Positive electrode active material for sodium-ion secondary battery, method for manufacturing positive electrode active material for sodium-ion secondary battery, and sodium-ion secondary battery

Abstract

The present invention addresses the problem of providing a positive electrode active material for a sodium-ion secondary battery, said positive electrode active material having a large discharge capacity and a high average operating voltage in the obtained sodium-ion secondary battery. The positive electrode active material for a sodium-ion secondary battery according to the present invention contains a composite oxide that is expressed by a specific formula (1). The crystal structure of the composite oxide belongs to a space group R-3 m, and in a diffraction chart that is obtained by performing X-ray diffraction measurement, the ratio of the peak intensity of 003 reflection of the composite oxide to the peak intensity of 104 reflection of the composite oxide is 1.00 or more.

Description

Positive electrode active material for sodium ion secondary battery, method for producing positive electrode active material for sodium ion secondary battery, and sodium ion secondary battery

[0001] The present invention relates to a positive electrode active material for a sodium ion secondary battery and a method for producing the same, and also to a sodium ion secondary battery having a positive electrode containing the positive electrode active material for a sodium ion secondary battery.

[0002] In recent years, the electrification of automobiles and the performance improvement of electronic devices have progressed rapidly, and secondary batteries used as power sources for these devices are required to have even higher energy densities. Lithium-ion secondary batteries are widely used as such secondary batteries due to their high energy density, and demand for them is expected to further expand in the future. Lithium-ion secondary batteries require lithium because lithium ions act as charge carriers. However, because lithium is a rare metal and its production areas are unevenly distributed, widespread use of lithium is not necessarily desirable in terms of stable supply.

[0003] In view of the above, sodium ion secondary batteries, in which sodium ions serve as charge carriers instead of lithium ions, have been studied as secondary batteries to replace lithium ion secondary batteries. Sodium ion secondary batteries usually include a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and an electrolyte that is interposed between the positive electrode and the negative electrode and conducts sodium ions. Various compounds have been studied as positive electrode active materials for sodium ion secondary batteries. For example, Patent Document 1 discloses Na ｘ MO ２ (wherein M is two or more elements selected from the group consisting of metal elements excluding alkali metal elements, and x is a value exceeding 0 and not exceeding 1) has been proposed.

[0004] JP 2010-080424 A

[0005] Recently, there has been a demand for achieving higher energy densities in sodium ion secondary batteries. The present inventors have studied the positive electrode active material described in Patent Document 1 and found that the energy density of a sodium ion secondary battery formed using the positive electrode active material does not meet the recent required level and there is room for improvement. That is, there is room for improvement in the discharge capacity and average operating voltage of the sodium ion secondary battery.

[0006] Therefore, an object of the present invention is to provide a positive electrode active material for a sodium ion secondary battery that provides a large discharge capacity and a high average operating voltage in the resulting sodium ion secondary battery. Another object of the present invention is to provide a method for producing the positive electrode active material for a sodium ion secondary battery. A further object of the present invention is to provide a sodium ion secondary battery using the above positive electrode active material for a sodium ion secondary battery.

[0007] The present inventors have conducted extensive research to solve the above problems and have completed the present invention. That is, they have found that the above problems can be solved by the following configuration.

[0008] [1] A positive electrode active material for a sodium ion secondary battery, comprising a complex oxide represented by the following formula (1): The crystal structure of the complex oxide belongs to the space group R-3m, and in a diffraction chart obtained by X-ray diffraction measurement, the ratio of the peak intensity of the 003 reflection of the complex oxide to the peak intensity of the 104 reflection of the complex oxide is 1.00 or more. ａ Ni ｂ Mn ｃ Ti ｄ O ｅIn formula (1), a to e are positive real numbers, and b, c, and d satisfy the following relationships: 0.80≦b / c≦1.20 0<d / (b+c+d)≦0.30 b+c+d=1.00 [2] The positive electrode active material for a sodium ion secondary battery according to [1], having a lattice constant in the c-axis direction of 1.605 to 1.630 nm. [3] The positive electrode active material for a sodium ion secondary battery according to [1] or [2], having an average particle size of the secondary particles of 2 to 20 μm. [4] A method for producing a positive electrode active material for a sodium ion secondary battery according to any one of [1] to [3], comprising: introducing a nickel source and a manganese source into a reaction vessel liquid maintained at a pH of 9 or more and 12 or less to form a precipitate; mixing the precipitate with a sodium source and a titanium source to obtain a mixture; and heating the mixture at a temperature of 400° C. or more and 1200° C. or less. [5] A sodium ion secondary battery comprising: a positive electrode containing the positive electrode active material for a sodium ion secondary battery according to any one of [1] to [3]; a negative electrode; and a non-aqueous electrolyte interposed between the positive electrode and the negative electrode and conducting sodium ions. [6] A sodium ion secondary battery comprising: a positive electrode containing the positive electrode active material for a sodium ion secondary battery according to any one of [1] to [3]; a negative electrode; and a solid electrolyte interposed between the positive electrode and the negative electrode and conducting sodium ions.

[0009] According to the present invention, it is possible to provide a positive electrode active material for a sodium ion secondary battery, which has a large discharge capacity and a high average operating voltage in the resulting sodium ion secondary battery. Another object of the present invention is to provide a method for producing the positive electrode active material for a sodium ion secondary battery. A further object of the present invention is to provide a sodium ion secondary battery using the above positive electrode active material for a sodium ion secondary battery.

[0010] 1 is a diffraction chart obtained by performing X-ray diffraction measurement in Example 1 and Comparative Example 1. FIG. 2 is a charge / discharge curve of the sodium ion secondary batteries fabricated in Example 1 and Comparative Example 1.

[0011] The present invention will be described in detail below. The following description of the components may be based on representative embodiments of the present invention, but the present invention is not limited to such embodiments.

[0012] The meaning of each description in this specification is explained below. Hereinafter, an embodiment of the present invention will be described in detail. However, the embodiment described below is an example, and the present invention is not limited to the embodiment described below. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​described before and after "to" as the lower and upper limits.

[0013] <Cathode Active Material for Sodium-Ion Secondary Battery> The cathode active material for sodium-ion secondary batteries of the present invention (hereinafter also simply referred to as "cathode active material") is a cathode active material for sodium-ion secondary batteries containing a composite oxide represented by formula (1) described below. The crystal structure of the composite oxide represented by formula (1) described below belongs to the space group R-3m. Furthermore, in a diffraction chart obtained by performing X-ray diffraction measurement on the cathode active material of the present invention, the ratio of the peak intensity of the 003 reflection of the composite oxide to the peak intensity of the 104 reflection of the composite oxide is 1.00 or more. It is believed that when the composite oxide has the above-described composition and crystal structure and the peak intensity ratio is 1.00 or more, sodium ions in the composite oxide are easily desorbed and inserted. As a result, when a sodium-ion secondary battery is fabricated using a cathode active material containing the above-described composite oxide, the discharge capacity is large and the average operating voltage is high. Each item will be explained below.

[0014] [Composition] The composite oxide contained in the positive electrode active material has a composition represented by the following formula (1): Formula (1) Na ａ Ni ｂ Mn ｃ Ti ｄ O ｅIn formula (1), a to e are positive real numbers, and b, c, and d satisfy the following relationships: 0.80≦b / c≦1.20 0<d / (b+c+d)≦0.30 b+c+d=1.00 In formula (1), Na, Ni, Mn, Ti, and O represent sodium, nickel, manganese, titanium, and oxygen, respectively. In formula (1), the values ​​of a and e are not particularly limited. Among these, a is preferably 0.40 or more, more preferably 0.60 or more, and even more preferably 0.90 or more. Furthermore, a is preferably 1.30 or less, more preferably 1.20 or less, and even more preferably 1.10 or less. b is preferably 0.35 or more, and more preferably 0.40 or more. Furthermore, b is preferably 0.55 or less, and more preferably 0.50 or less. c is preferably 0.35 or more, and more preferably 0.40 or more. d is preferably 0.03 or more, more preferably 0.05 or more, and even more preferably 0.10 or more. d is preferably 0.30 or less, more preferably 0.25 or less, and even more preferably 0.20 or less. The value of b / c is preferably 0.85 or more, more preferably 0.90 or more, and even more preferably 0.95 or more. The value of b / c is preferably 1.15 or less, more preferably 1.10 or less, and even more preferably 1.05 or less. The value of d / (b+c+d) is preferably 0.03 or more, more preferably 0.05 or more, and even more preferably 0.10 or more. The value of d / (b+c+d) is preferably 0.25 or less, and even more preferably 0.20 or less. In this specification, the composition ratios of the composite oxides (the values ​​of a to e above) are determined by inductively coupled plasma optical emission spectroscopy (ICP-OES).

[0015] [Space Group and Lattice Constant] The crystal structure of the composite oxide contained in the positive electrode active material of the present invention belongs to the space group R-3m. The space group R-3m means that the composite oxide belongs to a space group having one three-fold reciprocal axis and one mirror plane. The space group belongs to a trigonal system and is No. 166. The lattice constant in the c-axis direction of the composite oxide contained in the positive electrode active material of the present invention is preferably 1.605 to 1.630 nm. The lattice constant in the c-axis direction is determined by X-ray diffraction measurement, which will be described later. The lattice constant in the c-axis direction is more preferably 1.606 nm or more, and even more preferably 1.607 nm or more. The lattice constant in the c-axis direction is more preferably 1.628 nm or less, and even more preferably 1.626 nm or less. The composite oxide contained in the positive electrode active material of the present invention has layers in which octahedral units, each of which has six oxygen atoms arranged to one of nickel, manganese, and titanium, are bonded together by sharing the faces of the octahedral units, and sodium is inserted between the layers. The unit lattice of the composite oxide contains three of the above layers and two layers into which sodium is inserted. The normal direction of the layers is parallel to the c-axis direction, and the layers are stacked in the c-axis direction. Here, the distance from the center of one layer to the center of the next adjacent layer is 1 / 3 of the lattice constant in the c-axis direction.

[0016] [XRD] In the diffraction chart obtained by X-ray diffraction (XRD) measurement of the positive electrode active material of the present invention, the ratio of the peak intensity of the 003 reflection of the composite oxide to the peak intensity of the 104 reflection of the composite oxide is 1.00 or more. In this specification, the XRD measurement method follows the method of the Examples shown later. In the diffraction chart, whether the composite oxide belongs to the above space group and whether the diffraction lines are from the crystal planes with the above indexes are analyzed by conventional methods. The diffraction line of the 003 reflection of the composite oxide typically appears in the range of 2θ = 15 to 20° in the diffraction chart obtained by so-called θ / 2θ measurement. Furthermore, the diffraction line of the 104 reflection of the composite oxide typically appears in the range of 2θ = 40 to 50° in the diffraction chart obtained by so-called θ / 2θ measurement. The term "θ / 2θ measurement" refers to a measurement method in which, when the incident direction of X-rays on a sample is θ°, a diffracted X-ray detector is scanned on a plane including the incident direction of the X-rays while maintaining a positional relationship of 2θ° with respect to the incident direction of the X-rays. The θ / 2θ measurement is performed by a measurement method commonly used in this field.

[0017] A peak intensity ratio of 1.00 or more is considered to correspond to relatively small disorder in the crystal structure in the c-axis direction, and thus easy absorption and desorption of sodium ions from the composite oxide when used in a sodium ion secondary battery. The peak intensity ratio is preferably 1.10 or more, more preferably 1.20 or more, and even more preferably 1.25 or more. The peak intensity ratio is usually 2.00 or less.

[0018] [Positive Electrode Active Material Powder] The positive electrode active material of the present invention is usually in powder form. As described above, the positive electrode active material of the present invention contains the above-described composite oxide. The content of the composite oxide relative to the total mass of the positive electrode active material is preferably 95% by mass or more, more preferably 98% by mass or more. The upper limit of the content is not particularly limited, and may be 100% by mass relative to the total mass of the positive electrode active material.

[0019] The particles constituting the powder of the positive electrode active material preferably form secondary particles formed by aggregation of primary particles. The shape of the secondary particles is not particularly limited, but is preferably approximately spherical. The average particle size of the secondary particles is preferably 2 μm or more, more preferably 4 μm or more, and even more preferably 8 μm or more. The average particle size of the secondary particles is preferably 20 μm or less, more preferably 15 μm or less.

[0020] In this specification, the average particle size of the secondary particles is measured by a laser diffraction method using a Malvern Mastersizer 3000. For the measurement, a powder of the positive electrode active material is dispersed in water stirred at 1000 rpm, and subjected to ultrasonic treatment.

[0021] The particle size distribution of the secondary particles may be unimodal or multimodal, but a multimodal distribution is preferred in that it improves the packing property of the positive electrode active material and facilitates the improvement of the energy density. As a multimodal particle size distribution, for example, a bimodal particle size distribution is preferred, and it is more preferred that the secondary particles exhibit a bimodal particle size distribution and the preferred average particle size.

[0022] <Method for producing a positive electrode active material for a sodium ion secondary battery> The method for producing a positive electrode active material for a sodium ion secondary battery of the present invention includes introducing a nickel source and a manganese source into a reaction tank solution maintained at a pH of 9 or more and 12 or less to form a precipitate, mixing the precipitate with a sodium source and a titanium source to obtain a mixture, and heating the mixture at 400° C. or more and 1200° C. or less. According to the above production method, the positive electrode active material of the present invention is easily obtained. Each step will be described below.

[0023] [Precipitate Formation] First, a nickel source and a manganese source are introduced into a reaction bath solution having a pH of 9 or more and 12 or less to form a precipitate. Nickel and manganese form precipitates at the above pH, so by carrying out the above procedure, nickel and manganese co-precipitate to obtain a precipitate. In the precipitate obtained by co-precipitation, nickel and manganese are uniformly dispersed at the atomic level, and it is believed that the resulting positive electrode active material satisfies the above-mentioned characteristics.

[0024] The above-mentioned "reactor liquid" refers to the liquid in the reaction vessel. Furthermore, "introducing into the reaction vessel liquid whose pH is maintained at 9 or more and 12 or less" means introducing the nickel source and manganese source into the reaction vessel liquid while maintaining the pH of the reaction vessel liquid at 9 or more and 12 or less. As a method for maintaining the pH of the reaction vessel liquid at 9 or more and 12 or less, for example, the nickel source and manganese source may be introduced into the reaction vessel liquid while adding an alkaline aqueous solution described below to the reaction vessel liquid. Furthermore, an ammonium source described below may be introduced into the reaction vessel liquid together with the nickel source and manganese source.

[0025] Examples of nickel sources include nickel salts such as nickel sulfate, nickel carbonate, nickel nitrate, nickel acetate, and nickel chloride, with nickel sulfate being preferred. Examples of manganese sources include manganese salts such as manganese sulfate, manganese carbonate, manganese nitrate, manganese acetate, and manganese chloride, with manganese sulfate being preferred. The nickel source and manganese source may be solutions (e.g., aqueous solutions) containing the nickel source and manganese source, respectively. When using solutions containing the nickel source and manganese source as the nickel source and manganese source, it is preferable to premix the solution containing the nickel source and the solution containing the manganese source to form a mixed solution, which is then introduced into the reaction vessel liquid. The ratio of the nickel content of the nickel source to the manganese content of the manganese source (Ni / Mn) can be appropriately adjusted so as to achieve a composition within the range of the above-mentioned formula (1), but a molar ratio of 0.8 to 1.2 is preferred.

[0026] Examples of the ammonium source include ammonium salts such as ammonium sulfate, ammonium chloride, ammonium nitrate, and ammonium carbonate, with ammonium sulfate being preferred. The ammonium source may be a solution (e.g., an aqueous solution) containing the ammonium source. Alternatively, aqueous ammonia may be used as the ammonium source. Addition of the ammonium source can suppress nucleation during coprecipitation and promote particle growth of the generated nuclei. The ratio of the ammonium ion content of the ammonium source to the total of the nickel content of the nickel source and the manganese content of the manganese source (hereinafter referred to as "NH ４＋ / (Ni+Mn)" is a molar ratio, and is preferably 0 or more and 1 or less.

[0027] The alkaline aqueous solution is not particularly limited, but examples thereof include an aqueous sodium hydroxide (NaOH) solution and an aqueous potassium hydroxide (KOH) solution, with an aqueous sodium hydroxide solution being preferred.

[0028] The introduction of the reactant into the reaction vessel liquid is preferably carried out while stirring the reaction vessel liquid. The reaction vessel liquid can be stirred, for example, by using a stirring blade. The temperature of the reaction vessel liquid is preferably 30°C or higher and 60°C or lower, more preferably 35°C or higher and 45°C or lower. The atmosphere in which the reaction vessel is placed, i.e., the atmosphere above the reaction vessel liquid, may be either an air atmosphere or an inert atmosphere. Examples of inert gases that can be used to create an inert atmosphere include nitrogen gas and argon gas. The inert gas may also be bubbled through the reaction vessel liquid to suppress oxidation of the precipitate. The generation of the precipitate may be carried out in a batch-type reaction vessel or a continuous reaction vessel. By carrying out the above procedure, a reaction vessel liquid in which the precipitate is dispersed can be obtained.

[0029] [Mixing and Heating] Next, the precipitate, a sodium source, and a titanium source are mixed to obtain a mixture, and the mixture is heated at 400°C or higher and 1200°C or lower. Examples of methods for obtaining the mixture include a method of separating the precipitate from the reaction bath liquid in which the precipitate is dispersed, and then mixing the separated precipitate with the sodium source and the titanium source. Known methods can be used to separate the precipitate from the reaction bath liquid. Examples include a method of filtering the reaction bath liquid to recover the precipitate, a method of separating the precipitate from the reaction bath liquid using centrifugal force, and a method of allowing the reaction bath liquid to stand to settle the precipitate and then removing the supernatant liquid.

[0030] The precipitate obtained by separating the precipitate from the reaction vessel liquid may be subjected to a drying treatment. A known method can be used for the drying treatment. Examples of the drying method include air drying, hot air drying, and vacuum drying. Separation and drying of the precipitate may be performed simultaneously, and for example, spray drying may be used as the method.

[0031] The precipitate may be washed before the drying treatment. For example, the washing method may involve contacting the precipitate with water. More specifically, the precipitate may be dispersed in water and then separated from the water by the above-described separation method. The water used in the above procedure is preferably purified water such as ion-exchanged water. After washing the precipitate, the washed precipitate may be dried by the above-described drying method.

[0032] The mixing method for mixing the separated precipitate, the sodium source, and the titanium source to obtain a mixture is not particularly limited, and examples thereof include a method of mixing powders with a stirring blade or the like, and a method of mixing while pulverizing the powder using a mortar, a ball mill, a jet mill, or the like.

[0033] The ratio of the titanium content of the titanium source to the total amount of the nickel content and manganese content in the precipitate (hereinafter also referred to as "Ti / (Ni+Mn)") can be appropriately adjusted so as to achieve a composition within the range of the above-mentioned formula (1). In particular, Ti / (Ni+Mn) is preferably 0.05 or more and 0.30 or less in molar ratio. The ratio of the sodium content of the sodium compound to the total amount of the nickel content, manganese content, and titanium content of the titanium source in the precipitate (hereinafter also referred to as "Na / (Ni+Mn+Ti)") can be appropriately adjusted so as to achieve a composition within the range of the above-mentioned formula (1). In particular, Na / (Ni+Mn+Ti) is preferably 0.60 or more and 1.10 or less in molar ratio, and more preferably 0.80 or more and 1.10 or less.

[0034] Examples of titanium sources include titanium oxide, titanium chloride, titanium nitride, and titanium fluoride, with titanium oxide being preferred. Examples of sodium sources include sodium carbonate and sodium oxide, with sodium carbonate being preferred.

[0035] Next, the resulting mixture is heated at 400°C or higher and 1200°C or lower. By carrying out the heating, the positive electrode active material of the present invention is obtained. The heating temperature is preferably 600°C or higher and 1200°C or lower. The atmosphere in which the heating is carried out is not particularly limited, and examples thereof include an oxidizing atmosphere (e.g., air atmosphere) and a non-oxidizing atmosphere (e.g., nitrogen atmosphere, argon atmosphere, etc.).

[0036] The temperature profile when performing the heat treatment can be adjusted as appropriate. For example, the heat treatment may be performed by maintaining a predetermined temperature for a predetermined time, or the heat treatment may be performed in two or more separate steps. When the heat treatment is performed in two or more separate steps, it is preferable that the maximum temperature of the first heat treatment is lower than the maximum temperature of the second heat treatment. The heating time is not particularly limited, but, for example, the maintenance time at the above temperature is preferably 0.5 hours or more, more preferably 5 hours or more, and even more preferably 12 hours or more. On the other hand, this maintenance time may be, for example, 72 hours or less, or may be 48 hours or less. When the heat treatment is performed in two or more separate steps, it is also preferable that the total heating time is the above-mentioned preferred heating time. Furthermore, when the heat treatment is performed in two or more separate steps, it is also preferable that the atmosphere of the first heat treatment is a non-oxidizing atmosphere (e.g., a nitrogen atmosphere, an argon atmosphere, etc.) and the atmosphere of the second heat treatment is an oxidizing atmosphere (e.g., an air atmosphere, etc.).

[0037] While the above describes a method in which the precipitate is separated from the reaction vessel liquid and then mixed, the order, timing, and method of mixing can be adjusted as appropriate. For example, a titanium source may be added to the reaction vessel liquid, and the precipitate and titanium source may be separated from the reaction vessel liquid by the above method and mixed with a sodium source. Alternatively, the precipitate may be separated by the above method, washed by the above method, and the titanium source may be added to a slurry containing the precipitate and the liquid used for washing (e.g., water), and the precipitate and titanium source may be separated from the slurry to which the titanium source has been added and mixed with a sodium source. Furthermore, in the above procedure, a sodium source may be further added to the slurry to which the titanium source has been added, and solids may be separated from the slurry to which the sodium source has been added to obtain a mixture.

[0038] [Other Steps] The method for producing a positive electrode active material for a sodium ion secondary battery of the present invention may include steps other than those described above. For example, the positive electrode active material obtained by the above steps may be washed. The washing method can be the method described above. Washing the obtained positive electrode active material makes it possible to remove unreacted raw materials (e.g., sodium source). The washed positive electrode active material may be further dried. The drying method can be the method described above. Furthermore, after drying, heating may be performed. The heating temperature is preferably 200°C or higher and 800°C or lower. The heating time can be adjusted as appropriate, and may be, for example, 0.5 to 12 hours.

[0039] <Sodium-ion secondary battery> The sodium-ion secondary battery of the present invention comprises a positive electrode containing the above-described positive electrode active material of the present invention, a negative electrode, and an electrolyte interposed between the positive electrode and the negative electrode to conduct sodium ions. Other components of the sodium-ion secondary battery of the present invention can be conventionally known. For example, the sodium-ion secondary battery of the present invention may further comprise a separator. Furthermore, the form of the sodium-ion secondary battery of the present invention can be arbitrarily selected from cylindrical, prismatic, coin, sheet, laminate, button, and other types depending on the application, mounted equipment, required charge / discharge capacity, and the like. Each component of the sodium-ion secondary battery of the present invention will be described below.

[0040] [Positive Electrode] The positive electrode of the sodium secondary battery of the present invention typically includes the above-mentioned positive electrode active material (or a positive electrode mixture containing the positive electrode active material) and a positive electrode current collector supporting the positive electrode active material (or the positive electrode mixture). The positive electrode current collector may be a metal foil or a porous metal body (e.g., a nonwoven fabric of metal fibers, a porous metal sheet, etc.). As the porous metal body, a porous metal body having a three-dimensional mesh-like skeleton (particularly a hollow skeleton) can also be used. The material of the positive electrode current collector is not particularly limited, but aluminum or an aluminum alloy is preferred from the viewpoint of stability at the positive electrode potential. The thickness of the metal foil serving as the positive electrode current collector is, for example, 10 to 50 μm, and the thickness of the porous metal body serving as the positive electrode current collector is, for example, 100 to 2000 μm.

[0041] The positive electrode mixture contains a positive electrode active material and other components. The positive electrode mixture may further contain, for example, at least one of a conductive additive and a binder in addition to the positive electrode active material. Examples of conductive additives include, but are not limited to, carbon black, acetylene black, graphite, carbon fiber (such as vapor-grown carbon fiber), and carbon nanotubes. A single conductive additive may be used, or two or more may be used simultaneously. To enhance conductivity, the conductive additive may be disposed so as to coat the surfaces of the positive electrode active material particles. Coating of the positive electrode active material particles with the conductive additive can be achieved by contacting the conductive additive with the positive electrode active material particles and adhering the conductive additive to the surfaces of the positive electrode active material particles, for example, by mechanochemical treatment (including mechanofusion treatment). The amount of conductive additive in the positive electrode mixture is preferably 1 to 25 parts by mass per 100 parts by mass of the positive electrode active material.

[0042] The binder is not particularly limited, but examples thereof include fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene; polyolefin resins; rubbery polymers such as styrene butadiene rubber; polyamide resins (aromatic polyamides, etc.); polyimides and polyamideimides; polyvinylpyrrolidone; polyvinyl alcohol; and cellulose ethers (carboxyalkyl celluloses and salts thereof, such as carboxymethyl cellulose and its sodium salts); etc. One type of binder may be used alone, or two or more types may be used simultaneously. The amount of binder in the positive electrode mixture is not particularly limited, but is preferably 0.5 to 15 parts by mass per 100 parts by mass of the positive electrode active material, in that it exhibits high binding properties while easily increasing the discharge capacity.

[0043] The positive electrode can be formed, for example, by applying or filling a positive electrode mixture onto a positive electrode current collector, drying it, and, if necessary, compressing (or rolling) it in the thickness direction. The positive electrode mixture is usually used in the form of a slurry (or paste) containing a dispersion medium. As the dispersion medium, for example, an organic solvent such as N-methyl-2-pyrrolidone (NMP) or water is used. Two or more solvents may be mixed together to form the dispersion medium.

[0044] [Negative Electrode] The negative electrode of the sodium secondary battery of the present invention typically includes a negative electrode active material (or a negative electrode mixture containing the negative electrode active material) and a negative electrode current collector supporting the negative electrode active material (or the negative electrode mixture). The negative electrode current collector may be a metal foil or a porous metal body (e.g., a nonwoven fabric of metal fibers, a porous metal sheet, etc.). As the porous metal body, a porous metal body having a three-dimensional mesh-like skeleton (particularly a hollow skeleton) can also be used. The material of the negative electrode current collector is not particularly limited, but examples include copper, nickel, aluminum, an aluminum alloy, and stainless steel, with aluminum being preferred. The thickness of the metal foil serving as the negative electrode current collector is, for example, 10 to 50 μm, and the thickness of the porous metal body serving as the negative electrode current collector is, for example, 100 to 2000 μm.

[0045] Examples of the negative electrode active material for the sodium secondary battery of the present invention include sodium metal, sodium alloys, and materials capable of doping and dedoping sodium ions. Examples of materials capable of doping and dedoping sodium ions include carbonaceous materials, chalcogen compounds such as oxides and sulfides that can be doped and dedoped with sodium ions at a lower potential than the positive electrode, and borates. Examples of carbonaceous materials include graphitizable carbon (soft carbon) and non-graphitizable carbon (hard carbon), with hard carbon being preferred.

[0046] The negative electrode may contain a thermoplastic resin as a binder as needed. That is, the negative electrode composite may contain a thermoplastic resin as a binder. Examples of the thermoplastic resin include polyvinylidene fluoride, polyethylene, and polypropylene.

[0047] The negative electrode can be formed, for example, by pressure molding a negative electrode current collector and the above-mentioned negative electrode active material or negative electrode composite, or by forming a paste using the above-mentioned dispersion medium or the like, applying it to the negative electrode current collector, drying it, and then pressing it to fix it.

[0048] [Electrolyte] The electrolyte is interposed between the positive electrode and the negative electrode and conducts sodium ions. The electrolyte may be a non-aqueous electrolyte or a solid electrolyte.

[0049] (Non-aqueous electrolyte) The non-aqueous electrolyte typically contains a solvent other than water and a solute capable of conducting sodium ions. Examples of solvents used in the non-aqueous electrolyte solution include carbonates such as propylene carbonate, ethylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, and 1,2-di(methoxycarbonyloxy)ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of solvents that can be used include ethers such as sulfolane; esters such as methyl formate, methyl acetate, and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, 1,3-propane sultone, ethylene sulfite, propylene sulfite, dimethyl sulfite, and diethyl sulfite; and solvents in which a fluorine substituent has been further introduced into the above solvents. Typically, two or more of the above solvents are used in combination. Among these, mixed solvents containing carbonates are preferred, mixed solvents containing a cyclic carbonate and an acyclic carbonate are preferred, and mixed solvents containing a cyclic carbonate and an ether are even more preferred. Here, as a mixed solvent containing a cyclic carbonate and an acyclic carbonate, a mixed solvent containing ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is preferred in terms of a wide operating temperature range and excellent load characteristics.

[0050] As a solute capable of conducting sodium ions, a sodium salt is preferably used. The type of anion (first anion) constituting the sodium salt is not particularly limited, and examples thereof include anions of fluorine-containing acids (fluorine-containing phosphate anions such as hexafluorophosphate anions; fluorine-containing borate anions such as tetrafluoroborate anions; etc.), anions of chlorine-containing acids (perchlorate ions, etc.), anions of oxyacids having an oxalate group (bis(oxalato)borate ions (B(C ２ O ４ ) ２ － oxalatoborate ions such as tris(oxalato)phosphate ions (P(C ２ O ４ ) ３ － oxalatophosphate ions such as fluoroalkanesulfonic acid anions (trifluoromethanesulfonate ions (CF ３ SO ３ － ) and the like), and bissulfonylamide anions. Examples of the bissulfonylamide anions include bis(fluorosulfonyl)amide anions (FSA: bis(fluorosulfonyl)amide anions), bis(trifluoromethylsulfonyl)amide anions (TFSA: bis(trifluoromethylsulfonyl)amide anions), and (fluorosulfonyl)(perfluoroalkylsulfonyl)amide anions ((FSO ２ ) (CF ３ SO ２ ) N － etc.), bis(perfluoroalkylsulfonyl)amide anion (N(SO ２ CF ３ ) ２ － , N(SO ２ C ２ F ５ ) ２ －Among these, at least one of FSA and TFSA is particularly preferred. The sodium salt may be used alone, or two or more sodium salts having different first anions may be used in combination. Specific examples of the solute of the non-aqueous electrolyte solution include NaClO ４ , NaPF ６ , NaBF ４ , NaCF ３ SO ３ , NaN(CF ３ SO ２ ) ２ , NaN(C ２ F ５ SO ２ ) ２ , and NaC(CF ３ SO ２ ) ３ Examples include:

[0051] The mixing ratio of the solvent and the solute is not particularly limited and may be appropriately set depending on the purpose.

[0052] (Solid Electrolyte) A solid electrolyte refers to a solid electrolyte capable of conducting sodium ions. Examples of the solid electrolyte include sulfide-based and oxide-based materials. Examples of sulfide-based solid electrolytes include Na ３ P.S. ４ , Na ３ P.S. ４ -Na ４ SiS ４ , Na ２ S-P ２ S ５ , Na ２ S-SiS ２ , and Na ２ S-GeS ２ Examples of oxide-based solid electrolytes include β-alumina, β"-alumina, and oxide materials containing NASICON (Na Super Ionic Conductor) type crystals. NASICON type crystals are generally made of Na １＋ｘ Zr ２ Si ｘ P ３－ｘ O １２where x is greater than 0 and less than 3. At least one of Zr and Si in the above composition formula may be partially substituted with another element.

[0053] [Separator] The sodium ion secondary battery of the present invention may include a separator. A separator is a material disposed between the positive electrode and the negative electrode, preventing them from contacting each other while allowing sodium ions to pass through it. The material of the separator is not particularly limited, but examples thereof include woven fabric, nonwoven fabric, and synthetic resin microporous membranes. As synthetic resin microporous membranes, polyolefin microporous membranes are preferred in terms of thickness, membrane strength, and membrane resistance. Examples of polyolefin microporous membranes include polyethylene microporous membranes, polypropylene microporous membranes, and microporous membranes combining these. Glass fiber separators are also preferred because they are effective in preventing micro-short circuits.

[0054] The present invention will be described in more detail below with reference to examples. The materials, amounts used, ratios, treatment details, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the examples shown below.

[0055] Example 1 Positive electrode active material 1 was produced in the following manner, the positive electrode active material 1 was evaluated, a sodium ion secondary battery was fabricated using the positive electrode active material 1, and the charge / discharge characteristics of the sodium ion secondary battery were evaluated.

[0056] [Precipitate Formation] 5 L of pure water and aqueous sodium hydroxide solution were added to a reaction vessel and mixed to obtain a reaction vessel liquid adjusted to pH 11. A mixed aqueous solution of a 3.0 mol / L nickel sulfate solution and a 3.0 mol / L manganese sulfate solution was added to the reaction vessel liquid at a rate of 200 mL / h, respectively. Simultaneously, a 3.0 mol / L aqueous ammonium sulfate solution was added at a rate of 100 mL / h. During the addition of these aqueous solutions, aqueous sodium hydroxide solution was added to the reaction vessel liquid so that the pH of the reaction vessel liquid was maintained at 11. The reaction vessel liquid was stirred at 600 rpm with a stirrer under atmospheric conditions, while the temperature of the reaction vessel liquid was controlled at 40°C. A precipitate was formed according to the above procedure.

[0057] [Separation and Drying] The precipitate obtained by the above procedure was filtered off, washed with water and dried.

[0058] [Mixing and Heating] 120 g of the precipitate dried in the above procedure, 12 g of titanium oxide, and 80 g of sodium carbonate were placed in a ball mill and mixed by rotating at 280 rpm for 4 hours to obtain a mixture.

[0059] The mixture obtained by the above mixing was subjected to two heat treatments to obtain a fired product. More specifically, it was heated at 650°C for 24 hours in a nitrogen atmosphere, and then heated at 950°C for 15 hours in an air atmosphere. The fired product obtained was crushed using a mortar and pestle to obtain cathode active material 1, which is a composite oxide. The molar ratios of sodium, nickel, manganese, and titanium in the obtained cathode active material 1 (composite oxide) were determined by fusion coupled plasma atomic emission spectroscopy. As a result, the ratios a to d in the above formula (1) were a:b:c:d = 1:0.45:0.45:0.1. That is, cathode active material 1 was obtained by the formula Na １ Ni ０．４５ Mn ０．４５ Ti ０．１ O ｅ (e is a positive real number), where b / c = 1.00, d / (b + c + d) = 0.10, and b + c + d = 1.00. The average particle size of the secondary particles of positive electrode active material 1 was 9.3 μm.

[0060] [XRD] XRD measurement was performed on the positive electrode active material 1 to obtain a diffraction chart. The XRD measurement was performed using a SmartLab manufactured by Rigaku Corporation, and CuK α wire (CuK α１ wire and CuK α２ The tube voltage was 40 kV and the tube current was 30 mA. βA Ni filter was used to remove the X-rays. The obtained diffraction chart is shown in FIG. 1. From the diffraction chart, it was confirmed that the space group of the crystal structure of the composite oxide contained in the positive electrode active material 1 belongs to R-3m (No. 166), and the lattice constant in the c-axis direction was 1.608 nm. In addition, the ratio of the peak intensity of the 003 reflection to the peak intensity of the 104 reflection was 1.28. The wavelength of the X-rays used to calculate the lattice constant was CuK α１ Line wavelength and CuK α２ The arithmetic mean value calculated from the wavelengths of the rays and their intensity ratios is CuK α The wavelength of the line was used.

[0061] [Battery Fabrication] N-methyl-2-pyrrolidone was added to the positive electrode active material 1 (80% by mass), acetylene black (10% by mass), and polyvinylidene fluoride (10% by mass), and the mixture was kneaded to obtain a mixture. The resulting mixture was applied to an aluminum current collector (20 μm thick) to form a coating film. The laminate of the coating film and the aluminum current collector was pressed using a roll press. The pressed laminate was punched into a disk with a diameter of 14 mm. The punched disk was vacuum dried at 150°C for 15 hours to obtain a positive electrode. A sodium metal foil with a diameter of 16 mm was used as the negative electrode. A glass fiber separator with a diameter of 20 mm was used as the separator. The electrolyte was prepared by dissolving 1 mol of NaPF in 1 L of a mixed solution containing ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio as the solvent. ６ A non-aqueous electrolyte solution in which the above-mentioned positive electrode, negative electrode, separator, and non-aqueous electrolyte solution were dissolved was used. A sodium ion secondary battery was fabricated in a glove box purged with argon using the above-mentioned positive electrode, negative electrode, separator, and non-aqueous electrolyte solution. The fabricated sodium ion secondary battery included a positive electrode, a separator, and a negative electrode in this order, with a non-aqueous electrolyte interposed between the positive electrode and the negative electrode.

[0062] [Charge / Discharge Test] The fabricated sodium ion secondary battery was charged and discharged at a constant current value of 0.075 C in the voltage range of 2.2 to 4.0 V to obtain a charge / discharge curve. The obtained charge / discharge curve is shown in Figure 2. The discharge capacity of the sodium ion secondary battery obtained using positive electrode active material 1 was 134 mAh / g, and the average operating voltage was 3.07 V.

[0063] Example 2 A precipitate was obtained in the same manner as for cathode active material 1, followed by washing with water and drying. 110 g of the dried precipitate, 24 g of titanium oxide, and 82 g of sodium carbonate were mixed in the same manner as in Example 1. Except for the above, cathode active material 2 was obtained in the same manner as for cathode active material 1, and XRD measurement, battery fabrication, and charge / discharge tests were carried out in the same manner as in Example 1, except that cathode active material 2 was used instead of cathode active material 1. For the obtained cathode active material 2 (composite oxide), the ratios a to d in the above formula (1) were a:b:c:d=1:0.40:0.40:0.20. That is, cathode active material 2 was obtained by mixing 110 g of the dried precipitate, 24 g of titanium oxide, and 82 g of sodium carbonate in the same manner as in Example 1. １ Ni ０．４ Mn ０．４ Ti ０．２ O ｅ (e is a positive real number), where b / c = 1.00, d / (b + c + d) = 0.20, and b + c + d = 1.00. The average particle size of the secondary particles of positive electrode active material 2 was 9.1 μm. From the diffraction chart, it was confirmed that the space group of the crystal structure of the complex oxide contained in positive electrode active material 2 belonged to R-3m (No. 166), and the lattice constant in the c-axis direction was 1.625 nm. In addition, the ratio of the peak intensity of the 003 reflection to the peak intensity of the 104 reflection was 1.03. The discharge capacity of the sodium ion secondary battery obtained using positive electrode active material 2 was 118 mAh / g, and the average operating voltage was 3.15 V.

[0064] Example 3 A precipitate was obtained in the same manner as for Cathode Active Material 1, and was then washed with water and dried. 105 g of the dried precipitate, 39.4 g of titanium oxide, and 89.5 g of sodium carbonate were mixed in the same manner as in Example 1. Except for the above, Cathode Active Material 3 was obtained in the same manner as for Cathode Active Material 1, and XRD measurement, battery fabrication, and charge / discharge testing were carried out in the same manner as in Example 1, except that Cathode Active Material 3 was used instead of Cathode Active Material 1. For the obtained Cathode Active Material 3 (composite oxide), the ratios a to d in the above formula (1) were a:b:c:d=1:0.35:0.35:0.30. That is, Cathode Active Material 3 was obtained by the procedure of the formula Na １ Ni ０．３５ Mn ０．３５ Ti０．３ O ｅ (e is a positive real number), where b / c = 1.00, d / (b + c + d) = 0.30, and b + c + d = 1.00. The average particle size of the secondary particles of positive electrode active material 3 was 9.3 μm. From the diffraction chart, it was confirmed that the space group of the crystal structure of the complex oxide contained in positive electrode active material 3 belonged to R-3m (No. 166), and the lattice constant in the c-axis direction was 1.630 nm. In addition, the ratio of the peak intensity of the 003 reflection to the peak intensity of the 104 reflection was 1.00. The discharge capacity of the sodium ion secondary battery obtained using positive electrode active material 3 was 116 mAh / g, and the average operating voltage was 3.21 V.

[0065] Example 4: 5 L of pure water and aqueous sodium hydroxide solution were mixed in a reactor to obtain a reactor liquid adjusted to pH 11. A mixed aqueous solution of 2.4 mol / L nickel sulfate solution and a 3.0 mol / L manganese sulfate solution were added to the reactor liquid at a rate of 200 mL / h. Simultaneously, a 3.0 mol / L ammonium sulfate solution was added at a rate of 100 mL / h. During the addition of these aqueous solutions, aqueous sodium hydroxide solution was added to the reactor liquid so that the pH of the reactor liquid was maintained at 11. The reactor liquid was stirred at 600 rpm with a stirrer under atmospheric conditions, while the temperature of the reactor liquid was controlled at 40°C. A precipitate was formed by the above procedure. The precipitate obtained by the above procedure was filtered, washed with water, and dried.

[0066] 105 g of the dried precipitate, 1.9 g of titanium oxide, and 62.5 g of sodium carbonate were mixed in the same manner as in Example 1. Other than the above, cathode active material 4 was obtained in the same manner as cathode active material 1, and XRD measurement, battery fabrication, and charge / discharge tests were carried out in the same manner as in Example 1, except that cathode active material 4 was used instead of cathode active material 1. For the obtained cathode active material 4 (composite oxide), the ratios a to d in the above formula (1) were a:b:c:d=1:0.44:0.54:0.02. That is, cathode active material 4 was a composite oxide having the formula Na １ Ni ０．４４ Mn ０．５４ Ti ０．０２ O ｅ(e is a positive real number), where b / c = 0.80, d / (b + c + d) = 0.02, and b + c + d = 1.00. The average particle size of the secondary particles of positive electrode active material 4 was 9.0 μm. From the diffraction chart, it was confirmed that the space group of the crystal structure of the complex oxide contained in positive electrode active material 4 belonged to R-3m (No. 166), and the lattice constant in the c-axis direction was 1.611 nm. In addition, the ratio of the peak intensity of the 003 reflection to the peak intensity of the 104 reflection was 1.01. The discharge capacity of the sodium ion secondary battery obtained using positive electrode active material 4 was 115 mAh / g, and the average operating voltage was 3.11 V.

[0067] Example 5: 5 L of pure water and aqueous sodium hydroxide solution were mixed in a reactor to obtain a reactor liquid adjusted to pH 11. A mixed aqueous solution of 3.0 mol / L nickel sulfate solution and 2.5 mol / L manganese sulfate solution was added to the reactor liquid at a rate of 200 mL / h. Simultaneously, a 3.0 mol / L aqueous ammonium sulfate solution was added at a rate of 100 mL / h. During the addition of these aqueous solutions, aqueous sodium hydroxide solution was added to the reactor liquid so that the pH of the reactor liquid was maintained at 11. The reactor liquid was stirred at 600 rpm with a stirrer under atmospheric conditions, while the temperature of the reactor liquid was controlled at 40°C. A precipitate was formed by the above procedure. The precipitate obtained by the above procedure was filtered, washed with water, and dried.

[0068] 105 g of the dried precipitate, 39.2 g of titanium oxide, and 89.5 g of sodium carbonate were mixed in the same manner as in Example 1. Other than the above, cathode active material 5 was obtained in the same manner as cathode active material 1, and XRD measurement, battery fabrication, and charge / discharge tests were carried out in the same manner as in Example 1, except that cathode active material 5 was used instead of cathode active material 1. For the obtained cathode active material 5 (composite oxide), the ratios a to d in the above formula (1) were a:b:c:d=1:0.38:0.32:0.30. That is, cathode active material 5 was obtained by the method represented by the formula Na １ Ni ０．３８ Mn ０．３２ Ti ０．３ O ｅ(e is a positive real number), where b / c = 1.20, d / (b + c + d) = 0.30, and b + c + d = 1.00. The average particle size of the secondary particles of positive electrode active material 5 was 9.2 μm. From the diffraction chart, it was confirmed that the space group of the crystal structure of the complex oxide contained in positive electrode active material 5 belonged to R-3m (No. 166), and the lattice constant in the c-axis direction was 1.628 nm. In addition, the ratio of the peak intensity of the 003 reflection to the peak intensity of the 104 reflection was 1.12. The discharge capacity of the sodium ion secondary battery obtained using positive electrode active material 5 was 120 mAh / g, and the average operating voltage was 3.13 V.

[0069] Comparative Example 1 A precipitate was obtained in the same manner as for Cathode Active Material 1, and was then washed with water and dried. 126.5 g of the dried precipitate and 75.8 g of sodium carbonate were mixed in the same manner as in Example 1. Except for the above, Cathode Active Material 6 was obtained in the same manner as for Cathode Active Material 1, and XRD measurement, battery fabrication, and charge / discharge testing were carried out in the same manner as in Example 1, except that Cathode Active Material 3 was used instead of Cathode Active Material 1. For the obtained Cathode Active Material 6 (composite oxide), the ratios a to d in the above formula (1) were a:b:c:d=1:0.5:0.5:0. That is, Cathode Active Material 6 was obtained from the compound represented by the formula Na １ Ni ０．５ Mn ０．５ O ｅ (e is a positive real number), where b / c = 1.00, d / (b + c + d) = 0.00, and b + c + d = 1.00. The average particle size of the secondary particles of positive electrode active material 6 was 9.2 μm. The obtained diffraction chart is shown in FIG. 1. From the diffraction chart, it was confirmed that the space group of the crystal structure of the complex oxide contained in positive electrode active material 6 belonged to R-3m (No. 166), and the lattice constant in the c-axis direction was 1.602 nm. In addition, the ratio of the peak intensity of the 003 reflection to the peak intensity of the 104 reflection was 0.67. The obtained charge / discharge curve is shown in FIG. 2. The discharge capacity of the sodium ion secondary battery obtained using positive electrode active material 6 was 117 mAh / g, and the average operating voltage was 2.99 V.

[0070] Comparative Example 2 A precipitate was obtained in the same manner as for Cathode Active Material 1, and was then washed with water and dried. 105 g of the dried precipitate, 43.5 g of titanium oxide, and 92.0 g of sodium carbonate were mixed in the same manner as in Example 1. Except for the above, Cathode Active Material 7 was obtained in the same manner as for Cathode Active Material 1, and XRD measurement, battery fabrication, and charge / discharge testing were carried out in the same manner as in Example 1, except that Cathode Active Material 7 was used instead of Cathode Active Material 1. For the obtained Cathode Active Material 7 (composite oxide), the ratios a to d in the above formula (1) were a:b:c:d=1:0.34:0.34:0.32. That is, Cathode Active Material 7 was obtained by the procedure of the formula Na １ Ni ０．３４ Mn ０．３４ Ti ０．３２ O ｅ (e is a positive real number), where b / c = 1.00, d / (b + c + d) = 0.32, and b + c + d = 1.00. The average particle size of the secondary particles of positive electrode active material 7 was 9.4 μm. From the diffraction chart, it was confirmed that the space group of the crystal structure of the complex oxide contained in positive electrode active material 7 belonged to R-3m (No. 166), and the lattice constant in the c-axis direction was 1.633 nm. In addition, the ratio of the peak intensity of the 003 reflection to the peak intensity of the 104 reflection was 0.72. The discharge capacity of the sodium ion secondary battery obtained using positive electrode active material 7 was 108 mAh / g, and the average operating voltage was 3.18 V.

[0071] Comparative Example 3: 5 L of pure water and aqueous sodium hydroxide solution were mixed in a reaction vessel to obtain a reaction vessel liquid adjusted to pH 11. A mixed aqueous solution of a 2.25 mol / L nickel sulfate solution and a 3.0 mol / L manganese sulfate solution was added to the reaction vessel liquid at a rate of 200 mL / h. Simultaneously, a 3.0 mol / L ammonium sulfate solution was added at a rate of 100 mL / h. During the addition of these aqueous solutions, aqueous sodium hydroxide solution was added to the reaction vessel liquid so that the pH of the reaction vessel liquid was maintained at 11. The reaction vessel liquid was stirred at 600 rpm with a stirrer under atmospheric conditions, while the temperature of the reaction vessel liquid was controlled at 40°C. A precipitate was formed by the above procedure. The precipitate obtained by the above procedure was filtered, washed with water, and dried.

[0072] 110 g of the dried precipitate, 2.0 g of titanium oxide, and 66.0 g of sodium carbonate were mixed in the same manner as in Example 1. Other than the above, cathode active material 8 was obtained in the same manner as cathode active material 1, and XRD measurement, battery production, and charge / discharge tests were carried out in the same manner as in Example 1, except that cathode active material 8 was used instead of cathode active material 1. For the obtained cathode active material 8 (composite oxide), the ratios a to d in the above formula (1) were a:b:c:d=1:0.42:0.56:0.02. That is, cathode active material 8 was obtained by the procedure of the formula Na １ Ni ０．４２ Mn ０．５６ Ti ０．０２ O ｅ (e is a positive real number), where b / c = 0.75, d / (b + c + d) = 0.02, and b + c + d = 1.00. The average particle size of the secondary particles of positive electrode active material 8 was 8.9 μm. From the diffraction chart, it was confirmed that the space group of the crystal structure of the complex oxide contained in positive electrode active material 8 belonged to R-3m (No. 166), and the lattice constant in the c-axis direction was 1.609 nm. In addition, the ratio of the peak intensity of the 003 reflection to the peak intensity of the 104 reflection was 0.61. The discharge capacity of the sodium ion secondary battery obtained using positive electrode active material 8 was 102 mAh / g, and the average operating voltage was 3.12 V.

[0073] Comparative Example 4: 5 L of pure water and aqueous sodium hydroxide solution were mixed in a reaction vessel to obtain a reaction vessel liquid adjusted to pH 11. A mixed aqueous solution of 3.0 mol / L nickel sulfate solution and 2.4 mol / L manganese sulfate solution was added to the reaction vessel liquid at a rate of 200 mL / h. Simultaneously, a 3.0 mol / L aqueous ammonium sulfate solution was added at a rate of 100 mL / h. During the addition of these aqueous solutions, aqueous sodium hydroxide solution was added to the reaction vessel liquid so that the pH of the reaction vessel liquid was maintained at 11. The reaction vessel liquid was stirred at 600 rpm with a stirrer under atmospheric conditions, while the temperature of the reaction vessel liquid was controlled at 40°C. A precipitate was formed by the above procedure. The precipitate obtained by the above procedure was filtered, washed with water, and dried.

[0074] 110 g of the dried precipitate, 40.2 g of titanium oxide, and 91.3 g of sodium carbonate were mixed in the same manner as in Example 1. Other than the above, cathode active material 9 was obtained in the same manner as cathode active material 1, and XRD measurement, battery fabrication, and charge / discharge tests were carried out in the same manner as in Example 1, except that cathode active material 9 was used instead of cathode active material 1. For the obtained cathode active material 5 (composite oxide), the ratios a to d in the above formula (1) were a:b:c:d=1:0.39:0.31:0.3. That is, cathode active material 5 was a composite oxide having the formula Na １ Ni ０．３９ Mn ０．３１ Ti ０．３ O ｅ (e is a positive real number), where b / c = 1.25, d / (b + c + d) = 0.32, and b + c + d = 1.00. The average particle size of the secondary particles of positive electrode active material 9 was 9.1 μm. From the diffraction chart, it was confirmed that the space group of the crystal structure of the complex oxide contained in positive electrode active material 5 belongs to R-3m (No. 166), and the lattice constant in the c-axis direction was 1.632 nm. In addition, the ratio of the peak intensity of the 003 reflection to the peak intensity of the 104 reflection was 0.90. The discharge capacity of the sodium ion secondary battery obtained using positive electrode active material 9 was 111 mAh / g, and the average operating voltage was 3.02 V.

[0075] Comparative Example 5 62 g of nickel hydroxide, 51 g of trimanganese tetroxide, 80 g of sodium carbonate, and 12 g of titanium oxide were mixed in the same manner as in Example 1. Other than the above, a cathode active material 10 was obtained in the same manner as in Cathode Active Material 1, and XRD measurement, battery fabrication, and charge / discharge tests were carried out in the same manner as in Example 1, except that cathode active material 10 was used instead of cathode active material 1. For the obtained cathode active material 10 (composite oxide), the ratios a to d in the above-mentioned formula (1) were a:b:c:d=1:0.45:0.45:0.10. That is, cathode active material 10 was a composite oxide having the formula Na １ Ni ０．４５ Mn ０．４５ Ti ０．１０ O ｅ(e is a positive real number), where b / c = 1.00, d / (b + c + d) = 0.10, and b + c + d = 1.00. The average particle size of the secondary particles of the positive electrode active material 3 was 8.8 μm. From the diffraction chart, it was confirmed that the space group of the crystal structure of the complex oxide contained in the positive electrode active material 10 belongs to R-3m (No. 166), and the lattice constant in the c-axis direction was 1.604 nm. In addition, the ratio of the peak intensity of the 003 reflection to the peak intensity of the 104 reflection was 0.58. The discharge capacity of the sodium ion secondary battery obtained using the positive electrode active material 10 was 95 mAh / g, and the average operating voltage was 3.04 V.

[0076] <Results> Table 1 shows the compositions, secondary particle diameters (average particle diameters of secondary particles), lattice constants in the c-axis direction, and XRD peak intensity ratios of the positive electrode active materials produced in Examples 1 to 5 and Comparative Examples 1 to 5. Table 1 also shows the discharge capacities and average operating voltages of sodium ion secondary batteries produced using the positive electrode active materials produced in Examples 1 to 5 and Comparative Examples 1 to 5.

[0077]

[0078] From the above results, it was confirmed that when the space group of the composite oxide belongs to R-3m (No. 166), the lattice constant in the c-axis direction is 1.605 to 1.630 nm, and the ratio of the peak intensity of the 003 reflection to the peak intensity of the 104 reflection in a diffraction chart obtained by performing X-ray diffraction measurement is 1.00 or more, a sodium ion secondary battery obtained using a positive electrode active material for a sodium ion secondary battery containing the above composite oxide has a large discharge capacity and a high average operating voltage.

Claims

1. A positive electrode active material for a sodium ion secondary battery, comprising a complex oxide represented by the following formula (1), wherein the crystal structure of the complex oxide belongs to the space group R-3m, and in a diffraction chart obtained by performing X-ray diffraction measurement, the ratio of the peak intensity of the 003 reflection of the complex oxide to the peak intensity of the 104 reflection of the complex oxide is 1.00 or more. Formula (1) Na ａ Ni ｂ Mn ｃ Ti ｄ O ｅ In formula (1), a to e are positive real numbers, and b, c, and d satisfy the following relationships: 0.80≦b / c≦1.20 0<d / (b+c+d)≦0.30 b+c+d=1.00 2. The sodium ion secondary battery positive electrode active material according to claim 1, wherein the lattice constant in the c-axis direction is 1.605 to 1.630 nm.

3. The positive electrode active material for a sodium ion secondary battery according to claim 1, wherein the average particle size of the secondary particles is 2 to 20 μm.

4. A method for producing a positive electrode active material for a sodium ion secondary battery according to any one of claims 1 to 3, comprising: introducing a nickel source and a manganese source into a reaction tank liquid maintained at a pH of 9 to 12 to produce a precipitate; mixing the precipitate with a sodium source and a titanium source to obtain a mixture; and heating the mixture at a temperature of 400°C to 1200°C.

5. A sodium ion secondary battery comprising: a positive electrode containing the positive electrode active material for sodium ion secondary batteries according to any one of claims 1 to 3; a negative electrode; and a non-aqueous electrolyte interposed between the positive electrode and the negative electrode and conducting sodium ions.

6. A sodium ion secondary battery comprising: a positive electrode containing the positive electrode active material for sodium ion secondary batteries according to any one of claims 1 to 3; a negative electrode; and a solid electrolyte interposed between the positive electrode and the negative electrode for conducting sodium ions.

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