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

Abstract

The present invention provides a method for producing a positive electrode active material for a sodium secondary battery, a positive electrode active material for a sodium secondary battery, and a sodium secondary battery, comprising the steps of: a) introducing 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 b) mixing the composite transition metal hydroxide precursor and a sodium compound on which the second cobalt compound has been formed on the surface of the particles, and then heat-treating the mixture to produce a sodium manganese oxide doped with cobalt (Co), wherein the sodium manganese oxide contains at least nickel (Ni), manganese (Mn), and cobalt (Co), and contains 55 mol% or more of manganese among the total metals other than sodium.

Description

[Technical Field]

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

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

[0003] Layered transition metal oxides, a typical form of positive electrode active material, have the advantages of a simple structure, excellent electrochemical performance, and ease of synthesis. They can be classified into O3 type and P2 type depending on the position of the sodium ions. Of these, positive electrode active materials with a P2 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, when sodium ions with a large ionic radius are inserted / deinserted within the layered structure, the lattice structure changes, causing the positive electrode active material to undergo continuous phase transitions. The irreversible phase generated during this process reduces the cycle life and high-rate characteristics of sodium-ion secondary batteries.

[0004] Among layered transition metal oxides, high-manganese sodium nickel manganese oxide (NNMO) has advantages over other transition metal oxides, such as high capacity, price competitiveness (superior in terms of embedded volume), and environmental friendliness. However, it has the disadvantage of relatively low electrical conductivity due to the excess manganese contained in the oxide, resulting in a low capacity rate for sodium secondary batteries. When the capacity rate is low, there is a problem of reduced charge / discharge capacity and capacity retention during the cycling of sodium secondary batteries.

[0005] To address the aforementioned problems, attempts have been made to improve the high-manganese oxide issue by modifying the particle structure and surface, such as adjusting the particle size of the positive electrode active material or coating the particle surface. However, these attempts have not yet reached a level suitable for commercialization. [Overview of the project] [Problems that the invention aims to solve]

[0006] In order to improve the decrease in electrical conductivity caused by the excess manganese in high-manganese oxides, it is preferable to produce ternary transition metal oxides that further contain transition metals other than manganese. However, when a coprecipitation reaction is performed to produce ternary transition metal hydroxide precursors, the high reactivity of manganese leads to the synthesis of a large amount of by-products, such as manganese oxide (MnO2, etc.), nickel oxide (NiO), and other transition metal oxides, resulting in a non-uniform metal composition within the oxide particles. Consequently, problems arise during battery charging and discharging, such as a decrease in energy density, an increase in electrolyte side reactions, and a decrease in high-rate stability.

[0007] Therefore, the present invention aims to provide a sodium manganese-based oxide in which the transition metal composition within the positive electrode active material particles is uniform by applying a Co coating to the surface of a transition metal hydroxide precursor under specific conditions.

[0008] Furthermore, the present invention aims to provide a sodium secondary battery with improved electrochemical properties (discharge capacity, initial efficiency, high-rate characteristics, lifespan characteristics, etc.) by suppressing the formation of heterogeneous phases (MnO2, NiO, NaCoO2) on the surface and inside the positive electrode material, synthesizing a high-purity P2-type layered sodium manganese oxide. [Means for solving the problem]

[0009] One embodiment of the present invention provides a method for producing a positive electrode active material for a sodium secondary battery, comprising the steps of: a) introducing 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 b) mixing the composite transition metal hydroxide precursor and a sodium compound on which the second cobalt compound has been formed on the surface of the particles, and then heat-treating the mixture to produce a sodium manganese oxide doped with cobalt (Co), wherein the sodium manganese oxide contains at least nickel (Ni), manganese (Mn), and cobalt (Co), and contains 55 mol% or more of manganese among the total metals other than sodium.

[0010] Step a) may include a1) adding the composite transition metal hydroxide precursor and solvent to a reactor and stirring, and a2) adding the first cobalt compound to the reactor and stirring to form cobalt hydroxide on the surface of the composite transition metal hydroxide precursor particles.

[0011] The composite transition metal hydroxide precursor in step a) above may contain manganese in an amount of 55 mol% or more of the total metal.

[0012] The first cobalt compound in step a) above may be Co(OH)2, CoOOH, Co(OCOCH3)2, Co(NO3)2, CoSO4, Co(SO4)2, or a combination thereof.

[0013] The heat treatment in step b) above can be carried out at 800 to 1,100°C for 5 to 40 hours.

[0014] The sodium manganese-based oxide in the step b) may have a P2-type layered structure.

[0015] Another embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, which contains a sodium manganese-based oxide produced by the method for producing a positive electrode active material for a sodium secondary battery.

[0016] The sodium manganese-based oxide contains at least sodium (Na), nickel (Ni), manganese (Mn), and cobalt (Co). Among the total metals (M) other than sodium, the content of manganese is 55 mol% or more. The sodium manganese-based oxide is secondary particles in which at least one primary particle is aggregated, and can satisfy the following relational expression 1. [Relational Expression 1] (Dc - Ds) / Dt ≤ ±10% In the relational expression 1, based on the cross-section of the secondary particles, a region corresponding to 0 to 50% of the length (R) from the center to the surface is defined as the first region (R1), and a region corresponding to 50 to 100% is defined as the second region (R2). Ds, Dc, and Dt are respectively measured in the first region (R1), the second region (R2), and the entire cross-section of the particle (R1 + R2), and are respectively the molar concentrations of cobalt (Co) with respect to the total metals (M) other than sodium.

[0017] The sodium manganese-based oxide is secondary particles in which at least one primary particle is aggregated, and the primary particles can include plate-like primary particles having an aspect ratio of 1 to 5.

[0018] The average particle size of the secondary particles of the sodium manganese-based oxide may be 5 to 15 μm.

[0019] The sodium manganese-based oxide can be represented by the following chemical formula 2. [Chemical Formula 2] Na a Ni x Co yM1 z Mn 1-x-y-z O2 In the above chemical formula 2, M1 is 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 the following inequalities apply: 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.

[0020] The sodium manganese oxide can be observed by X-ray diffraction analysis (XRD) to show at least one peak selected from the group consisting of (004), (100), (101), (102), (103), (104), and (002) peaks.

[0021] The sodium manganese oxide may have a NiO content of 3% or less, as measured by the Rietveld refinement method based on X-ray diffraction analysis.

[0022] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery containing the positive electrode active material.

[0023] Another embodiment of the present invention provides a sodium secondary battery including the positive electrode. [Effects of the Invention]

[0024] In this invention, a uniform Co compound is formed on the surface of the composite transition metal hydroxide precursor particles, enabling uniform Co doping in a subsequent process. This minimizes surface defects in the positive electrode active material particles, improves surface stability, and enhances electrochemical properties such as high-rate characteristics and charge / discharge efficiency.

[0025] In this invention, by using sodium (Na) instead of lithium (Li), which is an expensive metal, and increasing the proportion of manganese (Mn), which is relatively cheaper than nickel (Ni), it is possible to provide a high-Mn-based (NCM) cathode active material for sodium secondary batteries that has improved price competitiveness.

[0026] In this invention, surface side reactions caused by the electrolyte can be improved during precursor Co coating and Co doping into the interior of the positive electrode active material, and phase transitions (P2-O2) that occur in the high-voltage region during charging and discharging can be suppressed, thereby improving cycle life characteristics. [Brief explanation of the drawing]

[0027] [Figure 1] This is an SEM image of the surface of the positive electrode active material particles according to Example 1. [Figure 2a] This is an SEM / EDS image of the surface of the positive electrode active material secondary particles according to Example 1. [Figure 2b] This is an SEM / EDS image of a cross-section of the positive electrode active material secondary particles according to Example 1. [Figure 2c] This graph (EDS line scanning) shows the change in the content of metal elements (Ni, Co, Mn) as determined by EDS analysis in the direction represented by the SEM image of the cross-section of the positive electrode active material secondary particles manufactured in Example 1. [Figure 3a] This is an SEM / EDS image of the surface of the positive electrode active material secondary particles according to Example 2. [Figure 3b] This is an SEM / EDS image of a cross-section of the positive electrode active material secondary particles according to Example 2. [Figure 3c] This graph (EDS line scanning) shows the change in the content of metal elements (Ni, Co, Mn) as determined by EDS analysis in the direction represented by the SEM image of the cross-section of the positive electrode active material secondary particles manufactured in Example 2. [Figure 4] This is an SEM / EDS image of the surface of the positive electrode active material secondary particles manufactured in Comparative Example 2. [Figure 5]These are the XRD analysis results of the positive electrode active material (bulk) produced in Example 1 and Comparative Example 1. [Modes for carrying out the invention]

[0028] The advantages and features of the present invention, as well as methods for achieving them, will become clearer with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but can be realized in a variety of different forms, provided that these embodiments complete the disclosure of the present invention and fully inform those who are ordinary skill in the art to which the invention pertains, and the present invention is defined only by the scope of the claims.

[0029] Unless otherwise defined, all terms used herein (including technical and scientific terms) should be used in a way that is commonly understood by a person of ordinary skill in the art to which the invention pertains. Throughout the specification, when a part of the specification “includes” a component, this means, unless otherwise stated, that it may include other components rather than excluding them. Also, singular forms include plural forms unless otherwise specified in the text.

[0030] One embodiment of the present invention provides a method for producing a positive electrode active material for a sodium secondary battery. The production method comprises the steps of: a) introducing 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 b) mixing the composite transition metal hydroxide precursor and sodium compound on which the second cobalt compound has been formed on the surface of the particles, and then heat-treating the mixture to produce a sodium manganese oxide doped with cobalt (Co). The sodium manganese oxide is characterized in that it contains at least nickel (Ni), manganese (Mn), and cobalt (Co), and contains 55 mol% or more of manganese among the total metals other than sodium.

[0031] According to the manufacturing method of the present invention, a high-manganese NCM oxide having a uniform ternary transition metal composition can be synthesized by coating a binary transition metal hydroxide precursor with cobalt and then passing it through a calcination process. Conventionally, the problem of low electrical conductivity of high-manganese oxides can be improved by cobalt doping, and the problem of large amounts of manganese oxide (such as MnO2) being synthesized during the coprecipitation reaction due to the high reactivity of manganese in the production of ternary transition metal hydroxide precursors can be solved.

[0032] Step a) above is a step of forming a second cobalt compound on the surface of the composite transition metal hydroxide precursor particles, and specifically includes a1) a step of introducing the composite transition metal hydroxide precursor and solvent into a reactor and stirring, and a2) a step of introducing a first cobalt compound into the reactor and stirring to form cobalt hydroxide (Co(OH)2) on the surface of the composite transition metal hydroxide precursor particles.

[0033] In conventional dry coating processes or coating processes applied to the surface of positive electrode active materials, if heat treatment is performed at high temperatures after coating, island-like coating layers may form on the particle surface, making it difficult to form a uniform secondary cobalt compound overall on the particle surface. Furthermore, it may be difficult to expect a sufficient amount of doping into the interior of the particles while simultaneously achieving uniform cobalt doping. Such coating layer formation or non-uniform distribution of cobalt on the surface and inside the particles can cause surface defects, increasing the contact area with the electrolyte (specific surface area), and potentially leading to problems such as battery swelling due to side reactions of the electrolyte and a decrease in cycle life. In the present invention, since a uniform Co compound is formed overall on the surface of the composite transition metal hydroxide precursor particles, uniform Co doping can be performed in a subsequent process, minimizing surface defects in the positive electrode active material particles, improving surface stability, and enhancing electrochemical properties such as high-rate characteristics and charge / discharge efficiency.

[0034] a1) Step may be the step of adding the composite transition metal hydroxide precursor and solvent to the reactor and stirring.

[0035] The composite transition metal hydroxide precursor and the solvent may be mixed in the reactor in a weight ratio of 0.5:1.5 to 1.5:0.5, or 0.7:1.2 to 1.2:0.7, or, for example, 1:1.

[0036] A basic solution can be added to the reactor to adjust the pH to 10-12, a small amount of ammonium solution can be used as a complexing agent, and the stirring speed may be 200-500 rpm, 200-400 rpm, or 300-400 rpm, but the present invention is not limited thereto. This allows the second cobalt compound to be uniformly formed on the surface of the composite transition metal hydroxide precursor particles in the subsequent step a2).

[0037] a2) Step may also be the step of adding the first cobalt compound to the reactor and stirring to form cobalt hydroxide (Co(OH)2) on the surface of the composite transition metal hydroxide precursor.

[0038] The first cobalt compound may be fed into the reactor so as to be mixed with the combined transition metal hydroxide precursor and the solvent in a weight ratio of 5:1 to 1.1:1, 4.5:1 to 1.5:1, preferably 4:1 to 1.5:1, or 3:1 to 1.5:1. Furthermore, the first cobalt compound may be added to the reactor such that the cobalt (Co) content ratio (Co / M) relative to the total metal (M) of 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. Here, the first cobalt compound can be added during the process for 0.5 to 5 hours, 0.5 to 4 hours, 0.5 to 3 hours, or 1 to 3 hours, such that the Co / M range is satisfied, by adding an aqueous solution of the first cobalt compound with a concentration of 1.5 to 2.5 M. More specifically, the addition of the aqueous solution of the first cobalt compound can be carried out at a rate of 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 conditions for introducing the first cobalt compound are exceeded, the cobalt aggregation phenomenon may prevent the secondary or primary oxide particles from growing into a uniform shape during the firing of the positive electrode active material.

[0039] If the input rate exceeds the aforementioned rate, it becomes difficult to uniformly form cobalt hydroxide on the surface of the composite transition metal hydroxide precursor particles. Conversely, if the input rate is less than the aforementioned rate, the cost increases as the process time lengthens, and Ni elution may occur inside the transition metal hydroxide precursor particles.

[0040] Step a) is carried out in an inert atmosphere such as nitrogen or argon at 40-70°C or 4 The reaction can be carried out at a temperature of 5 to 65°C, and the reaction time may be 60 to 90 minutes, or for example, 75 to 105 minutes. This can further improve the effects described above.

[0041] The aforementioned complex transition metal hydroxide precursor can be represented by the following chemical formula 1.

[0042] [Chemical formula 1] Ni x Mn 1-x (OH)2

[0043] In the above chemical formula 1, 0.05 ≤ x ≤ 0.45 and 0.55 ≤ 1-x ≤ 0.95 may also be used.

[0044] The nickel-manganese hydroxide precursor may have a molar ratio (Mn / M) of manganese (Mn) to total metal (M) of 0.55 to 0.95. When the manganese content is within the specified range, high capacity can be achieved under high-voltage operating conditions, ensuring price competitiveness. The Mn content may more preferably be 0.60 ≤ 1-x ≤ 0.85, 0.60 ≤ 1-x ≤ 0.80, or 0.60 ≤ 1-x ≤ 0.70 in chemical formula 1.

[0045] The nickel-manganese hydroxide precursor may have a molar ratio of nickel (Ni) to total metal (M) of 0.05 to 0.45 (Ni / M). Within the range of nickel content, it is possible to improve the problem of decreased structural and chemical stability of the active material due to changes in the oxidation state of nickel, which occurs as the nickel content increases. The Ni content may more preferably be 0.1 ≤ x ≤ 0.45, 0.2 ≤ x ≤ 0.45, 0.25 ≤ x ≤ 0.45, or 0.3 ≤ x ≤ 0.4 in chemical formula 1.

[0046] The solvent may be one or more selected from deionized water, distilled water, and ethanol, and preferably deionized water. 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.

[0047] The basic solution may be an alkali metal or alkaline earth metal hydroxide such as NaOH, KOH, or Ca(OH)2, a hydroxide thereof, or a combination thereof, and may be added to adjust the pH of the solution in the reactor.

[0048] The ammonium solution may contain NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, NH4CO3, or a combination thereof, and as a complexing agent, it can effectively co-precipitate the first cobalt compound in the reactor onto the surface of the composite transition metal hydroxide particles.

[0049] The first cobalt compound may be a solution containing a cobalt-containing compound, and can be produced by adding the first cobalt compound to water or a mixed solvent of an organic solvent such as an alcohol that can be homogeneously mixed with water. The first cobalt compound may be Co(OH)2, CoOOH, Co(OCOCH3)2, Co(NO3)2, CoSO4, Co(SO4)2, or a combination thereof, preferably CoSO4 and / or Co(SO4)2. Here, the first cobalt compound may include hydroxides and acid anhydrides of the above-mentioned substances. When CoSO4·7H2O or CoSO4·acid anhydride, preferably CoSO4·7H2O is used, it reacts easily with NaOH, and the second cobalt compound (Co(OH)2) can be uniformly formed on the surface of the precursor, and then Na During the source and firing process, cobalt can be uniformly doped. On the other hand, C When doping with metals other than o, for example, when using an Fe-containing compound as a doping source, the doping metal may be concentrated on only some of the secondary particles, resulting in a non-uniform distribution of Fe on the surface of the positive electrode active material particles. Furthermore, the Fe-containing compound may oxidize and dissolve during the feeding process to coat the surface of the hydroxide precursor particles.

[0050] The reactor can be any reactor used for coprecipitation reactions, which are generally used in the production of hydroxide precursors for positive electrode active materials, without any limitations.

[0051] The manufacturing method of the present invention may further include a drying step after step a). The drying step is a step for removing moisture from the composite transition metal hydroxide precursor on which the second cobalt compound produced in step a) is formed, and can be carried out under vacuum conditions and at a temperature of 100 to 300°C for 12 hours or more.

[0052] Step b) is a step in which the second cobalt compound is mixed with a sodium compound and a composite transition metal hydroxide precursor on which the particle surface is formed, in order to produce a cobalt-doped sodium manganese oxide, and then heat-treated.

[0053] The mixing of the composite transition metal hydroxide precursor and the sodium compound may be done in such a way that the transition metal:sodium ratio of the precursor is 1:0.5 to 1:0.8 molar ratio, 1:0.6 to 1:0.8 molar ratio, 1:0.6 to 1:0.75 molar ratio, or 1:0.65 to 1:0.75 molar ratio. When the amount of sodium compound mixed is within the above range, the resulting positive electrode active material may have a P2-type layered crystalline structure, which allows for high atmospheric and moisture stability and less sensitivity to synthesis conditions (temperature and atmosphere, etc.). Furthermore, within the above sodium content range, the battery discharge capacity can be improved and the amount of unreacted residual Na can be minimized.

[0054] The heat treatment can be carried out at a temperature of 800°C to 1,100°C. When the firing temperature is within this range, the reaction between the raw materials can proceed sufficiently, and the particles can grow uniformly. The heat treatment can more preferably be carried out at a temperature of 800 to 1,050°C, 850 to 1,050°C, or 900 to 1,000°C. The heat treatment can be carried out for 5 to 40 hours. When the firing time is within this range, a highly crystalline positive electrode active material can be obtained, the particle size can be appropriate, and production efficiency can be improved. The heat treatment can more preferably be carried out for 5 to 20 hours, 5 to 18 hours, 8 to 15 hours, or 10 to 14 hours.

[0055] The sodium compound may be at least one selected from the group consisting of Na2CO3, NaOH, NaNO3, CH3COONa, and Na2(COO)2, and preferably Na2CO3, NaOH, or a combination thereof.

[0056] The sodium manganese oxide may be uniformly doped with cobalt on the surface and inside of the particles, or it may have a P2-type layered structure. Even if transition metal compound coating and doping are performed by the same method as the manufacturing method, in the case of elements other than cobalt (Co), such as iron (Fe), titanium (Ti), and magnesium (Mg), the doping metal may be concentrated on the surface of the oxide secondary particles, forming a coating layer, or the transition metal coating / doping may be performed unevenly, such as showing a concentration gradient that continuously increases or decreases inside the oxide secondary particles. In other words, in the present invention, sodium manganese oxide particles can form a P2-type layered structure, and a ternary transition metal oxide with a uniform composition can be synthesized.

[0057] The manufacturing method of the present invention may further include a water washing step and a drying step after step b) above. The water washing step is a step for removing unreacted substances, impurities and residual sodium, and the positive electrode active material produced in step b) above is placed in a reactor into which one or more selected from deionized water, distilled water and ethanol is introduced, and the material can be washed with water at a temperature of 1 to 80°C or 5 to 50°C at a stirring speed of 200 to 500 rpm, 200 to 400 rpm, or 300 to 400 rpm for 0.5 to 5 hours, 0.5 to 4 hours, 0.5 to 3 hours, or 0.5 to 1.5 hours. The drying step is a step for removing moisture from the positive electrode active material that has contained moisture after the water washing step, and the material can be dried under vacuum conditions at a temperature of 100 to 300°C for 12 hours or more.

[0058] Another embodiment of the present invention provides a positive electrode active material for a sodium secondary battery comprising a sodium manganese oxide produced by the method described above. The sodium manganese oxide comprises at least sodium (Na), nickel (Ni), manganese (Mn), and cobalt (Co), and the manganese content of the total metals (M) other than sodium may be 55 mol% or more.

[0059] The sodium-manganese oxide is an NCM-based sodium-nickel-cobalt-manganese oxide containing at least sodium, nickel, cobalt, and manganese. The sodium-manganese oxide is a high-manganese (high-Mn) oxide containing 55 mol% or more of manganese among the total metals other than sodium. The manganese content among the metals other than sodium may be 55 mol% or more, 60 mol% or more, or 65 mol% or more, and there is no particular upper limit, but for example it may be 85 mol% or less, 80 mol% or less, or 75 mol% or less. The higher the manganese content among the metals other than sodium, the more advantageous it is to be able to exhibit high capacity under high-voltage operating environments, and the concentration of both nickel and cobalt can be reduced to enhance price competitiveness.

[0060] On the other hand, as mentioned above, when doping binary transition metal hydroxide precursor particles with Co coating is performed, the generation of manganese oxide (such as MnO2), a by-product that is synthesized in large quantities and is a problem in the production of high-manganese ternary hydroxide precursors, can be suppressed. Furthermore, compared to the technology in which Co is concentrated and distributed only on the surface of the oxide particles to form a coating layer, the present invention can further improve the above-mentioned effects by uniformly doping the interior and surface of the oxide secondary particles with Co. Moreover, when Co, an electrochemically active metal, is doped, it not only prevents surface degradation of the oxide particles but also participates in oxidation-reduction reactions during charging / discharging of sodium secondary batteries, thereby increasing the discharge capacity compared to coating / doping with other metals.

[0061] The sodium manganese oxide is a secondary particle formed by the aggregation of at least one primary particle, and can satisfy the following relational equation 1.

[0062] [Relationship 1] (Dc-Ds) / Dt ≤ ±10%

[0063] In relational equation 1, if we define the region corresponding to 0-50% of the length (R) from the center to the surface as the first region (R1) and the region corresponding to 50-100% as the second region (R2), based on the cross-section of the secondary particle, then Ds, Dc, and Dt are measured in the first region (R1), the second region (R2), and the entire cross-section of the particle (R1+R2), respectively, and may each be the molar concentration of cobalt (Co) relative to the total metal (M) other than sodium.

[0064] The sodium manganese oxides mentioned above are, for example, (Dc-Ds) / Dt ≤ ±9%, (Dc-Ds) / Dt ≤ ±8%, (Dc-Ds) / Dt ≤ ±7%, and (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 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 may also 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 cobalt concentration distribution within the sodium manganese oxide particles satisfies the design range, it can be understood that the Co element is uniformly distributed throughout with respect to the cross-section of the secondary particles, forming a ternary transition metal oxide. As a result, the present invention suppresses the formation of heterogeneous phases (MnO2, CoO2) on the surface and inside the positive electrode material, and by synthesizing a high-purity P2-type layered sodium manganese oxide, it is possible to provide a sodium secondary battery with improved electrochemical properties (discharge capacity, initial efficiency, high-rate characteristics, lifespan characteristics, etc.).

[0065] Here, the first region (R1) may be a region corresponding to 0-50%, 0-40%, 0-30%, 0-20%, 0-10%, 30-50%, 40-50%, or 20-30% of the length (100%) from the center of the secondary particle to the surface, based on the cross-section of the secondary particle, and the second region (R2) may be a region corresponding to 50-100%, 60-100%, 70-100%, 80-100%, 90-100%, 50-70%, 50-60%, or 70-80% of the length (100%) from the center of the particle to the surface.

[0066] On the one hand, the concentration of cobalt (Co) can be measured and calculated by X-ray photoelectron spectroscopy or SEM-EDS, TEM-EDS, but the present invention is not limited thereto.

[0067] The sodium manganese oxide can be represented by the following Chemical Formula 2.

[0068] [Chemical Formula 2] Na a Ni x Co y M1 z Mn 1-x-y-z O2

[0069] In 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 0.5 ≦ a ≦ 0.8, 0.05 ≦ x ≦ 0.45, 0.01 ≦ y ≦ 0.15, 0.55 ≦ 1 - x - y - z ≦ 0.85 may also be satisfied.

[0070] In the sodium manganese oxide of Chemical Formula 2, the molar ratio of sodium (Na) to the total metal (M) other than sodium (Na / M) may be 0.5 to 0.8. In Chemical Formula 2, when the content corresponding to a of Na is less than 0.5, the capacity may decrease; when it exceeds 0.8, the position of sodium ions may change, and an O3-type crystal structure can be shown. The O3-type cathode active material may be more sensitive to low air and moisture stability and synthesis conditions (such as temperature and atmosphere) compared to the P2-type cathode active material. The Na may more preferably be 0.60 ≦ a ≦ 0.80, 0.60 ≦ a ≦ 0.75, or 0.65 ≦ a ≦ 0.75.

[0071] In the sodium manganese oxide, the molar ratio of manganese (Mn) to the total metal (M) other than sodium (Mn / M) may be 0.55 to 0.85. When the content range containing manganese is satisfied, a high capacity can be achieved in a high-voltage operating environment, and the valence This ensures competitive pricing. The Mn content may more preferably be 0.60 ≤ 1-xyz ≤ 0.85, 0.60 ≤ 1-xyz ≤ 0.80, or 0.60 ≤ 1-xyz ≤ 0.70 in chemical formula 2.

[0072] The manganese-based sodium composite transition metal oxide may have a molar ratio of nickel (Ni) to the total metal (M) other than sodium (Ni / M) of 0.05 to 0.45. This can suppress the problem that arises as the nickel content increases within the range of nickel content, namely, the problem of decreased structural and chemical stability of the active material due to changes in the oxidation state of nickel. The Ni content may more preferably be 0.1 ≤ x ≤ 0.45, 0.2 ≤ x ≤ 0.45, 0.25 ≤ x ≤ 0.45, or 0.3 ≤ x ≤ 0.4 in chemical formula 2.

[0073] Furthermore, the sodium transition metal oxide may have a molar ratio of nickel to manganese (Ni / Mn) of 0.05-0.75, 0.1-0.7, 0.2-0.7, 0.3-0.6, or 0.4-0.6. Generally, a higher Mn content results in higher capacity, but early degradation may occur in the cell lifespan, and a higher Ni content improves lifespan. However, since Mn is more cost-effective than Ni, in sodium-ion secondary batteries, considering the cost, electrochemical properties can be improved by doping with cobalt (Co) at a high Mn content.

[0074] The sodium manganese oxide contains secondary particles formed by the aggregation of at least one primary particle. The primary particles may include plate-type particles, where the length in the thickness direction (c-axis) is relatively smaller than the lengths of the major axis (a-axis) and minor axis (b-axis) in the plane direction of the primary particle. Furthermore, the plane containing the major and minor axes of the primary particle may be rectangular, elliptical, hexagonal, or amorphous, with the major or minor axes being different from each other, or it may have the same circular or square shape, but the present invention is not limited thereto.

[0075] The sodium manganese oxide may have an aspect ratio of 1 to 5, 1 to 4, 2 to 5, or 2 to 4 for the primary particles. In one embodiment, the sodium manganese oxide may have an average particle size of 5 to 15 μm for the secondary particles. Also, the average length of the major axis of the primary particles may be 1 to 3.5 μm. By satisfying the above conditions, the primary and secondary particles contained in the positive electrode active material can improve the particle density (tap density) within the positive electrode active material. This can improve the electrochemical properties of the positive electrode active material.

[0076] Herein, the present invention allows 50% or more, for example 60% or 70% or more of the total number of primary particles constituting the secondary particles to have an aspect ratio, major axis length and / or minor axis length within the above-mentioned range, or at least 10 primary particles among the primary particles constituting the secondary particles to have an aspect ratio, major axis length and / or minor axis length within the above-mentioned range.

[0077] The sodium manganese oxide can exhibit at least one peak selected from the group consisting of (004), (100), (101), (102), (103), (104), and (002) peaks by X-ray diffraction analysis (XRD), preferably all of the (004), (100), (101), (102), (103), (104), and (002) peaks. These peaks form a P2-type layered structure, and a technical feature of the present invention is that the sodium manganese oxide stably forms and maintains a P2-type layered structure even after being doped with Fe. As described above, the P2-type layer Cathode active materials with a crystalline structure have the advantages of relatively high atmospheric and moisture stability, and being less sensitive to synthesis conditions such as temperature and atmosphere.

[0078] The sodium manganese oxide may have a NiO ratio of 3% or less, measured by the Rietveld refinement method based on X-ray diffraction analysis, for example, 2.9% or less, 2.7% or less, 2.5% or less, or 2.2% or less. This suppresses the generation of oxide by-products such as NiO and makes it possible to produce a transition metal oxide with a uniform composition. The NiO ratio can be measured by the Rietveld refinement method based on X-ray diffraction analysis, but the present invention is not limited to this.

[0079] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery and a sodium secondary battery, both containing the positive electrode active material.

[0080] The positive electrode includes a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector, and the positive electrode active material according to one aspect of the present invention is present in the positive electrode active material layer.

[0081] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with surface treatment using carbon, nickel, titanium, silver, etc., can be used. The positive electrode current collector can usually have a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. Such positive electrode current collectors can be provided in various forms, such as films, sheets, foils, meshes, porous materials, foams, and nonwoven fabrics.

[0082] Furthermore, the positive electrode active material layer may also contain a conductive material and a binder along with the positive electrode active material described above.

[0083] Here, the conductive material is used to impart conductivity to the electrode and can be used without particular limitations as long as it does not cause a chemical change in the positive electrode active material and is conductive. Non-restrictive examples of conductive materials include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. The conductive material can usually be included in an amount of 1% to 30% by weight relative to the total weight of the positive electrode active material layer.

[0084] Furthermore, the binder is a substance that improves the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Non-limiting examples of binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. The binder can usually be present in an amount of 1% to 30% by weight relative to the total weight of the positive electrode active material layer.

[0085] The positive electrode according to one embodiment of the present invention uses the above-mentioned positive electrode active material, except that it is a conventional sodium Positive electrodes for secondary batteries can be manufactured by a method for manufacturing positive electrodes. For example, a positive electrode can be manufactured by applying a slurry for forming a positive electrode active material layer, which selectively contains a positive electrode active material, a binder, and a conductive material, onto a positive electrode current collector, followed by drying and rolling. In another example, a positive electrode can be manufactured by casting a slurry for forming a positive electrode active material layer onto another support, peeling the positive electrode active material layer from the support, and then laminating the resulting film onto a positive electrode current collector.

[0086] In yet another aspect of the present invention, an electrochemical element including the above-described positive electrode is provided. Here, the electrochemical element may be a battery, a capacitor, and more specifically, a sodium secondary battery.

[0087] A sodium-based secondary battery includes a positive electrode, a negative electrode positioned opposite the positive electrode, a separator interposed between the positive and negative electrodes, and an electrolyte. The sodium-based secondary battery may also include a battery container (case) for housing the electrode assembly, including the positive electrode, negative electrode, and separator, and a sealing member for sealing the battery container.

[0088] Here, depending on the shape of the battery container (case), sodium secondary batteries can be divided into can-type sodium secondary batteries, in which the electrode assembly is housed in a metal can, and pouch-type sodium secondary batteries, in which the electrode assembly is housed in a pouch made of a sheet such as aluminum laminate.

[0089] In particular, in the case of a pouch-type sodium secondary battery using a positive electrode containing a positive electrode active material according to various embodiments of the present invention, there is a low possibility of side reactions between the positive electrode active material and the electrolyte, which has the advantage of improving stability during storage and / or operation and reducing gas generation.

[0090] The present invention will be described in detail below with reference to examples, but these are for the purpose of explaining the present invention in more detail and the scope of the rights of the present invention is not limited by the following examples.

[0091] Examples Manufacturing Example 1. Manufacturing of positive electrode active material Example 1 (a) Production of precursors In the reactor, a mixed aqueous solution of NiSO4·6H2O and MnSO4·H2O in a molar ratio of 35:65 was added while stirring, along with NaOH and NH4OH. The temperature in the reactor was maintained at 45°C, and after the reaction was complete, the mixture was washed and dehydrated to obtain Ni with an average particle size of 10 μm. 0.35 Mn 0.65 (OH)2 precursor was obtained.

[0092] (b) Precursor coating NaOH was added to the reactor containing the precursor obtained in step (a) while it was being stirred. Then, an aqueous solution of CoSO4·7H2O was added to the stirred reactor at a rate of 2.5 ml / min for 60 minutes. Here, the CoSO4·7H2O was weighed so that the Co / (Ni+Co+Mn) ratio was 10 mol% before being added. After the reaction was complete, the precursor was filtered, washed and dehydrated, and then dried at 110°C for 12 hours to obtain the coated precursor.

[0093] (c) Heat treatment The coating precursor obtained in step (b) was mixed with NaOH (Na / (Na-excluded metal) mol ratio = 0.67) as the sodium compound to prepare the mixture.

[0094] Next, the furnace in an air atmosphere was heated at a rate of 4°C / min, then maintained at 950°C, and the mixture was heat-treated for 12 hours. After furnace cooling, the mixture was finally obtained as sodium manganese oxide (Na) with an average particle size of 7-10 μm. 0.67 Ni 0.315 Co 0.1 Mn 0.585 We obtained a positive electrode active material containing O2.

[0095] Referring to the SEM image in Figure 1, the obtained sodium manganese oxides had a ratio of the length of the major axis to the minor axis of the primary particles (La / Lb) of 2 to 5.

[0096] Example 2 (b) In the precursor coating step, CoSO4·7H2O was added in the same manner as in Example 1, except that the ratio of Co / (Ni+Co+Mn) was 5 mol%, to obtain a sodium manganese oxide Na with an average particle size of 7-10 μm. 0.67 Ni 0.3325 Co 0.05 Mn 0.6175 We obtained a positive electrode active material containing O2.

[0097] The sodium manganese oxides obtained here had a ratio of the length of the major axis to the minor axis of the primary particles (La / Lb) of 1 to 4.

[0098] Comparative Example 1 (b) The procedure is the same as in Example 1, except that the precursor coating step is omitted, and sodium manganese oxide Na has an average particle size of 7-10 μm. 0.67 Ni 0.35 Mn 0.65 We obtained a positive electrode active material containing O2.

[0099] Comparative Example 2 (b) The same procedure as in Example 1 was followed, except that the precursor coating step was performed as described below, to obtain a sodium manganese oxide with an average particle size of 7-10 μm. 0.67 Ni 0.315 Fe 0.1 Mn 0.585 We obtained a positive electrode active material containing O2.

[0100] (b) Precursor coating NaOH and NH4OH were added to the reactor in which the precursor obtained in step (a) was being stirred. Subsequently, an aqueous solution of FeSO4·7H2O was added to the stirred reactor at a rate of 2.5 cc / min for 60 minutes. Here, the FeSO4·7H2O was weighed to a concentration of 10 mol% before being added. After the reaction was complete, the mixture was filtered, washed, and dehydrated, then dried at 110°C for 12 hours to obtain the coated precursor.

[0101] Comparative Example 3 400g of DIW was added to a 5L batch-type reactor, followed by 23.9g of 1M NaOH solution. The mixture was then maintained at 25°C with a stirring speed of 300 rpm for 15 minutes until the pH reached 12.8. Subsequently, Na 0.67 Ni 0.35 Mn 0.65 O2 (the positive electrode active material of Comparative Example 1) was added and stirred at 300 rpm.

[0102] Subsequently, a 1.7 M cobalt sulfate aqueous solution was added at a rate of 51.18 ml / hr, adjusting the rate, and reacted for 60 minutes while maintaining a pH of 11.5-12 to form cobalt hydroxide on the surface of the sodium manganese oxide particles. The obtained particles were separated and dried in a vacuum oven at 110°C for 24 hours. Then, the particle powder was placed in an alumina crucible and heat-treated in an air atmosphere at 600°C for 12 hours, followed by Na 0.67 Ni 0.315 Co 0.1 Mn 0.585 A positive electrode active material containing O2 was manufactured.

[0103] Comparative Example 4 Except for the heat treatment for cobalt doping being performed at 700°C in Comparative Example 3, the procedure was carried out in the same manner as in Comparative Example 3, and Na 0.67 Ni 0.315 Co 0.1 Mn 0.585 O2-containing positive electrode active material Quality was manufactured.

[0104] Manufacturing Example 2: Manufacturing of Sodium Rechargeable Batteries A positive electrode slurry was prepared by dispersing 90 wt% of each of the positive electrode active materials produced by Production 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 onto a 15 μm thick aluminum thin film and vacuum-dried at 135°C to produce a positive electrode for a sodium secondary battery.

[0105] A coin cell was manufactured using a sodium foil as the counter electrode for the positive electrode, a porous polyethylene film (Celgard 2300, thickness: 25 μm) as the separator, and an electrolyte containing ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:7 with NaPF6 present at a concentration of 1.15 M.

[0106] Experimental example Experimental Example 1. Confirmation of the uniformity of the distribution of metallic elements on the surface and cross-section of positive electrode active material particles. SEM / EDS analysis was performed to confirm changes in the content of metallic elements. SEM / EDS images of sodium manganese oxide were obtained after cross-sectional treatment with FIB.

[0107] Figures 2a and 2b are SEM / EDS images of the surface (Figure 2a) and cross-section (Figure 2b) of the positive electrode active material secondary particles according to Example 1, respectively.

[0108] Figure 2c is a graph (EDS line scanning) showing the change in the content of metal elements (Ni, Co, Mn) as determined by EDS analysis in the direction represented by the SEM image of the cross-section of the positive electrode active material secondary particles manufactured in Example 1.

[0109] Figures 3a and 3b are SEM / EDS images of the surface (Figure 3a) and cross-section (Figure 3b) of the positive electrode active material secondary particles according to Example 2, respectively.

[0110] Figure 3c is a graph (EDS line scanning) showing the change in the content of metal elements (Ni, Co, Mn) as determined by EDS analysis in the direction represented by the SEM image of the cross-section of the positive electrode active material secondary particles manufactured in Example 2.

[0111] Figure 4 shows an SEM / EDS image of the surface of the positive electrode active material secondary particles produced in Comparative Example 2.

[0112] Table 1 below shows the concentrations (mol%) of Ni, Co, Fe, and Mn in the central and surface regions of the positive electrode active material secondary particles. The concentrations (mol%) of Ni, Co, Fe, and Mn in the central and surface regions of the secondary particles were calculated from line EDS spectra, while the concentrations of Ni, Co, Fe, and Mn within the whole particle (bulk) were calculated by ICP analysis.

[0113] [Table 1]

[0114] (In Table 1, the region corresponding to 0-50% of the length (R) from the center to the surface is defined as the central region, and the region corresponding to 50-100% is defined as the surface region, based on the cross-section of the positive electrode active material secondary particles.)

[0115] In the case of the Co-doped cathode active materials produced in Examples 1 and 2, uniform cobalt doping was confirmed on the surface and inside of the secondary particles, confirming that the doping technology of the present invention can produce transition metal ternary oxides with a uniform composition.

[0116] Referring to Figure 4, in the case of the Fe-doped positive electrode active material produced in Comparative Example 2, Fe aggregation was observed, and the doping uniformity was significantly lower compared to Examples 1 and 2. Specifically, it was confirmed that the Fe was concentrated on the surface of the secondary particles of the positive electrode active material, forming an island-shaped coating layer, or that some of the secondary particles were selectively over-coated / doped. This confirmed that Fe doping is not easy when applying the manufacturing method of the present invention.

[0117] Experimental Example 2. Confirmation of P2-type layered structure and heterogeneous phases by XRD analysis. Figure 5 shows the XRD analysis results of the positive electrode active material (bulk) produced in Example 1 and Comparative Example 1. Table 2 below shows the ratio of NiO (heterogeneous phase) formed within the positive electrode active material, measured using the TOPAS program.

[0118] [Table 2]

[0119] Referring to Figure 5 and Table 2, XRD analysis of the positive electrode active material produced in Example 1 revealed peaks at (004), (100), (101), (102), (103), (104), and (002), confirming that it possesses a P2-type layered structure. Furthermore, referring to Figures 2b and 2c, it was confirmed that in Examples 1 and 2, although a small amount of NiO was synthesized during Co doping, heterogeneous phases related to Co and Mn (such as CoO2 and MnO2) were not formed.

[0120] Experimental Example 3. Evaluation of the electrochemical properties of a sodium secondary battery. For the sodium secondary battery (coin cell) manufactured in Manufacturing Example 2, charge-discharge experiments were conducted using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, with a voltage range of 2.0V to 4.6V and a discharge rate of 0.1C to 5.0C. The initial charge capacity, initial discharge capacity, initial reversibility efficiency, and rate characteristics (ratio of discharge capacity; rate capability (C-rate)) were measured.

[0121] Furthermore, for the same sodium secondary battery, after performing 50 charge / discharge cycles under 1C / 1C conditions at 25°C and within a driving voltage range of 2.0V to 4.6V, the ratio of the discharged capacity at the 50th cycle to the initial capacity (cycle capacity retention) was measured.

[0122] The measurement results are shown in Table 3 below.

[0123] [Table 3]

[0124] Referring to Table 3, it was confirmed that the sodium secondary batteries manufactured in Examples 1 and 2 showed improved initial discharge capacity, rate characteristics, and charge / discharge efficiency compared to Comparative Examples 1 and 2, while maintaining a cycle life at the same level as Comparative Example 1. In the case of Comparative Example 2, it was confirmed that the presence of Fe coating between the primary particles increased the side reactions of the electrolyte, resulting in an overall deterioration of the cell characteristics.

[0125] As described above, the present invention has been illustrated and explained with respect to specific embodiments, but it will be obvious to those of ordinary skill in the art that the present invention can be improved and modified in various ways without departing from the technical spirit of the invention provided by the following claims.

Claims

1. a) A step of introducing 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, b) The process includes the step of mixing the second cobalt compound with a composite transition metal hydroxide precursor formed on the surface of the particles and a sodium compound, and then heat-treating the mixture to produce a sodium manganese oxide doped with cobalt (Co), A method for producing a positive electrode active material for a sodium secondary battery, wherein the sodium manganese oxide contains at least nickel (Ni), manganese (Mn), and cobalt (Co), and contains 55 mol% or more of manganese among the total metals other than sodium.

2. Step a) above is, a1) The step of adding the composite transition metal hydroxide precursor and solvent to the reactor and stirring, a2) A method for producing a positive electrode active material for a sodium secondary battery according to claim 1, comprising the step of introducing 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 producing a positive electrode active material for a sodium secondary battery according to claim 1, wherein the composite transition metal hydroxide precursor in step a) contains 55 mol% or more of manganese from the total metal.

4. The first cobalt compound in step a) above is Co(OH) ２ , CoOOH, Co(OCOCH ３ ) ２ Co(NO ３ ) ２ CoSO ４ Co(SO ４ ) ２ A method for producing a positive electrode active material for a sodium secondary battery according to claim 1, or a combination thereof.

5. The method for producing a positive electrode active material for a sodium secondary battery according to claim 1, wherein the heat treatment in step b) is performed at 800 to 1,100°C for 5 to 40 hours.

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

7. A positive electrode active material for a sodium secondary battery, comprising a sodium manganese oxide produced by the method described in claim 1.

8. The aforementioned sodium-manganese oxide contains at least sodium (Na), nickel (Ni), manganese (Mn), and cobalt (Co), and the manganese content among the total metals (M) other than sodium is 55 mol% or more. The sodium manganese oxide is a secondary particle formed by the aggregation of at least one primary particle, and satisfies the following relational expression 1, as described in claim 7, for a positive electrode active material for a sodium secondary battery. [Relationship 1] (Dc-Ds) / Dt≦±10% In the relational expression 1, based on the cross-section of the secondary particle, a region corresponding to 0 to 50% of the length (R) from the center to the surface is defined as the first region (R １ ), and a region corresponding to 50 to 100% is defined as the second region (R ２ ), then Ds, Dc, and Dt are each in the first region (R １ ), second region (R ２ ) and the entire cross-section of the particle (R １ +R ２ These values ​​were measured using the ) method and represent the molar concentration of cobalt (Co) relative to the total metal (M) other than sodium.

9. The sodium manganese oxide is a secondary particle formed by the aggregation of at least one primary particle. can be, The positive electrode active material for a sodium secondary battery according to claim 7, wherein the primary particles include plate-shaped primary particles having an aspect ratio of 1 to 5.

10. The sodium manganese oxide is a positive electrode active material for a sodium secondary battery according to claim 7, wherein the average particle size of the secondary particles is 5 to 15 μm.

11. The sodium manganese oxide is represented by the following chemical formula 2, and is the positive electrode active material for a sodium secondary battery according to claim 7. [Chemical formula 2] Na ａ Ni ｘ Co ｙ M1 ｚ Mn １－ｘ－ｙ－ｚ O ２ In the aforementioned chemical formula 2, M1 is 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. 0.5 ≤ a ≤ 0.8, 0.05 ≤ x ≤ 0.45, 0.01 ≤ y ≤ 0.15, 0 ≤ z ≤ 0.05, and 0.55 ≤ 1 - x - y - z ≤ 0.

85.

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

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

14. A positive electrode for a sodium secondary battery, comprising the positive electrode active material described in claim 7.

15. A sodium secondary battery comprising the positive electrode described in claim 14.

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