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

Abstract

The method comprises the following steps: a) introducing a composite transition metal hydroxide precursor and a first cobalt compound into a reactor, and forming a second cobalt compound on the particle surface of the composite transition metal hydroxide precursor; and b) a step of mixing the composite transition metal hydroxide precursor with a sodium compound in which a second cobalt compound is formed on the surfaces of the particles, and then heat-treating the mixture to prepare a cobalt (Co)-doped sodium manganese-based oxide; wherein the sodium-manganese-based oxide contains at least nickel (Ni), manganese (Mn), and cobalt (Co), and the content of manganese is 55 mol% or more among all metals other than sodium. The invention provides a method for preparing a positive electrode active material for a sodium secondary battery, a positive electrode comprising the positive electrode active material, and a sodium secondary battery.

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. According to the position of sodium ions, they are divided into O3 type and P2 type. Among them, the positive electrode active materials with P2 type crystal structure have the advantages of relatively high atmospheric and moisture stability and are not very sensitive to synthesis conditions such as temperature and atmosphere. However, since sodium ions with a larger ionic radius will change the lattice structure when embedded / de-embedded in the layered structure, the positive electrode active material will continuously undergo phase changes, and irreversible phases will occur in this process, resulting in the problem of reduced cycle life and high-rate characteristics of sodium ion secondary batteries.

[0004] Among the layered transition metal oxides, high-manganese-based (high-Mn) sodium nickel manganese oxide (NNMO) has advantages over other transition metal oxides, such as high capacity, strong price competitiveness (superior reserves), and environmental friendliness. However, due to the excessive manganese contained in the oxide, its conductivity is relatively low, resulting in the disadvantage of low rate characteristics (capability rate) of sodium secondary batteries. In this way, if the rate characteristics are low, there is a problem of reduced charge / discharge capacity and life efficiency (capacity retention) of sodium secondary batteries during cycling.

[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 2The by-products such as nickel oxide (NiO), other transition metal oxides, etc., cause the metal composition inside the oxide particles to be uneven, which in turn leads to the problems of reduced energy density during battery charging and discharging, increased side reactions of the electrolyte, and decreased high-rate stability.

[0008] Therefore, the object of the present invention is to provide a sodium manganese-based oxide having a uniform transition metal composition in positive electrode active material particles by performing Co coating on the surface of a transition metal hydroxide precursor under specific conditions.

[0009] In addition, the present invention aims to suppress the surface and internal heterogeneity of the positive electrode material (MnO 2 、NiO、NaCoO 2 ) is formed and a sodium manganese-based oxide with a high-purity P2-type layered structure is synthesized, thereby providing a sodium secondary battery with improved electrochemical characteristics (discharge capacity, initial efficiency, high rate characteristics, life characteristics, etc.).

[0010] Solutions for solving technical problems

[0011] An 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) adding a composite transition metal hydroxide precursor and a first cobalt compound into a reactor to form a second cobalt compound on the surface of the composite transition metal hydroxide precursor particles; and step b) mixing the composite transition metal hydroxide precursor having the second cobalt compound formed on the particle surface with a sodium compound, and then heat treating to prepare a sodium manganese-based oxide doped with cobalt (Co), wherein the sodium manganese-based oxide contains at least nickel (Ni), manganese (Mn) and cobalt (Co), and the manganese content in all metals except sodium is 55 mol% or more.

[0012] The step a) may include: step a1) adding the composite transition metal hydroxide precursor and a solvent into a reactor and stirring; and step a2) adding a first cobalt compound into the reactor and stirring to form cobalt hydroxide on the surface of the composite transition metal hydroxide precursor particles.

[0013] In the composite transition metal hydroxide precursor in step a), the manganese content may be greater than 55 mol % of all metals.

[0014] The first cobalt compound in step a) may be Co(OH) 2 、CoOOH、Co(OCOCH 3 ) 2 、Co(NO 3 ) 2 、CoSO 4 、Co(SO4 ) 2 Or a combination of these.

[0015] The heat treatment in step b) may be performed at a temperature of 800 to 1100° C. for 5 to 40 hours.

[0016] The sodium manganese-based oxide in step b) may have a P2 type layered structure.

[0017] Another embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, wherein the positive electrode active material comprises a sodium manganese-based oxide prepared by the method for preparing the positive electrode active material for a sodium secondary battery.

[0018] The sodium manganese-based oxide may contain at least sodium (Na), nickel (Ni), manganese (Mn) and cobalt (Co), and the content of manganese in all metals (M) except sodium is 55 mol% or more,

[0019] The sodium manganese-based oxide is a secondary particle formed by agglomeration of at least one primary particle and satisfies the following relational formula 1:

[0020] [Equation 1]

[0021] (Dc-Ds) / Dt≤±10%

[0022] In the above relational expression 1, the area corresponding to 0 to 50% of the length (R) from the center to the surface based on the cross section of the secondary particle is defined as the first area (R 1 ), the area corresponding to 50 to 100% is defined as the second area (R 2 )hour,

[0023] Ds, Dc and Dt are respectively in the first region (R 1 ), the second region (R 2 ) and the entire particle cross section (R 1 +R 2 ) are measured and are the molar concentrations of cobalt (Co) relative to all metals (M) except sodium.

[0024] The sodium manganese-based oxide is a secondary particle formed by agglomeration of at least one primary particle, and the primary particle may include a plate-like primary particle having an aspect ratio of 1 to 5.

[0025] An average particle size of the secondary particles in the sodium manganese-based oxide may be 5 to 15 μm.

[0026] The sodium manganese-based oxide can be represented by the following chemical formula 2:

[0027] [Chemical formula 2]

[0028] Na a Nix Co y M1 z Mn 1-x-y-z O 2

[0029] In the chemical formula 2, M1 can be at least one selected from P, Sr, Ba, B, Ti, Mg, Zr, Al, W, Fe, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, Gd and Cu, and can be 0.5≤a≤0.8, 0.05≤x≤0.45, 0.01≤y≤0.15, 0≤z≤0.05, and 0.55≤1-xyz≤0.85.

[0030] The sodium manganese-based oxide may show at least one peak selected from the group consisting of (004), (100), (101), (102), (103), (104), and (002) peaks through X-ray diffraction (XRD).

[0031] In the sodium manganese-based oxide, the NiO content measured by the Rietveld refinement method based on X-ray diffraction analysis may be 3% or less.

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

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

[0034] Effects of the Invention

[0035] In the present invention, since a uniform Co compound is formed on the entire surface of the composite transition metal hydroxide precursor particles, uniform Co doping can be performed in the subsequent process, and the surface defects of the positive electrode active material particles can be minimized, thereby improving the surface stability and improving the electrochemical properties such as high rate characteristics and charge and discharge efficiency.

[0036] The present invention uses sodium (Na) instead of expensive metal lithium (Li) and increases the ratio of manganese (Mn) which is relatively cheaper than nickel (Ni), thereby providing a high manganese-based (NCM) positive electrode active material for sodium secondary batteries with improved price competitiveness.

[0037] When the precursor is coated with Co and the positive electrode active material is doped with Co, the surface side reactions caused by the electrolyte can be improved, and the phase change (P2-O2) occurring in the high voltage region during the charge and discharge process can be suppressed, thereby improving the cycle life characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 : is a surface SEM photograph of the positive electrode active material particles according to Example 1.

[0039] Figure 2a and Figure 2b Surface SEM / EDS photos of the secondary particles of the positive electrode active material according to Example 1 ( Figure 2a ) and cross-sectional SEM / EDS images ( Figure 2b ).

[0040] Figure 2c The EDS Line scanning is a graph showing the change in the content of metal elements (Ni, Co, Mn) in the direction shown in the SEM image of the cross section of the secondary particles of the positive electrode active material prepared in Example 1 through EDS analysis.

[0041] Figure 3a and Figure 3b Surface SEM / EDS images of the secondary particles of the positive electrode active material according to Example 2 ( Figure 3a ) and cross-sectional SEM / EDS images ( Figure 3b ).

[0042] Figure 3c This is a curve chart (EDS Line scanning) showing the change in the content of metal elements (Ni, Co, Mn) in the direction shown in the SEM image of the cross section of the secondary particles of the positive electrode active material prepared in Example 2 through EDS analysis.

[0043] Figure 4 This is a SEM / EDS photograph of the surface of the secondary particles of the positive electrode active material prepared in Comparative Example 2.

[0044] Figure 5 1 and 2 are XRD analysis results of the positive electrode active materials (bulk) prepared in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0045] 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.

[0046] 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.

[0047] An implementation example of the present invention provides a method for preparing a positive electrode active material for a sodium secondary battery. The preparation method comprises the following steps: step a) putting a composite transition metal hydroxide precursor and a first cobalt compound into a reactor to form a second cobalt compound on the surface of the composite transition metal hydroxide precursor particles; and step b) mixing the composite transition metal hydroxide precursor having the second cobalt compound formed on the particle surface with a sodium compound, and then heat treating to prepare a sodium manganese-based oxide doped with cobalt (Co), wherein the sodium manganese-based oxide contains at least nickel (Ni), manganese (Mn) and cobalt (Co), and the content of manganese in all metals except sodium is 55 mol% or more.

[0048] According to the preparation method of the present invention, a high manganese NCM oxide having a uniform ternary transition metal composition can be synthesized by coating cobalt on a binary transition metal hydroxide precursor and then performing a sintering process. The problem of low conductivity of the previous high manganese-based oxide can be improved by doping with cobalt elements, and the problem of synthesizing a large amount of manganese oxide (MnO) in the coprecipitation reaction for preparing a ternary transition metal hydroxide precursor due to the high reactivity of manganese can be solved. 2 etc.) issues.

[0049] The step a) is a step of forming a second cobalt compound on the surface of the composite transition metal hydroxide precursor particles, which may specifically include: step a1) adding the composite transition metal hydroxide precursor and a solvent into a reactor and stirring; and step a2) adding a first cobalt compound into the reactor and stirring to form cobalt hydroxide (Co(OH)) on the surface of the composite transition metal hydroxide precursor particles. 2 ).

[0050] In the existing dry coating process or the coating process performed on the surface of the positive electrode active material, if a high-temperature heat treatment is performed after coating, it is difficult to form a uniform second cobalt compound on the entire surface of the particle, and an island coating will be formed on the surface of the particle. In addition, it may be difficult to expect to uniformly dope cobalt with a sufficient doping amount inside the particle. The formation of such a coating or the uneven distribution of cobalt on the surface and inside of the particle may cause surface defects, thereby increasing the contact area (specific surface area) with the electrolyte, and causing battery expansion due to side reactions of the electrolyte, reduced cycle life and other problems. In the present invention, since a uniform Co compound is formed on the entire surface of the composite transition metal hydroxide precursor particles, uniform Co doping can be performed in the subsequent process, and the surface defects of the positive electrode active material particles can be minimized, thereby improving the surface stability and improving the electrochemical characteristics such as high rate characteristics and charge and discharge efficiency.

[0051] Step a1) may be a step of adding the composite transition metal hydroxide precursor and a solvent into a reactor and stirring them.

[0052] The composite transition metal hydroxide precursor and the solvent may be mixed in a reactor at a weight ratio of 0.5:1.5 to 1.5:0.5, or at a weight ratio of 0.7:1.2 to 1.2:0.7, for example, at a weight ratio of 1:1.

[0053] An alkaline solution may be added to the reactor to adjust the pH value to 10 to 12, a small amount of ammonium solution may be used as a complexing agent, and the stirring speed may be 200 to 500 rpm, 200 to 400 rpm, or 300 to 400 rpm, but the present invention is not limited thereto. Therefore, in the post-process step a2), the second cobalt compound may be uniformly formed on the surface of the composite transition metal hydroxide precursor particles.

[0054] Step a2) may be to add a first cobalt compound into the reactor and stir it to form cobalt hydroxide (Co(OH)) on the surface of the composite transition metal hydroxide precursor. 2 ) steps.

[0055] The first cobalt compound may be fed into the reactor so that the sum of the composite transition metal hydroxide precursor and the solvent: the first cobalt compound is mixed in a weight ratio of 5:1 to 1.1:1, a weight ratio of 4.5:1 to 1.5:1, preferably a weight ratio of 4:1 to 1.5:1, or a weight ratio of 3:1 to 1.5:1. In addition, the first cobalt compound may be fed into the reactor so that the content ratio (Co / M) of cobalt (Co) relative to all metals (M) in the composite transition metal hydroxide precursor is 0.01 to 0.15, preferably 0.01 to 0.12, 0.01 to 0.1, 0.02 to 0.15, or 0.03 to 0.12. At this time, the first cobalt compound may be fed into the reactor after the first cobalt compound aqueous solution with a concentration of 1.5 to 2.5 M is performed for 0.5 to 5 hours, 0.5 to 4 hours, 0.5 to 3 hours, or 1 to 3 hours, so as to meet the Co / M range. More specifically, the first cobalt compound aqueous solution may be added at 2 to 20 ml / min, 2 to 18 ml / min, or 2 to 16 ml / min for 0.5 to 5 hours, 0.5 to 4 hours, 0.5 to 3 hours, or 0.5 to 1.5 hours. If the first cobalt compound is added at a rate exceeding the conditions, during the sintering process of the positive electrode active material, the secondary particles or primary particles of the oxide may not grow into a uniform shape due to the agglomeration of cobalt.

[0056] If the input speed exceeds the above-mentioned speed, it is difficult to uniformly form cobalt hydroxide on the surface of the composite transition metal hydroxide precursor particles. Conversely, if the input speed is lower than the above-mentioned speed, the process time becomes longer, the cost increases, and Ni dissolution may occur inside the transition metal hydroxide precursor particles.

[0057] The step a) can be carried out under an inert atmosphere such as nitrogen or argon at a temperature of 40 to 70° C. or 45 to 65° C., and the reaction time can be 60 to 90 minutes, such as 75 to 105 minutes. Therefore, the above effects can be further improved.

[0058] The composite transition metal hydroxide precursor may be represented by Chemical Formula 1 below.

[0059] [Chemical formula 1]

[0060] Ni x Mn 1-x (OH) 2

[0061] In the chemical formula 1, 0.05≤x≤0.45, 0.55≤1-x≤0.95 may be satisfied.

[0062] In the nickel manganese hydroxide precursor, the molar ratio (Mn / M) of manganese (Mn) to all metals (M) may be 0.55 to 0.95. When the content range of manganese is satisfied, high capacity can be achieved under high voltage working environment and price competitiveness can be ensured. In Chemical Formula 1, 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.

[0063] 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 as the nickel content increases can be improved. 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.

[0064] The solvent may be one or more selected from deionized water, distilled water, and ethanol, preferably deionized water. At this time, the temperature of the solvent used in the mixing may be 1 to 80° C. or 5 to 50° C., but the present invention is not limited thereto.

[0065] The alkaline solution can be NaOH, KOH or Ca(OH) 2 The pH value of the solution in the reactor can be controlled by adding hydroxides of alkali metals or alkaline earth metals, hydrates thereof or combinations thereof.

[0066] The above ammonium solution may include NH 4 OH, (NH 4 ) 2 SO 4 NH 4 NO 3 NH 4 Cl, CH 3 COONH 4 NH 4 CO 3 or a combination thereof, can act as a complexing agent to effectively co-precipitate the first cobalt compound in the reactor onto the surface of the composite transition metal hydroxide particles.

[0067] The first cobalt compound may be a solution containing a cobalt-containing compound, which may be prepared by adding the first cobalt compound to water or a mixed solvent of an organic solvent (such as alcohol) that can be uniformly mixed with water. The first cobalt compound may be Co(OH) 2 、CoOOH、Co(OCOCH 3 ) 2、Co(NO 3 ) 2 、CoSO 4 、Co(SO 4 ) 2 or a combination thereof, preferably CoSO 4 and / or Co(SO 4 ) 2 Here, the first cobalt compound may include hydrates and anhydrous substances of the above substances. When CoSO 4 7H 2 O or CoSO 4 anhydride, preferably CoSO 4 7H 2 O, it is easy to react with NaOH to evenly generate a second cobalt compound (Co(OH) 2 ), and then sintered with a sodium source (Na source), cobalt can be uniformly doped. On the other hand, when doping metals other than Co, for example, when using Fe-containing compounds as doping sources, the doping metal may only be concentrated on a portion of the secondary particles, resulting in uneven distribution of Fe on the surface of the positive electrode active material particles. In addition, in the process of feeding the Fe-containing compound in order to coat the surface of the hydroxide precursor particles, the problem of oxidation dissolution occurs.

[0068] The reactor may be any reactor generally used for a coprecipitation reaction of a hydroxide precursor for preparing a positive electrode active material, and can be used without limitation.

[0069] The preparation method of the present invention may further include a drying process after step a). The drying process is a process for removing moisture from the composite transition metal hydroxide precursor having the second cobalt compound prepared in step a), and the drying process may be performed at a temperature of 100 to 300° C. under vacuum conditions for more than 12 hours.

[0070] The step b) is a step of performing heat treatment after forming a composite transition metal hydroxide precursor of the second cobalt compound and a sodium compound on the surface of the mixed particles to prepare a sodium manganese-based oxide doped with cobalt.

[0071] The mixing of the composite transition metal hydroxide precursor and the sodium compound can make the molar ratio of the transition metal: sodium of the precursor be 1:0.5 to 1:0.8, 1:0.6 to 1:0.8, 1:0.6 to 1:0.75 or 1:0.65 to 1:0.75. When the mixing amount of the sodium compound is within the above range, the prepared positive electrode active material can have a P2-type layered crystal structure, and therefore can 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 increased within the sodium content range, and the unreacted residual Na can be minimized.

[0072] The heat treatment can be carried out at a temperature of 800°C to 1100°C. When the sintering temperature is within the above range, sufficient reaction can occur between the raw materials and the particles can grow uniformly. The heat treatment is more preferably carried out at a temperature of 800 to 1050°C, 850 to 1050°C or 900 to 1000°C. The heat treatment can be carried out for 5 to 40 hours. When the sintering time is within the above range, a highly crystalline positive electrode active material can be obtained, and the particle size is suitable, which can improve production efficiency. The heat treatment is more preferably carried out for 5 to 20 hours, 5 to 18 hours, 8 to 15 hours or 10 to 14 hours.

[0073] 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 components, preferably Na 2 CO 3 , NaOH or a combination of these.

[0074] The sodium manganese-based oxide may be uniformly doped with cobalt on the surface and inside of the particles, and may have a P2-type layered structure. Even if the coating and doping of the transition metal compound are carried out by the same method as the above-mentioned manufacturing method, for elements other than cobalt (Co), such as iron (Fe), titanium (Ti), and magnesium (Mg), the coating / doping of the transition metal may be uneven, such as forming a coating with a concentrated distribution of the doped metal on the surface of the oxide secondary particles, or presenting a continuously increasing or continuously decreasing concentration gradient inside the oxide secondary particles. That is, in the present invention, the sodium manganese-based oxide particles can form a P2-type layered structure, while synthesizing a ternary transition metal oxide with a uniform composition.

[0075] The preparation method of the present invention may further include a cleaning process and a drying process after step b). The cleaning process is a process for removing unreacted substances, impurities and residual sodium. The positive electrode active material prepared in step b) can be put into a reactor added with at least one selected from deionized water, distilled water and ethanol, and 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.

[0076] Another embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, comprising a sodium manganese-based oxide prepared by the method. The sodium manganese-based oxide contains at least sodium (Na), nickel (Ni), manganese (Mn) and cobalt (Co), and the manganese content of all metals (M) except sodium can be 55 mol% or more.

[0077] The sodium manganese-based oxide is an NCM-based sodium nickel-cobalt-manganese oxide containing at least sodium, nickel, cobalt and manganese. The sodium manganese-based oxide is a high-manganese-based (high-Mn) oxide with a manganese content of 55 mol% or more in all metals except sodium. In metals other than sodium, the manganese content can be more than 55 mol%, more than 60 mol%, or more than 65 mol%, and the upper limit is not particularly limited, for example, it can be less than 85 mol%, less than 80 mol%, or less than 75 mol%. In metals other than sodium, the more the manganese content increases, the more it has the advantage of being able to exert high capacity under high-pressure working environments, and the price competitiveness can be improved by reducing the concentration of nickel and cobalt.

[0078] In addition, as described above, when doping is performed by coating Co on binary transition metal hydroxide precursor particles, the problem of synthesizing a large amount of manganese oxide (MnO 2 In addition, compared with the technology that Co is only concentrated on the surface of oxide particles and forms a coating, the present invention can further improve the above effect by uniformly doping Co inside and on the surface of oxide secondary particles. In addition, if the electrochemically active metal Co is doped, it can not only prevent the degradation of the surface of the oxide particles, but also participate in the redox reaction during the charge / discharge of the sodium secondary battery, so that the discharge capacity can be increased compared with coating / doping with other metals.

[0079] The sodium manganese-based oxide is a secondary particle formed by agglomeration of at least one primary particle, and can satisfy the following relational expression 1.

[0080] [Equation 1]

[0081] (Dc-Ds) / Dt≤±10%

[0082] In the above relational expression 1, based on the cross section of the secondary particle, the region corresponding to 0% to 50% of the length R from the center to the surface is defined as the first region R 1 , the area equivalent to 50% to 100% is defined as the second area R 2 When Ds, Dc and Dt are respectively in the first region R 1 , the second region R 2 and the entire particle cross section R 1 +R 2 are measured in and are the molar concentrations of cobalt (Co) relative to all metals (M) except sodium.

[0083] The sodium manganese-based oxide may, for example, be (Dc-Ds) / Dt≤±9%, (Dc-Ds) / Dt≤±8%, (Dc-Ds) / Dt≤±7%, (Dc-Ds) / Dt≤±6%, (Dc-Ds) / Dt≤±5%, (Dc-Ds) / Dt≤±4%, (Dc-Ds) / Dt≤±3%, (Dc-Ds) / Dt≤±2%, or (Dc-Ds) / Dt≤±1%, and may be 0≤(Dc-Ds) / Dt≤9%, 0≤(Dc-Ds) / Dt≤8%, 0≤(Dc-Ds) / Dt≤7%, 0≤(Dc-Ds) / Dt≤6%, 0≤(Dc-Ds) / Dt≤5%, 0≤(Dc-Ds) / Dt≤4%, 0≤(Dc-Ds) / Dt≤3%, 0≤(Dc-Ds) / Dt≤2%, or 0≤(Dc-Ds) / Dt≤1%, and can be 0≥(Dc-Ds) / Dt≥-9%, 0≥(Dc-Ds) / Dt≥-8%, 0≥(Dc-Ds) / Dt≥-7%, 0≥(Dc-Ds) / Dt≥-6%, 0≥(Dc-Ds) / Dt≥-5%, 0≥(Dc-Ds) / Dt≥-4%, 0≥(Dc-Ds) / Dt≥-3%, 0≥(Dc-Ds) / Dt≥-2% or 0≥(Dc-Ds) / Dt≥-1%. When the concentration distribution of cobalt in the sodium manganese-based oxide particles meets the above design range, it can be understood that the Co element is evenly distributed on the cross section of the secondary particles to form a ternary transition metal oxide. 2 、CoO 2) is formed to synthesize a high-purity P2-type layered sodium manganese-based oxide, thereby providing a sodium secondary battery with improved electrochemical properties (discharge capacity, initial efficiency, high rate characteristics, life characteristics, etc.).

[0084] At this time, the first region R 1 It may be a region corresponding to 0 to 50%, 0 to 40%, 0 to 30%, 0 to 20%, 0 to 10%, 30 to 50%, 40 to 50% or 20 to 30% of the length (100%) from the center of the particle to the surface based on the cross-section of the secondary particle, and the second region (R2) may be a region corresponding to 50 to 100%, 60 to 100%, 70 to 100%, 80 to 100%, 90 to 100%, 50 to 70%, 50 to 60% or 70 to 80% of the length (100%) from the center of the particle to the surface.

[0085] In addition, the cobalt (Co) concentration may be calculated by measuring using X-ray photoelectron spectroscopy, SEM-EDS, or TEM-EDS, but the present invention is not limited thereto.

[0086] The sodium manganese-based oxide can be represented by the following Chemical Formula 2.

[0087] [Chemical formula 2]

[0088] Na a Ni x Co y M1 z Mn 1-x-y-z O 2

[0089] In the chemical formula 2, M1 may be at least one selected from P, Sr, Ba, B, Ti, Mg, Zr, Mn, Al, W, Fe, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, Gd and Cu, and may be 0.5≤a≤0.8, 0.05≤x≤0.45, 0.01≤y≤0.15, and 0.55≤1-xyz≤0.85.

[0090] In the sodium manganese-based oxide of the chemical formula 2, 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 2, 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.

[0091] In the sodium manganese-based oxide, the molar ratio (Mn / M) of manganese (Mn) to all metals (M) except sodium 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-xyz≤0.85, 0.60≤1-xyz≤0.80 or 0.60≤1-xyz≤0.70.

[0092] The molar ratio (Ni / M) of nickel (Ni) to all metals (M) other than sodium in the manganese-based sodium composite transition metal oxide 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 is more preferably 0.1≤x≤0.45, 0.2≤x≤0.45, 0.25≤x≤0.45 or 0.3≤x≤0.4.

[0093] 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, the higher the Mn content, the higher the capacity, but the battery life may decay early, while 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, the electrochemical properties can be improved by doping cobalt (Co) at a high Mn content ratio.

[0094] The sodium manganese-based oxide comprises at least one secondary particle formed by agglomeration of primary particles. When the major axis direction (a-axis), minor axis direction (b-axis) and thickness direction (c-axis) of the primary particles are set, the primary particles may include plate-type particles, and the plate-type may mean that the length of the thickness direction (c-axis) of the primary particles is relatively smaller than the length of the major axis (a-axis) and minor axis (b-axis) of the plane direction of the primary particles. In addition, the surface containing the major axis and minor axis of the primary particles may be rectangular, elliptical, hexagonal plate-like or amorphous with different major axes or minor axes, or may be circular or square identical to each other, but the present invention is not limited thereto.

[0095] In the sodium manganese-based oxide, the aspect ratio of the primary particles may be 1 to 5, 1 to 4, 2 to 5 or 2 to 4. In one embodiment, the average particle size of the secondary particles in the sodium manganese-based oxide may be 5 to 15 μm. In addition, the average major axis length of the primary particles may be 1 to 3.5 μm. By satisfying the above conditions, the primary particles and the secondary particles contained in the positive electrode active material can increase the particle density (tap density) in the positive electrode active material. Thus, the electrochemical properties of the positive electrode active material can be improved.

[0096] At this time, the present invention may be that more than 50%, for example, more than 60% or 70% of the total number of primary particles constituting the secondary particles have an aspect ratio, major axis length and / or minor axis length within the above range, or at least 10 primary particles among the primary particles constituting the secondary particles have an aspect ratio, major axis length and / or minor axis length within the above range.

[0097] The sodium manganese-based oxide can show at least one peak selected from the group consisting of (004), (100), (101), (102), (103), (104) and (002) peaks by X-ray diffraction (XRD), and preferably, can show all (004), (100), (101), (102), (103), (104) and (002) peaks. The peaks are peaks for forming a P2-type layered structure. The technical feature of the present invention is that after the sodium manganese-based oxide is doped with Fe, it can still stably form and maintain a P2-type layered structure. As described above, the positive electrode active material having a P2-type layered structure has the advantages of high atmospheric and moisture stability and is not very sensitive to synthesis conditions such as temperature and atmosphere.

[0098] In the sodium manganese-based oxide, the NiO content measured by Rietveld refinement method based on X-ray diffraction analysis can be 3% or less, for example, 2.9% or less, 2.7% or less, 2.5% or less, or 2.2% or less. Thus, the generation of oxide byproducts such as NiO can be suppressed, and a transition metal oxide with a uniform composition can be prepared. The NiO ratio can be measured by Rietveld refinement method based on X-ray diffraction analysis, but the present invention is not limited thereto.

[0099] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery and a sodium secondary battery including the positive electrode active material.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] 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 the 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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).

[0109] 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.

[0110] 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.

[0111] Example

[0112] Preparation Example 1. Preparation of positive electrode active material

[0113] Example 1

[0114] (a) Preparation of precursor

[0115] In the reactor, NiSO 4 6H 2 O and MnSO 4 ·H 2 A mixed aqueous solution of NaOH and NH O in a molar ratio of 35:65 was added 4 OH, while stirring. The temperature in the reactor was maintained at 45°C. After the reaction was completed, Ni with an average particle size of 10 μm was obtained after washing and dehydration. 0.35 Mn 0.65 (OH) 2 Precursor.

[0116] (b) Coating precursor

[0117] NaOH is added to the reactor in which the precursor obtained in step (a) is stirred. Then, CoSO 4 7H 2 O aqueous solution was added to the stirring reactor at a rate of 2.5 ml / min for 60 minutes. At this time, the CoSO 4 7H 2 O was added so that the ratio of Co / (Ni+Co+Mn) became 10 mol%. After the reaction was completed, the mixture was filtered, washed, dehydrated, and then dried at 110° C. for 12 hours to obtain a coated precursor.

[0118] (c) Heat treatment

[0119] As the coating precursor obtained in step (b) and a sodium compound, NaOH (Na / (metal other than Na) molar ratio = 0.67) was mixed to prepare a mixture.

[0120] Next, the temperature of the sintering furnace in air atmosphere was raised at a rate of 4°C / min and maintained at 950°C. The mixture was heat treated for 12 hours and then cooled with furnace cooling to finally obtain sodium manganese-based oxide Na with an average particle size of 7 to 10 μm. 0.67 Ni 0.315 Co 0.1 Mn0.585 O 2 positive electrode active material.

[0121] At this time, refer to Figure 1 From the SEM photograph of , it can be seen that the length ratio (La / Lb) of the major axis / minor axis of the primary particles in the obtained sodium manganese-based oxide is 2 to 5.

[0122] Example 2

[0123] (b) In the precursor coating step, CoSO 4 7H 2 O, and the Co / (Ni+Co+Mn) ratio was 5 mol%, except that the same method as in Example 1 was used to obtain a sodium manganese-based oxide Na having an average particle size of 7 to 10 μm. 0.67 Ni 0.3325 Co 0.05 Mn 0.6175 O 2 positive electrode active material.

[0124] At this time, the length ratio (La / Lb) of the major axis / minor axis of the primary particles in the obtained sodium manganese-based oxide is 1 to 4.

[0125] Comparative Example 1

[0126] (b) The same method as in Example 1 was performed except that the precursor coating step was not performed to obtain a sodium manganese-based oxide Na having an average particle size of 7 to 10 μm. 0.67 Ni 0.35 Mn 0.65 O 2 positive electrode active material.

[0127] Comparative Example 2

[0128] (b) The same method as in Example 1 was performed except that the precursor coating step was not performed to obtain a sodium manganese-based oxide Na having an average particle size of 7 to 10 μm. 0.67 Ni 0.315 Fe 0.1 Mn 0.585 O 2 positive electrode active material.

[0129] (b) Precursor coating

[0130] NaOH and NH 4 OH. Then, FeSO 4 7H 2O aqueous solution was added to the stirring reactor at a rate of 2.5 cc / min for 60 minutes. At this time, FeSO 4 7H 2 O was added to make it 10 mol%. After the reaction was completed, it was filtered, washed, dehydrated, and then dried at a temperature of 110° C. for 12 hours to obtain a coated precursor.

[0131] Comparative Example 3

[0132] 400 g of DIW was placed in a 5 L batch reactor, and 23.9 g of 1 M NaOH solution was added, followed by stirring at 300 rpm at 25°C for 15 minutes to adjust the pH to 12.8. 0.67 Ni 0.35 Mn 0.65 O 2 (Positive electrode active material of Comparative Example 1), and stirred at a speed of 300 rpm.

[0133] Subsequently, a 1.7M cobalt sulfate aqueous solution was added at a rate of 51.18 ml / hr, the pH value was maintained at 11.5 to 12, and the reaction was continued for 60 minutes to form cobalt hydroxide on the surface of the sodium manganese-based oxide particles. The obtained particles were separated and dried in a vacuum oven at 110°C for 24 hours. Subsequently, the particle powder was put into an alumina crucible and then heat-treated at 600°C in an air atmosphere for 12 hours to prepare Na-Mn-based oxide particles. 0.67 Ni 0.315 Co 0.1 Mn 0.585 O 2 positive electrode active material.

[0134] Comparative Example 4

[0135] The same process as in Comparative Example 3 was performed except that the heat treatment for doping cobalt in Comparative Example 3 was performed at a temperature of 700° C. to prepare a Na 0.67 Ni 0.315 Co 0.1 Mn 0.585 O 2 positive electrode active material.

[0136] Preparation Example 2. Preparation of Sodium Secondary Battery

[0137] A positive electrode slurry was prepared by dispersing 90 wt % of the positive electrode active material prepared according to Preparation Example 1, 5.5 wt % of carbon black, and 4.5 wt % of PVDF binder in 30 g of N-methyl-2-pyrrolidone (NMP). The positive electrode slurry was uniformly coated 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.

[0138] Sodium foil was used as the counter electrode of the positive electrode, a porous polyethylene film (Celgard 2300, thickness: 25 μm) was used as a separator, and NaPF was present at a concentration of 1.15 M in a solvent of ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:7. 6 button cells were prepared using electrolyte.

[0139] Experimental example

[0140] Experimental Example 1. Confirmation of the uniformity of metal element distribution on the surface and cross section of positive electrode active material particles

[0141] SEM / EDS analysis was performed to confirm the change in the content of metal elements. The SEM / EDS image of the sodium manganese-based oxide was obtained by cross-section processing of the sodium manganese-based oxide using FIB.

[0142] Figure 2a and Figure 2b Surface SEM / EDS photos of the secondary particles of the positive electrode active material according to Example 1 ( Figure 2a ) and cross-sectional SEM / EDS images ( Figure 2b ).

[0143] Figure 2c The EDS Line scanning is a graph showing the change in the content of metal elements (Ni, Co, Mn) in the direction shown in the SEM image of the cross section of the secondary particles of the positive electrode active material prepared in Example 1 through EDS analysis.

[0144] Figure 3a and Figure 3b Surface SEM / EDS images of the secondary particles of the positive electrode active material according to Example 2 ( Figure 3a ) and cross-sectional SEM / EDS images ( Figure 3b ).

[0145] Figure 3c This is a curve chart (EDS Line scanning) showing the change in the content of metal elements (Ni, Co, Mn) through EDS analysis in the direction shown in the SEM image of the cross section of the secondary particles of the positive electrode active material prepared in Example 2.

[0146] Figure 4This is a SEM / EDS photograph of the surface of the secondary particles of the positive electrode active material prepared in Comparative Example 2.

[0147] Table 1 below shows the concentrations (mol%) of Ni, Co, Fe, and Mn in the center and surface of the secondary particles of the positive electrode active material. The concentrations (mol%) of Ni, Co, Fe, and Mn in the center and surface of the secondary particles were calculated by Line EDS spectrum, and the concentrations of Ni, Co, Fe, and Mn in the entire particle (bulk) were calculated by ICP analysis.

[0148]

Table 1

[0149]

[0150] (In Table 1, based on the cross section of the positive electrode active material secondary particle, the region corresponding to 0 to 50% of the length R from the center to the surface is defined as the central portion, and the region corresponding to 50 to 100% is defined as the surface portion.)

[0151] In the cobalt-doped positive electrode active materials prepared in Examples 1 and 2, it was confirmed that cobalt was uniformly doped on the surface and inside of the secondary particles, and it was confirmed that a transition metal ternary oxide with uniform composition can be prepared by the doping technology of the present invention.

[0152] refer to Figure 4 In the Fe-doped positive electrode active material prepared in Comparative Example 2, Fe aggregation was confirmed, and the doping uniformity was significantly reduced compared with Examples 1 and 2. Specifically, it can be confirmed that Fe is concentrated on the surface of the secondary particles of the positive electrode active material to form an island coating, or some particles among the multiple secondary particles are selectively over-coated / doped. Therefore, it can be confirmed that Fe doping is not easy when the preparation method of the present invention is adopted.

[0153] Experimental Example 2. Confirmation of P2-type layered structure and heterogeneous phase by XRD analysis

[0154] Figure 5 The XRD analysis results of the positive electrode active materials (bulk) prepared in Example 1 and Comparative Example 1 are shown. In addition, the following Table 2 shows the ratio of NiO (heterogeneous phase) formed in the positive electrode active material measured using the TOPAS program.

[0155]

Table 2

[0156] NiO(%) Example 1 2.49 Example 2 2.13 Comparative Example 1 0.81 Comparative Example 2 2.24 Comparative Example 3 2.66 Comparative Example 4 3.09

[0157] Reference Figure 5As shown in Table 2, the positive electrode active material prepared in Example 1 has peaks (004), (100), (101), (102), (103), (104) and (002) detected by XRD analysis, which confirms that it has a P2-type layered structure. Figure 2b and 2c It can be confirmed that in Examples 1 and 2, a small amount of NiO was synthesized when Co was doped, but no heterogeneous phase (CoO 2 、MnO 2 wait).

[0158] Experimental Example 3. Evaluation of electrochemical performance of sodium secondary battery

[0159] For the sodium secondary battery (button cell) prepared in Preparation Example 2, a charge and discharge experiment was carried out using an electrochemical analyzer (Toyo, Toscat-3100) at a temperature of 25°C, a voltage range of 2.0V to 4.6V, and a discharge rate of 0.1C to 5.0C to measure the initial charge capacity, initial discharge capacity, initial reversible efficiency and rate characteristics (discharge capacity ratio; rate capability (C-rate)).

[0160] In addition, for the same sodium secondary battery, 50 charge / discharge cycles were performed at 1C / 1C at a temperature of 25°C and an operating voltage range of 2.0V to 4.6V, and then the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention) was measured.

[0161] The above measurement results are shown in Table 3.

[0162]

Table 3

[0163]

[0164] Referring to Table 3, it can be confirmed that the sodium secondary batteries prepared in Examples 1 and 2 have higher initial discharge capacity, rate characteristics and charge and discharge efficiency than Comparative Examples 1 and 2, and the cycle life remains at a level similar to that of Comparative Example 1. In Comparative Example 2, due to the presence of the Fe coating between the primary particles, the side reactions of the electrolyte also increase, resulting in deterioration of the overall battery characteristics.

[0165] 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 method for preparing a positive electrode active material for a sodium secondary battery, in, include: Step a) adding a composite transition metal hydroxide precursor and a first cobalt compound into a reactor to form a second cobalt compound on the surface of the composite transition metal hydroxide precursor particles; as well as Step b) mixing a composite transition metal hydroxide precursor having the second cobalt compound formed on the surface of the particles with a sodium compound, and then heat treating to prepare a sodium manganese-based oxide doped with cobalt (Co), The sodium manganese-based oxide contains at least nickel (Ni), manganese (Mn) and cobalt (Co), and the content of manganese in all metals except sodium is 55 mol % or more.

2. The method for preparing the positive electrode active material for sodium secondary battery according to claim 1, in, The step a) comprises: Step a1) adding the composite transition metal hydroxide precursor and a solvent into a reactor and stirring; and Step a2) adding a first cobalt compound into the reactor and stirring to form cobalt hydroxide on the surface of the composite transition metal hydroxide precursor particles.

3. The method for preparing the positive electrode active material for sodium secondary battery according to claim 1, in, In the composite transition metal hydroxide precursor in step a), the content of manganese is more than 55 mol% of all metals.

4. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 1, in, The first cobalt compound in step a) is Co(OH) 2 、CoOOH、Co(OCOCH 3 ) 2 、Co(NO 3 ) 2 、CoSO 4 、Co(SO 4 ) 2 Or a combination of these.

5. The method for preparing the positive electrode active material for sodium secondary battery according to claim 1, in, The heat treatment in step b) is performed at a temperature of 800 to 1100° C. for 5 to 40 hours.

6. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 1, in, The sodium manganese-based oxide in step b) has a P2 type layered structure.

7. A positive electrode active material for a sodium secondary battery, in, The invention comprises a sodium manganese-based oxide prepared by the method according to claim 1.

8. The positive electrode active material for sodium secondary battery according to claim 7, in, The sodium manganese-based oxide contains at least sodium (Na), nickel (Ni), manganese (Mn) and cobalt (Co), and the content of manganese in all metals (M) except sodium is 55 mol% or more, The sodium manganese-based oxide is a secondary particle formed by agglomeration of at least one primary particle and satisfies the following relational formula 1: [Equation 1] (Dc-Ds) / Dt≤±10% In the above relational expression 1, the area corresponding to 0 to 50% of the length (R) from the center to the surface based on the cross section of the secondary particle is defined as the first area (R 1 ), the area corresponding to 50 to 100% is defined as the second area (R 2 )hour, Ds, Dc and Dt are respectively in the first region (R 1 ), the second area (R 2 ) and the entire particle cross section (R 1 +R 2 ) are measured and are the molar concentrations of cobalt (Co) relative to all metals (M) except sodium.

9. The positive electrode active material for sodium secondary battery according to claim 7, in, The sodium manganese-based oxide is a secondary particle formed by agglomeration of at least one primary particle. The primary particles include plate-like primary particles having an aspect ratio of 1 to 5.

10. The positive electrode active material for sodium secondary battery according to claim 7, in, The sodium manganese-based oxide has an average particle size of 5 to 15 μm for the secondary particles.

11. The positive electrode active material for sodium secondary battery according to claim 7, in, The sodium manganese-based oxide is represented by the following chemical formula 2: [Chemical formula 2] So a Ni x Co y M1 z Mr 1-x-y-z O 2 In the chemical formula 2, M1 may be at least one selected from P, Sr, Ba, B, Ti, Mg, Zr, Al, W, Fe, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, Gd and Cu, and It can be 0.5≤a≤0.8, 0.05≤x≤0.45, 0.01≤y≤0.15, 0≤z≤0.05, 0.55≤1-xyz≤0.

85.

12. The positive electrode active material for sodium secondary battery according to claim 7, in, The sodium manganese-based oxide shows at least one peak selected from the group consisting of (004), (100), (101), (102), (103), (104) and (002) peaks through X-ray diffraction (XRD).

13. The positive electrode active material for sodium secondary battery according to claim 7, in, In the sodium manganese-based oxide, the NiO content measured by Rietveld refinement method based on X-ray diffraction analysis is 3% or less.

14. A positive electrode for a sodium secondary battery, in, Comprising the positive electrode active material according to claim 7.

15. A sodium secondary battery, in, Comprising the positive electrode according to claim 14.

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