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

Abstract

The present invention provides a positive electrode active material for a sodium secondary battery, a method for preparing the same, and a sodium secondary battery comprising the same, the method for preparing the positive electrode active material for a sodium secondary battery comprising: a step a) of adding a sodium composite transition metal oxide and a washing solution to a reactor and stirring the reactants to obtain a reaction solution; the residual sodium on the surface of the sodium composite transition metal oxide is dissolved in the washing liquid; a step b) of adding a cobalt salt to the reactor while stirring, and co-precipitating a cobalt hydroxide into the sodium composite transition metal oxide particles; and c) forming a cobalt coating layer on the particles by heat-treating the particles of the sodium composite transition metal oxide on which the cobalt hydroxide has been formed.

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 are widely used as energy storage devices in various electronic technology fields. Recently, as the demand for lithium-ion secondary batteries has increased dramatically, sodium-ion secondary batteries have attracted much attention as a substitute for expensive metal lithium. Sodium-ion secondary batteries have a similar working principle of insertion / extraction reaction as lithium-ion secondary batteries, and therefore are one of the new generation materials with high potential for application in secondary batteries.

[0003] As the positive electrode active material of the sodium ion secondary battery, a layered transition metal oxide with a simple structure, excellent electrochemical performance and easy synthesis is typically used. In addition, when preparing the positive electrode active material, during the sintering process, the residual amount of sodium byproducts in the form of Na2CO3 and NaOH on the particle surface increases, thereby reducing the battery life and stability during battery operation, such as the generation of gas due to electrolyte side reactions, and reducing the capacity and power of the positive electrode material.

[0004] In order to remove the sodium byproducts remaining on the surface of the positive electrode active material, a water washing process is essential. However, if a water washing process is performed to remove the sodium byproducts, the life characteristics are sharply reduced due to the surface defects of the positive electrode active material. Therefore, after water washing, the surface of the particles of the positive electrode active material is coated. However, in the existing coating process, the coating portion is formed in an island shape on the particle surface of the positive electrode active material, and it is difficult to form a coating uniformly on the entire particle surface. In addition, the uneven coating acts as a resistance layer, so there is no capacity improvement effect before / after coating, and the electrochemical characteristics of the battery are deteriorated.

[0005] Therefore, there is a need for a solution to improve the performance of the positive electrode active material of the sodium ion battery and to improve the degradation of the life characteristics caused by the water washing process. Summary of the invention

[0006] Technical problem to be solved by the invention

[0007] The object of the present invention is to provide a method for preparing a positive electrode active material for a sodium secondary battery. In the process of preparing the positive electrode active material for a sodium secondary battery, a water washing process and a coating process are simultaneously performed, and then a uniform and capacity-rich coating is formed on the particle surface of the entire positive electrode active material, thereby improving battery performance such as surface stability, capacity and life characteristics.

[0008] In addition, in the present invention, residual sodium on the surface of the sodium transition metal composite oxide is used to reduce the additional amount of alkaline material (NaOH) used in the coating formation process, thereby simplifying the process and improving the process cost.

[0009] At the same time, in the present invention, when preparing the sodium transition metal composite oxide, the added sodium equivalent (Na / M) and the high-Mn transition metal composition are adjusted, thereby controlling the total amount of residual sodium on the surface of the sodium transition metal oxide and the ratio of residual sodium compounds (NaOH / Na2CO3) within a specific range, thereby optimizing the additional amount of alkaline substance (NaOH) added in the coating formation process.

[0010] In addition, the present invention provides a method for coating a positive electrode active material for a sodium secondary battery, wherein different processes are applied according to the coating amount of the positive electrode active material, thereby enabling uniform formation of a coating layer regardless of changes in the coating amount.

[0011] Meanwhile, the present invention provides a positive electrode active material for a sodium secondary battery, which is prepared by the above-mentioned preparation method and forms a uniform coating with capacity on the particle surface of the entire positive electrode active material.

[0012] Means for solving technical problems

[0013] An example of the present invention provides a method for preparing a positive electrode active material for a sodium secondary battery, characterized in that it includes: step a), adding a sodium composite transition metal oxide and a water washing liquid to a reactor, and stirring the reactants, thereby dissolving the residual sodium on the surface of the sodium composite transition metal oxide in the water washing liquid; step b), adding a cobalt salt to the reactor and stirring, and coprecipitating cobalt hydroxide into the sodium composite transition metal oxide particles; and step c), heat-treating the sodium composite transition metal oxide particles formed with the cobalt hydroxide to form a cobalt coating on the particles.

[0014] In the above step a), the sodium composite transition metal oxide is prepared by mixing a transition metal hydroxide precursor and a sodium compound so that the molar ratio of Na / M (M = all metals except Na) becomes 0.6 to 0.72, and calcining the mixture.

[0015] In the above step a), the sodium composite transition metal oxide is a sodium byproduct remaining on the surface of the oxide particles, comprising a combination of sodium hydroxide (NaOH) and sodium carbonate (Na2CO3), and the sodium composite transition metal oxide can contain residual sodium at a weight ratio of sodium hydroxide to sodium carbonate (NaOH / Na2CO3) of 50 to 110.

[0016] In the above step a), during the stirring process of the reactor, the pH of the reactants may be increased from pH 6 to 8 to pH 10 to 12.

[0017] In the above step a), the residual sodium content (total sodium (TTS), ppm) in the above sodium composite transition metal oxide may be 3000 ppm to 20000 ppm.

[0018] The above-mentioned step b) may include: step b1), when the content of cobalt element contained in the above-mentioned cobalt oxide coating layer is less than 2 mol% relative to the total metal (M) other than sodium of the above-mentioned sodium composite transition metal oxide, without adding sodium-containing alkaline substances, and stirring; and step b2), when the content of cobalt element contained in the above-mentioned cobalt oxide coating layer is greater than 2 mol% relative to the total metal (M) other than sodium of the above-mentioned sodium composite transition metal oxide, when the cumulative content of cobalt element added to the above-mentioned reactor (Co' / M) is 0 mol% to 2 mol%, without adding sodium-containing alkaline substances, and stirring, when the cumulative content of cobalt element added to the above-mentioned reactor (Co' / M) is greater than 2 mol%, cobalt salt (CS2) and sodium-containing alkaline substances are added simultaneously, and stirring.

[0019] In the above cobalt hydroxide coprecipitation step b), step b3) is to interrupt the addition of cobalt salt, or cobalt salt and sodium-containing alkaline substance after the above step b1) or step b2), and stir the substances added to the above reactor for additional 1 minute to 10 minutes.

[0020] The residual sodium in step a) and the sodium-containing alkaline substance in step b2) are substances that are partially or completely ionized in the water washing liquid and show alkalinity, and the cobalt salt in step b) can be a substance that is partially or completely ionized in the water washing liquid and shows acidity.

[0021] Another example of the present invention provides a positive electrode active material for a sodium secondary battery, characterized in that it comprises a plurality of sodium composite transition metal oxide particles, wherein the plurality of sodium composite transition metal oxide particles comprise a cobalt oxide coating formed on the surface and / or inside of the particles, and in the plurality of sodium composite transition metal oxide particles, at any 4 points selected by EDS mapping analysis, the relative standard deviation (RSD) of the atomic molar ratio (Co / M) of cobalt to all metals (M) other than sodium is less than 30.

[0022] In the plurality of sodium composite transition metal oxide particles, a relative standard deviation (RSD) of an atomic molar ratio (Co / M) of cobalt to all metals (M) excluding sodium may be 2 to 20 at any 4 points selected by EDS mapping analysis.

[0023] The sodium composite transition metal oxide may be a sodium-manganese-based oxide containing at least sodium, nickel, and manganese.

[0024] The sodium composite transition metal oxide may be represented by the following Chemical Formula 1.

[0025] [Chemical formula 1]

[0026] Na a Ni x M1 y M2 z Mn 1-x-y-z O2

[0027] In the above chemical formula 1,

[0028] M1 is Co or Fe,

[0029] M2 is at least one element selected from Co, P, Sr, Ba, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu, M1 and M2 are different elements,

[0030] 0.50≤a≤0.80, 0.05≤x≤0.45, 0≤y≤0.45, 0≤z≤0.1, 0.55≤1-xyz≤0.85.

[0031] The cobalt oxide coating may include sodium cobalt oxide (NaCoO 2 ), cobalt oxide (Co 2 O 3 ), or a combination thereof.

[0032] The cobalt content (Co) contained in the cobalt oxide coating layer may be 0.1 mol % to 10 mol % relative to the total metal (M) excluding sodium of the sodium composite transition metal oxide.

[0033] The plurality of sodium composite transition metal oxide particles including the cobalt oxide coating layer formed on the surface and / or inside of the particles may contain 10000 ppm or less of residual sodium.

[0034] Another example of the present invention provides a positive electrode for a sodium secondary battery including a positive electrode active material, and a sodium secondary battery including the positive electrode; a negative electrode; and an electrolyte.

[0035] Effects of the Invention

[0036] According to the present invention, in the positive electrode active material for sodium secondary batteries, a water washing process for removing sodium byproducts remaining on the surface and a coating process for improving surface defects and improving performance are simultaneously performed, thereby simplifying the process and saving production time and process costs.

[0037] Furthermore, according to the present invention, a uniform and high-capacity coating layer is formed on the entire surface of the positive electrode active material particles for sodium secondary batteries, thereby enhancing surface stability and improving battery performance such as capacity and life characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1a and Figure 1b The low-rate (1.000K) EDS mapping results and the high-rate (5.000K) EDS mapping results of the positive electrode active material prepared in Example 1 are shown.

[0039] Figure 2a and Figure 2b The low-rate (1.000K) EDS mapping results and the high-rate (5.000K) EDS mapping results of the positive electrode active material prepared in Comparative Example 1 are shown.

[0040] Figure 3 The first charge and discharge curves of the sodium secondary batteries prepared in Example 1, Comparative Example 1 and Reference Example (Reference) are shown. DETAILED DESCRIPTION

[0041] The advantages, features and methods of achieving the advantages and features of the present invention can be clearly seen by referring to the accompanying drawings and the embodiments described in detail below. However, the present invention is not limited to the embodiments disclosed below, but can be embodied in a variety of different forms. The embodiments are only used to make the disclosure of the present invention more complete and to fully inform the ordinary technicians in the technical field to which the present invention belongs of the scope of the invention. The present invention is only defined by the scope of the claims.

[0042] Unless otherwise defined, all terms (including technical terms and scientific terms) used in this specification can be used with the meanings commonly understood by ordinary technicians in the technical field to which the present invention belongs. Throughout the specification, when a part is mentioned to "include" a component, unless there is a special record to the contrary, it means that other components are also included, rather than excluding other components. In addition, unless otherwise mentioned, the singular form can also include the plural form in a sentence.

[0043] The present invention provides a method for preparing a positive electrode active material for a sodium secondary battery. When preparing the positive electrode active material for a sodium secondary battery, a water washing process and a coating process are simultaneously performed, and then a coating having a capacity and being formed uniformly on the surface of particles of the positive electrode active material is formed. The method for preparing the positive electrode active material for a sodium secondary battery comprises: step a), adding a sodium composite transition metal oxide and a water washing liquid to a reactor, and stirring the reactants, thereby dissolving the residual sodium on the surface of the sodium composite transition metal oxide in the water washing liquid; step b), adding a cobalt salt to the reactor and stirring, and coprecipitating cobalt hydroxide into the sodium composite transition metal oxide particles; and step c), heat-treating the sodium composite transition metal oxide particles formed with the cobalt hydroxide to form a cobalt coating on the particles. Thus, the surface stability of the positive electrode active material can be improved, and the electrochemical properties such as capacity and life characteristics can be improved.

[0044] In step a), in order to apply the optimal pH for the coprecipitation of cobalt hydroxide, the sodium byproduct remaining on the surface of the positive electrode active material is preferentially utilized without adding an alkaline substance, and the sodium composite transition metal oxide and the water washing liquid are added to the reactor, and the reactants are stirred, thereby dissolving the sodium byproduct (residual sodium) remaining on the surface of the above-mentioned sodium composite transition metal oxide in the above-mentioned water washing liquid. Thus, the sodium byproduct remaining on the particle surface of the sodium composite transition metal oxide can be dissolved in the water washing liquid to control the pH to 11-12 optimized for the coprecipitation of cobalt hydroxide, and the process can be simplified and the process cost can be reduced without adding an alkaline solution and an ammonium solution.

[0045] In the above step a), the stirring of the reactants is carried out at 100 rpm to 300 rpm for 1 minute to 10 minutes, specifically, at 200 rpm to 400 rpm for 3 minutes to 7 minutes, or at 300 rpm to 500 rpm for 4 minutes to 6 minutes. Thus, during the stirring of the reactor, the pH of the reactants increases from pH 6 to 8 to pH 10 to 12, and in the case of no additional addition of alkaline substances in the coprecipitation process of the subsequent step, only residual sodium is used to prepare the optimal pH for coprecipitation.

[0046] The above-mentioned sodium composite transition metal oxide is prepared by mixing a transition metal hydroxide precursor and a sodium compound so that the Na / M (M = all metals except Na) molar ratio becomes 0.6 to 0.72, specifically, the Na / M (M = all metals except Na) molar ratio becomes 0.63 to 0.72, 0.65 to 0.71, 0.66 to 0.71, preferably, the Na / M (M = all metals except Na) molar ratio becomes 0.68 to 0.71, and calcining the above-mentioned mixture.

[0047] The prepared sodium composite transition metal oxide may be high-Mn sodium nickel manganese oxide (NNMO) represented by the following Chemical Formula 1.

[0048] [Chemical formula 1]

[0049] Na a Ni x M1 y M2 z Mn 1-x-y-z O2

[0050] In the above chemical formula 1,

[0051] M1 is Co or Fe,

[0052] M2 is at least one element selected from Co, P, Sr, Ba, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu, M1 and M2 are different elements,

[0053] 0.50≤a≤0.80, 0.05≤x≤0.45, 0≤y≤0.45, 0≤z≤0.1, 0.55≤1-xyz≤0.85.

[0054] In the high-Mn transition metal composition, it is shown that the higher the proportion of Mn, the higher the residual sodium content on the surface of the oxide particles after preparing the sodium composite transition metal oxide. In the present invention, the sodium by-products remaining on the surface can be utilized to the maximum extent to reduce the amount of alkaline substances used in Co co-precipitation coating.

[0055] The sodium composite transition metal oxide is a sodium byproduct remaining on the surface of oxide particles, and may include a combination of sodium hydroxide (NaOH) and sodium carbonate (Na2CO3).

[0056] Referring to the reaction formula below, NaOH in the residual sodium is strongly alkaline. When coprecipitation coating is performed, it can be used instead of the alkaline substance (NaOH) added additionally to meet the optimal pH of coprecipitation 11-12. In this process, it also has the effect of removing residual sodium. However, Na2CO3 in the residual sodium is weakly alkaline. In order to meet the optimal pH of coprecipitation 11-12, a large amount of residual sodium is required. Therefore, if the overall residual sodium amount increases, it is difficult to evenly coat in the coprecipitation coating process. Therefore, the purpose of the present invention is to use NaOH that can be used as an alkaline substance in the coprecipitation process of residual sodium to perform the coprecipitation process, i) within the range of an appropriate total amount of residual sodium, ii) prepare / use the above-mentioned sodium composite transition metal oxide in a manner having a specific (NaOH / Na2CO3) weight ratio.

[0057] NaOH(aq.)------------>>(Na + )+(OH - )...............................(1)

[0058] Na2CO3+H2O<----->>(2Na + )+(HCO3 - )+(OH - )……………………(2)

[0059] (HCO3 - )+H2O<------>>(H2CO3)+(OH-)........................(3)

[0060] In reaction formula (1), NaOH is a strong base, so Na + and OH-, in reaction formula (2), Na2CO3 is a salt of a strong base and a weak acid, which dissolves in the water wash liquid and shows weak alkalinity, in reaction formula (3), HCO3 - It reacts with water again to generate OH - Weak alkalinity.

[0061] The sodium composite transition metal oxide may contain residual sodium at a weight ratio of sodium hydroxide to sodium carbonate (NaOH / Na2CO3) of 50 to 110, for example, may contain residual sodium at a weight ratio of 65 to 110, 65 to 105, 70 to 110 or 70 to 105.

[0062] When the (NaOH / Na2CO3) weight ratio is greater than the design range, the proportion of NaOH as a strong base is relatively increased, and thus, the pH of the water washing solution of the above-mentioned water washing step a) may be excessively increased (excessively exceeding the optimal pH of coprecipitation 11-12). Therefore, in the coprecipitation step b), even if only cobalt hydroxide is added alone (step b1), coprecipitation is performed in a state exceeding the optimal pH of coprecipitation 11-12, and therefore, it is difficult to uniformly apply cobalt. In addition, the overall residual sodium amount is excessively increased, and thus, it is difficult to uniformly apply with the increased residual sodium amount. On the contrary, when the (NaOH / Na2CO3) weight ratio is less than the design range, the proportion of Na2CO3 as a weak base is relatively increased, and thus, the pH of the water washing solution of the above-mentioned water washing step a) may be excessively reduced (less than the optimal pH of coprecipitation 11-12). In the coprecipitation step b), if cobalt hydroxide is added alone (step b1), coprecipitation is performed under conditions that do not reach the optimal pH of coprecipitation 11-12, and therefore, it is difficult to apply cobalt. Therefore, as in the prior art, the additional amount of alkaline substance (NaOH) used in the coating formation process increases, and it is difficult to use residual sodium, and there are problems of process complexity and increased process cost.

[0063] In the above-mentioned sodium composite transition metal oxide, the residual sodium content (total sodium (Total sodium, TTS), ppm) can be 3000ppm to 20000ppm, for example, it can be 5000ppm to 20000ppm, 8000ppm to 20000ppm, 10000ppm to 20000ppm, 12000ppm to 20000ppm, 13000ppm to 18000ppm or 14000ppm to 17000ppm. Thus, in order to carry out the coprecipitation process in the residual sodium, using NaOH which can be used as an alkaline substance in fact to carry out the coprecipitation process, i) within the range of the appropriate residual sodium total amount, ii) in a manner having a specific (NaOH / Na2CO3) weight ratio to prepare the above-mentioned sodium composite transition metal oxide.

[0064] The water washing liquid can be a water washing liquid commonly used for washing sodium composite transition metal oxides, for example, it can be ethanol, distilled water, or deionized water.

[0065] The above reactor can use a coprecipitation reaction reactor commonly used for preparing positive electrode active materials without limitation, for example, a batch type reactor, a Couette-talyor reactor, etc. When the above Couette-talyor reactor is used, the coprecipitation of cobalt hydroxide can be carried out quickly compared to conventional reactors such as batch reactors, thereby having the advantage of reducing process time.

[0066] Step b) is a step of uniformly coprecipitating cobalt hydroxide on the surface of the sodium composite transition metal oxide particles, and adding cobalt salt to the above-mentioned reactor and stirring to coprecipitate cobalt hydroxide on the above-mentioned sodium composite transition metal oxide particles. Therefore, different coprecipitation processes can be applied according to the cumulative coprecipitation amount (coating amount) of cobalt hydroxide.

[0067] Specifically, step b1) can be performed in the above step b), in which, when the content of cobalt element contained in the above cobalt oxide coating is less than 2 mol% relative to the total metal (M) other than sodium of the above sodium composite transition metal oxide, the above cobalt salt is added, and no sodium-containing alkaline substance is added, and stirring is performed.

[0068] In addition, step b2) may be performed in the above step b), in which, when the content of cobalt element contained in the above cobalt oxide coating is greater than 2 mol% relative to the total metal (M) other than sodium of the above sodium composite transition metal oxide, cobalt salt (CS1) is added when the cumulative content of cobalt element (Co' / M) added to the above reactor is 0 mol% to 2 mol%, and no sodium-containing alkaline substance is added, and the mixture is stirred; when the cumulative content of cobalt element (Co' / M) added to the above reactor is greater than 2 mol%, cobalt salt (CS2) and sodium-containing alkaline substance are added simultaneously, and the mixture is stirred.

[0069] The total metal (M) other than sodium in the sodium composite transition metal oxide may include a transition metal, and the transition metal is contained in the cobalt oxide coating layer.

[0070] In addition, the above-mentioned cobalt salt is a cobalt-containing raw material, and the above-mentioned cobalt-containing raw material can be a cobalt-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide or oxyhydroxide, etc., specifically, it can be Co(OH)2, CoOOH, Co(OCOCH3)2·4H2O, Co(NO3)2·6H2O, CoSO4, CoSO4·7H2O or a combination thereof, but is not limited to this.

[0071] The alkaline solution may be a hydroxide of an alkali metal or alkaline earth metal such as NaOH, KOH or Ca(OH)2, a hydrate thereof or a combination thereof. The alkaline compound may also be used in the form of an aqueous solution, in which case water or a mixture of an organic solvent (specifically, alcohol, etc.) uniformly miscible with water and water may be used as a solvent.

[0072] The alkaline solution is added to adjust the pH of the reaction solution in the reactor, and can be added in an amount such that the pH of the reaction solution in the reactor becomes pH 11 to 12 when the coprecipitation of the cobalt hydroxide is performed.

[0073] The above coprecipitation reaction can be performed under an inert atmosphere such as nitrogen or argon at a temperature of 40°C to 70°C, and the coprecipitation time of the above cobalt hydroxide coprecipitation can be 1 minute to 1 hour, more preferably, 5 minutes to 1 hour, and more preferably, 10 minutes to 50 minutes. The coprecipitation time of the above cobalt hydroxide is adjusted within the above range to improve the surface defects in the particle surface of the above sodium composite transition metal oxide, effectively form a coating for improving battery performance, and minimize the film resistance caused by the formation of the coating, and shorten the previous long coprecipitation process time of about 2 hours, thereby simplifying the process, saving process costs, and forming a uniform coating.

[0074] By the process as described above, the sodium composite transition metal oxide and the water washing liquid are stirred to form cobalt hydroxide on the surface of the sodium composite transition metal oxide particles. Thus, a water washing process for removing sodium by-products remaining on the particle surface and a coating process for improving surface defects and improving performance are simultaneously carried out to simplify the process, save production time and process costs, and thereby form a uniform coating on the entire particle surface.

[0075] Step c) is used to form a cobalt coating on the surface of the sodium composite transition metal oxide particles, and is a step of heat treating the sodium composite transition metal oxide particles formed with the above-mentioned cobalt hydroxide.

[0076] The heat treatment may be performed at a temperature of 500° C. to 900° C., more preferably, 600° C. to 900° C., and even more preferably, 700° C. to 800° C. The heat treatment may be performed at a temperature within a range, thereby forming a coating layer containing a sodium cobalt oxide layer structure on the particle surface.

[0077] When the heat treatment is performed, a coating source is also mixed and heat treated. The coating source can be a coating source for coating the surface of the particles of the sodium composite transition metal oxide, for example, it can include at least one selected from the group consisting of B, Al, F, W, Mo, Ti and Nb, but is not limited thereto.

[0078] By the above-described process, cobalt hydroxide is converted into (sodium) cobalt oxide having capacity on the surface of the sodium composite transition metal oxide particles, thereby improving not only the surface stability but also the capacity characteristics.

[0079] Another example of the present invention provides a positive electrode active material for a sodium secondary battery. The positive electrode active material comprises a plurality of sodium composite transition metal oxide particles, the plurality of sodium composite transition metal oxide particles comprise a cobalt oxide coating formed on the surface and / or inside of the particles, and the plurality of sodium composite transition metal oxide particles, in any 4 points selected by EDS mapping analysis, the relative standard deviation (RSD) of the atomic molar ratio (Co / M) of cobalt to all metals (M) other than sodium is less than 30, specifically 2 to 20, 2 to 15, 2 to 10, 2 to 5 or 2 to 4.

[0080] Thus, a uniform and capacity-rich coating can be formed on the surface of the particles of the entire positive electrode active material. When the relative standard deviation of the above-mentioned atomic molar ratio Co / M cannot meet the above-mentioned range, the uneven coating acts as a resistance layer, thereby, there is no capacity improvement effect before / after coating, and further, the battery characteristics can be reduced.

[0081] In addition, the relative standard deviation (RSD) is also called the coefficient of variation, which shows the relative size of the standard deviation relative to the average value, and can be calculated by dividing the standard deviation by the arithmetic mean value (standard deviation / average×100). In the present invention, as an indicator for confirming whether the cobalt coating formed on the surface of a plurality of sodium composite transition metal oxide particles is uniformly formed, the standard deviation and average of the atomic molar ratio (Co / M) of cobalt to the total metal (M) calculated at any 4 points including the plurality of oxide particles can be calculated by EDS mapping analysis, thereby calculating the relative standard deviation (standard deviation / average×100).

[0082] The sodium composite transition metal oxide may be at least a NNMO sodium-manganese oxide containing sodium, nickel and manganese. The sodium-manganese oxide is a high-manganese (high-Mn) oxide containing more than 55 mol% of manganese in all metals other than sodium. The manganese content in the metal other than sodium may 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 may be less than 85 mol%, less than 80 mol% or less than 75 mol%. Among metals other than sodium, as the manganese content increases, it has the advantage of high capacity in a high-voltage working environment, while reducing nickel and cobalt concentrations and improving price competitiveness.

[0083] The sodium composite transition metal oxide can exist in the form of secondary particles of at least one primary particle agglomeration, and the average particle size (D50) of the oxide particles can be 2 μm to 15 μm, for example, 4 μm to 10 μm, but is not limited thereto. In this case, the electrode density increases, thereby increasing the energy density per unit volume of the electrode.

[0084] The sodium composite transition metal oxide can be represented by the following Chemical Formula 1.

[0085] [Chemical formula 1]

[0086] Na a Ni x M1 y M2 z Mn 1-x-y-z O2

[0087] In the above chemical formula 1,

[0088] M1 is Co or Fe,

[0089] M2 is at least one element selected from Co, P, Sr, Ba, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu, M1 and M2 are different elements,

[0090] 0.50≤a≤0.80, 0.05≤x≤0.45, 0≤y≤0.45, 0≤z≤0.1, 0.55≤1-xyz≤0.85.

[0091] In the sodium composite transition metal oxide of the above chemical formula 1, the molar ratio (Na / M) of sodium (Na) to all metals (M) other than sodium can be 0.5 to 0.8. In the above chemical formula 1, if the content of a in Na is less than 0.5, the capacity can be reduced, and if it is greater than 0.8, the position of the sodium ion changes, thereby showing an O3 type crystalline structure. Compared with the P2 type, the O3 type positive electrode active material is more sensitive to lower atmospheric and moisture safety, as well as synthesis conditions (temperature and atmosphere, etc.). The above Na is more preferably 0.60≤a≤0.80, 0.60≤a≤0.75 or 0.65≤a≤0.75.

[0092] The cobalt oxide coating can be formed by surface Co coating and internal co-diffusion (Co diffusion) of the plurality of sodium composite transition metal oxide particles. Thus, a uniform and capacity-rich coating is formed on the surface of the entire positive electrode active material particle, thereby improving surface stability and improving battery performance such as capacity and life characteristics.

[0093] The content of cobalt (Co) contained in the above-mentioned cobalt oxide coating layer may be 0.1 mol% to 10 mol%, for example, 0.5 mol% to 7 mol%, 0.5 mol% to 5 mol%, 0.5 mol% to 4.5 mol% or 0.5 mol% to 4 mol%, relative to the total metal (M) other than sodium of the above-mentioned sodium composite transition metal oxide.

[0094] The plurality of sodium composite transition metal oxide particles comprising a cobalt oxide coating formed on the surface and / or inside of the above-mentioned particles can contain residual sodium (total sodium (TTS), ppm) at less than 10000 ppm, for example, can contain residual sodium (total sodium (TTS), ppm) at 2000 ppm to 10000 ppm, 2000 ppm to 7000 ppm or 2000 ppm to 5000 ppm.

[0095] Before forming (including) the above-mentioned cobalt oxide coating, the above-mentioned multiple sodium composite transition metal oxide particles can contain residual sodium (total sodium (TTS), ppm) at less than 20000 ppm, for example, can contain residual sodium (total sodium (TTS), ppm) at 8000 ppm to 20000 ppm, 8000 ppm to 15000 ppm or 8000 ppm to 10000 ppm.

[0096] Furthermore, before forming (including) the coating layer, the residual sodium content of the plurality of sodium composite transition metal oxide particles is usually measured without performing a water washing process.

[0097] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery including the positive electrode active material, and a sodium secondary battery.

[0098] The positive electrode comprises a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector. The positive electrode active material according to one aspect of the present invention is present in the positive electrode active material layer.

[0099] The positive electrode collector is not limited as long as it does not cause chemical changes in the battery and has conductivity. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, etc. can be used. In addition, the positive electrode collector can generally have a thickness of 3 μm to 500 μm, and fine concavoconvexities can be formed on the surface of the collector to improve the adhesion of the positive electrode active material. This positive electrode collector can be provided in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc.

[0100] In addition, the positive electrode active material layer may be a layer including the above-mentioned positive electrode active material, a conductive material, and a binder.

[0101] Among them, the conductive material is used to impart conductivity to the electrode, and can be used without restriction as long as it does not cause chemical changes in the positive electrode active material and has conductivity. As non-limiting examples of conductive materials, there are graphites such as natural graphite or artificial graphite; carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber and other carbon materials; 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. Usually, based on the total weight of the positive electrode active material layer, it can contain 1 weight percent to 30 weight percent of conductive material.

[0102] In addition, the binder is a substance that serves to improve the adhesion between the positive electrode active material particles and the bonding force between the positive electrode active material and the collector. As a non-limiting example of the binder, there are 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 polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber or their various copolymers. Usually based on the total weight of the positive electrode active material, etc., 1 weight percent to 30 weight percent of the binder may be included.

[0103] In addition to using the above-mentioned positive electrode active material, the positive electrode of one example of the present invention can be prepared according to a conventional positive electrode preparation method for sodium secondary batteries. For example, the positive electrode active material and the slurry for forming the positive electrode active material layer containing the binder and the conductive material are selectively applied to the positive electrode collector, and then dried and rolled to prepare the positive electrode. According to another example, after the slurry for forming the positive electrode active material layer is cast on an additional support, the film obtained by peeling the positive electrode active material layer from the support is laminated on the positive electrode collector, thereby preparing the positive electrode.

[0104] According to another example of the present invention, an electrochemical element comprising the above-mentioned positive electrode is provided, wherein the electrochemical element may be a battery, a capacitor, etc., and more specifically, may be a sodium secondary battery.

[0105] The sodium secondary battery includes: a positive electrode, a negative electrode disposed opposite to the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte (electrolyte). In addition, the sodium secondary battery may include: a battery container (housing) for accommodating an electrode assembly including the positive electrode, the negative electrode, and the separator; and a sealing member for sealing the battery container.

[0106] At this time, sodium secondary batteries can be divided into can type sodium secondary batteries in which an electrode assembly is placed in a metal can and pouch type sodium secondary batteries in which an electrode assembly is placed in a pouch formed of a sheet such as an aluminum layer, according to the shape of a battery container (casing).

[0107] In particular, in the case of a pouch-type sodium secondary battery using a positive electrode containing a positive electrode active material in various embodiments of the present invention, the possibility of a side reaction between the positive electrode active material and the electrolyte is low, thereby having the advantages of improving stability during storage and / or operation and reducing gas generation.

[0108] Hereinafter, the present invention will be described in detail by way of examples. This is for the purpose of further illustrating the present invention, but the scope of rights of the present invention is not limited to the following examples.

[0109] Example

[0110] Preparation Example 1: Preparation of positive electrode active material

[0111] (Example 1)

[0112] a) Preparation of sodium composite transition metal oxides

[0113] To you 0.35 Mn 0.65 The (OH)2 precursor was added with a sodium compound Na2CO3 in an amount of Na / M=0.70 equivalents, and heat treated at 950°C for 12 hours in an air atmosphere to obtain a sodium nickel manganese oxide positive electrode active material powder (Na 0.67 Ni 0.35 Mn 0.65 O2 powder).

[0114] b) Coprecipitation of cobalt hydroxide

[0115] DIW and Na were added to a 5 L batch reactor. 0.67 Ni 0.35 Mn 0.65 O2 and stirred at 300 rpm for 5 min.

[0116] Next, the reactor was maintained at 25° C., 300 rpm, and pH 11-12 until the atomic molar ratio of Co / M (=Ni+Mn) in the aqueous cobalt sulfate solution reached 2 mol %, and the amount added was adjusted to coprecipitate cobalt hydroxide for 10 minutes.

[0117] After that, the reactor was maintained at 25°C, 300 rpm, and pH 11-12 until the Co / M (=Ni+Mn) atomic molar ratio in the aqueous cobalt sulfate solution and the aqueous NaOH solution reached 3 mol%, the added amount was adjusted, and cobalt hydroxide was coprecipitated for 30 minutes.

[0118] After the reactor was additionally stirred at 300 rpm for 5 minutes, the obtained particles were separated and dried in a vacuum oven at 110° C. for 24 hours.

[0119] c) Formation of cobalt oxide coating

[0120] The dried particle powder was added to an alumina crucible and heat treated at 800° C. for 12 hours in an air atmosphere to prepare a sodium composite transition metal oxide positive electrode active material having a cobalt oxide coating formed on the particle surface.

[0121] (Comparative Example 1)

[0122] The sodium composite transition metal oxide positive electrode active material having a cobalt oxide coating layer formed thereon was prepared in the same manner as in Example 1 except that step b) was performed as follows.

[0123] b) Cobalt hydroxide coprecipitation process

[0124] DIW was added to 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 11-12. 0.67 Ni 0.35 Mn 0.65 O2 and stirred at 300 rpm.

[0125] Next, the reactor was maintained at 25° C., 300 rpm, and pH 11-12 until the Co / M (=Ni+Mn) atomic molar ratio in the aqueous cobalt sulfate solution reached 3.4 mol %, and the amount added was adjusted to coprecipitate cobalt hydroxide for 60 minutes.

[0126] After the reactor was additionally stirred at 300 rpm for 15 minutes, the obtained particles were separated and dried in a vacuum oven at 110° C. for 24 hours.

[0127] (Reference)

[0128] The Na prepared in step a) of Example 1 0.67 Ni 0.35 Mn 0.65 O2 powder is used as the positive electrode active material.

[0129] Preparation Example 2: Preparation of Sodium Secondary Battery

[0130] 90 wt % of the prepared positive electrode active material, 5.5 wt % of carbon black, and 4.5 wt % of PVdF binder were dispersed in 30 g of methyl pyrrolidone (NMP) to prepare a positive electrode slurry. The positive electrode slurry was evenly coated on an aluminum film with a thickness of 15 μm and vacuum dried at 135° C., thereby preparing a positive electrode for a sodium secondary battery.

[0131] For the above-mentioned positive electrode, a sodium metal plate was used as a counter electrode, a porous polyethylene film (Celgard 2300, thickness: 25 μm) was used as a separation membrane, and an electrolyte solution containing NaPF6 at a concentration of 1.15 M in a volume in which ethylene carbonate and ethyl methyl carbonate were mixed in a volume ratio of 3:7 was used to prepare a sodium secondary battery (button battery).

[0132] Experimental example

[0133] Experimental Example 1: Evaluation of Co coating uniformity (Relative standard deviation (RSD) analysis)

[0134] After sampling the sample on the carbon tape, the analysis was performed with a working distance of 15, an acceleration voltage of 15kV, and KCPS110-130. When performing the analysis, one cut at a low magnification (x1.000k) in any interval and four cuts at a high magnification (x5.000k) in any interval were analyzed, and after obtaining the atomic% values ​​of cobalt, Ni, and Mn, the atomic ratio of Co / (Ni+Mn) (Co mol%) was obtained. After that, the Co mol% RSD (relative standard deviation) of the four cuts at high magnification was calculated. The calculation results are shown in the following Table 1.

[0135] exist Figure 1a , Figure 1b , Figure 2a , Figure 2b EDS results are shown.

[0136] Table 1

[0137]

[0138] Reference Figure 1a , Figure 1b , Figure 2a , Figure 2bAs shown in Table 1, the coating uniformity of the Co-coated positive electrode active material applied by the preparation method of the present invention is improved, and thus, when EDS Co mol% analysis is performed, it can be confirmed that the 4-point relative standard deviation (RSD) is reduced to 2.1. On the contrary, in the case of Comparative Example 1 in which the conventional coating method is applied, the RSD value shown is 94.5, and the unevenly formed coating acts as a resistance layer, thus, it is difficult to show the capacity improvement effect before / after coating, and it is predicted that the battery characteristics will deteriorate.

[0139] Experimental Example 2: Evaluation of Co coating uniformity (relative to standard) based on the total amount of residual Na before coating and the ratio of compounds Deviation (RSD) analysis)

[0140] a) In the preparation process of sodium composite transition metal oxide, Ni 0.35 Mn 0.65 A positive electrode active material coated with Co was prepared in the same manner as in Example 1 except that the (OH) 2 precursor was mixed with the sodium compound Na 2 CO 3 in the (Na / M) equivalents shown in Table 2 below.

[0141] Table 2

[0142]

[0143] Referring to Table 2, in the case of a positive electrode active material prepared under the conditions of Na / M equivalent in the preferred range of 0.66 to 0.71, i) the total amount of residual Na is appropriate, ii) the weight ratio of sodium hydroxide to sodium carbonate (NaOH / Na2CO3) in the residual sodium is 67 to 104, and NaOH that can replace the alkaline substance (NaOH) additionally added to meet the optimal pH of 11 to 12 for coprecipitation can be used in the residual sodium. Thus, i') it can be evenly coated, and ii') the residual sodium can be utilized to the maximum extent, thereby simplifying the process and improving the process cost.

[0144] In addition, it was analyzed that: in the case of the positive electrode active material prepared under the condition of Na / M equivalent greater than 0.7, i) the total amount of residual Na increased excessively, and ii) the weight ratio of sodium hydroxide to sodium carbonate (NaOH / Na2CO3) in the residual sodium increased excessively to 185.6 and 268.4. As a result, the coprecipitation coating reaction was carried out at a state exceeding the optimal pH of 11 to 12 for coprecipitation, thereby reducing the coating uniformity.

[0145] In addition, it was analyzed that: in the case of the positive electrode active material prepared under the conditions of Na / M equivalent of 0.61 to 0.67, the (NaOH / Na2CO3) weight ratio was 51 and 61, and the proportion of the relatively weak base Na2CO3 increased. As a result, the additional amount of alkaline substance (NaOH) added in the co-precipitation coating process increased to an amount equal to the decrease in pH of the washing liquid in the above-mentioned washing step a), and it was difficult to utilize the residual sodium, which increased the process complexity and process cost.

[0146] Experimental Example 3: Analysis of unreacted residual Na content

[0147] In order to determine the amount of residual sodium, 1 g of sodium composite transition metal oxide was immersed in 5 g of distilled water, stirred for 5 minutes, and the filtrate was taken out and titrated with 0.1 M HCl until the pH of the filtrate reached 5. The volume of the added HCl was measured to analyze the unreacted sodium by-products remaining on the surface of the particles.

[0148] The residual sodium content is determined by measuring the compound containing residual Na (e.g., NaOH or Na2CO3) by potentiometric neutralization titration, and then calculating the total amount of Na alone (Total Sodium, TTS). The calculation method is as follows:

[0149] [Calculation formula 1]

[0150] TTS (Total Na) = NaOH analysis value (%) × Na / NaOH + Na2CO3 analysis value (%) × 2Na / Na2CO3

[0151] Table 3

[0152] Example 1 NaOH(ppm) <![CDATA[Na2CO3(ppm)]]> Total Na (ppm) Before Co coating 12311 5419 9427 After Co coating 450 2115 1176

[0153] Referring to Table 3, it is analyzed that when comparing before and after Co coating, NaOH is removed at a relatively larger ratio than Na2CO3, which reduces the usage of alkaline substances for Co co-precipitation coating by maximizing the utilization of NaOH in the sodium byproduct remaining on the surface.

[0154] Experimental Example 4: Evaluation of electrochemical performance of sodium secondary batteries

[0155] For the sodium secondary batteries prepared in Example 1, Comparative Example 1 and Reference Example, the first charge / discharge capacity was measured after one charge / discharge was performed at 25° C. and in the driving voltage range of 2.0 V to 4.6 V at 0.1 C / 0.5 C. Figure 3 The results are shown.

[0156] Reference Figure 3In the Co-coated positive electrode active material prepared by the method of the present invention, a uniform and capacity-containing coating is formed on the entire particle surface, and the surface stability is improved. It can be confirmed that the capacity and life (capacity maintenance rate) characteristics are improved compared with the reference example.

[0157] On the contrary, in the case of Comparative Example 1, the non-uniformly formed Co coating layer functioned as a resistance layer, and thus, there was no discharge capacity improvement effect, and the capacity retention ratio was deteriorated.

[0158] As described above, the present invention only shows and illustrates specific embodiments, and it is obvious to those skilled in the art that the present invention can be modified and varied in many ways without departing from the scope of the technical idea of ​​the present invention provided by the attached claims.

Claims

1. A method for preparing a positive electrode active material for a sodium secondary battery, characterized in that: include: Step a), adding a sodium composite transition metal oxide and a water washing liquid into a reactor, and stirring the reactants, thereby dissolving the residual sodium on the surface of the sodium composite transition metal oxide in the water washing liquid; Step b), adding a cobalt salt into the reactor and stirring, and coprecipitating cobalt hydroxide into the sodium composite transition metal oxide particles; as well as Step c) heat-treating the sodium composite transition metal oxide particles formed with the cobalt hydroxide to form a cobalt coating on the particles.

2. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 1, characterized in that: In the step a), the sodium composite transition metal oxide is prepared by mixing a transition metal hydroxide precursor and a sodium compound so that the Na / M molar ratio is 0.6 to 0.72, and calcining the mixture, wherein M is a full metal except Na.

3. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 1, characterized in that: In said step a), The sodium composite transition metal oxide is a sodium byproduct remaining on the surface of oxide particles, including a combination of sodium hydroxide NaOH and sodium carbonate Na2CO3. The sodium composite transition metal oxide may include residual sodium at a weight ratio of sodium hydroxide to sodium carbonate NaOH / Na2CO3 of 50 to 110.

4. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 1, characterized in that: In said step a), During the stirring process of the reactor, the reactants increased from pH 6-8 to pH 10-12.

5. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 1, characterized in that: In the step a), the residual sodium content in the sodium composite transition metal oxide is 3000 ppm to 20000 ppm.

6. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 1, characterized in that: The step b) comprises: Step b1), when the content of the cobalt element in the cobalt oxide coating is less than 2 mol% relative to the total metal M other than sodium in the sodium composite transition metal oxide, adding the cobalt salt without adding a sodium-containing alkaline substance, and stirring; and Step b2), when the content of the cobalt element in the cobalt oxide coating is greater than 2 mol% relative to the total metal M other than sodium in the sodium composite transition metal oxide, When the cumulative cobalt content Co' / M added to the reactor is 0 mol% to 2 mol%, cobalt salt CS1 is added without adding sodium-containing alkaline substances, and stirring is performed. When the cumulative cobalt content Co' / M added to the reactor is greater than 2 mol%, cobalt salt CS2 and sodium-containing alkaline substance are added simultaneously and stirred.

7. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 6, characterized in that: In the cobalt hydroxide coprecipitation step b), step b3) is to add a terminal cobalt salt, or a cobalt salt and a sodium-containing alkaline substance after step b1) or step b2), and stir the substances added to the reactor for an additional 1 minute to 10 minutes.

8. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 6, characterized in that: The residual sodium in step a) and the sodium-containing alkaline substance in step b2) are substances that are partially or completely ionized in the water washing liquid and show alkalinity. The cobalt salt in step b) is a substance that is partially or completely ionized in the water washing liquid and exhibits acidity.

9. A positive electrode active material for a sodium secondary battery, characterized in that: The invention comprises a plurality of sodium composite transition metal oxide particles, wherein the plurality of sodium composite transition metal oxide particles comprises a cobalt oxide coating formed on the surface and / or inside of the particles, In the plurality of sodium composite transition metal oxide particles, a relative standard deviation of an atomic molar ratio Co / M of cobalt to all metals M excluding sodium is less than 30 at any four points selected by EDS mapping analysis.

10. The positive electrode active material for sodium secondary battery according to claim 9, characterized in that In the plurality of sodium composite transition metal oxide particles, a relative standard deviation of an atomic molar ratio Co / M of cobalt to all metals M excluding sodium is 2 to 20 at any four points selected by EDS mapping analysis.

11. The positive electrode active material for sodium secondary battery according to claim 9, characterized in that: The sodium composite transition metal oxide is a sodium-manganese oxide including at least sodium, nickel and manganese.

12. The positive electrode active material for sodium secondary battery according to claim 9, characterized in that: The sodium composite transition metal oxide is represented by the following chemical formula 1: [Chemical formula 1] <h2 style=";text-align:left;direction:ltr">Na<h2 style=";text-align:left;direction:ltr"> a <h2 style=";text-align:left;direction:ltr"> Ni<h2 style=";text-align:left;direction:ltr"> x <h2 style=";text-align:left;direction:ltr"> M1<h2 style=";text-align:left;direction:ltr"> y <h2 style=";text-align:left;direction:ltr"> M2<h2 style=";text-align:left;direction:ltr"> z <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 1-x-y-z <h2 style=";text-align:left;direction:ltr"> O2 In the chemical formula 1, M1 is Co or Fe, M2 is at least one selected from Co, P, Sr, Ba, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu, M1 and M2 are different elements. 0.50≤a≤0.80, 0.05≤x≤0.45, 0≤y≤0.45, 0≤z≤0.1, 0.55≤1-xyz≤0.

85.

13. The positive electrode active material for sodium secondary battery according to claim 9, characterized in that: The cobalt oxide coating includes sodium cobalt oxide NaCoO2, cobalt oxide Co2O3 or a combination thereof.

14. The positive electrode active material for sodium secondary battery according to claim 9, characterized in that: The content Co of cobalt included in the cobalt oxide coating layer is 0.1 mol % to 10 mol % relative to the total metal M excluding sodium of the sodium composite transition metal oxide.

15. The positive electrode active material for sodium secondary battery according to claim 9, characterized in that: The plurality of sodium composite transition metal oxide particles including a cobalt oxide coating layer formed on the surface and / or inside of the particles include 10000 ppm or less of residual sodium.

16. A positive electrode for a sodium secondary battery, characterized in that: The positive electrode active material according to claim 9 is included.

17. A sodium secondary battery, characterized in that: include: The positive electrode according to claim 16; the negative electrode; and the electrolyte.

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