.660 Å or less, preferably 6.580 Å or more and 6.650 Å or less, more preferably 6.625 Å or more and 6.645 Å or less; c is 5.100 Å or more and 5.180 Å or less, preferably 5.110 Å or more and 5.170 Å or less, more preferably 5.129 Å or more and 5.165 Å or less, still more preferably 5.131 Å or more and 5.160 Å or less; α is 90.00°, γ is 90.00°, and β is 89.00° or more and 89.40° or less, preferably 89.10° or more and 89.30° or less, more preferably 89.15° or more and 89.29° or less, still more preferably 89.15° or more and 89.25° or less.

[0024] In this embodiment, the general formula Na 3-x The lattice constants of the Na polyanion represented by CoPO4CO3 are as follows: a is 8.875 Å or more and 8.940 Å or less, preferably 8.880 Å or more and 8.930 Å or less, more preferably 8.885 Å or more and 8.930 Å or less, still more preferably 8.900 Å or more and 8.925 Å or less; b is 6.605 Å or more and 6.660 Å or less, preferably 6.615 Å or more and 6.650 Å or less, more preferably 6.625 Å or more and 6.645 Å or less; c is 5.125 Å or more and 5.175 Å or less, preferably 5.130 Å or more and 5.165 Å or less, more preferably 5.135 Å or more and 5.160 Å or less, still more preferably 5.140 Å or more and 5.149 Å or less; α is 90°, γ is 90°, and β is 89.20° or more and 89.65° or less, preferably 89.30° or more and 89.55° or less, more preferably 89.35° or more and 89.52° or less, still more preferably 89.35° or more and 89.48° or less.

[0025] It is preferable that part or all of the Na polyanion has carbon, which tends to increase the conductivity. Examples of the state in which the Na polyanion has carbon (i.e., carbon-coated Na polyanion) include the case where part or all of the surface of the Na polyanion is coated with carbon, and the case where at least part of the Na polyanion has carbon in an island shape.

[0026] The method for making the Na polyanion have carbon is arbitrary, and a method of mixing the Na polyanion and a carbon material can be exemplified. The carbon material is a precursor of carbon and is at least one of conductive carbon and carbon compounds. Specific carbon materials as precursors of carbon include one or more selected from the group consisting of furnace black, channel black, acetylene black, and thermal black, further at least one of acetylene black and thermal black, and still further acetylene black. The atmosphere during mixing may be an atmosphere in which the conductivity of the carbon material is less likely to be impaired, and an inert atmosphere is preferable. Examples of the inert atmosphere include an atmosphere of at least one of nitrogen and argon. The mixing method of the Na polyanion and carbon is not particularly limited as long as the Na polyanion has carbon on part or all of its surface, but a method of mixing while pulverizing the carbon material is preferable, a mixing method using a ball mill is preferable, and a mixing method using a planetary ball mill is more preferable.

[0027] The Na polyanion having carbon on the surface (carbon-coated Na polyanion) may be subjected to an annealing treatment, for example, carbothermal treatment under an inert atmosphere. By performing these treatments, the electronic conductivity can be improved. Further, when the Na polyanion has the general formula Na 3-xWhen it is a Na polyanion represented by FePO4CO3, it is also conceivable that carbon on the surface of the Na polyanion deposits conductive iron phosphide as a reducing agent on the surface of the Na polyanion by carbothermal treatment, reducing the contact resistance between Na polyanion particles. The method of carbothermal treatment is not particularly limited, but a method of treating a Na polyanion having carbon on its surface in an inert atmosphere of at least one of nitrogen and argon at 200°C or higher and 700°C or lower for 1 hour or longer and 12 hours or shorter can be exemplified.

[0028] When carbothermally treating a carbon-coated Na polyanion, in addition to a carbon material such as acetylene black as a precursor of carbon, a compound that can be at least partially carbonized by pyrolysis may be used as a precursor. Examples of such compounds include alcohols and organic compounds, and one or more selected from the group consisting of cyclodextrin, ascorbic acid, glycolic acid, malic acid, citric acid, fructose, and sucrose are preferred.

[0029] The Na polyanion can be produced by any method. Examples of the method for producing the Na polyanion include, for example, a method of mechanically milling a composition containing sodium, a transition metal, phosphoric acid, and carbonic acid. Specifically, a method of mixing NaMPO4 (M is at least one selected from the group consisting of Fe, Mn, Ni, and Co) and Na2CO3 and mechanically milling in an inert atmosphere such as argon can be exemplified. Thereby, a solid-phase synthesized Na polyanion is obtained. As another method for producing the Na polyanion, for example, a method of hydrothermally treating a composition containing a sodium source, a transition metal source, a phosphoric acid source, a carbonic acid source, and water can be mentioned. Preferably, it has a step of hydrothermally treating a composition containing a sodium source, a transition metal source, a phosphoric acid source, a carbonic acid source, and water, the hydrothermal treatment temperature is 140 °C or higher and 280 °C or lower, and further, the pH of the composition is 7.5 or higher. It is preferably a production method characterized by this, and it is more preferable that the composition contains two or more water-soluble sodium salts. Thereby, a liquid-phase synthesized Na polyanion is obtained. Specifically, a method of hydrothermally treating a composition containing a transition metal sulfate, sodium phosphate, anhydrous sodium carbonate, and water and having a pH of 7.5 or higher and 10.0 or lower in an autogenous pressure atmosphere at 160 °C or higher and 250 °C or lower can be exemplified.

[0030] The positive electrode of the aqueous sodium ion secondary battery of this embodiment only needs to contain a Na polyanion as a positive electrode active material. The content of the Na polyanion in the positive electrode active material is preferably 80% by mass or more and 100% by mass or less, more preferably 90% by weight or more and 100% by weight or less, and particularly preferably 100% by weight (the positive electrode active material is only a Na polyanion, that is, substantially does not contain an active material other than the Na polyanion). Further, the positive electrode may contain a Na polyanion as a positive electrode mixture containing the positive electrode active material and at least one of a binder and a conductive material. Known binders and conductive materials can be used. For example, the binder is one or more selected from the group of fluorine-based resins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and ethylene tetrafluoroethylene (ETFE), polyethylene, polypropylene, SBR-based materials, and imide-based materials, and the conductive material is, for example, one or more selected from carbon materials, conductive fibers such as metal fibers, metal powders such as copper, silver, nickel, and aluminum, and organic conductive materials such as polyphenylene derivatives. The carbon material as the conductive material is exemplified by, but not limited to, mesoporous carbon obtained by firing graphite, soft carbon, hard carbon, carbon black, ketjen black, acetylene black, graphite, activated carbon, carbon nanotubes, carbon fibers, synthetic resins containing aromatic rings, petroleum pitch, etc. As a specific positive electrode mixture, a positive electrode mixture containing at least one of a Na polyanion, PVDF, and PTFE, and one or more selected from the group of graphite, graphite, soft carbon, hard carbon, and acetylene black can be exemplified. The positive electrode mixture can be produced by any method, and the Na polyanion, binder, and conductive material may be mixed by any method at a ratio suitable for the intended positive electrode mixture.

[0031] The aqueous sodium-ion secondary battery of this embodiment includes an aqueous electrolyte as the electrolyte solution. The aqueous electrolyte solution contains water as the solvent and an electrolyte. The electrolyte may be any water-soluble sodium salt, and examples thereof include one or more selected from the group consisting of Na2SO4, NaNO3, NaClO4, and NaOH. From the perspective of ease of handling, the electrolyte is preferably one or more selected from the group consisting of Na2SO4, NaNO3, and NaClO4, and more preferably NaClO4. Although the electrolyte concentration in the electrolyte solution is not particularly limited, from the perspective of increasing the energy density as a sodium-ion secondary battery, it is preferable that the electrolyte concentration (sodium salt concentration) in the electrolyte solution is high. Examples of the sodium salt concentration include 2 mol / L or more, and it can be a concentration below the saturation solubility. When using NaClO4 as the electrolyte, the electrolyte concentration (NaClO4 concentration) in the electrolyte solution can be 5 mol / kg or more, 7 mol / kg or more, or 10 mol / kg or more, and can also be 17 mol / kg or less.

[0032] In order to optimize physical properties and characteristics such as the storage stability and battery characteristics of the electrolyte solution, the electrolyte solution may contain an additive. Examples of the additive include, but are not limited to, succinic acid, glutamic acid, maleic acid, citraconic acid, gluconic acid, itaconic acid, diglycol, cyclohexanedicarboxylic acid, cyclopentanetetracarboxylic acid, 1,3-propanesultone, 1,4-butanesultone, methyl methanesulfonate, sulfolane, dimethyl sulfone, N,N-dimethylmethanesulfonamide, etc. The content in the electrolyte solution is preferably 0.01 to 10% by weight.

[0033] The negative electrode of the sodium-ion secondary battery of the embodiment may contain, as a negative electrode active material, a material that does not prevent the occlusion and release of sodium ions in the positive electrode active material in a sodium-ion secondary battery equipped with an aqueous electrolyte. Examples of the negative electrode active material include 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, and one or more selected from the group consisting of carbon materials, polyimides, transition metal-containing cyanide compounds, and transition metal-containing polyanion compounds are preferred. Specific examples of each negative electrode active material include activated carbon as the carbon material, Na2Mn[Mn(CN)6] as the transition metal-containing cyanide compound, and NaTi2(PO4)3 as the transition metal-containing polyanion compound. NaTi2(PO4)3 is particularly preferred as the negative electrode active material.

[0034] The negative electrode of the aqueous sodium-ion secondary battery of the present embodiment may contain a negative electrode active material as a negative electrode mixture containing at least one of a binder and a conductive material. Known binders and conductive materials can be used, and for example, the same binders and conductive materials as those that can be used in the above positive electrode mixture can be mentioned. The negative electrode mixture can be manufactured by any method, and the negative electrode active material, binder, and conductive material may be mixed at a ratio suitable for the target negative electrode mixture. Other components of the aqueous sodium-ion secondary battery, such as current collectors, can be those used in known sodium-ion secondary batteries.

[0035] Another aqueous sodium-ion secondary battery of the present embodiment includes a positive electrode, a negative electrode, and an aqueous electrolyte, wherein the positive electrode includes a positive electrode active material containing at least a sodium transition metal polyanion represented by the general formula Na 3-x MPO4CO3 (M is one or more selected from the group consisting of Fe, Mn, Ni, and Co, and x is 0 or more and 2 or less), the negative electrode includes at least NaTi2(PO4)3, and the aqueous electrolyte includes at least NaClO4, which is an aqueous sodium-ion secondary battery. Yet another aqueous sodium-ion secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, and an aqueous electrolyte, wherein the positive electrode contains at least a sodium transition metal polyanion represented by the general formula Na 3-x MPO4CO3 (M is one or more selected from the group consisting of Fe, Mn, and Co, and x is 0 or more and 2 or less), and the negative electrode contains at least NaTi2(PO4)3, and the aqueous electrolyte contains at least NaClO4. This is an aqueous sodium-ion secondary battery. Yet another aqueous sodium-ion secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, and an aqueous electrolyte, wherein the positive electrode contains at least a sodium transition metal polyanion represented by the general formula Na 3-x MPO4CO3 (M is Fe or Mn, and x is 0 or more and 2 or less), and the negative electrode contains at least NaTi2(PO4)3, and the aqueous electrolyte contains at least NaClO4. This is an aqueous sodium-ion secondary battery. Yet another aqueous sodium-ion secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, and an aqueous electrolyte, wherein the positive electrode contains at least a sodium transition metal polyanion represented by the general formula Na 3-x MnPO4CO3 (x is 0 or more and 2 or less), and the negative electrode contains at least NaTi2(PO4)3, and the aqueous electrolyte contains at least NaClO4. It is preferable that this is an aqueous sodium-ion secondary battery. Yet another aqueous sodium-ion secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, and an aqueous electrolyte, wherein the positive electrode contains at least a sodium transition metal polyanion represented by the general formula Na 3-x FePO4CO3 (x is 0 or more and 2 or less), and the negative electrode contains at least NaTi2(PO4)3, and the aqueous electrolyte contains at least NaClO4. This is an aqueous sodium-ion secondary battery.

Examples

[0036] Hereinafter, the present embodiment will be specifically described with reference to examples. However, the present embodiment is not limited to these examples.

[0037] <Identification of Sodium Polyanion> The XRD pattern of the sodium polyanion obtained in the synthesis example was measured under the following conditions. X-ray source: CuKα ray (λ = 1.5405 Å) Measurement mode: Step scan Scan condition: 20° / min Measurement time: 3 seconds 2θ: 5° to 90° The obtained XRD pattern was subjected to Rietveld refinement analysis using the data processing software PDXL-2 attached to an X-ray diffractometer (equipment name: SmartLab, manufactured by Rigaku Corporation) to identify and analyze the crystal phase of the product. <Fabrication of Sodium Ion Secondary Battery (Half Cell)> Sodium polyanion and acetylene black (AB) were carbon-coated at 400 - 500 rpm for 9 hours under an Ar atmosphere using a planetary ball mill so that the weight ratio was 70:30, and a positive electrode active material composed of powdery sodium polyanion with its surface coated with carbon (carbon-coated sodium polyanion) was obtained. The obtained positive electrode active material, AB, and PTFE were mixed at a weight ratio of 60:30:10 to obtain a pellet-shaped positive electrode mixture with a diameter of 10 mm. A beaker-type half cell-type sodium ion secondary battery shown in Figure 1 was fabricated using the positive electrode mixture for the working electrode (positive electrode), plate-shaped zinc metal (Zn) for the counter electrode (negative electrode), silver chloride electrode (Ag / AgCl) for the reference electrode, and 17 M (17 mol / kg) aqueous NaClO4 solution for the electrolyte.

[0038] <Fabrication of Sodium Ion Secondary Battery (Coin Cell)> (Fabrication of Negative Electrode) As the negative electrode active material, NaTi2(PO4)3 synthesized by the Pechini method shown below was used. First, 40 ml of a solution in which Ti(OCH2CH2CH2CH3)4 was dissolved in a 30% hydrogen peroxide solution, 15 ml of 28% aqueous ammonia, 10 ml of a nitric acid solution of citric acid in a molar amount twice that of Na2CO3 and Ti, 10 ml of an aqueous NH4H2PO4 solution, and ethylene glycol were mixed, and the resulting mixed solution was evaporated to dryness at 80 °C for 1 to 2 hours. Then, it was heated at 140 °C in the air to obtain a brown gel-like composition. This was calcined at 350 °C in the air and then at 800 °C in the air to obtain NaTi2(PO4)3.

[0039] The obtained NaTi2(PO4)3 was mixed at 400 rpm for 1 hour using a planetary ball mill so that the weight ratio of NaTi2(PO4)3 to AB was 70:25, and carbon coating treatment was performed to obtain a negative electrode active material composed of powdery NaTi2(PO4)3 with a carbon-coated surface. The obtained negative electrode active material and PTFE were mixed at a weight ratio of 95:5 and molded into a pellet with a diameter of 10 mm to obtain a negative electrode mixture.

[0040] (Fabrication of coin cell) For the positive electrode and the negative electrode, a positive electrode mixture and the above negative electrode mixture manufactured in the same manner as the positive electrode mixture used in the half cell were used, respectively. Using 17M aqueous NaClO4 solution as the electrolyte, a coin cell type sodium ion secondary battery shown in Figure 2 was fabricated. The positive electrode and the negative electrode in the sodium ion secondary battery were made such that the weight ratio of the positive electrode active material to the negative electrode active material was 1:1.5.

[0041] (Synthesis of Na polyanion) Synthesis Example 1 (Synthesis of sodium iron phosphate carbonate) Ferrous sulfate heptahydrate (FeSO4·7H2O), sodium phosphate dodecahydrate (Na3PO4·12H2O) and anhydrous sodium carbonate (Na2CO3) were pulverized and mixed, and then pure water was added thereto to obtain a composition having the following composition. Ferrous sulfate heptahydrate: 13.6 wt% Sodium phosphate dodecahydrate: 18.7 wt% Sodium carbonate anhydrous: 5.2% by weight Pure water: the balance

[0042] The pH of the composition was 11.0. After filling and sealing the composition in a container with a Teflon (registered trademark) resin lid, it was placed in a thermostatic bath and subjected to hydrothermal treatment under the following conditions. Hydrothermal treatment temperature: 180 °C Hydrothermal treatment time: 16 hours Hydrothermal treatment pressure: autogenous pressure

[0043] After hydrothermal treatment, the product cooled to room temperature was washed by adding a sufficient amount of pure water and stirring, and then sodium iron phosphate carbonate was obtained by suction filtration using a filter paper with a pore size of 0.2 μm.

[0044] The obtained sodium iron phosphate carbonate was dried at 110 °C for 4 hours under a vacuum atmosphere, and then further pulverized with a mortar to obtain the sodium iron phosphate carbonate of this synthesis example. The purity of the sodium iron phosphate carbonate Na3FePO4CO3 of this synthesis example was 100% by weight, and no crystallized substances other than Na3FePO4CO3 were confirmed in its XRD pattern. The Na3FePO4CO3 of this synthesis example had a crystallite diameter (WH diameter) of 270 Å, lattice constants of a = 8.953 Å, b = 6.633 Å, c = 5.152 Å, α = 90.00°, β = 89.58°, and γ = 90.00°, a monoclinic crystal system, a bonsiteite-type structure, and belonged to the space group P21 / m. The main XRD peaks were as follows. (020) plane: 2θ = 26.86° relative intensity = 81 (220) plane: 2θ = 33.57° relative intensity = 78 (002) plane: 2θ = 34.80° relative intensity = 100

[0045] Synthesis Example 2 (Synthesis of Sodium Iron Phosphate Carbonate) Sodium carbonate (Na2CO3) and sodium iron phosphate (maricite NaFePO4) were mixed in a stoichiometric ratio of 1:1, and then mixed in a planetary ball mill at 600 rpm for 12 hours under an Ar atmosphere. AB was added to the obtained powder to a weight ratio of 15 wt%, and mixed in a planetary ball mill at 600 rpm for 12 hours under an Ar atmosphere. Further, AB was added to a weight ratio of 15 wt%, and mixed in a planetary ball mill at 400 rpm for 3 hours under an Ar atmosphere to obtain sodium iron phosphate carbonate Na3FePO4CO3.

[0046] Synthesis Example 3 (Synthesis of Sodium Manganese Phosphate Carbonate) Manganese sulfate pentahydrate (MnSO4·5H2O), sodium phosphate dodecahydrate (Na3PO4·12H2O) and anhydrous sodium carbonate (Na2CO3) were pulverized and mixed, and then pure water was added thereto to obtain a composition having the following composition. Manganese sulfate pentahydrate: 12.0 wt% Sodium phosphate dodecahydrate: 18.9 wt% Anhydrous sodium carbonate: 5.2 wt% Pure water: the balance

[0047] The pH of the composition was 11.0. After filling and sealing the composition in a Teflon (registered trademark) resin container with a lid, it was placed in a thermostatic bath and subjected to hydrothermal treatment under the following conditions. Hydrothermal treatment temperature: 180 °C Hydrothermal treatment time: 72 hours Hydrothermal treatment pressure: autogenous pressure

[0048] After hydrothermal treatment, the product cooled to room temperature was washed by adding and stirring a sufficient amount of pure water, and then sodium manganese phosphate carbonate was obtained by suction filtration using a filter paper with a pore size of 0.2 μm. The obtained sodium manganese phosphate carbonate was dried at 110 °C for 4 hours in a vacuum atmosphere, and then further pulverized in a mortar to obtain the sodium manganese phosphate carbonate of this synthesis example. The purity of the sodium manganese phosphate carbonate Na3MnPO4CO3 of this synthesis example was 100% by weight, and no crystallized substances other than Na3MnPO4CO3 were confirmed in its XRD pattern. The Na3MnPO4CO3 of this synthesis example had a crystallite size (WH size) of 172 Å, lattice constants of a = 9.003 Å, b = 6.745 Å, c = 5.166 Å, α = 90.00°, β = 90.16°, and γ = 90.00°, was of the monoclinic crystal system, had a siderenite type (equivalent to the bonsite type) structure, and was a compound belonging to the space group P21 / m. The main XRD peaks were as follows. (020) plane: 2θ = 26.44° Relative intensity = 100 (220) plane: 2θ = 33.25° Relative intensity = 100 (002) plane: 2θ = 34.68° Relative intensity = 72

[0049] Synthesis Example 4 (Synthesis of Sodium Manganese Phosphate Carbonate) Sodium carbonate (Na2CO3) and sodium manganese phosphate (olivine NaMnPO4) were mixed in a stoichiometric ratio of 1:1, and then mixed in a planetary ball mill at 600 rpm for 12 hours under an Ar atmosphere. AB was added to the obtained powder to a weight ratio of 15 wt%, and the mixture was further mixed in a planetary ball mill at 600 rpm for 12 hours under an Ar atmosphere. Then, AB was added again to a weight ratio of 15 wt%, and the mixture was mixed in a planetary ball mill at 400 rpm for 3 hours under an Ar atmosphere to obtain sodium manganese phosphate carbonate Na3MnPO4CO3.

[0050] Synthesis Example 5 (Synthesis of Sodium Nickel Phosphate Carbonate) Nickel sulfate hexahydrate (NiSO4·6H2O), sodium phosphate dodecahydrate (Na3PO4·12H2O), anhydrous sodium carbonate (Na2CO3), and anhydrous sodium sulfite (Na2SO3) were pulverized and mixed, and then pure water was added thereto to obtain a composition having the following composition. Nickel(II) sulfate hexahydrate: 13.7 wt% Sodium phosphate dodecahydrate: 19.6 wt% Sodium carbonate anhydrous: 5.4 wt% Purified water: the balance

[0051] The pH of the composition was 11.0. After filling and sealing the composition in a container with a Teflon (registered trademark) resin lid, it was placed in a thermostatic bath and hydrothermally treated under the following conditions. Hydrothermal treatment temperature: 180 °C Hydrothermal treatment time: 16 hours Hydrothermal treatment pressure: autogenous pressure After hydrothermal treatment, the product cooled to room temperature was washed by adding a sufficient amount of purified water and stirring, and then sodium nickel phosphate carbonate was obtained by suction filtration using a filter paper with a pore size of 0.2 μm.

[0052] The obtained sodium nickel phosphate carbonate was dried at 110 °C for 4 hours under a vacuum atmosphere, and then further pulverized in a mortar to obtain the sodium nickel phosphate carbonate of this synthesis example. The purity of the sodium nickel phosphate carbonate Na3NiPO4CO3 of this synthesis example was 100 wt%, and no crystallized substances other than Na3NiPO4CO3 were confirmed in its XRD pattern. The Na3NiPO4CO3 of this synthesis example had a crystallite size (WH size) of 107 Å, lattice constants of a = 8.833 Å, b = 6.613 Å, c = 5.147 Å, α = 90.00°, β = 89.29°, and γ = 90.00°, had a monoclinic crystal system, had a structure equivalent to the bonsiteite type, and was a compound belonging to the space group P21 / m. The main XRD peaks were as follows. (020) plane: 2θ = 26.91° relative intensity = 96 (220) plane: 2θ = 33.68° relative intensity = 100 (002) plane: 2θ = 34.89° relative intensity = 88

[0053] Synthesis Example 6 (Synthesis of Sodium Cobalt Phosphate Carbonate) Cobalt sulfate heptahydrate (CoSO4·7H2O), sodium phosphate dodecahydrate (Na3PO4·12H2O), and anhydrous sodium carbonate (Na2CO3) were pulverized and mixed, and then pure water was added thereto to obtain a composition having the following composition. Cobalt sulfate heptahydrate: 14.4% by weight Sodium phosphate dodecahydrate: 19.4% by weight Anhydrous sodium carbonate: 5.4% by weight Pure water: the balance

[0054] The pH of the composition was 11.0. After the composition was filled and sealed in a Teflon (registered trademark) resin container with a lid, it was placed in a thermostatic bath and subjected to hydrothermal treatment under the following conditions. Hydrothermal treatment temperature: 180 °C Hydrothermal treatment time: 16 hours Hydrothermal treatment pressure: autogenous pressure

[0055] After the hydrothermal treatment, the product cooled to room temperature was washed by adding and stirring a sufficient amount of pure water, and then sodium cobalt phosphate carbonate was obtained by suction filtration using a filter paper with a pore size of 0.2 μm.

[0056] The obtained sodium cobalt phosphate carbonate was dried at 110 °C for 4 hours under a vacuum atmosphere, and then further pulverized with a mortar to obtain the sodium cobalt phosphate carbonate of this synthesis example. The purity of the sodium cobalt phosphate carbonate Na3CoPO4CO3 of this synthesis example was 100% by weight, and no crystallized substances other than Na3CoPO4CO3 were confirmed in its XRD pattern. The Na3CoPO4CO3 of this synthesis example had a crystallite diameter (WH diameter) of 220 Å, lattice constants of a = 8.903 Å, b = 6.637 Å, c = 5.148 Å, α = 90.00°, β = 89.51°, and γ = 90.00°, a crystal system of monoclinic system, a structure equivalent to the bonsiteite type, and belonged to the space group P21 / m. The main XRD peaks were as follows. Plane (020): 2θ = 26.88° Relative intensity = 89 Plane (220): 2θ = 33.63° Relative intensity = 94 (002) Plane: 2θ = 34.85°, Relative Intensity = 100

[0057] Example 1 Using sodium iron phosphate carbonate Na3FePO4CO3 of Synthesis Example 1 as the positive electrode active material, a sodium ion secondary battery (half cell) was fabricated, and a charge-discharge test was carried out at room temperature in the range of -1.2 V to 1.3 V with respect to the Ag / AgCl reference electrode at a current density of 2 mA / cm 2 The discharge capacity of the first cycle was 134.3 mAh / g.

[0058] Example 2 A sodium ion secondary battery (half cell) was fabricated and a charge-discharge test was carried out in the same manner as in Example 1, except that sodium iron phosphate carbonate Na3FePO4CO3 obtained in Synthesis Example 2 was used as the positive electrode active material. The discharge capacity of the first cycle was 159.4 mAh / g.

[0059] Example 3 Using sodium iron phosphate carbonate Na3FePO4CO3 obtained in Synthesis Example 2 as the positive electrode active material, a sodium ion secondary battery (coin cell) was fabricated. A charge-discharge test was carried out at room temperature in the range of -1.2 V to 1.3 V with respect to the cell voltage at a current density of 2 mA / cm 2 The discharge capacity of the first cycle was 160.5 mAh / g.

[0060] Example 4 A sodium ion secondary battery (half cell) was fabricated and a charge-discharge test was carried out in the same manner as in Example 1, except that sodium manganese phosphate carbonate Na3MnPO4CO3 obtained in Synthesis Example 4 was used as the positive electrode active material. The discharge capacity of the first cycle was 134.5 mAh / g.

[0061] Example 5 A sodium ion secondary battery (half cell) was fabricated and a charge-discharge test was carried out in the same manner as in Example 1, except that sodium cobalt phosphate carbonate Na3CoPO4CO3 obtained in Synthesis Example 6 was used as the positive electrode active material. The discharge capacity of the first cycle was 141.2 mAh / g.

[0062] Reference Example (Non-aqueous Sodium Ion Secondary Battery) The discharge capacity of a sodium ion secondary battery equipped with a known non-aqueous electrolyte was measured by a method according to Japanese Patent No. 6270056.

[0063] In the same manner as in Example 1, a non-aqueous electrolyte in which sodium hexafluorophosphate (NaPF6) with a concentration of 1 M was dissolved in a mixed solvent containing ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 1:1 as the electrolyte, a sodium iron phosphate carbonate Na3FePO4CO3 obtained in Synthesis Example 2 as the positive electrode active material, and a sodium ion secondary battery (coin cell) equipped with metallic sodium as the negative electrode was fabricated. At a current density of 2 mA / cm 2 A charge-discharge test was conducted at room temperature with the cell voltage in the range of 1.5 to 4.65 V. The discharge capacity in the first cycle was 128.0 mAh / g. The operating voltage range of the reference example is 3 V or higher, which is wider than that of the example. However, since the discharge capacity is lower than that of Example 3, it can be seen that the aqueous sodium ion secondary battery of this example exhibits a higher electrochemical capacity even compared to a known non-aqueous sodium ion secondary battery.

Explanation of Symbols

[0064] 1: Sodium ion secondary battery (coin cell) 2: Negative electrode cap 3: Positive electrode container 4: Negative electrode current collector 5: Negative electrode mixture 6: Positive electrode current collector 7: Positive electrode mixture 8: Separator 9: Space (electrolyte) 10: Gasket The entire contents of the specification, claims, abstract, and drawings of Japanese Patent Application No. 2019-098326 filed on May 27, 2019 are hereby incorporated by reference and made a part of the disclosure of this specification of the present disclosure.

Claims

1. A positive electrode, a negative electrode, an electrolytic solution, and a separator are provided. The positive electrode contains at least a sodium transition metal polyanion represented by the general formula Na ３－ｘ MPO ４ CO ３ (where M is at least one selected from the group consisting of Fe, Mn, Ni, and Co, and x is 0 or more and 2 or less), and has a crystallite diameter of 90 Å or more and 280 Å or less. The aqueous sodium ion secondary battery is characterized by comprising an aqueous electrolytic solution as the electrolytic solution.

2. The aqueous sodium-ion secondary battery according to claim 1, characterized in that part or all of the surface of the sodium transition metal polyanion has carbon.

3. The aqueous electrolyte is Na ２ SO ４ , NaNO ３ and NaClO ４ The aqueous sodium ion secondary battery according to claim 1 or 2, comprising one or more selected from the group of

4. The aqueous electrolyte is NaClO ４ The aqueous sodium ion secondary battery according to any one of claims 1 to 3, which contains

5. The aqueous sodium secondary battery according to claim 4, wherein the electrolyte concentration in the electrolyte is 5 mol / kg or more.

6. The negative electrode contains at least NaTi ２ (PO ４ ) ３ The aqueous sodium ion secondary battery according to any one of claims 1 to 5, which contains

7. wherein the positive electrode contains at least a sodium transition metal polyanion represented by the general formula Na ３－ｘ FePO ４ CO ３ (x is 0 or more and 2 or less), and has a crystallite diameter of 90 Å or more and 280 Å or less, and the negative electrode contains at least NaTi ２ (PO ４ ) ３ and the aqueous electrolyte contains at least NaClO ４ The aqueous sodium ion secondary battery according to any one of claims 1 to 6.

8. At least the general formula Na ３－ｘ MPO ４ CO ３ (M is at least any one selected from the group of Fe, Mn, Ni and Co, x is 0 or more and 2 or less), and a positive electrode active material for an aqueous sodium ion secondary battery containing a sodium transition metal polyanion having a crystallite size of 90 Å or more and 280 Å or less.

9. The positive electrode active material for an aqueous sodium-ion battery according to claim 8, wherein M contains at least Fe.

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