Active material and sodium ion battery

Abstract

To provide an active material and a sodium ion battery excellent in charge / discharge characteristics.SOLUTION: The active material includes a compound represented by Na2M1xFe (CN) 6. (1). In the formula (1), M1 is at least one selected from Mn, Ni, Co, and Cu, and x satisfies 0.95 <x <1.05. The compound has a rhombohedral crystal structure. In the active material, when the compound is subjected to powder X-ray diffraction measurement using CuK α rays, a diffraction peak is confirmed in a range of 16 ° ≤ 2 θ ≤ 18 °. The half-width of the first diffraction peak having the highest intensity among the diffraction peaks is 0.2 ° or more and 0.4 ° or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an active material and a sodium ion battery. [Background technology]

[0002] Lithium-ion secondary batteries are widely used as a power source for mobile devices such as mobile phones and laptops, as well as hybrid cars, etc. However, the production areas of key materials such as lithium are unevenly distributed, and stable development and sales of lithium-ion secondary batteries involve price hikes and procurement risks.

[0003] In contrast, sodium is found in greater quantities in the Earth's crust than lithium and is not unevenly distributed, making it a popular choice as a key battery material.

[0004] Prussian blue analogues have been reported as one of the positive electrode active materials for sodium ion batteries. For example, Non-Patent Document 1 describes the use of Na as a positive electrode active material. 1.45 Ni[Fe(CN)6] 0.87 It is disclosed that a sodium-ion battery using 3.02H2O and an aqueous electrolyte solution can be repeatedly charged and discharged 8,000 times. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Liuxue Shen et al., Chemical Engineering Journal, Volume 388, 15 May 2020, 124228. Summary of the Invention [Problem to be solved by the invention]

[0006] The development of new active materials is required for the advancement of sodium-ion batteries.

[0007] The present disclosure has been made in view of the above problems, and an object thereof is to provide an active material and a sodium ion battery having excellent charge and discharge characteristics.

Means for Solving the Problems

[0008] In order to solve the above problems, the following means are provided.

[0009] (1) The active material according to the first aspect contains a compound represented by Na2M1 x Fe(CN)6···(1). In formula (1), M1 is at least one selected from Mn, Ni, Co, and Cu, and x satisfies 0.95 < x < 1.05. The compound has a rhombohedral crystal structure. When the compound is subjected to powder X-ray diffraction measurement using CuKα rays for this active material, diffraction peaks are confirmed in the range of 16° ≤ 2θ ≤ 18°. The half-value width of the first diffraction peak with the strongest intensity among the diffraction peaks is 0.2° or more and 0.4° or less.

[0010] (2) When the compound is subjected to powder X-ray diffraction measurement using CuKα rays for the active material according to the above aspect, diffraction peaks may be confirmed in the range of 23.5° ≤ 2θ ≤ 25.5°. The intensity ratio obtained by dividing the intensity of the second diffraction peak attributable to the (220) plane among the diffraction peaks confirmed in the range of 23.5° ≤ 2θ ≤ 25.5° by the intensity of the first diffraction peak may be 0.16 or more and 0.40 or less.

[0011] (3) The active material according to the above aspect may contain a compound represented by Na2Co y M2 z Fe(CN)6···(2). In formula (2), M2 is at least one selected from Mn, Ni, and Cu, y satisfies 0.50 ≤ y < 1.05, z satisfies 0 < z ≤ 0.50, and y + z satisfies 0.95 < y + z < 1.05.

[0012] (4) The sodium ion battery according to the second aspect includes the positive electrode according to the above aspect and a negative electrode.

[0013] (5) The sodium ion battery according to the above aspect may further include a separator and a non-aqueous electrolyte. The separator separates the positive electrode and the negative electrode. The non-aqueous electrolyte includes an electrolyte salt and an electrolyte solvent. The electrolyte salt may include sodium perchlorate, and the electrolyte solvent may include ethylene carbonate and diethyl carbonate.

[0014] (6) In the sodium ion battery according to the above aspect, the negative electrode may be metallic sodium or hard carbon. [Effects of the Invention]

[0015] The active material and sodium ion battery according to the above embodiment have excellent charge / discharge characteristics. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a cross-sectional view of a sodium ion battery according to a first embodiment. [Figure 2] 1 shows the results of X-ray diffraction for Examples 1 to 5 and Reference Example 1. [Figure 3] 3 shows the charge / discharge characteristics of the sodium ion battery of Example 1. [Figure 4] 3 shows the charge / discharge characteristics of the sodium ion battery of Example 2. [Figure 5] 10 shows the charge / discharge characteristics of the sodium ion battery of Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, the embodiments will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may show characteristic portions enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate changes can be made within the scope of the present invention.

[0018] "Sodium-ion battery" FIG. 1 is a schematic diagram of a sodium ion battery according to a first embodiment. The sodium ion battery 100 shown in FIG. 1 includes a power generating element 40, an exterior body 50, and an electrolyte. The exterior body 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside via a pair of terminals 60, 62 connected to the power generating element 40. The electrolyte is contained in the exterior body 50. Although FIG. 1 illustrates a case where one power generating element 40 is contained in the exterior body 50, multiple power generating elements 40 may be stacked.

[0019] (power generating element) The power generating element 40 includes a separator 10 , a positive electrode 20 , and a negative electrode 30 .

[0020] <Positive electrode> The positive electrode 20 includes, for example, a positive electrode current collector 22 and a positive electrode active material layer 24 .

[0021] [Positive electrode current collector] The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 extends continuously in the xy plane. The positive electrode current collector 22 is, for example, a thin metal plate made of aluminum, copper, nickel, titanium, stainless steel, or the like. The positive electrode current collector 22 is not limited to a plate-shaped member and may be, for example, a sponge-like porous metal film. The surface of the metal film constituting the positive electrode current collector 22 may be coated with a conductive layer. The conductive layer may, for example, contain a conductive material, a binder, and, optionally, a thickener in a predetermined blend ratio. Examples of conductive materials include conductive carbon materials such as carbon black, acetylene black, graphite, carbon fiber, and carbon nanotubes. Examples of binders include acrylic binders and styrene butadiene rubber binders. Examples of thickeners include carboxymethyl cellulose. Lightweight aluminum is suitable for use as the positive electrode current collector 22. The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[0022] Further, the positive electrode current collector 22 may be a laminate in which a metal layer is formed on the surface of a resin layer. For the metal layer, for example, aluminum, copper, nickel, titanium, stainless steel, etc. can be used, and it is preferable to use aluminum which is light in weight. For the resin layer, for example, polyethylene terephthalate (PET), polyimide (PI), polyamideimide (PAI), polypropylene (PP), polyethylene (PE), etc. can be used.

[0023] [Positive electrode active material layer] The positive electrode active material layer 24 is in contact with at least one surface of the positive electrode current collector 22. The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may have a conductive assistant and a binder.

[0024] The positive electrode active material is Na2M1 x It contains a compound represented by Fe(CN)6···(1). In the formula (1), M1 is at least one selected from Mn, Ni, Co, and Cu, and x satisfies 0.95 < x < 1.05.

[0025] In the above formula (1), M1 may contain two elements selected from Mn, Ni, Co, and Cu. When M1 contains two or more elements, the cycle characteristics of the non-aqueous electrolyte battery are improved. For example, the positive electrode active material is Na2Co y M2 z It may contain a compound represented by Fe(CN)6···(2). In the formula (2), M2 is at least one selected from Mn, Ni, and Cu, y satisfies 0.50 ≤ y < 1.05, z satisfies 0 < z ≤ 0.50, and y + z satisfies 0.95 < y + z < 1.05. In the formula (2), the combination of Co and M2 corresponds to M1 in the formula (1).

[0026] When the compound constituting the positive electrode active material is measured by powder X-ray diffraction (XRD), a diffraction pattern is confirmed. The crystal structure of the compound constituting the positive electrode active material can be confirmed from the diffraction pattern. The crystal structure of the positive electrode active material is a rhombohedral crystal structure.

[0027] The diffraction pattern obtained by powder X-ray diffraction (XRD) measurement of the compound constituting the positive electrode active material has diffraction peaks in the range of 16°≦2θ≦18°. The peak with the strongest intensity among the diffraction peaks in the range of 16°≦2θ≦18° is hereinafter referred to as the first diffraction peak. The half-width of the first diffraction peak is 0.2 or more and 0.4 or less. The half-width of the diffraction peak is generally an index representing the crystallinity of the crystal.

[0028] The diffraction pattern may have a diffraction peak in the range of 23.5°≦2θ≦25.5°. The diffraction peaks occurring in the range of 23.5°≦2θ≦25.5° include a (220) peak due to reflection from the (220) plane of the compound. The (220) peak in the range of 23.5°≦2θ≦25.5° is hereinafter referred to as the second diffraction peak. The intensity ratio between the first diffraction peak and the second diffraction peak is preferably, for example, 0.16 or more and 0.40 or less. The intensity ratio between the first diffraction peak and the second diffraction peak is calculated by dividing the intensity of the second diffraction peak by the intensity of the first diffraction peak. When this intensity ratio is in the above range, the rate characteristics of the sodium-ion battery 100 tend to improve, although the reason is not clear.

[0029] The diffraction pattern may have a diffraction peak in the range of 34°≦2θ≦35°. The peak with the strongest intensity among the diffraction peaks in the range of 34°≦2θ≦35° is hereinafter referred to as the third diffraction peak. The intensity ratio between the first diffraction peak and the third diffraction peak is preferably, for example, 0.4 or more and 0.70 or less. The intensity ratio between the first diffraction peak and the third diffraction peak is determined by dividing the intensity of the third diffraction peak by the intensity of the first diffraction peak. When this intensity ratio is in the above range, the rate characteristics of the sodium-ion battery 100 tend to improve, although the reason is not clear.

[0030] Na2M1 x The compound represented by Fe(CN)6 (1) may have a composition ratio that deviates from the stoichiometric composition as long as the crystal structure can be confirmed by powder X-ray diffraction (XRD). For example, the composition ratio of Na and (CN) can vary to a certain extent based on the stoichiometric composition.

[0031] The conductive additive in the positive electrode active material layer 24 enhances electronic conductivity between the positive electrode active materials. Examples of the conductive additive include carbon powder, carbon nanotubes, carbon materials, metal powder, a mixture of carbon materials and metal powder, and conductive oxides. Examples of the carbon powder include carbon black, acetylene black, and ketjen black. Examples of the metal powder include powders of copper, nickel, stainless steel, iron, and the like.

[0032] The binder in the positive electrode active material layer 24 binds the positive electrode active material together. Known binders can be used. The binder is, for example, a fluororesin. Examples of the binder include polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), polyacrylic acid and its copolymers, metal ion crosslinked polyacrylic acid and its copolymers, maleic anhydride-grafted polypropylene (PP) or polyethylene (PE), and mixtures thereof. PVDF is particularly preferred as the binder used in the positive electrode active material layer.

[0033] A sintered plate or a compact may also be used for the positive electrode active material layer 24. When a sintered plate or a compact is used, it does not need to contain a conductive additive or a binder.

[0034] <Negative electrode> The negative electrode 30 includes, for example, a negative electrode current collector 32 and a negative electrode active material layer 34 .

[0035] [Negative electrode current collector] The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 is not limited to a plate-shaped member, and may be a sponge-like porous metal film, similar to the positive electrode current collector 22. The negative electrode current collector 32 may be the same as the positive electrode current collector 22.

[0036] [Negative electrode active material layer] The negative electrode active material layer 34 contains a negative electrode active material and may contain a conductive additive and a binder.

[0037] The negative electrode active material may be any compound capable of absorbing and releasing ions, and known active materials used in sodium-ion batteries can be used. Examples of the negative electrode active material include carbon materials, metals or alloys capable of reacting with sodium, composite materials of these metals or alloys with carbon materials, oxides, sulfur-modified polyacrylonitrile, and metallic sodium. Examples of carbon materials include natural graphite, artificial graphite, mesocarbon microbeads, mesocarbon fiber (MCF), cokes, glassy carbon, and fired organic compounds. The negative electrode active material is preferably metallic sodium or hard carbon.

[0038] The conductive additive in the negative electrode active material layer 34 enhances electronic conductivity between the negative electrode active materials. The conductive additive in the negative electrode active material layer 34 may be the same as the conductive additive in the positive electrode active material layer 24.

[0039] The binder in the negative electrode active material layer 34 binds the negative electrode active material together. The binder in the negative electrode active material layer 34 may be the same as the binder in the positive electrode active material layer 24.

[0040] <separator> The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 and the negative electrode 30 and prevents short-circuiting between the positive electrode 20 and the negative electrode 30. The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30. Sodium ions can pass through the separator 10.

[0041] The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or laminate of a polyolefin film. The separator 10 may also be a stretched membrane of a mixture of polyethylene, polypropylene, or the like. The separator 10 may also be a fibrous nonwoven fabric or a glass fiber nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may also be, for example, a solid electrolyte. Examples of the solid electrolyte include a polymer solid electrolyte, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, and a chloride-based solid electrolyte. The separator 10 may also be an inorganic-coated separator. The inorganic-coated separator is formed by coating the surface of the above-mentioned film with a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica. The inorganic-coated separator has excellent heat resistance and suppresses the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

[0042] <Electrolyte> The electrolytic solution is sealed in the exterior body 50 and impregnates the power generating element 40. The electrolytic solution includes, for example, an electrolyte solvent and an electrolyte salt. The electrolyte salt is dissolved in the electrolyte solvent.

[0043] The electrolyte may be either an aqueous electrolyte or a non-aqueous electrolyte. Specific examples of the non-aqueous electrolyte will be described below.

[0044] The electrolyte solvent is not particularly limited as long as it is a solvent generally used in nonaqueous electrolyte batteries. The electrolyte solvent may include, for example, a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, or a chain ester compound. The electrolyte solvent may include a mixture of these compounds in any ratio. Examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), vinylene carbonate, etc. Examples of the chain carbonate compound include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. Examples of the cyclic ester compound include γ-butyrolactone, etc. Examples of the chain ester compound include propyl propionate, ethyl propionate, ethyl acetate, etc. The electrolyte solvent may include, for example, ethylene carbonate (EC) and diethyl carbonate (DEC).

[0045] The electrolyte salt is, for example, a sodium salt. Examples of the electrolyte include NaPF6, NaClO4, NaBF4, Na2SO4, NaNO3, NaCF3SO3, NaCF3CF2SO3, NaC(CF3SO2)3, NaN(CF3SO2)2, NaN(CF3CF2SO2)2, NaN(CF3SO2)(C4F9SO2), NaN(CF3CF2CO)2, NaBOB, and NaN(FSO2)2. One type of sodium salt may be used alone, or two or more types may be used in combination. The electrolyte salt preferably contains, for example, sodium perchlorate (NaClO4). The concentration of NaClO4 is, for example, 1 mol / L.

[0046] <Exterior body> The exterior body 50 seals the power generating element 40 and the electrolyte therein. The exterior body 50 prevents the electrolyte from leaking to the outside and prevents moisture and the like from entering the sodium ion battery 100 from the outside.

[0047] 1, the exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

[0048] For example, aluminum foil can be used as the metal foil 52. A polymer film such as polypropylene can be used as the resin layer 54. The materials constituting the inner and outer resin layers 54 may be different. For example, the outer material may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the inner polymer film may be made of polyethylene (PE), polypropylene (PP), or the like.

[0049] <Terminal> Terminals 60 and 62 are connected to the negative electrode 30 and the positive electrode 20, respectively. The terminal 62 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 60 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 electrically connect the power generating element to the outside. The terminals 60 and 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screw fastening. It is preferable to protect the terminals 60 and 62 with insulating tape to prevent short circuits.

[0050] The sodium ion battery 100 is fabricated by preparing and assembling the negative electrode 30, the positive electrode 20, the separator 10, the electrolyte, and the exterior body 50. An example of a method for manufacturing the sodium ion battery 100 will be described below.

[0051] The positive electrode 20 is produced, for example, by sequentially carrying out a slurry production step, an electrode application step, a drying step, and a rolling step.

[0052] The slurry preparation process involves mixing a positive electrode active material, a conductive additive, and a binder in a solvent to produce a slurry. Examples of the solvent include water and N-methyl-2-pyrrolidone. The positive electrode active material can be obtained by ball milling NaFe(CN)6 and M1(CH3COO)2·4H2O, washing and drying the mixture, and then calcining it. M1 is at least one element selected from Mn, Ni, Co, and Cu. The ball milling is performed, for example, at 300 rpm for two hours. Compounds of formula (2) can also be obtained by using multiple types of M1(CH3COO)2·4H2O with different M1. The ratios of x, y, and z in formulas (1) and (2) can be altered by changing the mixing ratio. Calcination is performed, for example, at 170°C for eight hours in an argon atmosphere. The half-width and intensity of the diffraction peaks can be altered by changing the ball mill mixing and calcination conditions.

[0053] The electrode coating step is a step of coating the surface of the positive electrode current collector 22 with a slurry. The method of coating the slurry is not particularly limited. For example, a slit die coating method or a doctor blade method can be used as the method of coating the slurry. The slurry is coated, for example, at room temperature.

[0054] The drying step is a step of removing the solvent from the slurry. For example, the positive electrode current collector 22 coated with the slurry is dried in an atmosphere at 80°C to 350°C.

[0055] The rolling step is performed as necessary. The rolling step is a step of applying pressure to the positive electrode active material layer 24 to adjust the density of the positive electrode active material layer 24. The rolling step is performed, for example, using a roll press device or the like.

[0056] The negative electrode 30 differs only in the materials constituting the slurry, and can be produced in the same manner as the positive electrode 20. The separator 10 and the exterior body 50 can be commercially available products.

[0057] Next, the prepared positive electrode 20 and negative electrode 30 are stacked together with the separator 10 positioned between them, thereby preparing the power generating element 40.

[0058] Finally, the power generation element 40 is sealed in the exterior body 50. The electrolyte solution is poured into the exterior body 50. After the electrolyte solution is poured, the pressure is reduced, heating, etc. is performed, so that the electrolyte solution is impregnated into the power generation element 40. The sodium ion battery 100 is obtained by sealing the exterior body 50 by applying heat, etc. Note that the power generation element 40 may be impregnated with the electrolyte solution instead of pouring the electrolyte solution into the exterior body 50. After the electrolyte solution is poured into the power generation element, it is preferable to leave it to stand for 24 hours.

[0059] The sodium ion battery 100 according to the first embodiment has high rate characteristics due to the inclusion of the compound represented by the above formula (1) having a predetermined crystallinity and a predetermined crystal structure. The crystallinity and crystal structure of the compound affect ion diffusion within the active material. When the crystallinity and crystal structure of the compound satisfy predetermined conditions, ion diffusion within the active material becomes smooth, and it is believed that the rate characteristics of the sodium ion battery 100 are improved.

[0060] The above describes the embodiments of the present disclosure in detail with reference to the drawings. However, each configuration and combination thereof in each embodiment is an example, and addition, omission, substitution, and other modifications of the configuration are possible within the scope that does not deviate from the spirit of the present disclosure. [Example]

[0061] "Example 1" First, a positive electrode active material was prepared. 3 mmol of Na4Fe(CN)6 and 3 mmol of Co(CH3COO)2 4H2O were mixed in a ball mill. The ball mill was rotated at 300 rpm for 2 hours. The mixed sample was washed, dried, and calcined in an argon atmosphere at 170°C for 8 hours. The prepared sample was analyzed using a powder X-ray diffractometer. The results are shown in Figure 2. Figure 2 also shows the X-ray diffraction results for Examples 2 to 5 and Reference Example 1, which will be described later. Reference Example 1 is monoclinic Na2MnFe(CN)6. Note that monoclinic Na2MnFe(CN)6 was prepared by washing the mixed sample, drying it, and calcining it in air at 100°C for 24 hours.

[0062] X-ray diffraction analysis confirmed that the sample prepared in Example 1 was rhombohedral Na2CoFe(CN)6. The X-ray diffraction pattern of Example 1 had a first diffraction peak in the range of 16°≦2θ≦18° and a second diffraction peak in the range of 23.5°≦2θ≦25.5°. The intensity ratio between the first and second diffraction peaks was 0.23. The X-ray diffraction pattern of Example 1 also had a third diffraction peak in the range of 34°≦2θ≦35°, and the intensity ratio between the first and third diffraction peaks was 0.55.

[0063] Next, the positive electrode active material, conductive additive, binder, and solvent were mixed to prepare a positive electrode slurry. Carbon black was used as the conductive additive. Polyvinylidene fluoride (PVDF) was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. The mass ratio of the positive electrode active material, conductive additive, and binder was 70 wt %:10 wt %:20 wt %.

[0064] Next, the positive electrode slurry was applied to one side of an aluminum foil having a thickness of 15 μm. The solvent was removed from the positive electrode slurry in a drying furnace to prepare a positive electrode active material layer. The positive electrode active material layer was pressed with a roll press to prepare a positive electrode. After drying, the amount of positive electrode active material supported in the positive electrode active material layer was 22 mg / cm. 2 It was decided.

[0065] Next, a metallic sodium foil was attached to one side of the 10 μm thick copper foil to form a negative electrode active material layer. The copper foil served as a negative electrode current collector, and the metallic sodium foil served as a negative electrode active material layer.

[0066] Next, an electrolyte solution was prepared. The solvent for the electrolyte solution was ethylene carbonate (EC):diethyl carbonate (DEC) = 50% by volume:50% by volume. NaClO4 was added to the electrolyte solution as an electrolyte salt. The concentration of NaClO4 was 1.0 mol / L.

[0067] (Preparation of non-aqueous electrolyte battery for evaluation) The fabricated negative and positive electrodes were stacked with a glass fiber separator between them so that the positive and negative active material layers faced each other to obtain a laminate. This laminate was inserted into an aluminum laminate film exterior and heat-sealed except for one peripheral location to form a closed opening. Finally, the above-mentioned electrolyte solution was injected into the exterior, and the remaining location was heat-sealed while reducing the pressure using a vacuum sealer to produce a sodium-ion battery. The fabricated sodium-ion battery was left to stand for 24 hours.

[0068] (output characteristics evaluation) The fabricated sodium ion battery was charged and discharged under the following conditions using a secondary battery charge / discharge tester (manufactured by Hokuto Denko Corporation). The voltage range was 2.5 V to 4.2 V. Charging was performed at a constant current of 0.1 C. Discharging was performed at 0.1 C and 0.25 C, and the ratio of the discharge capacity at 0.25 C to the discharge capacity at 0.1 C, taken as 100%, (rate characteristics (unit: %)) was determined. Figure 3 shows the charge / discharge characteristics of the sodium ion battery of Example 1.

[0069] The cycle characteristics were measured using a CC charge-CC discharge format. Specifically, the battery was first charged at a constant current of 0.1C up to a maximum charge voltage of 4.2V. It was then discharged at a constant current of 0.1C until the battery voltage reached 2.5V. The discharge capacity after the end of charge and discharge was measured and counted as one cycle, and the battery capacity Q1 was calculated. This battery capacity Q1 was the initial discharge capacity.

[0070] Next, the battery was charged at a constant current of 0.1 C at a charge rate up to a maximum charge voltage of 4.2 V. It was then discharged at a constant current of 0.1 C at a discharge rate until the battery voltage reached 2.5 V. This charge / discharge cycle was counted as two cycles, and 10 charge / discharge cycles were performed. The discharge capacity after the end of the charge / discharge cycle was measured, and the battery capacity Q2 after the 10-cycle test was calculated.

[0071] The cycle characteristic is a ratio calculated by dividing the battery capacity Q2 after 10 cycles by the initial battery capacity Q1. The cycle characteristic of Example 1 was 72.7%.

[0072] "Examples 2 to 7" Examples 2 to 7 differ from Example 1 in that the material of the positive electrode active material was changed. The material of the positive electrode active material was changed by changing the type and mixing ratio of raw materials when preparing the positive electrode active material. The raw materials used in each example are shown below. The composition of the prepared positive electrode active material is summarized in Table 1, which will be described later. Example 2: 3 mmol NaFe(CN) + 2.25 mmol Co(CHCOO) 4H0 + 0.75 mmol Mn(CHCOO) 4H0 Example 3: 3 mmol NaFe(CN) + 1.5 mmol Co(CHCOO) 3H0 + 1.5 mmol Mn(CHCOO) 4H0 Example 4: 3 mmol NaFe(CN) + 0.75 mmol Co(CHCOO) 4H0 + 2.25 mmol Mn(CHCOO) 4H0 Example 5: 3 mmol NaFe(CN) + 3 mmol Mn(CHCOO) 4H0 Example 6: 3 mmol NaFe(CN) + 2.25 mmol Co(CHCOO) 4H0 + 0.75 mmol Ni(CHCOO) 4H0 Example 7: 3 mmol NaFe(CN) + 2.25 mmol Co(CHCOO) 4H0 + 0.75 mmol Cu(CHCOO) 4H0

[0073] In Examples 2 to 7, X-ray diffraction and charge / discharge characteristics were measured in the same manner as in Example 1. Figure 2 shows the X-ray diffraction results for Examples 2 to 5. Figure 4 shows the charge / discharge characteristics of Example 2. Figure 5 shows the charge / discharge characteristics of Example 3.

[0074] "Examples 8 and 9" Examples 8 and 9 differ from Example 2 in that the half width of the first diffraction peak of the positive electrode active material was changed. The half width of the first diffraction peak was changed by changing the mixing conditions and firing conditions of the raw materials when producing the positive electrode active material described below. In Example 8, the mixing was carried out in the ball mill at a rotation speed of 400 rpm for 2 hours. The firing in Example 9 was carried out in an argon atmosphere at 170° C. for 12 hours. In Examples 8 and 9, similarly to Example 1, X-ray diffraction and charge / discharge characteristics were measured.

[0075] "Examples 10 to 13" Examples 10 to 13 differ from Example 2 in that the intensity ratio between the first diffraction peak and the second diffraction peak of the positive electrode active material was changed. This intensity ratio was changed by changing the firing conditions when producing the positive electrode active material. In Example 12, the reaction was carried out in an argon atmosphere at 140° C. for 8 hours. In Example 13, the reaction was carried out in an argon atmosphere at 190° C. for 8 hours. Example 14 was carried out in an argon atmosphere at 120° C. for 8 hours. Example 15 was carried out in an argon atmosphere at 210° C. for 8 hours. In Examples 12 to 15, similarly to Example 1, X-ray diffraction and charge / discharge characteristics were measured.

[0076] "Comparative Examples 1 to 14" Comparative Examples 1 to 14 differ from any of Examples 1 to 7 in that the half width of the first diffraction peak of the positive electrode active material was changed. The half width of the first diffraction peak was changed by changing the mixing conditions and firing conditions of the raw materials when producing the positive electrode active material. In Comparative Examples 1 to 7, the mixing was carried out in a ball mill at a rotation speed of 500 rpm for 4 hours. The firing conditions for Comparative Examples 8 to 14 were 170° C. in an argon atmosphere for 36 hours. In Comparative Examples 1 to 8, similarly to Example 1, X-ray diffraction and charge / discharge characteristics were measured.

[0077] The measurement results of Examples 1 to 13 and Comparative Examples 1 to 14 are summarized in Table 1.

[0078] [Table 1]

[0079] Examples 1 to 13 had improved rate characteristics compared to Comparative Examples 1 to 14 with the same composition. Compounds with a small half-width of the first diffraction peak have high crystallinity. If the crystallinity of a compound is too high, the ion diffusion distance becomes long, inhibiting the free movement of ions, and it is thought that this reduces the rate characteristics of the nonaqueous electrolyte battery. On the other hand, compounds with a large half-width of the first diffraction peak have many crystal grain boundaries, and the orientation of each crystal grain is not aligned. It is thought that the disorder in the crystal grain boundaries and crystal orientation interrupts the ion diffusion path, thereby reducing the rate characteristics of the sodium-ion battery. [Explanation of symbols]

[0080] 10 Separator 20 positive electrode 22 Positive electrode current collector 24 Cathode active material layer 30 negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generating element 50 Exterior body 52 Metal foil 54 Resin layer 60, 62 terminals 100 sodium ion batteries

Claims

1. Na ２ M1 ｘ Fe(CN) ６ ... (1) In formula (1), M1 is at least one selected from Mn, Ni, Co, and Cu, and x satisfies 0.95<x<1.05; The compound has a rhombohedral crystal structure, When the compound is subjected to powder X-ray diffraction measurement using CuKα radiation, a diffraction peak is confirmed in the range of 16°≦2θ≦18°, The active material has a half-value width of a first diffraction peak having the highest intensity among the diffraction peaks of 0.2° or more and 0.4° or less.

2. When the compound is subjected to powder X-ray diffraction measurement using CuKα radiation, a diffraction peak is confirmed in the range of 23.5°≦2θ≦25.5°, 2. The active material according to claim 1, wherein the intensity ratio of the intensity of a second diffraction peak due to a (220) plane among diffraction peaks observed in the range of 23.5°≦2θ≦25.5° divided by the intensity of the first diffraction peak is 0.16 or more and 0.40 or less.

3. Na ２ Co ｙ M2 ｚ Fe(CN) ６ ... (2) 2. The active material according to claim 1, wherein, in formula (2), M2 is at least one selected from Mn, Ni, and Cu, y satisfies 0.50≦y<1.05, z satisfies 0<z≦0.50, and y+z satisfies 0.95<y+z<1.

05.

4. A sodium ion battery comprising a positive electrode containing the active material according to claim 1 and a negative electrode.

5. Further comprising a separator and a non-aqueous electrolyte; the separator separates the positive electrode and the negative electrode, the nonaqueous electrolyte solution contains an electrolyte salt and an electrolyte solvent, the electrolyte salt includes sodium perchlorate; 5. The sodium-ion battery of claim 4, wherein the electrolyte solvent comprises ethylene carbonate and diethyl carbonate.

6. 5. The sodium ion battery of claim 4, wherein the negative electrode comprises metallic sodium or hard carbon.

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