Positive electrode active material for sodium secondary batteries, method for producing the same, and sodium secondary battery containing the same

Abstract

This invention relates to at least sodium (Na), nickel (Ni), manganese (Mn), and doping metal (M D The present invention provides a positive electrode active material for a sodium secondary battery, a method for producing the positive electrode active material, a positive electrode for a sodium secondary battery containing the positive electrode active material, and a sodium secondary battery, wherein the positive electrode active material contains a sodium manganese oxide in which the manganese content among the total metals other than sodium is 55 mol% or more, the sodium manganese oxide is a secondary particle formed by the aggregation of at least one primary particle, and the aspect ratio of the primary particle is 1:1 to 1:2.5.

Description

[Technical Field]

[0001] This invention relates to a positive electrode active material for a sodium secondary battery, a method for producing the same, and a sodium secondary battery containing the same. [Background technology]

[0002] Lithium-ion rechargeable batteries have been widely used as energy storage devices in various fields of electronics technology. Recently, with the surge in demand for lithium-ion rechargeable batteries, sodium-ion rechargeable batteries are attracting attention as a substitute for lithium, an expensive metal. Sodium-ion rechargeable batteries have an insertion / deinsertion reaction operating principle similar to that of lithium-ion rechargeable batteries, making them one of the next-generation materials with high potential for application in rechargeable batteries.

[0003] Layered transition metal oxides, a typical form of cathode active material, have the advantages of a simple structure, excellent electrochemical performance, and ease of synthesis. In particular, high-manganese (high-Mn) sodium nickel manganese oxide (NNMO) holds a relative advantage over other layered transition metal oxides due to its high capacity, price competitiveness (superior in embedding volume), and environmental friendliness.

[0004] However, when sodium ions with large ionic radii are inserted / deinserted within the layered structure, they change the lattice structure, causing the positive electrode active material to undergo continuous phase transitions. The irreversible phase generated during this process reduces the cycle life and high-rate characteristics of sodium-ion secondary batteries. In particular, high-manganese (high-Mn) oxides with a P2-type layered structure have a problem where the electrochemical performance, such as high-rate characteristics and life characteristics, deteriorates due to the irreversible phase generated by Jahn-Teller strain during the charge-discharge process. Jahn-Teller strain is a phenomenon caused by the crystal field stabilization energy, and each time the oxidation state of Mn changes, (Mn 3+ →Mn 4+)Repeated expansion and contraction of the lattice structure cause deterioration of the lifespan of the positive electrode active material.

[0005] In order to solve the above problems, attempts have been made to improve the problems of high-manganese oxides by structurally improving and surface-modifying particles, such as adjusting the particle size of positive electrode active material particles or coating the particle surface, but the situation has not reached the level of commercialization.

Summary of the Invention

Problems to be Solved by the Invention

[0006] For high-manganese oxides, in order to improve the decrease in electrical conductivity due to excessive manganese, it is preferable to produce a ternary transition metal oxide further containing a transition metal other than manganese. However, when a coprecipitation reaction is carried out for the production of a ternary transition metal hydroxide precursor, there is a problem that a large amount of manganese oxide (such as MnO2) is synthesized due to the high reactivity of manganese.

[0007] An object of the present invention is to provide a high-manganese ternary transition metal oxide with a uniform composition by applying a technique of simultaneously performing an oxidation roasting process and a doping process of a precursor.

[0008] The P2-type Ni, High-Mn-based SIB positive electrode material has the merit of showing a higher energy density and capacity than the O3-type positive electrode material. However, in the voltage range of 4.0 V or higher a phase transition occurs, which has the demerit of reducing the reversible capacity. In order to solve the above problems, conventionally, a method of doping various transition metals into a precursor by a wet method has been used, and the transition metals are doped by heat treatment in a subsequent process. However, the wet method has the demerits that applicable doping sources are limited and productivity is not good due to additional heat treatment. In the present invention, in order to solve such problems, a technique of simultaneously performing doping and roasting of a precursor by a dry method is applied.

[0009] Furthermore, in this invention, primary particles within the secondary particles of the positive electrode active material can be grown by performing roasting and doping simultaneously under specific conditions. This has the effect of suppressing electrolyte side reactions caused by a decrease in BET specific surface area, and can provide a positive electrode active material with improved energy density, high voltage stability, lifetime characteristics, and high-rate characteristics.

[0010] Furthermore, the present invention aims to improve the reactivity with sodium by controlling the crystal structure of the doped roasting precursor and the roasting cathode active material produced thereby. [Means for solving the problem]

[0011] One embodiment of the present invention comprises at least sodium (Na), nickel (Ni), manganese (Mn), and doping metal (M D The present invention provides a positive electrode active material for a sodium secondary battery, comprising a sodium-manganese oxide in which the manganese content among the total metals other than sodium is 55 mol% or more, wherein the sodium-manganese oxide is a secondary particle formed by the aggregation of at least one primary particle, and the aspect ratio of the primary particle is 1:1 to 1:2.5.

[0012] The sodium manganese oxide may have a ratio (D2 / D1) of the particle size of the secondary particles (D2) to the particle size of the primary particles (D1) of 6 to 10.

[0013] The sodium manganese oxide may have an average particle size of 8 to 15 μm for the secondary particles and an average particle size of 1 to 3.5 μm for the primary particles.

[0014] The aforementioned sodium manganese oxide has a BET specific surface area of ​​0.1 to 0.45 m². 2 / g is also acceptable.

[0015] The positive electrode active material may have a c-axis length of 11.13 to 11.18 Å in its lattice structure.

[0016] The sodium manganese-based oxide can be represented by the following chemical formula 1. [Chemical formula 1] Na a Ni x (M D ) y M1 z Mn 1-x-y-z O2 In the chemical formula 1, M D may be at least one selected from Fe, Co, Al, Cu, Zn, Mg, and Ti, M1 may be at least one selected from P, Sr, Ba, Zn, Cu, Zr, W, Ce, Hf, Ta, Cr, F, Cr, V, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, M D and M1 may be different elements from each other, and 0.5 ≦ a ≦ 0.8, 0.05 ≦ x ≦ 0.45, 0.01 ≦ y ≦ 0.15, 0 ≦ z ≦ 0.05, 0.5 < 1 - x - y - z ≦ 0.85 may also be satisfied.

[0017] The doping metal (M D ) may be at least one selected from the group consisting of iron (Fe), cobalt (Co), aluminum (Al), copper (Cu), zinc (Zn), magnesium (Mg), and titanium (Ti).

[0018] The molar ratio (M D / M) of the doping metal (M D ) to the total metal (M) other than sodium in the sodium manganese-based oxide may be 0.01 to 0.15.

[0019] The molar ratio (Mn / M) of manganese (Mn) to the total metal (M) other than sodium in the sodium manganese-based oxide may be more than 0.5 and 0.85 or less.

[0020] The molar ratio (Ni / M) of nickel (Ni) to the total metal (M) other than sodium in the sodium manganese-based oxide may be 0.05 to 0.45.

[0021] The sodium manganese oxide may include a P2-type layered structure.

[0022] Another embodiment of the present invention provides a method for producing a positive electrode active material for a sodium secondary battery, comprising the steps of: a) dry mixing a nickel-manganese hydroxide precursor containing 55 mol% or more of manganese in the total metal with a doping compound, followed by roasting to produce an oxide precursor; and b) mixing the oxide precursor with a sodium compound, followed by heat treatment to produce a sodium-manganese oxide.

[0023] The roasting in step a) above can be carried out at a temperature of 750 to 1,050°C.

[0024] The doping compound may be at least one acetate compound, oxide, oxyhydroxide, hydroxide, or combination thereof selected from the group consisting of iron (Fe), cobalt (Co), aluminum (Al), copper (Cu), zinc (Zn), magnesium (Mg), and titanium (Ti).

[0025] The oxide precursor in step a) may include a (Ni-Mn-X)O4 crystal structure, where X in the crystal structure may be Fe, Co, Al, Cu, Zn, Mg, or Ti.

[0026] The heat treatment in step b) above can be carried out at a temperature of 800 to 1,100°C.

[0027] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery containing the positive electrode active material.

[0028] Another embodiment of the present invention provides a sodium secondary battery including the positive electrode. [Effects of the Invention]

[0029] In this invention, it is possible to suppress the phase transition that occurs in the high-voltage region of high-manganese oxides and increase the reversible capacitance. As a result, it is possible to provide a cathode active material with improved energy density, high-voltage stability, lifetime characteristics, and high-rate characteristics.

[0030] By applying dry doping through heat treatment, unlike conventional wet processes, it is possible to achieve economic benefits through a simplified process and reduced process costs in mass production. Furthermore, dry doping processes offer the advantage of being able to select a variety of doping sources that are not easily applicable in conventional wet processes.

[0031] Generally, compared to inserting sodium into a hydroxide precursor, the present invention improves reactivity with sodium (Na) by using an oxidative roasting precursor. ru. [Brief explanation of the drawing]

[0032] [Figure 1a] This is an SEM image of the surface of the precursor according to Example 1. [Figure 1b] This is an SEM image of the surface of the roasted precursor according to Example 1. [Figure 1c] This is an SEM image of the surface of the roasted cathode active material according to Example 1. [Figure 1d] This is an SEM image of a cross-section of the roasted cathode active material according to Example 1. [Figure 2a] This is an SEM image of the surface of the positive electrode active material according to Comparative Example 1. [Figure 2b] This is an SEM image of a cross-section of the positive electrode active material according to Comparative Example 1. [Figure 3a] This is an SEM image of the surface of the positive electrode active material according to Comparative Example 2-1. [Figure 3b] This is an SEM image of a cross-section of the positive electrode active material according to Comparative Example 2-1. [Figure 4] This is an SEM image of the surface of the positive electrode active material according to Comparative Example 2-2. [Figure 5a]This is the XRD analysis result of the roasted doping precursor (bulk) produced in Example 1. [Figure 5b] This shows the XRD analysis results of the roasted cathode active material (bulk) produced in Example 1. [Figure 6a] This shows the SEM-EDS analysis results of the cross-section of the roasted doping precursor particles produced in Example 1. [Figure 6b] This shows the SEM-EDS analysis results of the cross-section of the roasted cathode active material particles produced in Example 1. [Modes for carrying out the invention]

[0033] The advantages and features of the present invention, as well as methods for achieving them, will become clearer with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below and can be realized in a variety of different forms, provided that these embodiments complete the disclosure of the present invention and fully inform those who are ordinary skill in the art to which the invention pertains, and the present invention is defined only by the scope of the claims.

[0034] Unless otherwise defined, all terms used herein (including technical and scientific terms) should be used in a sense that is commonly understood by a person of ordinary skill in the art to which the invention pertains. Throughout the specification, when a part of the specification “includes” a component, this means, unless otherwise stated, that it may include other components rather than excluding them. Also, singular forms include plural forms unless otherwise specified in the text.

[0035] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery. The positive electrode active material comprises at least sodium (Na), nickel (Ni), manganese (Mn), and doping metal (M D It contains sodium-manganese oxides, which include sodium and have a manganese content of 55 mol% or more of the total metals other than sodium.

[0036] The sodium-manganese oxide is an NNMO-based sodium-manganese oxide containing at least sodium, nickel, and manganese. The sodium-manganese oxide is a high-manganese (high-Mn) oxide containing 55 mol% or more of manganese among the total metals other than sodium, and the manganese content among the metals other than sodium may be 55 mol% or more, 60 mol% or more, or 65 mol% or more. The upper limit is not particularly limited, but for example, it may be 85 mol% or less, 80 mol% or less, or 75 mol% or less. The higher the manganese content among the metals other than sodium, the greater the advantage of being able to exhibit high capacity under high-voltage operating environments, and the price competitiveness can be increased by reducing both nickel and cobalt concentrations.

[0037] The sodium manganese oxide of the present invention is a secondary particle formed by the aggregation of at least one primary particle, and the aspect ratio of the primary particle is 1:1 to 1:2.5. Specifically, the aspect ratio of the primary particle may be 1:1 to 1:2.4, 1:1 to 1:2.3, 1:1 to 1:2.2, 1:1 to 1:2.1, 1:1 to 1:2, 1:1 to 1:1.9, 1:1 to 1:1.8, 1:1 to 1:1.7, 1:1 to 1:1.6, preferably 1:1 to 1:1.5. In the present invention, by performing roasting and doping simultaneously under specific conditions, the particle size of the primary particles constituting the secondary particles in the positive electrode active material can be increased, and the aspect ratio can be decreased, thereby providing a positive electrode active material with improved energy density, high voltage stability, lifetime characteristics, and high-rate characteristics. In the present invention, the blending ratio of the doping metal dopant to the total metal other than sodium is optimized to adjust the doping metal content in the primary particles during the oxidation roasting process of the positive electrode active material precursor, the doping metal is dry-mixed to uniformly disperse on the surface of the precursor particles, and the oxidation roasting can be performed at a high temperature to achieve the desired aspect ratio range of the primary particles.

[0038] The term "aspect ratio" as used in this application refers to the ratio of the length to the width of the primary particle (Length / Width ratio), where the length indicates the direction of the relatively long region of the primary particle, and the width indicates the length of the relatively short region located on the same plane as the length. Here, the primary particle may be plate-shaped, meaning that the length in the thickness direction of the primary particle is significantly smaller than the length in the plane direction (long axis and width axis) of the primary particle. On the other hand, the width axis may be perpendicular to the long axis, and the "aspect ratio" of the primary particle can be calculated as the ratio of the long axis to the width axis of the primary particle measured from the surface of the primary particle.

[0039] The overall shape of the primary particle can be determined according to the lengths of the major axis and the minor axis. For example, if the aspect ratio, which is the ratio of the major axis to the minor axis of the primary particle, exceeds 2.5, the shape of the primary particle may be relatively closer to a rod shape than a plate shape, and the closer the aspect ratio of the primary particle is to 1, the closer the shape of the primary particle is to a plate shape. On the other hand, the primary particle may be a plate-shaped particle in which the shape of the surface including the major axis and minor axis is a circle, ellipse, polygon, or irregular shape, and specifically, it may be a disc shape.

[0040] The sodium manganese oxide may have a ratio (D2 / D1) of the particle size of secondary particles (D2) to the particle size of primary particles (D1) of 6 to 10, preferably 6 to 9, 6 to 8, 6.5 to 8, or 6.5 to 7.5. If the ratio (D2 / D1) of the primary particle size is greater than 10, the particle size of the primary particles is excessively small, and it is possible that primary particles are not formed in terms of shape, and therefore, the P2 structure is not present, and a large amount of by-products (impurities) may be generated. Such results may be due to the roasting reaction not proceeding smoothly or crystallization into a P2 structure, for example, due to roasting at a low temperature, roasting for a short time, and / or uneven roasting. On the other hand, the particle size of the primary particles may also be the length of the long axis.

[0041] Furthermore, the sodium manganese oxide may have an average particle size of 8 to 15 μm or 8 to 14 μm for the secondary particles, and an average particle size of 1 to 3.5 μm, 1 to 3 μm, 1 to 2.5 μm, 1 to 2 μm, or 1.5 to 2 μm for the primary particles. The primary and secondary particles contained in the positive electrode active material can improve the particle density within the positive electrode active material by satisfying at least the above conditions. This can improve the electrochemical properties of the positive electrode active material.

[0042] Herein, the present invention relates to 50% or more of the total number of primary particles constituting the secondary particles, for example, 60% or more of the particles may have an aspect ratio, particle size, and ratio of the particle size of secondary particles to the particle size of primary particles (D2 / D1) within the above-mentioned range, or at least 10 or at least 20 primary particles among the primary particles constituting the secondary particles may have an aspect ratio, particle size, and ratio of the particle size of secondary particles to the particle size of primary particles (D2 / D1) within the above-mentioned range.

[0043] The aforementioned sodium manganese oxide has a BET specific surface area of ​​0.1 to 0.45 m². 2 It may also be / g, specifically 0.1~0.4m 2 / g, 0.1~0.35m 2 / g, 0.1~0.3m 2 / g, 0.15~0.3m 2 / g or 0.2-0.3m 2 It may also be / g. The present invention can minimize electrolyte side reactions by reducing the BET specific surface area of ​​sodium manganese oxide particles, and can increase energy density by increasing the density of the positive electrode active material. Furthermore, because an oxidative roasting precursor is used, even if the BET specific surface area is reduced, the reactivity with sodium during the synthesis of the positive electrode active material can be improved, and the synthesis with sodium can be carried out sufficiently in a short time, thereby simplifying the process and improving productivity, as well as improving the structural and chemical stability of the precursor.

[0044] The positive electrode active material may have a c-axis length in its lattice structure of 11.13-11.18 (Å), 11.14-11.18 (Å), 11.16-11.18 (Å), or 11.17-11.18 (Å). In the present invention, the length of the c-axis in the lattice structure can be changed within a significant range as an effect of metal doping. In the present invention, when a precursor is metal-doped, the change in the length of the c-axis can be influenced by i) the atomic size of the doping element, where the atomic size of the doping element varies depending on the oxidation value at the time of doping. Furthermore, it can be influenced by ii) the crystal structure (e.g., P2, O3 structure) depending on the Na equivalent and iii) the content of Na inserted into the P2 structure, even with the same Na equivalent. In this invention, the length of the c-axis can be increased (changed) within the aforementioned range by roasting the hydroxide precursor and simultaneously doping it with a specific doping compound, and by adjusting the Na insertion content to produce a roasted cathode active material having a P2-type crystal structure.

[0045] Since the positive electrode active material of the present invention is manufactured from a precursor that has been oxidized and roasted at high temperature, even if the BET specific surface area is low, the length of the c axis in the lattice structure increases, and the insertion / desorption of Na can be improved.

[0046] The positive electrode active material may have a P2 crystal structure, which allows for high atmospheric and moisture stability, as well as less sensitivity to synthesis conditions (temperature, atmosphere, etc.).

[0047] On the other hand, the lengths of the a-axis and c-axis within the lattice structure can be measured by the Rietveld refinement method based on XRD analysis, but the present invention is not limited to this.

[0048] The sodium manganese oxide of the present invention can be represented by the following chemical formula 1.

[0049] [Chemical formula 1] Na a Ni x (MD ) y M1 z Mn 1-x-y-z O2

[0050] In the above chemical formula 1, M D M1 may be at least one selected from Fe, Co, Al, Cu, Zn, Mg and Ti, and M1 may be at least one selected from P, Sr, Ba, Zn, Cu, Zr, W, Ce, Hf, Ta, Cr, F, Cr, V, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb and Gd, M D M1 and M1 may be different elements, and may also satisfy the following conditions: 0.5≦a≦0.8, 0.05≦x≦0.45, 0.01≦y≦0.15, 0≦z≦0.05, and 0.5<1-xyz≦0.85.

[0051] The sodium manganese oxide of chemical formula 1 may have a molar ratio (Na / M) of sodium (Na) to total metal (M) other than sodium of 0.5 to 0.8. In chemical formula 1, if the content corresponding to a is less than 0.5, the capacity may decrease, and if it exceeds 0.8, the position of the sodium ions may change, and an O3 type crystal structure may be observed. The O3 type positive electrode active material may have lower atmospheric and moisture stability and be more sensitive to synthesis conditions (temperature and atmosphere, etc.) compared to the P2 type positive electrode active material. The Na may more preferably be 0.60 ≤ a ≤ 0.80, 0.60 ≤ a ≤ 0.75, or 0.65 ≤ a ≤ 0.75.

[0052] The sodium-manganese oxide may have a molar ratio (Mn / M) of manganese (Mn) to the total metal (M) other than sodium greater than 0.5 and less than or equal to 0.85. When the manganese content range is met, high capacity can be achieved under high-voltage operating conditions, and price competitiveness can be ensured. The Mn content may more preferably be 0.5 < 1-xyz ≤ 0.85, 0.55 ≤ 1-xyz ≤ 0.80, 0.55 ≤ 1-xyz ≤ 0.75, 0.55 ≤ 1-xyz ≤ 0.70, 0.55 ≤ 1-xyz ≤ 0.65, or 0.55 ≤ 1-xyz ≤ 0.60 in chemical formula 1.

[0053] The manganese-based sodium composite transition metal oxide may have a molar ratio of nickel (Ni) to the total metal (M) other than sodium (Ni / M) of 0.05 to 0.45. This can suppress the problem that arises as the nickel content increases within the range of nickel content, namely, the problem of decreased structural and chemical stability of the active material due to changes in the oxidation state of nickel. The Ni content may more preferably be 0.1 ≤ x ≤ 0.45, 0.2 ≤ x ≤ 0.45, 0.25 ≤ x ≤ 0.45, or 0.3 ≤ x ≤ 0.4 in chemical formula 1.

[0054] Furthermore, the sodium transition metal oxide may have a molar ratio of nickel to manganese (Ni / Mn) of 0.05-0.75, 0.1-0.7, 0.2-0.7, 0.3-0.6, or 0.4-0.6. Therefore, while a higher Mn content generally results in higher capacity, it may also lead to initial degradation in cell life. Generally, a higher Ni content results in better lifespan, but because Mn is more cost-effective than Ni, sodium-ion secondary batteries often use a high ratio of dissimilar transition metals (Mn) to take cost considerations into account. D ) can be doped to improve lifespan characteristics.

[0055] The sodium manganese oxide may contain a doping metal uniformly distributed inside and on the surface of the secondary particles. The sodium manganese oxide is distributed with respect to the cross-section of the secondary particles, with respect to the doping metal (M D It is not necessary for the concentration gradient to exhibit a predetermined trend, either continuously or discontinuously, over at least 50% of the cross-sectional area of ​​the secondary particle, such as a continuously increasing concentration gradient, a continuously decreasing concentration gradient, a continuously increasing followed by a decreasing concentration gradient, or a continuously decreasing followed by an increasing concentration gradient, as the concentration gradient moves from the surface to the center. Here, a discontinuous predetermined trend may mean, for example, a particle including a core-shell or a coating layer formed on the surface.

[0056] In this invention, by applying a technique that simultaneously performs the oxidation roasting process and the doping process of the precursor, the doping metal can be uniformly distributed inside and on the surface of the oxide particles, thereby providing a ternary transition metal oxide with a uniform composition. On the other hand, in conventional heterogeneous element doping / coating techniques, the heterogeneous elements are concentrated on the surface of the oxide particles to form a coating layer, and only a portion of the total heterogeneous elements contained in the coating layer diffuse into the interior of the oxide particles, resulting in a concentration gradient (doping metal) that decreases from the surface to the center of the particle. As a result, heterogeneous phases such as unreacted precursors like NiO... It can be synthesized. On the other hand, when doping a precursor with a transition metal using a dry method without conventional roasting, particle splitting may occur in the secondary or primary particles of the positive electrode active material, making it difficult to achieve the aspect ratio of the primary particles of the present invention.

[0057] The aforementioned doping metal (M DThe doping metal may be at least one selected from the group consisting of iron (Fe), cobalt (Co), aluminum (Al), copper (Cu), zinc (Zn), magnesium (Mg), and titanium (Ti), preferably iron (Fe), cobalt (Co), copper (Cu), zinc (Zn), or a combination thereof. Therefore, by producing a ternary transition metal oxide containing a doping metal, the structural stability of the positive electrode active material can be increased, Jahn-Teller strain can be suppressed, and the rate characteristics and cycle life characteristics of the battery can be improved.

[0058] The aforementioned sodium manganese oxide is a doping metal (M) to the overall metal (M) other than sodium. D ) molar ratio (M D The doping amount ( / M) may be 0.03 to 0.15, specifically 0.03 to 0.13, 0.03 to 0.11, 0.03 to 0.09, preferably 0.03 to 0.07. If the doping amount exceeds the above range, aggregation of the doped metal due to overdoping may occur. Conversely, if the doping amount is below the above range, the doping amount may be insufficient, making it difficult to ensure the structural / chemical stability of the oxide.

[0059] The following describes a method for producing a positive electrode active material for a sodium secondary battery according to the present invention.

[0060] The above manufacturing method includes the steps of: a) dry mixing a nickel-manganese hydroxide precursor containing 55 mol% or more of manganese in the total metal with a doping compound, and then roasting to produce an oxide precursor; and b) mixing the oxide precursor with a sodium compound, and then heat-treating to produce a sodium-manganese oxide.

[0061] a) Step involves dry mixing a nickel-manganese hydroxide precursor and a doping compound, followed by roasting, to produce a doped oxide precursor. In conventional dry doping of hydroxide precursors with transition metals without roasting, particle cracking may occur in the secondary or primary particles of the oxide after heat treatment for sodium insertion in a subsequent step. Analysis suggests that such phenomena are caused by organic elements such as acetates or oxygen elements such as oxides, oxyhydroxides, and hydroxides among the anionic groups contained in the doping compound. Furthermore, when doping with transition metals using a wet method, the selection of doping compounds is limited, leading to increased costs due to process complexity.

[0062] The roasting can be carried out in an oxidizing atmosphere at a temperature of 750 to 1,050°C, preferably 800 to 1,050°C, 800 to 950°C, 850 to 1,000°C, or 850 to 950°C. If roasting is carried out at a temperature higher than the above range, the particle size of the primary particles of the synthesized roasting precursor may become excessively large, limiting sodium ion diffusion on the particle surface. Conversely, if roasting is carried out at a temperature lower than the above range, the primary particles may not grow sufficiently, making it difficult to achieve the desired aspect ratio of the primary particles. Here, the roasting time is not particularly limited, but it is preferably carried out for 6 to 15 hours, 8 to 15 hours, or 8 to 13 hours.

[0063] The nickel-manganese hydroxyl precursor can be represented by the following chemical formula 2.

[0064] [Chemical formula 2] Ni x Mn 1-x (OH)2

[0065] In the above chemical formula 2, 0.05 ≤ x ≤ 0.45 and 0.55 ≤ 1-x ≤ 0.95 may also be applicable. stomach.

[0066] The nickel-manganese hydroxide precursor may have a molar ratio (Mn / M) of manganese (Mn) to total metal (M) of 0.55 to 0.85. When the manganese content is within the specified range, high capacity can be achieved under high-voltage operating conditions, ensuring price competitiveness. The Mn content may more preferably be 0.60 ≤ 1-x ≤ 0.85, 0.60 ≤ 1-x ≤ 0.80, or 0.60 ≤ 1-x ≤ 0.70 in chemical formula 2.

[0067] The nickel-manganese hydroxide precursor may have a molar ratio of nickel (Ni) to total metal (M) of 0.05 to 0.45 (Ni / M). Within the range of nickel content, it is possible to improve the problem of decreased structural and chemical stability of the active material due to changes in the oxidation state of nickel, which occurs as the nickel content increases. The Ni content may more preferably be 0.1 ≤ x ≤ 0.45, 0.2 ≤ x ≤ 0.45, 0.25 ≤ x ≤ 0.45, or 0.3 ≤ x ≤ 0.4 in chemical formula 2.

[0068] The doping compound may be at least one acetate compound, oxide, oxyhydroxide, hydroxide, or combination thereof selected from the group consisting of iron (Fe), cobalt (Co), aluminum (Al), copper (Cu), zinc (Zn), magnesium (Mg), and titanium (Ti), or at least one hydroxide selected from the group consisting of iron (Fe), cobalt (Co), aluminum (Al), copper (Cu), zinc (Zn), magnesium (Mg), and titanium (Ti), specifically at least one selected from the group consisting of Co(OH)2, Fe(CH3COO)2, Co(CH3COO)2, ZnO, and CuO, for example, Co(OH)2.

[0069] The doped cathode active material of the present invention can increase the length of the c-axis in the lattice structure as a doping effect, and this increase in c-axis length can improve Na insertion / desorption. In the present invention, when the doping compound is a cobalt (Co) and / or copper (Cu) hydroxide, a cobalt (Co) and / or copper (Cu) oxide, or a cobalt (Co) and / or copper (Cu) oxyhydroxide, it is preferable in terms of increasing the length of the c-axis in the lattice structure.

[0070] When roasting is performed simultaneously with doping the precursor using the dry method described above, the specific surface area and porosity of the roasted precursor decrease, the particle size of multiple primary particles within the secondary particles increases, and a phenomenon occurs in which the degree of aggregation increases. As a result, the tap density of the roasted precursor increases, and the particle cracking problem that occurs when dry doping is performed without roasting can be prevented.

[0071] The oxide precursor in step a) above may include a (Ni-Mn-X)O4 crystal structure, where X may be at least one selected from the group consisting of iron (Fe), cobalt (Co), aluminum (Al), copper (Cu), zinc (Zn), magnesium (Mg), and titanium (Ti).

[0072] b) Step is to mix the oxide precursor and the sodium compound, and then heat-treat them in order to produce a sodium manganese oxide.

[0073] The mixing of the oxide precursor and the sodium compound is performed such that the transition metal:sodium ratio of the precursor is 1:0.5 to 1:0.8 molar ratio, 1:0.6 to 1:0.8 molar ratio, 1:0.6 to 1:0.75 molar ratio, or 1:0.65 to 1:0.75 molar ratio. This is also acceptable. When the amount of sodium compound mixed is within the aforementioned range, the resulting positive electrode active material may have a P2-type layered crystalline structure, which allows for high atmospheric and moisture stability and less sensitivity to synthesis conditions (temperature and atmosphere, etc.). Furthermore, within the aforementioned sodium content range, the battery discharge capacity can be improved and unreacted residual Na can be minimized.

[0074] The heat treatment can be carried out at a temperature of 700°C to 1,100°C. When the firing temperature is within this range, the reaction between the raw materials can proceed sufficiently, and the particles can grow uniformly. The heat treatment can more preferably be carried out at a temperature of 750 to 1,050°C, 850 to 1,050°C, or 900 to 1,000°C. The heat treatment can be carried out for 5 to 40 hours. When the firing time is within this range, a highly crystalline positive electrode active material can be obtained, the particle size can be appropriate, and production efficiency can be improved. The heat treatment can more preferably be carried out for 5 to 20 hours, 5 to 18 hours, 8 to 15 hours, or 10 to 14 hours.

[0075] The sodium compound may be at least one selected from the group consisting of Na2CO3, NaOH, NaNO3, CH3COONa, and Na2(COO)2, and preferably Na2CO3, NaOH, or a combination thereof.

[0076] The manufacturing method of the present invention may further include a water washing step and a drying step after step b) above. The water washing step is a step for removing unreacted substances, impurities and residual sodium, and can be performed by adding the positive electrode active material produced in step b) above to a reactor into which one or more selected from deionized water, distilled water and ethanol have been added, and washing with water for 0.5 to 5 hours, 0.5 to 4 hours, 0.5 to 3 hours or 0.5 to 1.5 hours at a temperature of 1 to 80°C or 5 to 50°C at a stirring speed of 200 to 500 rpm, 200 to 400 rpm, or 300 to 400 rpm. The drying step is a step for removing moisture from the positive electrode active material that has contained moisture after the water washing step, and can be performed by drying under vacuum conditions at a temperature of 100 to 300°C for 12 hours or more.

[0077] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery and a sodium secondary battery, both containing the positive electrode active material.

[0078] The positive electrode includes a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector, and the positive electrode active material according to one aspect of the present invention is present in the positive electrode active material layer.

[0079] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with surface treatments such as carbon, nickel, titanium, or silver can be used. The positive electrode current collector can also typically have a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. Such positive electrode current collectors can be provided in various forms, such as films, sheets, foils, meshes, porous materials, foams, and nonwoven fabrics.

[0080] Furthermore, the positive electrode active material layer may also be a layer containing a conductive material and a binder, along with the positive electrode active material described above.

[0081] Here, the conductive material is used to impart conductivity to the electrode, and can be used without particular limitations as long as it does not cause a chemical change in the positive electrode active material and is conductive. Non-restrictive examples of conductive materials include graphite such as natural graphite and artificial graphite; carbon black, acetylene black, Ketjen black, channel black, furnace black, Examples include carbon-based materials such as lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. Conductive materials can typically be included in an amount of 1% to 30% by weight relative to the total weight of the positive electrode active material layer.

[0082] Furthermore, the binder is a substance that improves the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Non-limiting examples of binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. The binder can typically be present in an amount of 1% to 30% by weight relative to the total weight of the positive electrode active material layer.

[0083] A positive electrode according to one embodiment of the present invention can be manufactured by a conventional method for manufacturing positive electrodes for sodium secondary batteries, except that the positive electrode active material described above is used. For example, a positive electrode can be manufactured by applying a slurry for forming a positive electrode active material layer, which includes the positive electrode active material and selectively a binder and a conductive material, onto a positive electrode current collector, followed by drying and rolling. In another example, a positive electrode can be manufactured by casting the slurry for forming a positive electrode active material layer onto another support, peeling the positive electrode active material layer from the support, and then laminating the resulting film onto a positive electrode current collector.

[0084] In yet another aspect of the present invention, an electrochemical element including the above-described positive electrode is provided. Here, the electrochemical element may be a battery, a capacitor, and more specifically, a sodium secondary battery.

[0085] A sodium-based secondary battery includes a positive electrode, a negative electrode positioned opposite the positive electrode, a separator interposed between the positive and negative electrodes, and an electrolyte. The sodium-based secondary battery may also include a battery container (case) for housing the electrode assembly, including the positive electrode, negative electrode, and separator, and a sealing member for sealing the battery container.

[0086] Here, depending on the shape of the battery container (case), sodium secondary batteries can be divided into can-type sodium secondary batteries, in which the electrode assembly is housed in a metal can, and pouch-type sodium secondary batteries, in which the electrode assembly is housed in a pouch made of a sheet such as aluminum laminate.

[0087] In particular, in the case of a pouch-type sodium secondary battery using a positive electrode containing a positive electrode active material according to various embodiments of the present invention, there is a low possibility of side reactions between the positive electrode active material and the electrolyte, which has the advantage of improving stability during storage and / or operation and reducing gas generation.

[0088] The present invention will be described in detail below with reference to examples, but these are for the purpose of explaining the present invention in more detail and the scope of the rights of the present invention is not limited by the following examples.

[0089] Examples Manufacturing Example 1: Manufacturing of Cathode Active Material (Example 1) Step 1) Preparation of doping and roasting precursors Ni 0.35 Mn 0.65 (OH)2 high-manganese precursor and Co(OH)2 were mixed using a hand mixer in a molar ratio of Ni:Mn:Co=31.5:58.5:10. The mixture was then placed in an alumina crucible, oxidized and roasted at 950°C for 12 hours in an air atmosphere, and then cooled to room temperature to produce the roasted precursor (Ni-Mn-Co)O4.

[0090] Step 2) Manufacturing of the positive electrode active material The prepared roasting precursor and the sodium compound Na2CO3 were mixed at a ratio of Na / (Ni+Mn+Fe) = 0.67 equivalents to obtain a mixture. The prepared mixture was placed in an alumina crucible and calcined at 950°C for 12 hours in an O2 atmosphere, then cooled to room temperature to obtain P2-type Na 0.67 Ni 0.315 Co 0.1 Mn 0.585 We manufactured a positive electrode active material for O2 sodium secondary batteries.

[0091] (Examples 2-1 to 2-2) The roasting precursor and cathode active material were produced in the same manner as in Example 1, except that the oxidative roasting temperature was changed to 800°C (Example 2-1) and 900°C (Example 2-2) in Step 1).

[0092] (Examples 3-1 to 3-4) The procedure is carried out in the same manner as in Example 1, except that the doping source (Co(OH)2) in Step 1) is changed to Fe(CH3COO)2, Co(CH3COO)2, ZnO, and CuO respectively, to obtain the roasting precursor (Ni-Mn-X-O4) and the cathode active material (Na 0.67 Ni 0.315X 0.1 Mn 0.585 O2 was manufactured.

[0093] Here, X = Fe, Co, Zn, or Cu.

[0094] (Comparative Example 1) Step 1) is omitted, and Step 2) Ni 0.35 Mn 0.65 The procedure was carried out in the same manner as in Example 1, except that a (OH)2 high-manganese precursor was used. 0.67 Ni 0.35 Mn 0.65 We manufactured a positive electrode active material for O2 sodium secondary batteries.

[0095] (Comparative Example 2-1) The cathode active material was produced in the same manner as in Example 1 and Step 2), except that the coating precursor produced in Step 1) was used as shown below.

[0096] Step 1) Preparation of the precursor 400g of DIW was added to a batch-type 5L reactor, followed by 23.9g of 1M NaOH solution. The mixture was then maintained at 25°C with a stirring speed of 300 rpm for 15 minutes until the pH reached 12.8. 0.35 Mn 0.65 (OH)2 high-manganese precursor powder was added and stirred at 300 rpm.

[0097] Next, a 1.7 M aqueous solution of cobalt sulfate (CoSO4) was added at a rate of 51.18 ml / hr, and the mixture was reacted for 60 minutes while maintaining a pH of 11.5-12 to form cobalt hydroxide (Co(OH)2) on the surface of the high-manganese precursor particles. The formed particles were separated and dried in a vacuum oven at 110°C for 24 hours to produce the precursor.

[0098] (Comparative Example 2-2) Ni 0.35 Mn 0.65A mixture was obtained by mixing a (OH)2 high-manganese precursor with Fe(OH)2 and a sodium compound Na2CO3 in an amount of Na / (Ni+Mn+Fe) = 0.67 equivalents. The prepared mixture was placed in an alumina crucible and heated at 950°C in an air atmosphere. After oxidative roasting for 12 hours, it is cooled to room temperature and P2-type Na 0.67 Ni 0.315 Fe 0.1 Mn 0.585 We manufactured a positive electrode active material for O2 sodium secondary batteries.

[0099] (Comparative Example 3) Step 1) Production of the roasting precursor Ni 0.35 Mn 0.65 (OH)2 high-manganese precursor was placed in an alumina crucible, then oxidized and roasted in an air atmosphere at 600°C for 12 hours, and finally cooled to room temperature to produce the roasted precursor (Ni-Mn2)O4.

[0100] Step 2) Cathode Active Material Manufacturing The prepared roasting precursor and the sodium compound Na2CO3 were mixed at a ratio of Na / (Ni+Mn) = 0.67 equivalents to obtain a mixture. The prepared mixture was placed in an alumina crucible and calcined at 950°C for 12 hours in an O2 atmosphere, then cooled to room temperature to obtain P2-type Na 0.67 Ni 0.35 Mn 0.65 We manufactured a positive electrode active material for O2 sodium secondary batteries.

[0101] (Reference example) Step 1) Production of the roasting precursor Ni 0.35 Mn 0.65 (OH)2 high-manganese precursor was placed in an alumina crucible, then oxidized and roasted in an air atmosphere at 950°C for 12 hours, and finally cooled to room temperature to produce the roasted precursor (Ni-Mn2)O4.

[0102] Step 2) Manufacturing of the positive electrode active material The prepared roasting precursor and the sodium compound Na2CO3 were mixed at a ratio of Na / (Ni+Mn) = 0.67 equivalents to obtain a mixture. The prepared mixture was placed in an alumina crucible and calcined at 950°C for 12 hours in an O2 atmosphere, then cooled to room temperature to obtain P2-type Na 0.67 Ni 0.35 Mn 0.65 We manufactured a positive electrode active material for O2 sodium secondary batteries.

[0103] Manufacturing Example 2: Manufacturing of Sodium Rechargeable Batteries A cathode slurry was prepared by dispersing 30 g of 85 wt% of the manufactured cathode active material, 10 wt% of carbon black, and 5 wt% of PVdF binder in N-methyl-2-pyrrolidone (NMP). The cathode slurry was uniformly coated onto a 15 μm thick aluminum thin film and vacuum-dried at 135°C to produce a cathode for a sodium secondary battery.

[0104] A sodium secondary battery (coin cell) was manufactured using a sodium metal plate as the counter electrode for the positive electrode, a porous glass fiber (thickness: 200 μm) as the separator, and an electrolyte containing NaPF6 at a concentration of 1.0 M in a solvent mixed with propylene carbonate and fluoroethylene carbonate in a volume ratio of 98:2.

[0105] Experimental example Experimental Example 1: SEM analysis of surface and cross-sectional morphology of positive electrode active material particles Figures 1a, 1b, 1c, and 1d are surface SEM images of the precursor, surface SEM images of the roasted precursor, surface SEM images of the roasted cathode active material, and cross-sectional SEM images of the roasted cathode active material according to Example 1, respectively.

[0106] Figures 2a and 2b are SEM images of the surface and cross-section of the positive electrode active material according to Comparative Example 1, and Figures 3a and 3b are SEM images of the surface and cross-section of the positive electrode active material according to Comparative Example 2-1. Figure 4 is an SEM image of the surface of the positive electrode active material according to Comparative Example 2-2.

[0107] Table 1 below summarizes the particle morphology measured from SEM images of the surface and cross-section of positive electrode active material particles. The average aspect ratio and particle size (D1) of primary particles are average values ​​measured from at least 20 selected primary particles. The particle size of secondary particles was confirmed by SEM images and PSA (Particle size analysis).

[0108] On the other hand, cross-sectional SEM images were taken after the secondary particles were cross-sectioned using a cross-section polisher (accelerating voltage 5.0kV, 4-hour milling).

[0109] [Table 1]

[0110] Referring to Table 1, it was confirmed that the primary particle size of the cathode active material increased overall as the hydroxide precursor was roasted with the doping metal in the examples. This is expected to improve stability, such as bringing the aspect ratio of the primary particles close to 1:1, resulting in a higher crystal density of the primary particles, excellent crystal structure stability, improved energy density, and suppression of phase transitions during high-voltage driving.

[0111] On the other hand, the positive electrode active materials of Comparative Examples 1 and 2-1 had primary particle morphologies of rod-shape and plate-shape, respectively, and it was found that the particle size of the primary particles was not uniform, and the aspect ratio to particle size ratio (D2 / D1) exceeded the numerical range of the present invention. In the case of Comparative Example 3, the primary particle growth was insufficient because roasting was performed at a slightly lower temperature. On the other hand, in the positive electrode active material of Comparative Example 2-2, particle cracking occurred due to dry doping, making it impossible to measure the particle size of the primary and secondary particles.

[0112] Experimental Example 2: XRD analysis of the crystal structure of cathode active material particles Figures 5a and 5b show the XRD analysis results of the roasted doping precursor (bulk) and roasted cathode active material (bulk) produced in Example 1. In the case of the roasted precursor produced in Example 1, it was confirmed that all had the (Ni-Mn-X)O4 structure regardless of the doping source. In the case of the roasted cathode active material, it was found that the Na layer (002) plane in the crystal structure grew well, and the P2 structure was well formed.

[0113] Table 2 below shows the results of XRD analysis of the roasted cathode active materials produced in Example 1, Examples 3-1 to 3-4, Comparative Example 1, and Reference Example, based on the Rietveld method. This is a summary of the c-axis lengths within the lattice structure, measured using the finement method.

[0114] [Table 2]

[0115] The roasting precursor has a lattice structure in which the lengths of the axes a, b, and c are all the same. In this invention, the effect of metal doping was analyzed to be that there was no change in the length of the a axis in the lattice structure, but a significant change in the length of the c axis.

[0116] In Example 1, doping was performed using Co(OH)2, and it was analyzed that the metal ordering was best performed during doping. Comparing the results of Example 1 and Examples 3-4, it was analyzed that even though the same Co source was used as the doping metal, the doping effect was superior when using Co(OH)2 compared to Co(CH3COO)2.

[0117] Comparing Examples 3-1 and 3-4, it was found that Co source showed superior doping effect compared to Fe source, and comparing Examples 3-2 and 3-3, it was found that Zn source showed superior doping effect compared to Cu source.

[0118] Experimental Example 3: Confirmation of uniformity of doping metal distribution inside particles using SEM and EDS analysis Figures 6a and 6b show the SEM-EDS analysis results of cross-sections of the roasted doping precursor particles and roasted cathode active material particles produced in Example 1.

[0119] In the case of the Co-doped roasting precursor and roasting cathode active material produced in Example 1, uniform Co-doping into the interior of the secondary particles was confirmed, confirming that the roasting doping technology of the present invention can produce transition metal ternary roasting precursors (oxides) and cathode active materials with a uniform composition.

[0120] Experimental Example 4: Analysis of the electrochemical performance of sodium secondary batteries For the sodium secondary batteries (coin cells) manufactured in Example 1, Comparative Example 1, Comparative Example 2-1, and Comparative Example 3, charge-discharge experiments were conducted using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, with a voltage range of 2.2V to 4.4V and a discharge rate of 0.1C to 5.0C. The rate characteristics (rate capability (C-rate) of the discharge capacity) were measured, and the results are shown in Table 3 below.

[0121] Furthermore, for the same sodium secondary battery, after performing 50 charge / discharge cycles under conditions of 0.5C / 0.5C at 25°C and a drive voltage range of 2.2V to 4.4V, the ratio of the discharged capacity at the 50th cycle to the initial capacity (cycle capacity retention rate) was measured, and the results are shown in Table 3 below.

[0122] [Table 3]

[0123] Referring to Table 3, in the sodium secondary battery according to Example 1, the morphological specificity of the primary particles of the positive electrode active material and the uniform metal doping within the secondary particles suppress the phase transition that occurs in the high-voltage region, improving high-voltage stability and significantly improving cell performance as energy density increases.

[0124] As described above, the present invention has been illustrated and explained with respect to specific embodiments, but it will be obvious to those of ordinary skill in the art that the present invention can be improved and modified in various ways without departing from the technical spirit of the invention provided by the following claims.

Claims

1. At least sodium (Na), nickel (Ni), manganese (Mn), and doping metal (M Ｄ ) contains sodium manganese oxides in which the manganese content among the total metals other than sodium is 55 mol% or more, The sodium manganese oxide is a secondary particle formed by the aggregation of at least one primary particle, and the aspect ratio of the primary particle is 1:1 to 1:2.5, wherein it is a positive electrode active material for a sodium secondary battery.

2. The sodium manganese oxide has a particle size (D) of the primary particles. １ The particle size (D) of the secondary particles relative to ) ２ ) ratio (D ２ / D １ The positive electrode active material for a sodium secondary battery according to claim 1, wherein the ratio is 6 to 10.

3. The sodium manganese oxide is a positive electrode active material for a sodium secondary battery according to claim 1, wherein the average particle size of the secondary particles is 8 to 15 μm and the average particle size of the primary particles is 1 to 3.5 μm.

4. The aforementioned sodium manganese oxide has a BET specific surface area of ​​0.1 to 0.45 m². ２ The positive electrode active material for a sodium secondary battery according to claim 1, wherein the value is / g.

5. The positive electrode active material for a sodium secondary battery according to claim 1, wherein the length of the c axis in the lattice structure is 11.13 to 11.18 (Å).

6. The sodium manganese oxide is represented by the following chemical formula 1, and is a positive electrode active material for a sodium secondary battery according to claim 1. [Chemical formula 1] Na ａ Ni ｘ (M Ｄ ) ｙ M1 ｚ Mn １－ｘ－ｙ－ｚ O ２ In the above chemical formula 1, M Ｄ is at least one selected from Fe, Co, Al, Cu, Zn, Mg, and Ti. M1 is at least one selected from P, Sr, Ba, Zn, Cu, Zr, W, Ce, Hf, Ta, Cr, F, Cr, V, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd. M Ｄ M1 and M1 are different elements. 0.5 ≤ a ≤ 0.8, 0.05 ≤ x ≤ 0.45, 0.01 ≤ y ≤ 0.15, 0 ≤ z ≤ 0.05, and 0.5 < 1 - x - y - z ≤ 0.

85.

7. The aforementioned doping metal (M Ｄ The positive electrode active material for a sodium secondary battery according to claim 1, wherein the active material is at least one selected from the group consisting of iron (Fe), cobalt (Co), aluminum (Al), copper (Cu), zinc (Zn), magnesium (Mg), and titanium (Ti).

8. The aforementioned sodium manganese oxide is a doping metal (M) to the overall metal (M) other than sodium. Ｄ ) molar ratio (M Ｄ The positive electrode active material for a sodium secondary battery according to claim 1, wherein the ratio of ( / M) is 0.01 to 0.

15.

9. The sodium manganese oxide has a molar ratio (Mn / M) of manganese (Mn) to the total metal (M) other than sodium that is greater than 0.5 and less than or equal to 0.85, as described in claim 1.

10. The aforementioned sodium manganese oxide is nickel relative to the total metal (M) other than sodium. The positive electrode active material for a sodium secondary battery according to claim 1, wherein the molar ratio of Ni (Ni / M) is 0.05 to 0.

45.

11. The sodium manganese oxide comprises a P2-type layered structure, as described in claim 1, for a positive electrode active material for a sodium secondary battery.

12. a) A step of producing an oxide precursor by dry mixing a nickel-manganese hydroxide precursor containing 55 mol% or more of manganese in the total metal with a doping compound, followed by roasting, b) A method for producing a positive electrode active material for a sodium secondary battery, comprising the step of mixing the oxide precursor with a sodium compound and then heat-treating it to produce a sodium manganese oxide.

13. The method for producing a positive electrode active material for a sodium secondary battery according to claim 12, wherein the roasting in step a) is carried out at a temperature of 750 to 1,050°C.

14. The method for producing a positive electrode active material for a sodium secondary battery according to claim 12, wherein the doping compound is at least one acetate compound, oxide, oxyhydroxide, hydroxide, or combination thereof selected from the group consisting of iron (Fe), cobalt (Co), aluminum (Al), copper (Cu), zinc (Zn), magnesium (Mg), and titanium (Ti).

15. The oxide precursor in step a) is (Ni-Mn-X)O ４ Includes a crystalline structure, A method for producing a positive electrode active material for a sodium secondary battery according to claim 12, wherein X in the crystal structure is Fe, Co, Al, Cu, Zn, Mg, or Ti.

16. The method for producing a positive electrode active material for a sodium secondary battery according to claim 12, wherein the heat treatment in step b) is performed at a temperature of 800 to 1,100°C.

17. A positive electrode for a sodium secondary battery, comprising the positive electrode active material described in claim 1.

18. A sodium secondary battery comprising the positive electrode described in claim 17.

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