Positive electrode active material for sodium secondary battery, method for preparing same, and sodium secondary battery comprising same

Abstract

The present invention provides a positive electrode active material for a sodium secondary battery, a method for preparing the positive electrode active material, a positive electrode for a sodium secondary battery comprising the positive electrode active material, and a sodium secondary battery, in which the positive electrode active material for a sodium secondary battery comprises a sodium-manganese-based oxide containing at least sodium (Na), nickel (Ni), manganese (Mn), and a doped metal (MD), wherein the content of manganese in all metals except sodium is more than 55 mol%, the sodium-manganese-based oxide is a secondary particle formed by agglomerating at least one primary particle, and the aspect ratio of the primary particle is 1: 1 to 1: 2.5.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material for a sodium secondary battery, a preparation method thereof and a sodium secondary battery comprising the positive electrode active material. Background Art

[0002] Lithium-ion secondary batteries have been widely used in various electronic technology fields as energy storage devices. In recent years, with the surge in demand for lithium-ion secondary batteries, sodium-ion secondary batteries have attracted much attention as a substitute for expensive metallic lithium. Sodium-ion secondary batteries have a similar working principle of insertion / deinsertion reaction as lithium-ion secondary batteries, and are therefore one of the next-generation materials with potential for application as secondary batteries.

[0003] As a representative form of positive electrode active materials, layered transition metal oxides have the advantages of simple structure, excellent electrochemical performance, and easy synthesis. In particular, high-manganese-based (high-Mn) sodium nickel manganese oxide (NNMO) has relative advantages over other transition metal layered oxides due to its high capacity, price competitiveness (superior reserves), and environmental friendliness.

[0004] However, since sodium ions with a larger ionic radius will change the lattice structure when they are embedded / de-embedded in the layered structure, the positive electrode active material will continuously undergo phase changes, and irreversible phases will occur in the process, resulting in a decrease in the cycle life and high-rate characteristics of sodium-ion secondary batteries. In particular, high-manganese-based (high-Mn) oxides with a P2-type layered structure have problems with reduced electrochemical performance such as high-rate characteristics and life characteristics during the charge and discharge process due to the irreversible phases produced by Jahn-Teller distortion. Jahn-Teller distortion is a phenomenon caused by crystal field stabilization energy. Whenever the oxidation number of Mn changes (Mn 3+ →Mn 4+ ), the lattice structure will repeatedly expand and contract, causing the life of the positive electrode active material to deteriorate.

[0005] In order to solve the above problems, attempts have been made to improve the problems of high manganese-based oxides by adjusting the size of the positive electrode active material particles or coating the particle surface, etc., but none of them has reached the commercial level. Summary of the invention

[0006] Technical problem to be solved by the invention

[0007] In order to improve the decrease in conductivity of high manganese-based oxides due to excessive manganese, it is preferred to prepare ternary transition metal oxides containing transition metals other than manganese. However, when a ternary transition metal hydroxide precursor is prepared by coprecipitation reaction, a large amount of manganese oxide (MnO 2 etc.) issues.

[0008] The object of the present invention is to provide a high manganese ternary transition metal oxide with uniform composition by applying a technology of simultaneously performing an oxidation roasting process and a doping process of a precursor.

[0009] P2-type Ni and high Mn-based SIB positive electrode materials have the advantages of higher energy density and capacity than O3-type positive electrode materials. However, there is a disadvantage that a phase transition occurs in the voltage region above 4.0V, resulting in a decrease in reversible capacity. In order to solve the above problems, a method of doping various transition metals into precursors by a wet method has been used in the past, and the transition metals are doped by heat treatment in subsequent processes. However, the wet process has the disadvantages of limited applicable doping sources and the need for additional heat treatment, so the productivity is low. In order to solve the above problems, the present invention aims to apply the technology of simultaneously doping and calcining the precursor by a dry method.

[0010] In addition, the present invention can grow primary particles in secondary particles of positive electrode active materials by simultaneously performing calcination and doping under specific conditions. Therefore, it has the effect of suppressing electrolyte side reactions caused by reduced BET surface area, and can provide positive electrode active materials with improved energy density, high voltage stability, life characteristics and high rate characteristics.

[0011] Furthermore, the present invention aims to improve the reactivity with sodium by controlling the crystal structure of the doped calcined precursor and the calcined positive electrode active material prepared therefrom.

[0012] Solutions for solving technical problems

[0013] An embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, wherein the positive electrode active material comprises at least sodium (Na), nickel (Ni), manganese (Mn) and a doping metal (M D ) of a sodium manganese-based oxide, wherein the manganese content in all metals except sodium is 55 mol% or more, and the sodium manganese-based oxide is a secondary particle formed by agglomeration of at least one primary particle, and the aspect ratio of the primary particle is 1:1 to 1:2.5.

[0014] In the sodium manganese-based oxide, the secondary particle size (D 2 ) relative to the primary particle size (D 1 ) 2 / D 1 ) can be from 6 to 10.

[0015] In the sodium manganese-based oxide, the average particle size of the secondary particles may be 8 to 15 μm, and the average particle size of the primary particles may be 1 to 3.5 μm.

[0016] The BET specific surface area of ​​the sodium manganese-based oxide may be 0.1 to 0.45 m 2 / g.

[0017] The c-axis length in the lattice structure of the positive electrode active material can be 11.13 to

[0018] The sodium manganese-based oxide may be represented by the following Chemical Formula 1:

[0019] [Chemical formula 1]

[0020] Na a Ni x (M D ) y M1 z Mn 1-x-y-z O 2

[0021] In the chemical formula 1, M D It 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 M1 and M2 may be different elements from each other, and may satisfy 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.

[0022] 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).

[0023] The sodium manganese-based oxide is doped with metal (M D ) relative to all metals (M) except sodium D / M) can be from 0.01 to 0.15.

[0024] In the sodium manganese-based oxide, a molar ratio (Mn / M) of manganese (Mn) to all metals (M) except sodium may be greater than 0.5 and less than or equal to 0.85.

[0025] In the sodium manganese-based oxide, a molar ratio (Ni / M) of nickel (Ni) to all metals (M) except sodium may be 0.05 to 0.45.

[0026] The sodium manganese-based oxide may include a P2 type layered structure.

[0027] Another implementation example of the present invention provides a method for preparing a positive electrode active material for a sodium secondary battery, which comprises: step a) dry mixing a nickel manganese hydroxide precursor having a manganese content of 55 mol% or more in all metals with a doping compound, and then calcining to prepare an oxide precursor; and step b) mixing the oxide precursor with a sodium compound, and then heat treating to prepare a sodium manganese-based oxide.

[0028] The calcination in step a) may be performed at a temperature of 750 to 1050°C.

[0029] The dopant compound may be at least one acetate compound, oxide, oxyhydroxide, hydroxide or a combination thereof selected from the group consisting of iron (Fe), cobalt (Co), aluminum (Al), copper (Cu), zinc (Zn), magnesium (Mg) and titanium (Ti).

[0030] The oxide precursor in step a) may contain (Ni—Mn—X)O 4 Crystal structure, wherein X in the crystal structure may be Fe, Co, Al, Cu, Zn, Mg or Ti.

[0031] The heat treatment in step b) may be performed at a temperature of 800 to 1100°C.

[0032] Another implementation example of the present invention provides a positive electrode for a sodium secondary battery, which includes the positive electrode active material.

[0033] Another implementation example of the present invention provides a sodium secondary battery, which includes the positive electrode.

[0034] Effects of the Invention

[0035] In the present invention, the phase transition of high manganese-based oxide in the high voltage region is suppressed, thereby increasing the reversible capacity, thereby providing a positive electrode active material having improved energy density, high voltage stability, life characteristics and high rate characteristics.

[0036] Dry doping by heat treatment is different from the existing wet process, and can achieve economic benefits by simplifying the process and reducing the process cost in mass production. In addition, the dry doping process also has the advantage of being able to select various doping sources that are not easy to apply in the existing wet process.

[0037] Compared with the general insertion of sodium into a hydroxide precursor, the present invention can improve the reactivity with sodium (Na) by using an oxidatively roasted precursor. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1a , Figure 1b , Figure 1c and Figure 1d They are respectively a SEM photograph of the surface of the precursor according to Example 1, a SEM photograph of the surface of the calcined precursor, a SEM photograph of the surface of the calcined positive electrode active material, and a SEM photograph of the cross section of the calcined positive electrode active material.

[0039] Figure 2a and Figure 2b Surface SEM photographs and cross-sectional SEM photographs of the positive electrode active material according to Comparative Example 1.

[0040] Figure 3a and 3b Surface SEM photographs and cross-sectional SEM photographs of the positive electrode active material according to Comparative Example 2-1.

[0041] Figure 4 is a surface SEM photograph of the positive electrode active material according to Comparative Example 2-2.

[0042] Figure 5a and Figure 5b These are the XRD analysis results of the calcined doping precursor (bulk) and the calcined positive electrode active material (bulk) prepared in Example 1.

[0043] Figure 6a and Figure 6b These are the cross-sectional SEM-EDS analysis results of the calcined doped precursor particles and calcined doped positive electrode active material particles prepared in Example 1. DETAILED DESCRIPTION

[0044] The advantages, features and methods of achieving the same will become more apparent through the following embodiments described in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but can be implemented in various forms. These embodiments are provided only to complete the disclosure of the present invention and to enable ordinary technicians in the field to fully understand the scope of the present invention. The present invention is only defined by the scope of the claims.

[0045] Unless otherwise defined, all terms (including technical terms and scientific terms) used in this specification may be used in accordance with the meanings generally understood by those skilled in the art to which the present invention belongs. Throughout the specification, unless otherwise stated to the contrary, when a part is mentioned to "include" a component, it does not exclude other components, but may also include other components. In addition, unless otherwise clearly stated in the text, the singular form also includes the plural form.

[0046] An 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 a doping metal (Mn). D) and the manganese content in all metals except sodium is more than 55 mol%.

[0047] The sodium manganese-based oxide is an NNMO-based sodium manganese-based oxide containing at least sodium, nickel and manganese. The sodium manganese-based oxide is a high-manganese-based (high-Mn) oxide having a manganese content of 55 mol% or more in all metals except sodium. The manganese content in metals other than sodium can be 55 mol% or more, 60 mol% or more, or 65 mol% or more, and the upper limit is not particularly limited, for example, it can be 85 mol% or less, 80 mol% or less, or 75 mol% or less. Among metals other than sodium, the more manganese content, the more it can exert high capacity under high-pressure working environment, and the price competitiveness can be improved by reducing the concentration of nickel and cobalt.

[0048] The sodium manganese-based oxide of the present invention is a secondary particle formed by the agglomeration 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 can 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 simultaneously performing calcination and doping under specific conditions, it is possible to provide a positive electrode active material that increases the size of the primary particles constituting the secondary particles in the positive electrode active material, reduces the aspect ratio, and improves the energy density, high voltage stability, life characteristics and high rate characteristics. The present invention optimizes the matching ratio of the doped metal dopant to all metals except sodium, so that the content of the doped metal in the primary particles is adjusted during the oxidative roasting process of the positive electrode active material precursor, and the doped metal is evenly dispersed on the surface of the precursor particles through dry mixing. Moreover, oxidative roasting can be performed at a high temperature to achieve an aspect ratio range of the primary particles.

[0049] The term "aspect ratio" used herein refers to the ratio (Length / Width ratio) of the major axis (Length) and the minor axis (Width) of the primary particle, and when the major axis represents the direction of the relatively long region of the primary particle, the minor axis is located on the same plane as the major axis and represents the length of the relatively short region. At this time, the primary particle can have a plate-like shape (plate), which means that the length of the primary particle in the thickness direction is significantly smaller than the length of the primary particle in the plane direction (major axis and minor axis). In addition, the minor axis can be in a direction perpendicular to the major axis, and the "aspect ratio" of the primary particle can be calculated as the ratio of the major axis to the minor axis of the primary particle measured from the surface of the primary particle.

[0050] The overall shape of the primary particle can be determined according to the length of the major axis and the minor axis. For example, when the aspect ratio of the primary particle, i.e., the ratio of the major axis to the minor axis, exceeds 2.5, the shape of the primary particle may be relatively closer to a rod rather than a plate. In addition, the closer the aspect ratio of the primary particle is to 1, the closer the shape of the primary particle is to a plate. In addition, the primary particle can be a plate-like particle whose shape of the face including the major axis and the minor axis is circular, elliptical, polygonal or irregular, specifically, it can be a disc.

[0051] The secondary particle size (D 2 ) and the primary particle size (D 1 ) 2 / D 1 ) may be 6 to 10, preferably 6 to 9, 6 to 8, 6.5 to 8 or 6.5 to 7.5. 2 / D 1 ) exceeds 10, the primary particle size is too small, so the primary particle may not be able to form a shape, and therefore, does not have a P2 structure, and a large amount of by-products (impurity) may be generated. This result may be caused by an unsmooth calcination reaction or failure to crystallize into a P2 structure, for example, due to low temperature calcination, short time calcination and / or uneven calcination. In addition, the primary particle size may be the major axis length.

[0052] In addition, the average size of the secondary particles of the sodium manganese-based oxide may be 8 to 15 μm or 8 to 14 μm, and the average size of the primary particles may be 1 to 3.5 μm, 1 to 3 μm, 1 to 2.5 μm, 1 to 2 μm or 1.5 to 2 μm. The primary particles and the secondary particles contained in the positive electrode active material may increase the particle density in the positive electrode active material by at least satisfying the above conditions. Thus, the electrochemical characteristics of the positive electrode active material may be improved.

[0053] In this case, the present invention can have an aspect ratio, a primary particle size, and a ratio of the secondary particle size to the primary particle size (D) within the above range in more than 50%, for example, more than 60% or 70% of the total number of primary particles constituting the secondary particles. 2 / D 1 ), or at least 10 or at least 20 of the primary particles constituting the secondary particles have an aspect ratio, a primary particle size, and a ratio of the secondary particle size to the primary particle size (D 2 / D 1 ).

[0054] The BET specific surface area of ​​the sodium manganese-based oxide may be 0.1 to 0.45 m 2 / g, specifically, it can be 0.1 to 0.4 m 2 / g, 0.1 to 0.35m 2 / g, 0.1 to 0.3 m 2 / g, 0.15 to 0.3 m 2 / g or 0.2 to 0.3 m 2 / g. The present invention can reduce the side reaction of the electrolyte to the maximum extent by reducing the BET specific surface area of ​​the sodium manganese-based oxide particles, and can increase the energy density by increasing the density of the positive electrode active material. In addition, since an oxidative roasting precursor is used, even if the BET surface area is reduced, the reactivity with sodium during the synthesis of the positive electrode active material can be improved, and since the synthesis with sodium is fully carried out in a short time, the process can be simplified, the productivity can be improved, and the structural stability and chemical stability of the precursor can be improved.

[0055] The c-axis length in the lattice structure of the positive electrode active material may be 11.13 to 11.14 to 11.16 to or 11.17 to In the present invention, due to the effect of metal doping, the c-axis length in the lattice structure can be changed in a wide range. In the present invention, the change in the c-axis length when the metal is doped in the precursor is affected by i) the atomic size of the doping element, and at this time, the atomic size of the doping element is different according to the oxidation number when doping. In addition, it may be affected by ii) the crystal structure according to the Na equivalent (for example, P2, O3 structure), and iii) the Na content inserted into the P2 structure even at the same Na equivalent. In the present invention, a specific doping compound is used for doping while the hydroxide precursor is calcined. In addition, by adjusting the insertion content of Na, a calcined positive electrode active material having a P2 type crystal structure is manufactured, thereby increasing (changing) the c-axis length within the above range.

[0056] Since the cathode active material of the present invention is made from a precursor that is oxidatively calcined at a high temperature, even if the BET specific surface area is reduced, the c-axis length in the lattice structure is increased, thereby improving the insertion / deinsertion of Na.

[0057] The positive electrode active material may have a P2 type crystal structure, and therefore may have higher atmospheric and moisture stability and is less sensitive to synthesis conditions (temperature, atmosphere, etc.).

[0058] In addition, the a-axis length and the c-axis length in the lattice structure may be measured by a Rietveld refinement method based on XRD analysis, but the present invention is not limited thereto.

[0059] The sodium manganese-based oxide of the present invention may be represented by the following Chemical Formula 1.

[0060] [Chemical formula 1]

[0061] Na a Ni x (M D ) y M1 z Mn 1-x-y-z O 2

[0062] In the chemical formula 1, M D M1 may be at least one selected from Fe, Co, Al, Cu, Zn, Mg and Ti, M2 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, and M D M1 and M2 may be different elements from each other, and may satisfy 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.

[0063] In the sodium manganese-based oxide of the chemical formula 1, the molar ratio (Na / M) of sodium (Na) to all metals (M) other than sodium may be 0.5 to 0.8. In the chemical formula 1, when the content of Na equivalent to a is less than 0.5, the capacity may be reduced, and when it exceeds 0.8, the position of the sodium ion will change and present an O3 type crystal structure, and the atmospheric and moisture stability of the O3 type positive electrode active material will be lower than that of the P2 type, and it is more sensitive to the synthesis conditions (temperature and atmosphere, etc.). The Na is more preferably 0.60≤a≤0.80, 0.60≤a≤0.75, or 0.65≤a≤0.75.

[0064] In the sodium manganese-based oxide, the molar ratio (Mn / M) of manganese (Mn) to all metals (M) except sodium may be greater than 0.5 and less than 0.85. When the content range of manganese is satisfied, high capacity can be achieved under a high voltage working environment and price competitiveness can be ensured. In the chemical formula 1, the content of Mn is more preferably 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.

[0065] In the manganese-based sodium composite transition metal oxide, the molar ratio (Ni / M) of nickel (Ni) to all metals (M) except sodium may be 0.05 to 0.45. Within the content range of the nickel, the problem of decreased structural stability and chemical stability of the active material due to changes in the oxidation number of nickel, which occurs as the nickel content increases, can be suppressed. In Chemical Formula 1, the content of Ni is more preferably 0.1≤x≤0.45, 0.2≤x≤0.45, 0.25≤x≤0.45 or 0.3≤x≤0.4.

[0066] In addition, in the sodium transition metal oxide, the molar ratio of nickel to manganese (Ni / Mn) can be 0.05 to 0.75, 0.1 to 0.7, 0.2 to 0.7, 0.3 to 0.6 or 0.4 to 0.6. Therefore, although the higher the Mn content, the higher the capacity, the battery life may decay early. In addition, generally, the higher the Ni content, the better the life. However, since Mn is more effective than Ni in terms of price, in sodium ion secondary batteries, from a cost perspective, it is possible to dope heterogeneous transition metals (Mn) at a ratio with a high Mn content. D ) to improve life characteristics.

[0067] The sodium manganese-based oxide may include a doping metal uniformly distributed inside and on the surface of the secondary particle. The sodium manganese-based oxide, based on the cross section of the secondary particle, has a doping metal (M D ) When reaching the central part from the surface part, at least 50% or more of the cross-sectional area of ​​the secondary particle may not present a concentration gradient that continuously or discontinuously exhibits a constant trend, such as a continuously increasing concentration gradient, a continuously decreasing concentration gradient, a concentration gradient that continuously increases and then decreases, a concentration gradient that continuously decreases and then increases, etc. Here, the discontinuous constant trend may mean, for example, particles comprising a core-shell or a coating formed on the surface.

[0068] The present invention can evenly distribute the doped metal inside and on the surface of the oxide particles by applying the technology of simultaneously performing the oxidation roasting process and the doping process of the precursor, thereby providing a ternary transition metal oxide with uniform composition. On the other hand, in the existing heterogeneous element doping / coating technology, the heterogeneous elements are enriched on the surface of the oxide particles to form a coating, and only a part of all the heterogeneous elements contained in the coating diffuses into the interior of the oxide particles. Therefore, it is easy to have a (doped metal) concentration gradient that gradually decreases from the surface of the particle to the center. Therefore, heterogeneous phases such as unreacted precursors (such as NiO) can be synthesized. In addition, when the precursor is doped with transition metals by a dry method without roasting in the past, particle crushing occurs in the secondary particles 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.

[0069] 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), preferably Fe (iron), cobalt (Co), copper (Cu), zinc (Zn) or a combination thereof. Therefore, by preparing a ternary transition metal oxide containing a doped metal, the structural stability of the positive electrode active material can be improved, and the Jahn-Teller distortion can be suppressed, thereby improving the rate characteristics and cycle life characteristics of the battery.

[0070] The sodium manganese-based oxide is doped with metal (M D ) relative to all metals (M) except sodium D / M) can 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. When the doping amount exceeds the above range, the phenomenon of doped metal agglomeration may occur due to overdoping. On the contrary, when the doping amount is lower than the above range, the doping amount may be insufficient, making it difficult to ensure the structural / chemical stability of the oxide.

[0071] Next, a method for preparing the positive electrode active material for a sodium secondary battery of the present invention is provided.

[0072] The preparation method comprises: step a) dry-mixing a nickel manganese hydroxide precursor having a manganese content of more than 55 mol% in all metals with a doping compound and then calcining the mixture to prepare an oxide precursor; and step b) mixing the oxide precursor with a sodium compound and then heat-treating the mixture to prepare a sodium manganese-based oxide.

[0073] Step a) is a step of dry-mixing the nickel manganese hydroxide precursor with the doping compound and then calcining to prepare a doped oxide precursor. In the past, when the transition metal was doped into the hydroxide precursor by a dry method without roasting, after the heat treatment and sodium insertion in the subsequent process, particle breakage may occur in the secondary particles or primary particles of the oxide. It is believed that this phenomenon is caused by organic elements such as acetate or oxygen elements such as oxides, oxyhydroxides, and hydroxides in the anion groups contained in the doping compound. In addition, when the transition metal is doped by a wet method, the choice of doping compounds is limited, and there is a problem of increased costs due to complex processes.

[0074] The calcination can be carried out in an oxidizing atmosphere at a temperature of 750 to 1050°C, preferably 800 to 1050°C, 800 to 950°C, 850 to 1000°C or 850 to 950°C. When calcined at a high temperature higher than the above range, the size of the synthesized calcined precursor primary particles becomes too large, which may limit the diffusion of sodium ions (Naion) on the particle surface. On the contrary, when 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. At this time, the calcination time is not particularly limited, but it can be preferably carried out for 6 to 15 hours, 8 to 15 hours or 8 to 13 hours.

[0075] The nickel manganese hydroxide precursor may be represented by the following Chemical Formula 2.

[0076] [Chemical formula 2]

[0077] Ni x Mn 1-x (OH) 2

[0078] In the chemical formula 2, 0.05≤x≤0.45, 0.55≤1-x≤0.95 may be satisfied.

[0079] In the nickel manganese hydroxide precursor, the molar ratio (Mn / M) of manganese (Mn) to all metals (M) may be 0.55 to 0.85. When the content range of manganese is satisfied, high capacity can be achieved under a high voltage working environment and price competitiveness can be ensured. In Chemical Formula 2, the content of Mn is more preferably 0.60≤1-x≤0.85, 0.60≤1-x≤0.80 or 0.60≤1-x≤0.70.

[0080] In the nickel manganese hydroxide precursor, the molar ratio (Ni / M) of nickel (Ni) to all metals (M) may be 0.05 to 0.45. Within the content range of the nickel, the problem of decreased structural stability and chemical stability of the active material due to changes in the oxidation number of nickel, which occurs as the nickel content increases, can be suppressed. In Chemical Formula 2, the content of Ni 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.

[0081] 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), and may be 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), and may specifically be at least one hydroxide selected from the group consisting of Co(OH) 2 、Fe(CH 3 COO 2 、Co(CH 3 COO 2 , ZnO, CuO, for example, Co(OH) 2 .

[0082] The doped positive active material of the present invention can increase the c-axis length in the lattice structure through the doping effect, and the increase in the c-axis length can improve the insertion / deinsertion of Na. In the present invention, when the doping compound is a hydroxide of cobalt (Co) and / or copper (Cu), an oxide of cobalt (Co) and / or copper (Cu), or a hydroxyl oxide of cobalt (Co) and / or copper (Cu), it may be preferred from the perspective of increasing the c-axis length in the lattice structure.

[0083] When the precursor is doped by the dry method and calcined at the same time, the specific surface area and porosity of the calcined precursor decrease, the particle size of the multiple primary particles in the secondary particles increases, and the degree of agglomeration increases. Therefore, the tap density of the calcined precursor increases, which can prevent the particle breakage problem that occurs when dry doping is performed without calcination.

[0084] The oxide precursor of step a) may include (Ni—Mn—X)O 4 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).

[0085] Step b) is a step of mixing the oxide precursor and the sodium compound and then heat treating to prepare the sodium manganese-based oxide.

[0086] The mixing of the oxide precursor and the sodium compound can make the molar ratio of the transition metal: sodium of the precursor become 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. When the mixing amount of the sodium compound is within the above range, the prepared positive electrode active material may have a P2-type layered crystal structure, and therefore, may have higher atmospheric and moisture stability and is less sensitive to synthesis conditions (temperature and atmosphere, etc.). In addition, the battery discharge capacity can be improved within the above sodium content range, and the unreacted residual Na can be minimized.

[0087] The heat treatment may be performed at a temperature of 700°C to 1100°C. When the sintering temperature is within the above range, sufficient reaction may occur between the raw materials, and the particles may grow uniformly. More preferably, the heat treatment may be performed at a temperature of 750 to 1050°C, 850 to 1050°C or 900 to 1000°C. The heat treatment may be performed for 5 to 40 hours. When the sintering time is within the above range, a highly crystalline positive electrode active material may be obtained, and the particle size is appropriate, which may improve production efficiency. The heat treatment is more preferably performed for 5 to 20 hours, 5 to 18 hours, 8 to 15 hours or 10 to 14 hours.

[0088] The sodium compound may be selected from the group consisting of Na 2 CO 3 , NaOH, NaNO 3 , CH 3 COONa, and Na 2 (COO) 2 At least one of the group consisting of, preferably Na 2 CO 3 , NaOH or a combination of these.

[0089] The preparation method of the present invention may further include a cleaning process and a drying process after the step b). The cleaning process is a process for removing unreacted substances, impurities and residual sodium. The positive electrode active material prepared in the step b) can be put into a reactor to which at least one selected from deionized water, distilled water and ethanol has been added, and the positive electrode active material can be cleaned 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 500rpm, 200 to 400rpm or 300 to 400rpm. The drying process is a process for removing moisture from the positive electrode active material containing moisture by a cleaning process, and can be dried at a temperature of 100 to 300°C under vacuum conditions for more than 12 hours.

[0090] Other implementation examples of the present invention provide a positive electrode for a sodium secondary battery and a sodium secondary battery comprising the positive electrode active material.

[0091] The positive electrode includes a positive electrode collector and a positive electrode active material layer located on the positive electrode collector, and the positive electrode active material according to one aspect of the present invention exists in the positive electrode active material layer.

[0092] The positive electrode current collector is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or surface treatment of aluminum or stainless steel surfaces with carbon, nickel, titanium, silver, etc. can be used. In addition, the thickness of the positive electrode current collector can generally be 3μm to 500μm, and fine concavoconvexities can be formed on the surface of the current collector to improve the adhesion of the positive electrode active material. The positive electrode current collector can be provided in various forms, such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, etc.

[0093] In addition, the positive electrode active material layer may be a layer containing a conductive material and a binder in addition to the above-mentioned positive electrode active material.

[0094] Here, the conductive material is used to impart conductivity to the electrode. As long as it has conductivity and does not cause chemical changes in the positive electrode active material, it can be used without special restrictions. As non-limiting examples of conductive materials, it can include: graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, silver, etc.; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium dioxide; or conductive polymers, such as polyphenylene derivatives. Based on the total weight of the positive electrode active material layer, the content of the conductive material can generally be 1% by weight to 30% by weight.

[0095] In addition, the binder is a material for improving the bonding between the positive active material particles and the bonding force between the positive active material and the current collector. As a non-limiting example of a binder, polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile (polyacrylonitrile), carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber or its various copolymers, etc. Based on the total weight of the positive active material layer, the content of the binder can generally be 1% to 30% by weight.

[0096] According to the positive electrode of an implementation example of the present invention, in addition to using the above-mentioned positive electrode active material, it can be prepared by a common method for preparing a positive electrode for a sodium secondary battery. For example, the positive electrode can be prepared by applying a slurry for forming a positive electrode active material layer including a positive electrode active material and an optional binder and a conductive material to a positive electrode collector, and then drying and rolling. According to another example, the slurry for forming the positive electrode active material layer is cast on another support, and then the positive electrode active material layer is peeled off from the support, and the resulting film is pressed onto the positive electrode collector to prepare the positive electrode.

[0097] According to another aspect of the present invention, an electrochemical device comprising the above-mentioned positive electrode is provided. Here, the electrochemical device may specifically be a battery, a capacitor, etc., and more specifically may be a sodium secondary battery.

[0098] The sodium secondary battery includes a positive electrode, a negative electrode disposed opposite to the positive electrode, a separator between the positive electrode and the negative electrode, and an electrolyte (electrolyte). In addition, the sodium secondary battery may include: a battery container (shell) that accommodates an electrode assembly including a positive electrode, a negative electrode and a separator; and a sealing component for sealing the battery container.

[0099] At this time, sodium secondary batteries can be divided into can-type sodium secondary batteries in which the electrode assembly is built into a metal can and pouch-type sodium secondary batteries in which the electrode assembly is built into a pouch made of a sheet such as an aluminum laminate, depending on the shape of the battery container (casing).

[0100] In particular, for a pouch-type sodium secondary battery using a positive electrode including a positive electrode active material according to various embodiments of the present invention, since the possibility of a side reaction between the positive electrode active material and the electrolyte is low, it has the advantage of improving stability during storage and / or operation while reducing gas generation.

[0101] The present invention is described in detail below by way of examples. However, these examples are provided to illustrate the present invention in more detail, and the scope of the present invention is not limited to the following examples.

[0102] Example

[0103] Preparation Example 1: Preparation of positive electrode active material

[0104] (Example 1)

[0105] Step 1) Preparation of doping and calcining precursors

[0106] Ni 0.35 Mn 0.65 (OH) 2 High manganese-based precursor and Co(OH) 2 The mixture was mixed with a hand mixer at a molar ratio of (Ni:Mn:Co=31.5:58.5:10), and then placed in an alumina crucible, oxidized and calcined at 950°C for 12 hours in an air atmosphere, and then cooled to room temperature to prepare a calcined precursor (Ni-Mn-Co)O 4 .

[0107] Step 2) Preparation of positive electrode active material

[0108] The prepared calcined precursor is mixed with a sodium compound Na 2 CO 3 The mixture was mixed in an equivalent ratio of Na / (Ni+Mn+Fe)=0.67 to obtain a mixture. The prepared mixture was placed in an alumina crucible and heated in 0 2 atmosphere, sintered at 950 ° C for 12 hours, and then cooled to room temperature to prepare P2 type Na 0.67 Ni 0.315 Co 0.1 Mn 0.585 O 2 Positive electrode active materials for sodium secondary batteries.

[0109] (Examples 2-1 to 2-2)

[0110] The calcined precursor and the positive electrode active material were prepared in the same manner as in Example 1, except that the oxidative calcination temperature in step 1) was changed to 800° C. (Example 2-1) and 900° C. (Example 2-2).

[0111] (Examples 3-1 to 3-4)

[0112] In addition to the doping source (Co(OH) 2 ) were changed to Fe(CH 3 COO2 、Co(CH 3 COO 2 , ZnO and CuO, the calcined precursor (Ni-Mn-XO 4 ) and positive electrode active materials (Na 0.67 Ni 0.315 X 0.1 Mn 0.585 O 2 ).

[0113] Here, X = Fe, Co, Zn or Cu.

[0114] (Comparative Example 1)

[0115] In addition to not performing step 1), Ni 0.35 Mn 0.65 (OH) 2 Except for the high manganese-based precursor, the same method as in Example 1 was used to prepare Na 0.67 Ni 0.35 Mn 0.65 O 2 Positive electrode active materials for sodium secondary batteries.

[0116] (Comparative Example 2-1)

[0117] As described below, a positive electrode active material was prepared in the same manner as in Example 1 and step 2), except that the coating precursor prepared in step 1) was used.

[0118] Step 1) Preparation of precursor

[0119] 400 g of DIW was placed in a 5 L batch reactor, and 23.9 g of 1 M NaOH solution was added. The mixture was stirred at 300 rpm at 25°C for 15 minutes until the pH value reached 12.8. 0.35 Mn 0.65 (OH) 2 The high manganese-based precursor powder was stirred at a speed of 300 rpm.

[0120] Subsequently, 1.7 M cobalt sulfate (CoSO 4 ) aqueous solution at a rate of 51.18 ml / hr, maintaining a pH value of 11.5 to 12, and reacting for 60 minutes to form cobalt hydroxide (Co(OH) 2 The formed particles were separated and dried in a vacuum oven at 110° C. for 24 hours to prepare a precursor.

[0121] (Comparative Example 2-2)

[0122] Ni 0.35 Mn 0.65 (OH) 2 High manganese-based precursor and Fe(OH) 2 and sodium compounds Na 2 CO 3 The mixture was mixed with Na / (Ni+Mn+Fe)=0.67 equivalents. The prepared mixture was put into an alumina crucible, oxidized and roasted at 950°C for 12 hours in an air atmosphere, and then cooled to room temperature to prepare P2 type Na 0.67 Ni 0.315 Fe 0.1 Mn 0.585 O 2 Positive electrode active materials for sodium secondary batteries.

[0123] (Comparative Example 3)

[0124] Step 1) Preparation of calcined precursor

[0125] Ni 0.35 Mn 0.65 (OH) 2 The high manganese-based precursor was put into an alumina crucible, oxidized and calcined at 600°C for 12 hours in an air atmosphere, and then cooled to room temperature to prepare a calcined precursor (Ni-Mn 2 ) 4 .

[0126] Step 2) Preparation of positive electrode active material

[0127] The prepared calcined precursor and the sodium compound Na 2 CO 3 The mixture was mixed at an equivalent ratio of Na / (Ni+Mn)=0.67. The prepared mixture was put into an alumina crucible and heated at 0 2 atmosphere, sintered at 950 ° C for 12 hours, and then cooled to room temperature to prepare P2 type Na 0.67 Ni 0.35 Mn 0.65 O 2 Positive electrode active materials for sodium secondary batteries.

[0128] (Reference example)

[0129] Step 1) Preparation of calcined precursor

[0130] Ni 0.35 Mn 0.65 (OH) 2The high manganese-based precursor was put into an alumina crucible, oxidized and calcined at 950°C for 12 hours in an air atmosphere, and then cooled to room temperature to prepare a calcined precursor (Ni-Mn 2 ) 4 .

[0131] Step 2) Preparation of positive electrode active material

[0132] The prepared calcined precursor is mixed with a sodium compound Na 2 CO 3 The mixture was mixed at an equivalent ratio of Na / (Ni+Mn)=0.67. The prepared mixture was put into an alumina crucible and heated at 0 2 atmosphere, sintered at 950 ° C for 12 hours, and then cooled to room temperature to prepare P2 type Na 0.67 Ni 0.35 Mn 0.65 O 2 Positive electrode active materials for sodium secondary batteries.

[0133] Preparation Example 2: Preparation of Sodium Secondary Battery

[0134] 85 wt % of the prepared positive electrode active material, 10 wt % of carbon black and 5 wt % of PVdF binder were dispersed in 30 g of N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry. The positive electrode slurry was evenly applied on a 15 μm thick aluminum film and vacuum dried at a temperature of 135° C. to prepare a positive electrode for a sodium secondary battery.

[0135] A sodium metal plate was used as a counter electrode for the positive electrode, a porous glass fiber (thickness: 200 μm) was used as a separator, and a NaPF solution with a concentration of 1.0 M in a solvent of a mixture of propylene carbonate and fluoroethylene carbonate in a volume ratio of 98:2 was used. 6 Preparation of sodium secondary batteries (button batteries) using electrolytes.

[0136] Experimental example

[0137] Experimental Example 1: SEM analysis of the surface and cross-sectional morphology of positive electrode active material particles

[0138] Figure 1a , Figure 1b , Figure 1c and Figure 1d They are respectively a SEM photograph of the surface of the precursor according to Example 1, a SEM photograph of the surface of the calcined precursor, a SEM photograph of the surface of the calcined positive electrode active material, and a SEM photograph of the cross section of the calcined positive electrode active material.

[0139] Figure 2a and Figure 2bSurface SEM photographs and cross-sectional SEM photographs of the positive electrode active material according to Comparative Example 1, Figure 3a and Figure 3b These are surface SEM photographs and cross-sectional SEM photographs of the positive electrode active material according to Comparative Example 2-1. Figure 4 is a surface SEM photograph of the positive electrode active material according to Comparative Example 2-2.

[0140] Table 1 below shows the particle morphology measured from the surface and cross-sectional SEM photos of the positive electrode active material particles. The average aspect ratio of the primary particles and the primary particle diameter (D1) are the average values ​​of at least 20 primary particles. The particle size of the secondary particles is confirmed by SEM photos and PSA (Particle size analysis).

[0141] In addition, the cross-section SEM photograph was obtained by measuring the cross-section of the secondary particles using a cross-section polisher (accelerating voltage 5.0 kV, grinding for 4 hours).

[0142]

Table 1

[0143]

[0144] Referring to Table 1, it can be confirmed that the overall particle size of the primary particles of the positive electrode active material according to the embodiment is increased by calcining the hydroxide precursor and the doping metal together. Therefore, it is predicted that the aspect ratio of the primary particles will be close to 1:1, and the crystal density of the primary particles will be very high, thereby bringing excellent crystal structure stability, improving energy density, improving stability such as suppressing phase transition during high voltage driving, etc.

[0145] On the other hand, it can be seen that the positive electrode active materials of Comparative Examples 1 and 2-1 have rod-shaped and plate-shaped primary particle shapes, respectively, and the primary particle size is uneven, and the aspect ratio and particle size ratio (D2 / D1) exceed the numerical range of the present invention. In the case of Comparative Example 3, the growth of the primary particles is insufficient due to the calcination at a relatively low temperature. In addition, the positive electrode active material according to Comparative Example 2-2 has particle crushing due to dry doping, and the size of the primary particles and the secondary particles cannot be measured.

[0146] Experimental Example 2: XRD analysis of the crystal structure of positive electrode active material particles

[0147] Figure 5a and Figure 5bThe XRD analysis results of the calcined doped precursor (bulk) and the calcined positive electrode active material (bulk) prepared in Example 1. For the calcined precursor prepared in Example 1, it was confirmed that the (Ni-Mn-X)O 4 In addition, when the positive electrode active material is calcined, it can be seen that the Na layer (002) plane in the crystal structure grows precisely and forms a good P2 structure.

[0148] Table 2 below shows c-axis lengths in the lattice structure measured by the Rietveld refinement method based on XRD analysis of the positive electrode active materials prepared in Example 1, Examples 3-1 to 3-4, Comparative Example 1, and Reference Example.

[0149]

Table 2

[0150]

[0151] The calcined precursor has a structure in which the axis lengths in the lattice structure are all the same, that is, a-axis = b-axis = c-axis. The present invention analyzes that the effect of metal doping does not cause a change in the length of the a-axis in the lattice structure, while the length of the c-axis changes significantly.

[0152] In Example 1, it was analyzed that Co(OH) 2 The doping was performed, and the ordering of the metal was most effective during the doping process. Comparing the results of Example 1 with those of Examples 3 and 4, it was found that even if the same Co source was used as the doping metal, the use of Co(OH) 2 The doping effect is better than that of Co(CH 3 COO 2 The doping effect.

[0153] Comparing Examples 3-1 and 3-4, it can be seen that as a doping metal, the doping effect of the Co source is better than that of the Fe source. Comparing Examples 3-2 and 3-3, it can be seen that as a doping metal, the doping effect of the Zn source is better than that of the Cu source.

[0154] Experimental Example 3: Confirming the uniformity of the distribution of doped metals inside the particles through SEM and EDS analysis

[0155] Figure 6a and Figure 6b These are the cross-sectional SEM-EDS analysis results of the calcined doped precursor particles and the calcined doped positive electrode active material particles prepared in Example 1.

[0156] In the Co-doped calcined precursor and calcined positive electrode active material prepared in Example 1, it was confirmed that Co was uniformly doped into the interior of the secondary particles, and it was confirmed that a transition metal ternary precursor (oxide) and a positive electrode active material with uniform composition can be prepared by the calcination doping technology of the present invention.

[0157] Experimental Example 4: Electrochemical Performance Analysis of Sodium Secondary Batteries

[0158] Using an electrochemical analyzer (Toyo, Toscat-3100), at a temperature of 25°C, a voltage range of 2.2V to 4.4V, and a discharge rate of 0.1C to 5.0C, the rate characteristics (discharge capacity ratio; rate capability (C-rate)) of the sodium secondary batteries (button batteries) prepared in Example 1, Comparative Example 1, Comparative Example 2-1 and Comparative Example 3 were measured by charge and discharge experiments, and the results are shown in Table 3 below.

[0159] In addition, for the same sodium secondary battery, after 50 charge / discharge cycles at 0.5C / 0.5C under the conditions of 25°C, within the driving voltage range of 2.2V to 4.4V, the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention) was measured, and the results are shown in Table 3 below.

[0160]

Table 3

[0161]

[0162] Referring to Table 3, it is confirmed that the sodium secondary battery according to Example 1 suppresses the phase transition occurring in the high voltage region through the shape specificity of the primary particles of the positive electrode active material and the uniform metal doping in the secondary particles, thereby improving the high voltage stability and increasing the energy density, thereby greatly improving the battery performance.

[0163] The present invention has been described above with reference to specific embodiments, but it should be apparent to those skilled in the art that various modifications and changes may be made to the present invention without departing from the technical spirit of the present invention provided by the appended claims.

Claims

1. A positive electrode active material for a sodium secondary battery, in, Contains at least sodium (Na), nickel (Ni), manganese (Mn) and doping metal (M D ) of a sodium-manganese-based oxide, wherein the manganese content of all metals other than sodium is greater than 55 mol %, The sodium manganese-based oxide is a secondary particle formed by agglomeration of at least one primary particle, and the aspect ratio of the primary particle is 1:1 to 1:2.

5.

2. The positive electrode active material for sodium secondary battery according to claim 1, in, In the sodium manganese-based oxide, the secondary particle size (D 2 ) relative to the primary particle size (D 1 ) 2 / D 1 ) is 6 to 10.

3. The positive electrode active material for sodium secondary battery according to claim 1, in, In the sodium manganese-based oxide, 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 positive electrode active material for sodium secondary battery according to claim 1, in, The sodium manganese-based oxide has a BET specific surface area of ​​0.1 to 0.45 m 2 / g.

5. The positive electrode active material for sodium secondary battery according to claim 1, in, The length of the c-axis in the lattice structure of the positive electrode active material is 11.13 to 6. The positive electrode active material for sodium secondary battery according to claim 1, in, The sodium manganese-based oxide is represented by the following chemical formula 1: [Chemical formula 1] So a Ni x (M D ) y M1 z Mr 1-x-y-z O 2 In the 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 It is a different element from M1. 0.5≤a≤0.8, 0.05≤x≤0.45, 0.01≤y≤0.15, 0≤z≤0.05, 0.5<1-xyz≤0.

85.

7. The positive electrode active material for sodium secondary battery according to claim 1, in, The doping metal (M D ) 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 positive electrode active material for sodium secondary battery according to claim 1, in, The sodium manganese-based oxide is doped with metal (M D ) relative to all metals (M) except sodium D / M) is 0.01 to 0.

15.

9. The positive electrode active material for sodium secondary battery according to claim 1, in, In the sodium manganese-based oxide, a molar ratio (Mn / M) of manganese (Mn) to all metals (M) except sodium exceeds 0.5 and is less than or equal to 0.

85.

10. The positive electrode active material for sodium secondary battery according to claim 1, in, In the sodium manganese-based oxide, a molar ratio (Ni / M) of nickel (Ni) to all metals (M) except sodium is 0.05 to 0.

45.

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

12. A method for preparing a positive electrode active material for a sodium secondary battery, in, include: Step a) dry-mixing a nickel-manganese hydroxide precursor having a manganese content of more than 55 mol% in all metals with a doping compound, and then calcining to prepare an oxide precursor; as well as Step b) mixing the oxide precursor with a sodium compound and then performing heat treatment to prepare a sodium manganese-based oxide.

13. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 12, in, The calcination in step a) is performed at a temperature of 750 to 1050°C.

14. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 12, in, The dopant 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 method for preparing a positive electrode active material for a sodium secondary battery according to claim 12, in, The oxide precursor in step a) contains (Ni—Mn—X)O 4 Crystal structure, X in the crystal structure is Fe, Co, Al, Cu, Zn, Mg or Ti.

16. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 12, in, The heat treatment in step b) is performed at a temperature of 800 to 1100°C.

17. A positive electrode for a sodium secondary battery, in, The positive electrode active material according to claim 1 is included.

18. A sodium secondary battery, in, Comprising the positive electrode according to claim 17.

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