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

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

Technical Field

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

Background Art

[0002] In recent years, the electrification of automobiles and the performance improvement of electronic devices have been rapidly progressing, and further higher energy density is required for secondary batteries used as power sources for these. As such secondary batteries, lithium ion secondary batteries are widely used in terms of having a high energy density, and further demand expansion is expected in the future. In a lithium ion secondary battery, since lithium ions serve as charge carriers, lithium is essential. Lithium is a rare metal, and moreover, since the production areas are unevenly distributed, it can be said that the widespread use of lithium is not necessarily desirable in terms of stable supply.

[0003] From the above points, as a secondary battery replacing the lithium ion secondary battery, a sodium ion secondary battery in which sodium ions serve as charge carriers instead of lithium ions has been studied. A sodium ion secondary battery usually includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and an electrolyte interposed between the positive electrode and the negative electrode to conduct sodium ions. As positive electrode active materials for sodium ion secondary batteries, various compounds have been studied. For example, in Patent Document 1, as a positive electrode active material for a sodium ion secondary battery, Na x MO2 (M is two or more elements selected from the group consisting of metal elements excluding alkali metal elements, x is a value greater than 0 and less than or equal to 1) has been proposed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Recently, in sodium-ion secondary batteries as well, achieving a higher energy density has been demanded. When the present inventors examined the positive electrode active material described in Patent Document 1, they found that the energy density of the sodium-ion secondary battery formed using the above positive electrode active material did not meet the recent required levels and there was room for improvement. That is, it was found that there was room for improvement in terms of 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 has a large discharge capacity and a high average operating voltage in the obtained sodium-ion secondary battery. Another object of the present invention is to provide a method for manufacturing a positive electrode active material for a sodium-ion secondary battery. Furthermore, an 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.

Means for Solving the Problems

[0007] As a result of intensive studies to solve the above problems, the present inventors have completed the present invention. That is, it has been found that the above problems are solved by the following configuration.

[0008] 〔1〕 A positive electrode active material for a sodium-ion secondary battery containing a composite oxide represented by the following formula (1), The crystal structure of the above composite oxide belongs to the space group R-3m, In the diffraction chart obtained by performing X-ray diffraction measurement, the ratio of the peak intensity of the 003 reflection of the above composite oxide to the peak intensity of the 104 reflection of the above composite oxide is 1.00 or more, a positive electrode active material for a sodium-ion secondary battery. Formula (1) Naa Ni b Mn c Ti d O e In equation (1), a to e are positive real numbers, and b, c, and d satisfy the following relationship. 0.80 ≤ b / c ≤ 1.20 0 <d / (b+c+d)≦0.30 b+c+d=1.00 [2] A sodium-ion secondary battery positive electrode active material as described in [1], wherein the lattice constant in the c-axis direction is 1.605 to 1.630 nm. [3] A positive electrode active material for a sodium-ion secondary battery according to [1] or [2], 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 as described in any one of [1] to [3], A nickel source and a manganese source are introduced into a reaction vessel solution whose pH is maintained between 9 and 12 to generate a precipitate. The above precipitate is mixed with a sodium source and a titanium source to obtain a mixture. A method for producing a positive electrode active material for a sodium-ion secondary battery, comprising heating the above mixture at a temperature of 400°C to 1200°C. [5] A positive electrode containing a positive electrode active material for sodium ion secondary batteries as described in any one of [1] to [3], The negative electrode and, A sodium-ion secondary battery comprising a non-aqueous electrolyte interposed between the positive electrode and the negative electrode to conduct sodium ions. [6] A positive electrode containing a positive electrode active material for sodium ion secondary batteries as described in any one of [1] to [3], The negative electrode and, A sodium-ion secondary battery comprising a solid electrolyte interposed between the positive electrode and the negative electrode to conduct sodium ions. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a positive electrode active material for a sodium-ion secondary battery that has a large discharge capacity and a high average operating voltage. Furthermore, the present invention also aims to provide a method for producing a positive electrode active material for sodium-ion secondary batteries. Furthermore, the present invention also aims to provide a sodium-ion secondary battery using the above-mentioned positive electrode active material for sodium-ion secondary batteries. [Brief explanation of the drawing]

[0010] [Figure 1] This is a diffraction chart obtained by performing X-ray diffraction measurements in Example 1 and Comparative Example 1. [Figure 2] These are the charge and discharge curves of the sodium-ion secondary batteries prepared in Example 1 and Comparative Example 1. [Modes for carrying out the invention]

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

[0012] The following definitions are used within this specification. The embodiments of the present invention will be described in detail below. However, the embodiments described below are examples only, and the present invention is not limited to the embodiments described below. In this specification, numerical ranges represented by "~" mean a range that includes the numbers written before and after "~" as the lower and upper limits, respectively.

[0013] <Positive electrode active material for sodium-ion secondary batteries> The positive electrode active material for sodium-ion secondary batteries of the present invention (hereinafter also simply referred to as "positive electrode active material") is a positive electrode active material for sodium-ion secondary batteries that contains a composite oxide represented by formula (1) described later. In addition, the crystal structure of the composite oxide represented by the formula (1) described below belongs to the space group R-3m. Furthermore, in the diffraction chart obtained by performing X-ray diffraction 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. When the composite oxide has the above composition and the above crystal structure, and the ratio of the peak intensities is 1.00 or more, it is considered that sodium ions in the composite oxide are easily desorbed and inserted smoothly. As a result, when a sodium ion secondary battery is manufactured using the positive electrode active material containing the above composite oxide, the discharge capacity is large and the average operating voltage is high. Hereinafter, each item will be described.

[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 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 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 them, a is preferably 0.40 or more, more preferably 0.60 or more, and even more preferably 0.90 or more. Also, 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, more preferably 0.40 or more. Also, b is preferably 0.55 or less, more preferably 0.50 or less. c is preferably 0.35 or more, more preferably 0.40 or more. d is preferably 0.03 or higher, more preferably 0.05 or higher, and even more preferably 0.10 or higher. Furthermore, d is preferably 0.30 or lower, more preferably 0.25 or lower, and even more preferably 0.20 or lower. The b / c value is preferably 0.85 or higher, more preferably 0.90 or higher, and even more preferably 0.95 or higher. Furthermore, the b / c value is preferably 1.15 or lower, more preferably 1.10 or lower, and even more preferably 1.05 or lower. The value of d / (b+c+d) is preferably 0.03 or greater, more preferably 0.05 or greater, and even more preferably 0.10 or greater. Furthermore, 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 ratio of the composite oxide (values ​​a to e above) is determined by fusion-coupled plasma atomic emission spectroscopy (ICP-OES).

[0015] [Space group and lattice constants] The crystal structure of the composite oxide contained in the positive electrode active material of the present invention belongs to space group R-3m. Being in space group R-3m means belonging to a space group that has one triple-fold reciprocal axis and one mirror plane. This space group belongs to the trigonal system and is No. 166. Furthermore, the c-axis lattice constant of the composite oxide contained in the positive electrode active material of the present invention is preferably 1.605 to 1.630 nm. The c-axis lattice constant is determined by X-ray diffraction measurement as described later. The lattice constant in the c-axis direction is more preferably 1.606 nm or greater, and even more preferably 1.607 nm or greater. Furthermore, 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. Furthermore, the composite oxide contained in the positive electrode active material of the present invention has layers in which octahedral units, each having six oxygen atoms arranged around nickel, manganese, and titanium, are bonded together by sharing faces, and sodium elements are inserted between these layers. In the unit cell of the composite oxide, there are three of the above-mentioned layers, and two layers in which sodium elements are inserted. Furthermore, the normal direction of the above layers is parallel to the c-axis direction, and the above layers are stacked in the c-axis direction. Here, the distance from the center position of the above layer to the center position of the next adjacent layer is 1 / 3 of the lattice constant in the c-axis direction.

[0016] [XRD] The positive electrode active material of the present invention has a diffraction chart obtained by performing X-ray diffraction (XRD) measurement in which 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 originate from the crystal planes of the above indices are analyzed by conventional methods. The diffraction lines of 003 reflection from composite oxides typically appear in the range of 2θ = 15 to 20° in the diffraction chart obtained by so-called θ / 2θ measurement. Similarly, the diffraction lines of 104 reflection from composite oxides typically appear in the range of 2θ = 40 to 50° in the diffraction chart obtained by so-called θ / 2θ measurement. "θ / 2θ measurement" refers to a measurement method in which, when the X-ray incidence direction to the sample is θ°, the detector for diffracted X-rays is scanned while maintaining a positional relationship of 2θ° with respect to the X-ray incidence direction on the plane that includes the X-ray incidence direction. θ / 2θ measurement is performed using the measurement method commonly used in this field.

[0017] The fact that the peak intensity ratio is 1.00 or higher suggests that the disorder in the crystal structure along the c-axis is relatively small, which is thought to correspond to the ease with which sodium ions can be taken in and out of the composite oxide when used as a sodium-ion secondary battery. The ratio of peak intensities is preferably 1.10 or higher, more preferably 1.20 or higher, and even more preferably 1.25 or higher. Furthermore, the ratio of peak intensities is typically 2.00 or lower.

[0018] [Positive electrode active material powder] The positive electrode active material of the present invention is usually in powder form. Furthermore, as described above, the positive electrode active material of the present invention contains the composite oxide described above. The content of the composite oxide relative to the total mass of the positive electrode active material is preferably 95% by mass or more, and more preferably 98% by mass or more. There is no particular upper limit to the content, and it may be 100% by mass relative to the total mass of the positive electrode active material.

[0019] The particles constituting the positive electrode active material powder preferably form secondary particles that are aggregated from primary particles. The shape of the secondary particles is not particularly limited, but it 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, and more preferably 15 μm or less.

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

[0021] The particle size distribution of the secondary particles described above may be unimodal or multimodal, but a multimodal distribution is preferred because it improves the packing efficiency of the positive electrode active material and makes it easier to improve the energy density. As for the multimodal particle size distribution, for example, a bimodal particle size distribution is preferred, and it is more preferable that it exhibits a bimodal particle size distribution and also exhibits the preferred average particle size described above.

[0022] <Method for manufacturing positive electrode active material for sodium-ion secondary batteries> The present invention provides a method for producing a positive electrode active material for a sodium-ion secondary battery. This method involves introducing a nickel source and a manganese source into a reaction vessel solution with a pH of 9 to 12 to generate a precipitate, mixing the precipitate with a sodium source and a titanium source to obtain a mixture, and then heating the mixture at a temperature of 400°C to 1200°C. According to the above manufacturing method, the positive electrode active material of the present invention can be easily obtained. The following explains each step.

[0023] [Formation of precipitate] First, the nickel source and manganese source are introduced into a reaction vessel solution with a pH between 9 and 12 to generate a precipitate. Since nickel and manganese form precipitates at the above pH, when the above procedure is performed, nickel and manganese co-precipitate to obtain a precipitate. The precipitate obtained by co-precipitation has nickel and manganese uniformly dispersed at the atomic level, and it is thought that the resulting positive electrode active material will satisfy the above-mentioned properties.

[0024] The term "reaction vessel liquid" above refers to the liquid inside the reaction vessel. Furthermore, "introducing to a reaction vessel liquid with a pH of 9 to 12" means introducing the nickel and manganese sources into the reaction vessel liquid while maintaining a pH of 9 to 12. One way to maintain the pH of the reaction vessel liquid at 9 to 12 is, for example, to introduce the nickel and manganese sources into the reaction vessel liquid while adding the alkaline aqueous solution described later. In addition, an ammonium source, described later, may be introduced into the reaction vessel liquid along with the nickel and manganese sources.

[0025] Examples of nickel sources include nickel sulfate, nickel carbonate, nickel nitrate, nickel acetate, and nickel chloride, with nickel sulfate being preferred. Examples of manganese sources include manganese sulfate, manganese carbonate, manganese nitrate, manganese acetate, and manganese chloride, with manganese sulfate being preferred. The nickel source and manganese source may be a solution (e.g., an aqueous solution) containing the respective nickel source and manganese source. Furthermore, when using a solution containing a nickel source and a solution containing a manganese source as the nickel source and manganese source, it is preferable to pre-mix the nickel source solution and the manganese source solution to form a mixed solution and introduce it 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 adjusted as appropriate to fall within the range of the above-mentioned formula (1), but a molar ratio of 0.8 to 1.2 is preferred.

[0026] Examples of ammonium sources include ammonium sulfate, ammonium chloride, ammonium nitrate, and ammonium carbonate salts, with ammonium sulfate being preferred. The ammonium source may be a solution (e.g., an aqueous solution) containing the above-mentioned ammonium source. Alternatively, aqueous ammonia may be used as the ammonium source. By adding an ammonium source, nucleation during coprecipitation can be suppressed, and particle growth of the generated nuclei can be promoted. The ratio of the ammonium ion content of the ammonium source to the sum of the nickel content of the nickel source and the manganese content of the manganese source (hereinafter referred to as "NH4") + It is also written as " / (Ni+Mn)". ) represents the molar ratio, preferably between 0 and 1.

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

[0028] The introduction of the above-mentioned substance into the reaction vessel liquid is preferably carried out while stirring the reaction vessel liquid. Stirring of the reaction vessel liquid can be done, for example, by using a stirring blade. The temperature of the reaction vessel liquid is preferably between 30°C and 60°C, and more preferably between 35°C and 45°C. The atmosphere in which the reaction vessel is placed, that is, the atmosphere above the reaction vessel liquid, may be an atmospheric atmosphere or an inert atmosphere. Examples of inert gases used to create an inert atmosphere include nitrogen gas and argon gas. Alternatively, the above-mentioned inert gas may be bubbled into the reaction vessel liquid to suppress oxidation of the precipitate. The formation of the above precipitate may be carried out in a batch-type reactor or a continuous-type reactor. Furthermore, by following the above procedure, a reaction vessel liquid in which the precipitate is dispersed will be obtained.

[0029] [Mixing and heating] Next, the precipitate, a sodium source, and a titanium source are mixed to obtain a mixture, and this mixture is heated at a temperature between 400°C and 1200°C. One method for obtaining the mixture is to separate the precipitate from the reaction vessel liquid in which the precipitate is dispersed, and then mix the separated precipitate with a sodium source and a titanium source. When separating the precipitate from the reaction vessel liquid, known methods can be applied. For example, methods include filtering the reaction vessel liquid to recover the precipitate, separating the precipitate from the reaction vessel liquid by centrifugal force, and allowing the reaction vessel liquid to stand to settle the precipitate and then removing the supernatant of the reaction vessel liquid.

[0030] Furthermore, the precipitate obtained by separating it from the reaction vessel liquid may be subjected to drying. Known methods can be used for the drying process. Examples of drying methods include air drying, hot air drying, and vacuum drying. Furthermore, the separation and drying of the precipitate may be carried out simultaneously; for example, spray drying may be used as the method described above.

[0031] Furthermore, the precipitate may be washed before carrying out the drying process described above. One washing method is to bring the precipitate into contact with water, and more specifically, to disperse the precipitate in water and then separate it from the water using the separation method described above. It is preferable to use purified water, such as deionized water, in the above procedure. After washing the precipitate, the washed precipitate may be dried using the drying method described above.

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

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

[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 a temperature between 400°C and 1200°C. This heating process yields the positive electrode active material of the present invention. The heating temperature is preferably between 600°C and 1200°C. The atmosphere in which heating is performed is not particularly limited and can include an oxidizing atmosphere (e.g., an atmospheric atmosphere) and a non-oxidizing atmosphere (e.g., a nitrogen atmosphere and an argon atmosphere).

[0036] The temperature profile during the heat treatment can be adjusted as needed. For example, a heat treatment may be performed in which the material is held at a predetermined temperature for a predetermined time, or the heat treatment may be divided into two or more stages. When the heat treatment is divided into two or more stages, 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 holding 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 holding time may be, for example, 72 hours or less, or 48 hours or less. If the heat treatment is divided into two or more stages, it is also preferable that the total heating time is the above preferred heating time. Furthermore, when the heat treatment is divided into two or more stages, it is preferable that the atmosphere for the first heat treatment is a non-oxidizing atmosphere (e.g., a nitrogen atmosphere and an argon atmosphere), and the atmosphere for the second heat treatment is an oxidizing atmosphere (e.g., an air atmosphere).

[0037] The above describes a method for separating and mixing the precipitate from the reaction vessel liquid, but 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, the precipitate and titanium source may be separated from the reaction vessel liquid using the method described above, and then mixed with a sodium source. Alternatively, the precipitate may be separated using the method described above, the precipitate may be washed using the method described above, and a titanium source may be added to the slurry containing the washing liquid (e.g., water) and the precipitate. The precipitate and titanium source may then be separated from the slurry to which the titanium source has been added and mixed with the sodium source. Furthermore, in the above procedure, a sodium source may be added to the slurry to which the titanium source has been added, and the solid content 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 other steps besides those described above. For example, the positive electrode active material obtained by the above procedure may be washed. The washing method can be the one described above. Washing the obtained positive electrode active material removes unreacted raw materials (e.g., sodium source). The positive electrode active material that has undergone the above cleaning may be further dried. The drying method can be the one described above. Furthermore, heating may be performed after drying. The heating temperature is preferably between 200°C and 800°C. The heating time can be adjusted as appropriate, but for example, it can be between 0.5 and 12 hours.

[0039] <Sodium-ion secondary battery> The sodium-ion secondary battery of the present invention comprises a positive electrode containing the 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 those of conventionally known configurations. For example, the sodium-ion secondary battery of the present invention may further include a separator. Furthermore, the form of the sodium-ion secondary battery of the present invention can be arbitrarily selected from cylindrical, prismatic, coin-type, sheet-type, laminate-type, and button-type batteries, depending on the application, the equipment it is mounted on, and the required charge / discharge capacity. The following describes the various components of the sodium-ion secondary battery of the present invention.

[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 that supports the positive electrode active material (or positive electrode mixture). The positive electrode current collector may be a metal foil or a porous metal (e.g., a nonwoven fabric of metal fibers, or a porous metal sheet). As the porous metal, a porous metal having a three-dimensional mesh-like framework (especially a hollow framework) can also be used. The material of the positive electrode current collector is not particularly limited, but from the viewpoint of stability at the positive electrode potential, aluminum or an aluminum alloy is preferred. The thickness of the metal foil used as the positive electrode current collector is, for example, 10 to 50 μm, and the thickness of the porous metal used 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 composite material may, for example, further include at least one of a conductive additive and a binder in addition to the positive electrode active material. The conductive additive is not particularly limited, but examples include carbon black, acetylene black, graphite, carbon fibers (such as vapor-processed carbon fibers), and carbon nanotubes. One conductive additive may be used alone, or two or more may be used simultaneously. Furthermore, from the viewpoint of enhancing conductivity, the conductive additive may be arranged to coat the surface of the positive electrode active material particles. The coating of the positive electrode active material particles with the conductive additive can be achieved by bringing the conductive additive into contact with the positive electrode active material particles to adhere the conductive additive to the surface 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 positive electrode active material.

[0042] The binder is not particularly limited, but examples include fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene; polyolefin resins; rubbery polymers such as styrene-butadiene rubber; polyamide resins (aromatic polyamides, etc.); polyimides and polyimide resins such as polyamideimide; polyvinylpyrrolidone; polyvinyl alcohol; and cellulose ethers (carboxyalkyl cellulose such as carboxymethylcellulose and its sodium salts and their salts, etc.). The binder may be used alone or two or more at the same time. The amount of binder in the positive electrode composite is not particularly limited, but it is preferable to use 0.5 to 15 parts by mass per 100 parts by mass of positive electrode active material, for example, as it exhibits high binding properties while easily increasing the discharge capacity.

[0043] The positive electrode can be formed, for example, by coating 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 can be used. The dispersion medium may be a mixture of two or more solvents.

[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 that supports the negative electrode active material (or negative electrode mixture). The negative electrode current collector may be a metal foil or a porous metal (e.g., a nonwoven fabric of metal fibers, or a porous metal sheet). As the porous metal, a porous metal having a three-dimensional mesh-like skeleton (especially 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, aluminum alloy, or stainless steel, with aluminum being preferred. The thickness of the metal foil used as the negative electrode current collector is, for example, 10 to 50 μm, and the thickness of the porous metal used as the negative electrode current collector is, for example, 100 to 2000 μm.

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

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

[0047] For example, the negative electrode can be formed by press-molding a negative electrode current collector with the negative electrode active material or negative electrode composite material, or by creating a paste using the aforementioned dispersion medium, applying it to the negative electrode current collector, drying it, and then pressing it to fix it in place.

[0048] [Electrolyte] The electrolyte acts as an intermediary between the positive and negative electrodes, conducting sodium ions. The electrolyte may be a non-aqueous electrolyte or a solid electrolyte.

[0049] (Non-aqueous electrolytes) Non-aqueous electrolytes typically consist of a solvent other than water and a solute capable of conducting sodium ions. Examples of solvents used in non-aqueous electrolyte solutions 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. Other solvents that can be used include ethers such as lanes; 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-propanesalton, ethylene sulfite, propylene sulfite, dimethyl sulfite, and diethyl sulfite; and solvents to which fluorine substituents have been further introduced. Typically, two or more of the above solvents are used in mixture form. Among these, mixed solvents containing carbonates are preferred, mixed solvents containing cyclic carbonates and acyclic carbonates are preferred, and mixed solvents containing cyclic carbonates and ethers are even more preferred. Here, as a mixed solvent containing cyclic carbonates and acyclic carbonates, a mixed solvent containing ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is preferred in that it has a wide operating temperature range and excellent load characteristics.

[0050] Sodium salts are preferred as solutes capable of conducting sodium ions. The type of anion (first anion) constituting the sodium salt is not particularly limited, and can include, for example, anions of fluorine-containing acids (fluorine-containing phosphate anions such as hexafluorophosphate ions; fluorine-containing borate anions such as tetrafluoroborate ions; etc.), anions of chlorine-containing acids (perchlorate ions, etc.), and anions of oxygen acids having an oxalate group (bis(oxalato)borate ions (B(C2O4)2) - ) and other oxalatoborate ions, tris(oxalato)phosphate ions (P(C2O4)3 - ) and other oxalatophosphate ions, fluoroalkanesulfonic acid anions (trifluoromethanesulfonate ion (CF3SO3 - Examples include ) and bissulfonylamide anions. Examples of the above-mentioned bissulfonylamide anions include bis(fluorosulfonyl)amide anion (FSA), bis(trifluoromethylsulfonyl)amide anion (TFSA), and (fluorosulfonyl)(perfluoroalkylsulfonyl)amide anion ((FSO2)(CF3SO2)N - (etc.), bis(perfluoroalkylsulfonyl)amide anion (N(SO2CF3)2 - , N(SO2C2F5)2 - Examples include the following. Of these, FSA and TFSA, at least one of them, are particularly preferred. A single sodium salt may be used alone, or two or more sodium salts with different types of first anions may be used in combination. Examples of solutes in non-aqueous electrolyte solutions include NaClO4, NaPF6, NaBF4, NaCF3SO3, NaN(CF3SO2)2, NaN(C2F5SO2)2, and NaC(CF3SO2)3.

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

[0052] (solid electrolyte) Solid electrolytes refer to solid electrolytes that can conduct sodium ions. Examples of solid electrolytes include sulfide-based and oxide-based materials. Examples of sulfide-based solid electrolytes include Na3PS4, Na3PS4-Na4SiS4, Na2S-P2S5, Na2S-SiS2, and Na2S-GeS2. Examples of oxide-based solid electrolytes include β-alumina, β''-alumina, and oxide materials containing NASICON (Na Super Ionic Conductor) type crystals. Note that NASICON type crystals are generally Na 1+x Zr2Si x P 3-x O 12 It is represented by the empirical formula , where x is greater than 0 and less than 3. At least one of Zr and Si in the above empirical formula may be partially substituted with other elements.

[0053] [Separator] The sodium-ion secondary battery of the present invention may be equipped with a separator. A separator is a device placed between the positive and negative electrodes to prevent them from coming into contact, while still allowing sodium ions to pass through it. The material of the separator is not particularly limited, but for example, woven fabrics, nonwoven fabrics, and microporous membranes made of synthetic resins can be used. As for microporous membranes made of synthetic resins, polyolefin-based microporous membranes are preferred in terms of thickness, membrane strength, and membrane resistance. Examples of polyolefin-based microporous membranes include polyethylene microporous membranes, polypropylene microporous membranes, and microporous membranes made by combining these. Glass fiber separators are also preferred as they are effective in preventing minute short circuits. [Examples]

[0054] The present invention will be described in more detail below based on examples. The materials, quantities, proportions, processing details, and processing procedures shown in the following examples can be modified as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the following examples.

[0055] <Example 1> The positive electrode active material 1 was manufactured using the following procedure, evaluated, and a sodium-ion secondary battery was fabricated using the positive electrode active material 1. The charge-discharge characteristics of the sodium-ion secondary battery were then evaluated.

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

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

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

[0059] The mixture obtained from the above mixing was subjected to two heat treatments to obtain a calcined product. More specifically, the material was heated at 650°C for 24 hours under a nitrogen atmosphere, and then heated at 950°C for 15 hours under an air atmosphere. The resulting calcined material 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, a to d in equation (1) above were a:b:c:d = 1:0.45:0.45:0.1. In other words, the positive electrode active material 1 is given by formula Na1Ni 0.45 Mn 0.45 Ti 0.1 O e The composite oxide (where e is a positive real number) had the following composition: 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 measurements were performed on cathode active material 1 to obtain a diffraction chart. XRD measurements were performed using a Rigaku SmartLab, with CuK as the X-ray source. α Line (CuK α1 Line and CuK α2 X-rays containing a 2:1 intensity ratio were used, with a tube voltage of 40kV and a tube current of 30mA. β A Ni filter was used to remove the lines. The resulting diffraction chart is shown in Figure 1. From the diffraction chart, it was confirmed that the space group of the crystal structure of the composite oxide contained in 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. Note that the wavelength of the X-rays used when calculating the lattice constant is CuK α1 Wavelength of the line and CuK α2 The arithmetic mean value calculated from the wavelengths of the lines and their intensity ratios is CuK α The wavelength of the line was used.

[0061] [Battery making] A mixture was obtained by adding N-methyl-2-pyrrolidone to positive electrode active material 1 (80% by mass), acetylene black (10% by mass), and polyvinylidene fluoride (10% by mass) and kneading them together. The obtained mixture was applied to an aluminum current collector (20 μm thick) to form a coating. The laminate of the coating and the aluminum current collector was pressed using a roll press. The pressed laminate was punched out into a disc shape with a diameter of 14 mm. The punched disc was vacuum dried at 150°C for 15 hours to form the positive electrode. Furthermore, a 16 mm diameter sodium metal foil was used as the negative electrode. A 20 mm diameter glass fiber separator was used as the separator. As the electrolyte, a non-aqueous electrolyte solution was used, prepared by dissolving 1 mole of NaPF6 in 1 L of a mixed solution of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1 as the solvent. A sodium-ion secondary battery was fabricated in a glove box purged with argon using the positive electrode, negative electrode, separator, and non-aqueous electrolyte solution described above. The fabricated sodium-ion secondary battery had a positive electrode, a separator, and a negative electrode in that order, with a non-aqueous electrolyte interposed between the positive and negative electrodes.

[0062] [Charge / Discharge Test] The fabricated sodium-ion secondary battery was charged and discharged at a constant current of 0.075C within a voltage range of 2.2 to 4.0V to obtain charge-discharge curves. Figure 2 shows the obtained charge-discharge curves. 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 using the same procedure as for positive electrode active material 1, and then washed with water and dried. 110 g of the dried precipitate, 24 g of titanium dioxide, and 82 g of sodium carbonate were mixed in the same manner as in Example 1. Except as described above, positive electrode active material 2 was obtained in the same manner as positive electrode active material 1, and XRD measurement, battery fabrication, and charge / discharge tests were performed in the same manner as in Example 1, except that positive electrode active material 2 was used instead of positive electrode active material 1. For the obtained positive electrode active material 2 (composite oxide), a to d in the above-mentioned equation (1) were a:b:c:d = 1:0.40:0.40:0.20. In other words, the positive electrode active material 2 is, formula Na1Ni 0.4 Mn 0.4 Ti 0.2 O e The composite oxide (where e is a positive real number) had the following composition: 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 the 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 composite oxide contained in the positive electrode active material 2 belongs 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 using the same procedure as for positive electrode active material 1, and then washed with water and dried. 105 g of the dried precipitate, 39.4 g of titanium dioxide, and 89.5 g of sodium carbonate were mixed in the same manner as in Example 1. Except as described above, positive electrode active material 3 was obtained in the same manner as positive electrode active material 1, and XRD measurement, battery fabrication, and charge / discharge tests were performed in the same manner as in Example 1, except that positive electrode active material 3 was used instead of positive electrode active material 1. For the obtained positive electrode active material 3 (composite oxide), a to d in the above-mentioned equation (1) were a:b:c:d = 1:0.35:0.35:0.30. In other words, the positive electrode active material 3 is, formula Na1Ni 0.35 Mn 0.35 Ti 0.3 O eThe composite oxide (where e is a positive real number) had the following composition: 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 the 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 composite oxide contained in the positive electrode active material 3 belongs 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 an aqueous sodium hydroxide solution were added to the reaction vessel and mixed to obtain a reaction vessel solution adjusted to pH 11. A mixed aqueous solution of 2.4 mol / L nickel sulfate and a mixed aqueous solution of 3.0 mol / L manganese sulfate were added to the reaction vessel liquid at a rate of 200 mL / h each. Simultaneously, a 3.0 mol / L aqueous solution of ammonium sulfate was added at a rate of 100 mL / h. During the addition of the above aqueous solution, sodium hydroxide solution was added to the reaction vessel liquid to maintain the pH of the reaction vessel liquid at 11. The reaction vessel liquid was stirred at 600 rpm using a stirrer under an atmospheric environment, while the temperature of the reaction vessel liquid was controlled to 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 dioxide, and 62.5 g of sodium carbonate were mixed in the same manner as in Example 1. Except as described above, positive electrode active material 4 was obtained in the same manner as positive electrode active material 1, and XRD measurement, battery fabrication, and charge / discharge tests were performed in the same manner as in Example 1, except that positive electrode active material 4 was used instead of positive electrode active material 1. For the obtained positive electrode active material 4 (composite oxide), a to d in the above-mentioned equation (1) were a:b:c:d = 1:0.44:0.54:0.02. In other words, the positive electrode active material 4 is, formula Na1Ni 0.44 Mn 0.54 Ti 0.02 O e The composite oxide (where e is a positive real number) had the following composition: 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 the 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 composite oxide contained in the positive electrode active material 4 belongs 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 an aqueous sodium hydroxide solution were added to the reaction vessel and mixed to obtain a reaction vessel solution adjusted to pH 11. A mixed aqueous solution of 3.0 mol / L nickel sulfate and a mixed aqueous solution of 2.5 mol / L manganese sulfate were added to the reaction vessel liquid at a rate of 200 mL / h each. Simultaneously, a 3.0 mol / L aqueous solution of ammonium sulfate was added at a rate of 100 mL / h. During the addition of the above aqueous solution, sodium hydroxide solution was added to the reaction vessel liquid to maintain the pH of the reaction vessel liquid at 11. The reaction vessel liquid was stirred at 600 rpm using a stirrer under an atmospheric environment, while the temperature of the reaction vessel liquid was controlled to 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 dioxide, and 89.5 g of sodium carbonate were mixed in the same manner as in Example 1. Except as described above, positive electrode active material 5 was obtained in the same manner as positive electrode active material 1, and XRD measurement, battery fabrication, and charge / discharge tests were performed in the same manner as in Example 1, except that positive electrode active material 5 was used instead of positive electrode active material 1. For the obtained positive electrode active material 5 (composite oxide), a to d in the above-mentioned equation (1) were a:b:c:d = 1:0.38:0.32:0.30. In other words, the positive electrode active material 5 is of the formula Na1Ni 0.38 Mn 0.32 Ti 0.3 O e The composite oxide (where e is a positive real number) had the following composition: 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 the 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 composite oxide contained in the positive electrode active material 5 belongs 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 using the same procedure as for positive electrode active material 1, and 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 as described above, positive electrode active material 6 was obtained in the same manner as positive electrode active material 1, and positive electrode active material 3 was used instead of positive electrode active material 1. XRD measurement, battery fabrication, and charge / discharge tests were then performed in the same manner as in Example 1. For the obtained positive electrode active material 6 (composite oxide), a to d in the above-mentioned equation (1) were a:b:c:d = 1:0.5:0.5:0. In other words, the positive electrode active material 6 is, formula Na1Ni0.5 Mn 0.5 O e The composite oxide (where e is a positive real number) had the composition represented by 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 the positive electrode active material 6 was 9.2 μm. The resulting diffraction chart is shown in Figure 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 6 belongs 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. Furthermore, the charge-discharge curve obtained is shown in Figure 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 using the same procedure as for positive electrode active material 1, and then washed with water and dried. 105 g of the dried precipitate, 43.5 g of titanium dioxide, and 92.0 g of sodium carbonate were mixed in the same manner as in Example 1. Except as described above, positive electrode active material 7 was obtained in the same manner as positive electrode active material 1, and XRD measurement, battery fabrication, and charge / discharge tests were performed in the same manner as in Example 1, except that positive electrode active material 7 was used instead of positive electrode active material 1. For the obtained positive electrode active material 7 (composite oxide), a to d in the above-mentioned equation (1) were a:b:c:d = 1:0.34:0.34:0.32. In other words, the positive electrode active material 7 is of the formula Na1Ni 0.34 Mn 0.34 Ti 0.32 O e The composite oxide (where e is a positive real number) had the following composition: 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 the 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 composite oxide contained in the positive electrode active material 7 belongs 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 an aqueous sodium hydroxide solution were added to the reaction vessel and mixed to obtain a reaction vessel solution adjusted to pH 11. A mixed aqueous solution of 2.25 mol / L nickel sulfate and a mixed aqueous solution of 3.0 mol / L manganese sulfate were added to the reaction vessel liquid at a rate of 200 mL / h each. Simultaneously, a 3.0 mol / L aqueous solution of ammonium sulfate was added at a rate of 100 mL / h. During the addition of the above aqueous solution, sodium hydroxide solution was added to the reaction vessel liquid to maintain the pH of the reaction vessel liquid at 11. The reaction vessel liquid was stirred at 600 rpm using a stirrer under an atmospheric environment, while the temperature of the reaction vessel liquid was controlled to 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 dioxide, and 66.0 g of sodium carbonate were mixed in the same manner as in Example 1. Except as described above, positive electrode active material 8 was obtained in the same manner as positive electrode active material 1, and XRD measurement, battery fabrication, and charge / discharge tests were performed in the same manner as in Example 1, except that positive electrode active material 8 was used instead of positive electrode active material 1. For the obtained positive electrode active material 8 (composite oxide), a to d in the above-mentioned equation (1) were a:b:c:d = 1:0.42:0.56:0.02. In other words, the positive electrode active material 8 is, formula Na1Ni 0.42 Mn 0.56 Ti 0.02 O eThe composite oxide (where e is a positive real number) had the following composition: 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 the 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 composite oxide contained in the positive electrode active material 8 belongs 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 an aqueous sodium hydroxide solution were added to the reaction vessel and mixed to obtain a reaction vessel solution adjusted to pH 11. A mixed aqueous solution of 3.0 mol / L nickel sulfate and a mixed aqueous solution of 2.4 mol / L manganese sulfate were added to the reaction vessel liquid at a rate of 200 mL / h each. Simultaneously, a 3.0 mol / L aqueous solution of ammonium sulfate was added at a rate of 100 mL / h. During the addition of the above aqueous solution, sodium hydroxide solution was added to the reaction vessel liquid to maintain the pH of the reaction vessel liquid at 11. The reaction vessel liquid was stirred at 600 rpm using a stirrer under an atmospheric environment, while the temperature of the reaction vessel liquid was controlled to 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 dioxide, and 91.3 g of sodium carbonate were mixed in the same manner as in Example 1. Except as described above, positive electrode active material 9 was obtained in the same manner as positive electrode active material 1, and XRD measurement, battery fabrication, and charge / discharge tests were performed in the same manner as in Example 1, except that positive electrode active material 9 was used instead of positive electrode active material 1. For the obtained positive electrode active material 5 (composite oxide), a to d in the above-mentioned equation (1) were a:b:c:d = 1:0.39:0.31:0.3. In other words, the positive electrode active material 5 is of the formula Na1Ni 0.39 Mn 0.31 Ti 0.3 O e The composite oxide (where e is a positive real number) had the following composition: 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 the 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 composite oxide contained in the 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> 62g of nickel hydroxide, 51g of trimanganese tetroxide, 80g of sodium carbonate, and 12g of titanium dioxide were mixed in the same manner as in Example 1. Except as described above, the positive electrode active material 10 was obtained in the same manner as positive electrode active material 1, and XRD measurement, battery fabrication, and charge / discharge tests were performed in the same manner as in Example 1, except that positive electrode active material 10 was used instead of positive electrode active material 1. For the obtained positive electrode active material 10 (composite oxide), a to d in the above-mentioned equation (1) were a:b:c:d = 1:0.45:0.45:0.10. In other words, the positive electrode active material 10 is of the formula Na1Ni 0.45 Mn 0.45 Ti 0.10 O e The composite oxide (where e is a positive real number) had the following composition: 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 composite 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] <Result> Table 1 summarizes the composition, secondary particle size (average particle size of secondary particles), lattice constant in the c-axis direction, and XRD peak intensity ratio of the positive electrode active materials prepared in Examples 1-5 and Comparative Examples 1-5. Table 1 also shows the discharge capacity and average operating voltage of sodium-ion secondary batteries prepared using the positive electrode active materials prepared in Examples 1-5 and Comparative Examples 1-5.

[0077] [Table 1]

[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 the diffraction chart obtained by X-ray diffraction measurement is 1.00 or more, then a sodium-ion secondary battery obtained using a positive electrode active material for sodium-ion secondary batteries containing the above composite oxide will have 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 composite oxide represented by the following formula (1), The crystal structure of the aforementioned composite oxide belongs to space group R-3m, A positive electrode active material for a sodium-ion secondary battery, wherein, in a diffraction chart obtained by performing X-ray diffraction measurements, 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 and 1.28 or less. Formula (1) Na ａ Ni ｂ Mn ｃ Ti ｄ O ｅ In equation (1), a to e are positive real numbers, and b, c, and d satisfy the following relationship. 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, A nickel source and a manganese source are introduced into a reaction vessel solution whose pH is maintained between 9 and 12 to generate a precipitate. The precipitate is mixed with a sodium source and a titanium source to obtain a mixture. A method for producing a positive electrode active material for a sodium-ion secondary battery, comprising heating the aforementioned mixture at a temperature of 400°C to 1200°C.

5. A positive electrode comprising a positive electrode active material for a sodium-ion secondary battery as described in any one of claims 1 to 3, The negative electrode and, A sodium-ion secondary battery comprising a non-aqueous electrolyte interposed between the positive electrode and the negative electrode to conduct sodium ions.

6. A positive electrode comprising a positive electrode active material for a sodium-ion secondary battery as described in any one of claims 1 to 3, The negative electrode and, A sodium-ion secondary battery comprising a solid electrolyte interposed between the positive electrode and the negative electrode to conduct sodium ions.

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