Positive electrode active material for sodium secondary battery, method for producing the same, positive electrode for sodium secondary battery, and sodium secondary battery including the same

Abstract

To achieve a simplified process and an improved process cost.SOLUTION: The present invention provides a method for preparing a positive electrode active material for a sodium secondary battery. The method includes the steps of: a) inputting a sodium composite transition metal oxide and a water washing solution into a reactor and stirring the reactants to dissolve residual sodium on the surface of the sodium composite transition metal oxide in the water washing solution; b) inputting a cobalt salt into the reactor and performing stirring to coprecipitate cobalt hydroxide onto sodium composite transition metal oxide particles; and c) heat-treating the sodium composite transition metal oxide particles on which the cobalt hydroxide is formed to form a cobalt coating layer on the particles.SELECTED DRAWING: Figure 1a

Description

[Technical Field]

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

[0002] Lithium-ion secondary batteries have been widely used as energy storage devices in various electronic technology fields. In recent years, the demand for lithium-ion secondary batteries has skyrocketed, and sodium-ion secondary batteries have attracted attention as an alternative to the expensive metal lithium. Sodium-ion secondary batteries are one of the next-generation materials with great potential for application in secondary batteries, as they have an intercalation / extraction reaction operating principle similar to that of lithium-ion secondary batteries.

[0003] The cathode active material for sodium-ion secondary batteries typically uses layered transition metal oxides, which have a simple structure, excellent electrochemical performance, and are easy to synthesize. However, during the calcination process in the manufacture of the cathode active material, the amount of residual sodium by-products present on the particle surface in the form of Na2CO3 and NaOH increases. This leads to gas generation due to electrolyte side reactions during battery operation, reducing the capacity and output of the cathode material and reducing the battery's lifespan and stability.

[0004] A water washing process is essential to remove sodium by-products remaining on the surface of the positive electrode active material. However, this process can cause surface defects in the positive electrode active material, resulting in a rapid decline in lifespan characteristics. To address this issue, surface coating of the positive electrode active material particles is applied after washing. However, conventional coating processes result in island-like coatings on the surface of the positive electrode active material particles, making it difficult to form a uniform coating layer across the entire particle surface. Furthermore, an uneven coating layer acts as a resistive layer, resulting in no capacity improvement before or after coating and degrading the electrochemical characteristics of the cell.

[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 alleviate the deterioration of the life characteristics due to the water washing process. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] China Patent Registration CN 115028215 [Patent Document 2] Korean Patent Publication KR 10-2021-0118684 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a manufacturing method for a positive electrode active material for a sodium secondary battery, which improves cell performance such as surface stability, capacity, and life characteristics by simultaneously performing a water-washing step and a coating step and forming a uniform and capacitive coating layer on the entire surface of positive electrode active material particles. In addition, the present invention aims to simplify the process and improve process costs by reducing the amount of basic substance (NaOH) added during the coating layer formation process using the residual sodium on the surface of the sodium transition metal composite oxide.

[0008] Furthermore, in the present invention, by adjusting the sodium equivalent (Na / M) and the composition of the high-Mn transition metal added when producing the sodium transition metal composite oxide, the total amount of residual sodium on the surface of the sodium transition metal oxide and the ratio of residual sodium compounds (NaOH / Na2CO3) can be controlled within a specific range, thereby optimizing the amount of basic substance (NaOH) added in the coating layer formation process.

[0009] In addition, the present invention provides a method for coating a positive electrode active material for a sodium secondary battery, which can apply different processes depending on the coating amount of the positive electrode active material, thereby forming a uniform coating amount regardless of changes in the coating amount.

[0010] The present invention also provides a positive electrode active material for a sodium secondary battery, which is produced by the above-mentioned production method and has a uniform and capacitive coating layer formed on the entire surface of the positive electrode active material particles. [Means for solving the problem]

[0011] One embodiment of the present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery, the method comprising the steps of: a) adding a sodium composite transition metal oxide and a washing solution to a reactor and stirring the reactants to dissolve residual sodium on the surface of the sodium composite transition metal oxide in the washing solution; b) adding a cobalt salt to the reactor and stirring the reactants to coprecipitate cobalt hydroxide on the sodium composite transition metal oxide particles; and c) heat-treating the sodium composite transition metal oxide particles on which the cobalt hydroxide has been formed to form a cobalt coating layer on the particles.

[0012] In step a), the sodium composite transition metal oxide may be prepared by mixing a transition metal hydroxide precursor and a sodium compound so that the molar ratio of Na / M (M=total metals excluding Na) is 0.6 to 0.72, and then calcining the mixture.

[0013] In step a), the sodium composite transition metal oxide may contain a combination of sodium hydroxide (NaOH) and sodium carbonate (Na2CO3) as sodium by-products remaining on the surface of the oxide particles, and the sodium composite transition metal oxide may contain residual sodium in a weight ratio of sodium hydroxide to sodium carbonate (NaOH / Na2CO3) of 50 to 110.

[0014] In step a), the pH of the reaction mixture may be increased from pH 6-8 to pH 10-12 while stirring the reactor.

[0015] In step a), the sodium composite transition metal oxide may have a residual sodium content (TTS: Total sodium, ppm) of 3,000 to 20,000 ppm.

[0016] The step b) may include the steps of: b1) adding the cobalt salt and stirring without adding a sodium-containing basic material when the content of cobalt element contained in the cobalt oxide coating layer is 2 mol% or less based on the total metals (M) excluding sodium of the sodium composite transition metal oxide; and b2) adding the cobalt salt (CS1) and stirring without adding a sodium-containing basic material when the cumulative content of cobalt element introduced into the reactor (Co' / M) is 0 to 2 mol% when the content of cobalt element contained in the cobalt oxide coating layer is more than 2 mol% based on the total metals (M) excluding sodium of the sodium composite transition metal oxide; and adding the cobalt salt (CS2) and the sodium-containing basic material together and stirring when the cumulative content of cobalt element introduced into the reactor (Co' / M) exceeds 2 mol%.

[0017] b3) In the step b1) or b2), the cobalt hydroxide coprecipitation step may include interrupting the addition of the cobalt salt, or the cobalt salt and the sodium-containing basic material, and further stirring the materials added to the reactor for 1 to 10 minutes.

[0018] The residual sodium in step a) and the sodium-containing basic substance in step b2) may be a substance that is partially or completely ionized in the washing solution to exhibit basicity, and the cobalt salt in step b) may be a substance that is partially or completely ionized in the washing solution to exhibit acidity.

[0019] Another embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, comprising a plurality of sodium composite transition metal oxide particles each having a cobalt oxide coating layer formed on the surface and / or interior of the particle, wherein the plurality of sodium composite transition metal oxide particles have a relative standard deviation (RSD) of less than 30 in atomic molar ratio of cobalt to total metals excluding sodium (M) (Co / M) at any four points selected through EDS mapping analysis.

[0020] The plurality of sodium composite transition metal oxide particles may have a relative standard deviation (RSD) of 2 to 20 of the atomic molar ratio (Co / M) of cobalt to all metals (M) excluding sodium at any four points selected through EDS mapping analysis.

[0021] The sodium composite transition metal oxide may be a sodium manganese-based oxide containing at least sodium, nickel, and manganese.

[0022] The sodium composite transition metal oxide may be represented by the following chemical formula 1:

[0023] [ka] ...chemical formula 1

[0024] In the above 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, The range may be 0.50≦a≦0.80, 0.05≦x≦0.45, 0≦y≦0.45, 0≦z≦0.1, and 0.55≦1−xyz≦0.85.

[0025] The cobalt oxide coating layer may include sodium cobalt oxide (NaCoO2), cobalt oxide (Co2O3), or a combination thereof.

[0026] The cobalt content (Co) in the cobalt oxide coating layer may be 0.1 to 10 mol % based on the total metal (M) of the sodium composite transition metal oxide excluding sodium.

[0027] 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 may contain residual sodium at 10,000 ppm or less.

[0028] Another embodiment 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. [Effects of the Invention]

[0029] According to the present invention, in a positive electrode active material for a sodium secondary battery, a water washing process for removing sodium by-products remaining on the surface and a coating process for improving surface defects and performance can be simultaneously performed, thereby simplifying the process and saving production time and process costs.

[0030] Furthermore, according to the present invention, by forming a uniform and capacitive coating layer on the entire surface of positive electrode active material particles for a sodium secondary battery, surface stability can be improved, and battery performance such as capacity and life characteristics can be improved. [Brief explanation of the drawings]

[0031] [Figure 1a] 1 shows the results of low-magnification (1,000 K) EDS mapping of the positive electrode active material prepared in Example 1. [Figure 1b] 1 shows the results of high-magnification (5,000 K) EDS mapping of the positive electrode active material prepared in Example 1. [Figure 2a] 1 shows the results of low-magnification (1,000 K) EDS mapping of the positive electrode active material prepared in Comparative Example 1. [Figure 2b] 1 shows the results of high-magnification (5,000 K) EDS mapping of the positive electrode active material prepared in Comparative Example 1. [Figure 3] 1 is a graph showing the first charge / discharge cycles of sodium secondary batteries manufactured in Example 1, Comparative Example 1, and Reference. DETAILED DESCRIPTION OF THE INVENTION

[0032] The advantages and features of the present invention, as well as methods for achieving them, will become more apparent from the following detailed description of the embodiments in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various different forms. However, the present invention is defined only by the scope of the claims, so that the disclosure of the present invention will be complete and those skilled in the art will be able to fully understand the scope of the invention.

[0033] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in the same manner as commonly understood by a person of ordinary skill in the art to which this invention belongs. Throughout this specification, when a part is referred to as "comprising" a certain element, this does not mean that it excludes other elements, but that it may further include other elements, unless otherwise specified. Furthermore, the singular form includes the plural form unless otherwise specified in the context.

[0034] The present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery, which simultaneously performs a water washing process and a coating process to form a coating layer that is uniformly and capacitively formed on the surface of positive electrode active material particles, the method comprising: (a) adding a sodium composite transition metal oxide and a washing solution to a reactor and stirring the reactants to dissolve residual sodium on the surface of the sodium composite transition metal oxide in the washing solution; (b) adding a cobalt salt to the reactor and stirring the mixture to coprecipitate cobalt hydroxide on the sodium composite transition metal oxide particles; and (c) heat-treating the sodium composite transition metal oxide particles with the cobalt hydroxide formed thereon to form a cobalt coating layer on the particles. This improves the surface stability of the positive electrode active material and improves electrochemical performance, such as capacity and life characteristics.

[0035] Step a) preferentially uses the sodium by-product remaining on the surface of the positive electrode active material without adding an additional basic substance to achieve the optimal pH for the coprecipitation of cobalt hydroxide. This step involves adding a sodium composite transition metal oxide and a washing solution to a reactor, stirring the reactants, and dissolving the sodium by-product (residual sodium) remaining on the surface of the sodium composite transition metal oxide into the washing solution. This allows the sodium by-product remaining on the surface of the sodium composite transition metal oxide particles to be dissolved in the washing solution, thereby controlling the pH (11-12) optimized for the coprecipitation of cobalt hydroxide. This simplifies the process and reduces process costs without the need for additional addition of a basic solution or ammonium solution.

[0036] In step a), the reaction mixture may be stirred at 100 to 300 rpm for 1 to 10 minutes, specifically at 200 to 400 rpm for 3 to 7 minutes or at 300 to 500 rpm for 4 to 6 minutes. As a result, the pH of the reaction mixture increases from 6 to 8 to 10 to 12 while the reactor is being stirred, and the optimum pH for coprecipitation can be achieved using only residual sodium without adding an additional basic substance in the subsequent coprecipitation step.

[0037] The sodium composite transition metal oxide may be produced by mixing a transition metal hydroxide precursor and a sodium compound so that the Na / M (M=total metals excluding Na) molar ratio is 0.6 to 0.72, specifically 0.63 to 0.72, 0.65 to 0.71, 0.66 to 0.71, or preferably 0.68 to 0.71, and then calcining the mixture.

[0038] The produced sodium composite transition metal oxide may be a high-Mn sodium nickel manganese oxide (NNMO) represented by the following Chemical Formula 1.

[0039] [ka] ...chemical formula 1

[0040] In the above 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, The range may be 0.50≦a≦0.80, 0.05≦x≦0.45, 0≦y≦0.45, 0≦z≦0.1, and 0.55≦1−xyz≦0.85.

[0041] As the proportion of Mn in a high-Mn transition metal composition increases, the amount of residual sodium on the surface of oxide particles after the production of a sodium composite transition metal oxide tends to increase. In the present invention, the amount of basic substance used in Co co-precipitation coating can be reduced by making maximum use of the sodium by-product remaining on the surface.

[0042] The sodium composite transition metal oxide may contain a combination of sodium hydroxide (NaOH) and sodium carbonate (Na2CO3) as sodium by-products that remain on the surface of the oxide particles.

[0043] Referring to the following reaction formula, NaOH, which is a strong basic component of the residual sodium, can be used in place of the basic substance (NaOH) that is added during coprecipitation coating to achieve the optimal coprecipitation pH of 11 to 12, and this process also has the effect of removing residual sodium. However, Na2CO3, which is a weak basic component of the residual sodium, requires a large amount of residual sodium to achieve the optimal coprecipitation pH of 11 to 12. If the total amount of residual sodium increases, it becomes difficult to achieve uniform coating during the coprecipitation coating process. Therefore, the present invention aims to produce / use the sodium composite transition metal oxide so that i) the total amount of residual sodium is within an appropriate range, and ii) it has a specific (NaOH / Na2CO3) weight ratio, in order to perform the coprecipitation process using NaOH, which can essentially be used as a basic component of the residual sodium during the coprecipitation process.

[0044] NaOH(aq.)------------>>(Na + )+(OH - )·······(1) Na2CO3+H2O<----->>(2Na + )+(HCO3 - )+(OH - )··(2) (HCO3 - )+(H2O)<------>>(H2CO3)+(OH - )······(3)

[0045] In reaction formula (1), NaOH is a strong base, so Na + and OH - It exists as an ion, and in reaction (2) Na2CO3 is a salt of a strong base and a weak acid. It dissolves in the washing solution and exhibits weak basicity, and in reaction (3) HCO3 - reacts with water again to form OH - It exhibits weak basicity, generating

[0046] The sodium composite transition metal oxide may contain sodium hydroxide to sodium carbonate containing residual sodium in a weight ratio (NaOH / Na2CO3) of 50 to 110, for example, 65 to 110, 65 to 105, 70 to 110, or 70 to 105.

[0047] If the (NaOH / Na2CO3) weight ratio exceeds the design range, the proportion of NaOH, a relatively strong base, increases, which can cause the pH of the washing solution in the (a) washing step to increase excessively (exceeding the optimal coprecipitation pH of 11-12). Therefore, even if cobalt hydroxide is added alone in the (b) coprecipitation step (step b1), the coprecipitation occurs at a pH above the optimal coprecipitation pH of 11-12, making uniform cobalt coating difficult. Furthermore, the overall residual sodium content increases excessively, making it difficult to achieve uniform coating. Conversely, if the (NaOH / Na2CO3) weight ratio is below the design range, the proportion of Na2CO3, a relatively weak base, increases, which can cause the pH of the washing solution in the (a) washing step to decrease excessively (below the optimal coprecipitation pH of 11-12). b) If only cobalt hydroxide is added in the coprecipitation step (step b1), the coprecipitation occurs under conditions that do not reach the optimum pH for coprecipitation, 11-12, making it difficult to perform cobalt coating. This increases the additional input of basic substances (NaOH) used in the coating layer formation process, as in conventional technology, making it difficult to utilize the remaining sodium, leading to problems such as increased process complexity and cost.

[0048] The sodium composite transition metal oxide may have a residual sodium content (TTS: total sodium, ppm) of 3,000 to 20,000 ppm, for example, 5,000 to 20,000 ppm, 8,000 to 20,000 ppm, 10,000 to 20,000 ppm, 12,000 to 20,000 ppm, 13,000 to 18,000 ppm, or 14,000 to 17,000 ppm. This allows the coprecipitation step to be performed using NaOH, which can be used as a basic substance in the coprecipitation step, from the residual sodium, so that the sodium composite transition metal oxide can be prepared i) within an appropriate range of the total residual sodium content and ii) with a specific (NaOH / Na2CO3) weight ratio.

[0049] The washing liquid may be a washing liquid that is usually used when washing a sodium composite transition metal oxide, and may be, for example, ethanol, distilled water, or deionized water.

[0050] The reactor may be any co-precipitation reactor commonly used in the preparation of a positive electrode active material, such as a batch reactor, a Couette-Taylor reactor, etc. The use of the Couette-Taylor reactor has the advantage of reducing process time by allowing the co-precipitation of cobalt hydroxide to proceed more quickly than in a typical reactor such as a batch reactor.

[0051] Step b) is for uniformly coprecipitating cobalt hydroxide on the surface of the sodium composite transition metal oxide particles, and is a step of adding a cobalt salt to the reactor and stirring to coprecipitate cobalt hydroxide on the sodium composite transition metal oxide particles. Thus, the coprecipitation process may be applied differently depending on the cumulative coprecipitated amount (coating amount) of cobalt hydroxide.

[0052] Specifically, in step b), if the content of cobalt element contained in the cobalt oxide coating layer is 2 mol% or less based on the total metal (M) excluding sodium of the sodium composite transition metal oxide, step b1) of adding the cobalt salt and stirring without adding the sodium-containing basic material may be performed.

[0053] In addition, in step b), step b2) may include the steps of: adding a cobalt salt (CS1) and stirring without adding a sodium-containing basic material when the cumulative cobalt content (Co' / M) of the reactor is 0 to 2 mol% if the content of cobalt element contained in the cobalt oxide coating layer is more than 2 mol% relative to the total metal (M) excluding sodium in the sodium composite transition metal oxide; and adding a cobalt salt (CS2) and a sodium-containing basic material together and stirring when the cumulative cobalt content (Co' / M) of the reactor is more than 2 mol%.

[0054] Here, all metals (M) except for sodium in the sodium composite transition metal oxide may include the transition metals contained in the cobalt oxide coating layer.

[0055] Meanwhile, the cobalt salt is a cobalt-containing raw material, and the cobalt-containing raw material may be a cobalt-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, or oxyhydroxide, etc., specifically, Co(OH)2, CoOOH, Co(OCOCH3)2·4H2O, Co(NO3)2·6H2O, CoSO4, CoSO4·7H2O, or a combination thereof, but is not limited thereto.

[0056] The basic solution may be a hydroxide of an alkali metal or alkaline earth metal, such as NaOH, KOH, or Ca(OH), a hydrate thereof, or a combination thereof. The basic compound may also be used in the form of an aqueous solution, and in this case, the solvent may be water or a mixture of water and an organic solvent (e.g., alcohol) that is uniformly miscible with water.

[0057] The basic solution is added to adjust the pH of the reaction solution in the reactor, and may be added in an amount such that the pH of the reaction solution in the reactor becomes 11 to 12 during coprecipitation of the cobalt hydroxide.

[0058] The coprecipitation reaction may be carried out in an inert atmosphere such as nitrogen or argon at a temperature of 40°C to 70°C, and the coprecipitation time for coprecipitating the cobalt hydroxide may be 1 minute to 1 hour, more preferably 5 minutes to 1 hour, and even more preferably 10 minutes to 50 minutes. By adjusting the coprecipitation time for the cobalt hydroxide within this range, it is possible to effectively form a coating layer on the surface of the sodium composite transition metal oxide particles to improve surface defects and battery performance, while minimizing film resistance due to the formation of the coating layer. Furthermore, it is possible to shorten the coprecipitation process time from approximately 2 hours in the past, thereby simplifying the process, reducing process costs, and forming a uniform coating layer.

[0059] Through the above process, the sodium composite transition metal oxide is stirred with a washing solution to form cobalt hydroxide on the surface of the sodium composite transition metal oxide particles. This allows the washing process for removing sodium by-products remaining on the particle surface and the coating process for improving surface defects and performance to be performed simultaneously, thereby simplifying the process, reducing production time and process costs, and forming a uniform coating layer over the entire particle surface.

[0060] Step c) is a step of heat-treating the sodium composite transition metal oxide particles on which the cobalt hydroxide is formed, in order to form a cobalt coating layer on the surface of the sodium composite transition metal oxide particles.

[0061] The heat treatment may be carried out at 500 to 900° C., more preferably 600 to 900° C., and even more preferably 700 to 800° C. By carrying out the heat treatment within this temperature range, a coating layer containing sodium cobalt oxide with a layered structure may be formed on the surface of the particles.

[0062] The heat treatment may be performed by further mixing a coating source. The coating source may be a coating source used for surface coating of the sodium composite transition metal oxide particles, and may include, for example, at least one selected from the group consisting of B, Al, F, W, Mo, Ti, and Nb, but is not limited thereto.

[0063] By the above-mentioned process, the cobalt hydroxide on the surface of the sodium composite transition metal oxide particles is converted into (sodium) cobalt oxide having capacity, thereby improving not only the surface stability but also the capacity characteristics.

[0064] Another embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, comprising a plurality of sodium composite transition metal oxide particles each having a cobalt oxide coating layer formed on the surface and / or interior of the particle, wherein the sodium composite transition metal oxide particles have a relative standard deviation (RSD) of the atomic molar ratio of cobalt to total metals (M) excluding sodium (Co / M) of less than 30 at any four points selected through EDS mapping analysis, and may be, specifically, 2 to 20, 2 to 15, 2 to 10, 2 to 5, or 2 to 4.

[0065] This allows a uniform and capacitive coating layer to be formed over the entire surface of the positive electrode active material particles. If the relative standard deviation of the atomic molar ratio Co / M does not satisfy the above range, the non-uniform coating layer may act as a resistance layer, resulting in no improvement in capacity before and after coating and further deterioration of battery characteristics.

[0066] Meanwhile, the relative standard deviation (RSD), also known as the coefficient of variation, indicates the relative magnitude of the standard deviation with respect to the mean value, and can be calculated by dividing the standard deviation by the arithmetic mean (standard deviation / mean x 100). In the present invention, as an index for determining whether the cobalt coating layer formed on the surface of the plurality of sodium composite transition metal oxide particles is uniformly formed, the relative standard deviation (standard deviation / mean x 100) may be calculated by calculating the standard deviation and mean of the atomic molar ratio of cobalt to the total metal (M) (Co / M) calculated at any four points including the plurality of oxide particles through EDS mapping analysis.

[0067] The sodium composite transition metal oxide may be an NNMO-based sodium manganese-based oxide containing at least sodium, nickel, and manganese. The sodium manganese-based oxide is a high-Mn oxide containing 55 mol% or more of manganese among all metals excluding sodium. The manganese content among all metals excluding sodium may be 55 mol% or more, 60 mol% or more, or 65 mol% or more. The upper limit is not particularly limited, and may be, for example, 85 mol% or less, 80 mol% or less, or 75 mol% or less. Increasing the manganese content among all metals excluding sodium has the advantage of providing high capacity under high-voltage operating conditions and reducing the concentration of nickel and cobalt, thereby improving price competitiveness.

[0068] The sodium composite transition metal oxide may exist in the form of secondary particles formed by aggregation of at least one primary particle, and the oxide particles may have an average particle size (D50) of 2 to 15 μm, for example, but not limited to, 4 to 10 μm. In this case, the electrode density is increased, and the energy density per unit volume of the electrode can be improved.

[0069] The sodium composite transition metal oxide is represented by the following chemical formula 1.

[0070] [ka] ...chemical formula 1

[0071] In the above 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, The range may be 0.50≦a≦0.80, 0.05≦x≦0.45, 0≦y≦0.45, 0≦z≦0.1, and 0.55≦1−xyz≦0.85.

[0072] The sodium composite transition metal oxide of Formula 1 may have a molar ratio (Na / M) of sodium (Na) to all metals excluding sodium (M) of 0.5 to 0.8. In Formula 1, if the content of Na, corresponding to a, is less than 0.5, the capacity may decrease, while if it exceeds 0.8, the position of the sodium ion may change, resulting in an O3-type crystal structure. O3-type positive electrode active materials have lower air and moisture stability and are more sensitive to synthesis conditions (temperature, atmosphere, etc.) than P2-type positive electrode active materials. The Na content may more preferably be 0.60≦a≦0.80, 0.60≦a≦0.75, or 0.65≦a≦0.75.

[0073] The cobalt oxide coating layer may be formed by surface Co coating and internal Co diffusion of the sodium composite transition metal oxide particles, thereby forming a uniform and capacitive coating layer on the entire surface of the positive electrode active material particles, thereby improving surface stability and battery performance such as capacity and life characteristics.

[0074] The content of cobalt (Co) in the cobalt oxide coating layer may be 0.1 to 10 mol%, for example, 0.5 to 7 mol%, 0.5 to 5 mol%, 0.5 to 4.5 mol%, or 0.5 to 4 mol%, based on the total metals (M) excluding sodium in the sodium composite transition metal oxide.

[0075] 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 may contain residual sodium (TTS: total sodium, ppm) of 10,000 ppm or less, for example, 2,000 to 10,000 ppm, 2,000 to 7,000 ppm, or 2,000 to 5,000 ppm.

[0076] The plurality of sodium composite transition metal oxide particles may contain 20,000 ppm or less of residual sodium (TTS: total sodium, ppm) before forming (including) the cobalt oxide coating layer, for example, 8,000 to 20,000 ppm, 8,000 to 15,000 ppm, or 8,000 to 10,000 ppm.

[0077] Meanwhile, the residual sodium content of the plurality of sodium composite transition metal oxide particles before forming (including) the coating layer may generally be measured without undergoing a water washing process.

[0078] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery and a sodium secondary battery comprising the positive electrode active material.

[0079] The positive electrode includes a positive electrode current collector and a positive electrode active material layer disposed 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.

[0080] 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 surface-treated with carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector typically has a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to enhance the adhesive strength of the positive electrode active material. Such positive electrode current collectors may be provided in various forms, such as films, sheets, foils, nets, porous bodies, foams, nonwoven fabrics, etc.

[0081] The positive electrode active material layer may be a layer containing a conductive material and a binder in addition to the above-mentioned positive electrode active material.

[0082] Here, the conductive material is used to impart conductivity to the electrode and can be any material that is conductive and does not cause chemical changes in the positive electrode active material. Non-limiting examples of conductive materials include graphite such as natural graphite and artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon-based materials such as carbon fiber, metal powder or metal fiber such as copper, nickel, aluminum, or silver, conductive whiskers such as zinc oxide or potassium titanate, conductive metal oxides such as titanium oxide, and conductive polymers such as polyphenylene derivatives. The conductive material may typically be included in an amount of 1 wt % to 30 wt % based on the total weight of the positive electrode active material layer.

[0083] The binder is a substance that improves adhesion between positive electrode active material particles and 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, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. The binder may typically be included in an amount of 1 wt% to 30 wt% based on the total weight of the positive electrode active material layer.

[0084] The positive electrode according to an embodiment of the present invention may be manufactured by a conventional method for manufacturing a positive electrode for a sodium secondary battery, except that the positive electrode active material described above is used. For example, the positive electrode may be manufactured by coating a positive electrode active material layer-forming slurry containing the positive electrode active material and, optionally, a binder and a conductive material, on a positive electrode current collector, followed by drying and rolling. In another example, the positive electrode may be manufactured by casting the positive electrode active material layer-forming slurry on a separate support, peeling the positive electrode active material layer from the support, and laminating the resulting film on a positive electrode current collector.

[0085] According to yet another aspect of the present invention, there is provided an electrochemical device including the above-described positive electrode. The electrochemical device may be, for example, a battery or a capacitor, and more specifically, a sodium secondary battery.

[0086] A sodium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte (electrolytic solution). The sodium secondary battery may also include a battery container (case) that houses the electrode assembly including the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0087] Depending on the shape of the battery container (case), sodium secondary batteries can be classified 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 an aluminum laminate.

[0088] In particular, in the case of a pouch-type sodium secondary battery using a cathode including the cathode active material according to various embodiments of the present invention, there is a low possibility of a side reaction occurring between the cathode active material and the electrolyte, which has the advantage of improving stability during storage and / or operation and reducing gas generation.

[0089] The present invention will be described in detail below with reference to examples. However, these examples are for the purpose of explaining the present invention in more detail, and the scope of the present invention is not limited to the following examples.

[0090] Example Production Example 1: Production of positive electrode active material Example 1 a) Preparation of sodium composite transition metal oxide Ni 0.35 Mn 0.65 The sodium compound Na2CO3 was added to the (OH)2 precursor in an amount of Na / M = 0.70 equivalents, and the mixture was heat-treated at 950°C in an air atmosphere for 12 hours to produce sodium nickel manganese oxide positive electrode active material powder (Na 0.67 Ni 0.35 Mn 0.65 O2 powder was obtained.

[0091] b) Cobalt hydroxide coprecipitation DIW and Na were added to a 5 L batch reactor. 0.67 Ni 0.35 Mn 0.65 O2 was introduced and the mixture was stirred at 300 rpm for 5 minutes.

[0092] Next, while maintaining the reactor at 25°C, 300 rpm, and pH 11-12, the amount of cobalt sulfate aqueous solution added was adjusted until the Co / M (=Ni+Mn) atomic molar ratio became 2 mol%, and cobalt hydroxide was coprecipitated for 10 minutes.

[0093] Next, while maintaining the reactor at 25°C, 300 rpm, and pH 11-12, the amounts of cobalt sulfate aqueous solution and NaOH aqueous solution added were adjusted until the Co / M (= Ni + Mn) atomic molar ratio reached 3 mol%, and cobalt hydroxide was co-precipitated for 30 minutes.

[0094] After the reactor was further stirred at 300 rpm for 5 minutes, the resulting particles were separated and dried in a vacuum oven at 110° C. for 24 hours.

[0095] c) Formation of a cobalt oxide coating layer The dried particle powder was placed in an alumina crucible and heat-treated at 800°C for 12 hours in an air atmosphere to produce a sodium composite transition metal oxide positive electrode active material in which a cobalt oxide coating layer was formed on the surface of the particles.

[0096] (Comparative Example 1) A 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.

[0097] b) Cobalt hydroxide coprecipitation process DIW was added to a 5 L batch reactor, and 23.9 g of 1 M NaOH solution was added. The mixture was maintained at 25°C and a stirring speed of 300 rpm for 15 minutes until the pH reached 11-12. 0.67 Ni 0.35 Mn 0.65 O2 was added and the mixture was stirred at 300 rpm.

[0098] Next, while maintaining the reactor at 25°C, 300 rpm, and pH 11-12, the amount of cobalt sulfate aqueous solution added was adjusted until the Co / M (=Ni+Mn) atomic molar ratio became 3.4 mol%, and cobalt hydroxide was coprecipitated for 60 minutes.

[0099] After the reactor was further stirred at 300 rpm for 15 minutes, the resulting particles were separated and dried in a vacuum oven at 110° C. for 24 hours.

[0100] (Reference) Na prepared in step a) of Example 1 0.67 Ni 0.35 Mn 0.65 O2 powder was used as the positive electrode active material.

[0101] Manufacturing Example 2: Manufacturing of sodium secondary battery 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 N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry. The positive electrode slurry was uniformly coated on a 15 μm-thick aluminum foil and dried under vacuum at 135°C to prepare a positive electrode for a sodium secondary battery.

[0102] A sodium metal plate was used as a counter electrode for the positive electrode, a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) was used as a separator, and an electrolyte solution containing 1.15 M NaPF6 in a solvent containing ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7 was used to fabricate a sodium secondary battery (coin battery).

[0103] Experimental Example Experimental Example 1: Evaluation of Co coating uniformity (analysis of relative standard deviation RSD) After sampling the sample onto carbon tape, the analysis proceeded with a working distance of 15, an accelerating voltage of 15kV, and KCPS settings of 110-130. During the analysis, one cut was performed at low magnification (x1,000k) and four cuts were performed at high magnification (x5,000k). The atomic % values ​​of Cobalt, Ni, and Mn were then calculated, followed by the Co / (Ni + Mn) atomic ratio (Co mol%). The Co mol% RSD (relative standard deviation) was then calculated for four cuts at high magnification. The results are shown in Table 1 below.

[0104] The results of EDS are shown in Figures 1a, 1b, 2a and 2b.

[0105] [Table 1]

[0106] 1a, 1b, 2a, 2b, and Table 1, the Co-coated cathode active material prepared by the present invention had improved coating uniformity, with a 4-point relative standard deviation (RSD) of 2.1 in EDS Co mol% analysis. In contrast, the RSD value for Comparative Example 1, which used a conventional coating method, was 94.5, suggesting that the unevenly formed coating layer acts as a resistive layer, making it difficult to achieve capacity improvement before and after coating, resulting in degradation of cell characteristics.

[0107] Experimental Example 2: Evaluation of Co coating uniformity by adjusting the total amount of residual Na before coating and compound ratio (Relative Standard Deviation RSD analysis) a) In the sodium composite transition metal oxide manufacturing process, Ni 0.35 Mn 0.65 A Co-coated positive electrode active material was prepared in the same manner as in Example 1, except that the (OH) precursor was mixed with a sodium compound Na2CO3 in an amount (Na / M) equivalent as shown in Table 2 below.

[0108] [Table 2]

[0109] Referring to Table 2, in the case of a cathode active material manufactured with a 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) of the residual sodium is 67 to 104, and NaOH can be used as the residual sodium instead of the basic substance (NaOH) added to achieve the optimal co-precipitation pH of 11 to 12, i') uniform coating is possible, and ii') the residual sodium can be utilized to the maximum extent, resulting in simplified processes and improved process costs.

[0110] In addition, in the case of a cathode active material manufactured with a Na / M equivalent exceeding 0.7, i) the total amount of residual Na increases excessively, and ii) the weight ratio of sodium hydroxide to sodium carbonate (NaOH / Na2CO3) of the residual sodium increases excessively to 185.6 and 268.4, respectively, resulting in a co-precipitation coating reaction occurring at a pH exceeding the optimum co-precipitation pH of 11-12, which is analyzed to result in a decrease in coating uniformity.

[0111] In addition, in the case of a cathode active material prepared with a Na / M equivalent of 0.61 to 0.67, the (NaOH / Na2CO3) weight ratio is 51 and 61, which means that the proportion of Na2CO3, a relatively weak base, is high. As a result, the pH of the washing solution in the a) washing step decreases, which increases the amount of basic material (NaOH) added to the co-precipitation coating process. This makes it difficult to utilize the remaining sodium, which is analyzed to be a problem in terms of process complexity and increased process costs.

[0112] Experimental Example 3: Analysis of Unreacted Residual Na Content To measure the amount of residual sodium, 1 g of the sodium composite transition metal oxide was immersed in 5 g of distilled water and stirred for 5 minutes, and the filtrate was collected and titrated with 0.1 M HCl. The volume of HCl added until the pH of the filtrate reached 5 was measured, and the amount of unreacted sodium by-product remaining on the surface of the particles was analyzed.

[0113] The residual sodium content was determined by measuring each compound containing residual sodium (e.g., NaOH or Na2CO3) by potentiometric titration, and then separately calculating the total amount of sodium only (TTS, Total Sodium). The calculation method is the same as that of Equation 1 below.

[0114] [Formula 1] TTS (Total Na) = NaOH analysis value (%) × Na / NaOH + Na2CO3 analysis value (%) × 2Na / Na2CO3

[0115] [Table 3]

[0116] Referring to Table 3, when comparing before and after Co coating, NaOH was removed at a relatively higher rate than Na2CO3, which is analyzed as being due to the maximum use of NaOH among the sodium by-products remaining on the surface, thereby reducing the amount of basic substance used in Co co-precipitation coating.

[0117] Experimental Example 4: Evaluation of the electrochemical performance of sodium secondary batteries The sodium secondary batteries manufactured in Example 1, Comparative Example 1, and Reference were charged / discharged once at 25°C and 0.1C / 0.5C within a driving voltage range of 2.0V to 4.6V, and then the first charge / discharge capacity was measured. The results are shown in Figure 3.

[0118] Referring to Figure 3, it was confirmed that the Co-coated positive electrode active material manufactured by the manufacturing method of the present invention has a uniform and capacitive coating layer formed on the entire surface of the particles, improving surface stability and resulting in improved capacity and lifespan (capacity retention rate) characteristics compared to Ref.

[0119] On the other hand, in the case of Comparative Example 1, the unevenly formed Co coating layer acted as a resistance layer, resulting in no improvement in discharge capacity and a deterioration in capacity retention rate.

[0120] As mentioned above, although the present invention has been illustrated and described with reference to specific embodiments, it will be obvious to those skilled in the art that the present invention can be modified and changed in various ways without departing from the technical spirit of the present invention as defined by the following claims.

Claims

1. a) adding a sodium composite transition metal oxide and a washing solution to a reactor and stirring the reactants to dissolve residual sodium on the surface of the sodium composite transition metal oxide into the washing solution; b) adding a cobalt salt to the reactor and stirring to co-precipitate cobalt hydroxide with the sodium composite transition metal oxide particles; and c) heat-treating the sodium composite transition metal oxide particles having the cobalt hydroxide formed thereon to form a cobalt coating layer on the particles.

2. In the step a), 2. The method for producing a positive electrode active material for a sodium secondary battery according to claim 1, wherein the sodium composite transition metal oxide is produced by mixing a transition metal hydroxide precursor and a sodium compound so that a molar ratio of Na / M (M=total metals excluding Na) is 0.6 to 0.72, and then calcining the mixture.

3. In the step a), The sodium composite transition metal oxide contains sodium hydroxide (NaOH) and sodium carbonate (Na) as sodium by-products remaining on the surface of the oxide particles. ２ CO ３ ) combinations, The sodium composite transition metal oxide is prepared by dissolving residual sodium in a weight ratio of sodium hydroxide to sodium carbonate (NaOH / Na ２ CO ３ 2. The method for producing a positive electrode active material for a sodium secondary battery according to claim 1, wherein the content of SiO2 is 50 to 110.

4. In the step a), 2. The method of claim 1, wherein the pH of the reactants is increased from 6 to 8 to 10 to 12 while stirring the reactor.

5. In the step a), 2. The method for manufacturing a positive electrode active material for a sodium secondary battery according to claim 1, wherein the sodium composite transition metal oxide has a residual sodium content (TTS: Total sodium, ppm) of 3,000 to 20,000 ppm.

6. The step b) comprises: b1) When the content of the cobalt element contained in the cobalt oxide coating layer is 2 mol% or less based on the total metal (M) excluding sodium of the sodium composite transition metal oxide, adding the cobalt salt and stirring without adding a sodium-containing basic substance; and b2) When the content of the cobalt element contained in the cobalt oxide coating layer exceeds 2 mol% based on the total metal (M) excluding sodium of the sodium composite transition metal oxide, A cobalt salt (CS1) is added to the reactor so that the cumulative cobalt content (Co' / M) is 0 to 2 mol%, and the mixture is stirred without adding a sodium-containing basic material; 2. The method of claim 1, further comprising the step of adding and stirring a cobalt salt (CS2) and a sodium-containing basic material together when the cumulative cobalt element content (Co' / M) added to the reactor exceeds 2 mol%.

7. The step b) of cobalt hydroxide coprecipitation is b3) After step b1) or b2), the addition of the cobalt salt or the cobalt salt and the sodium-containing basic material is stopped, and the materials added to the reactor are further stirred for 1 to 10 minutes.

8. The residual sodium in step a) and the sodium-containing basic substance in step b2) are substances that are partially or completely ionized in the washing solution and exhibit basicity, 7. The method of claim 6, wherein the cobalt salt in step b) is a material that is partially or completely ionized in the washing solution and exhibits acidity.

9. The present invention relates to a method for producing a cobalt oxide coating layer comprising: forming a cobalt oxide coating layer on the surface and / or the interior of a sodium composite transition metal oxide particle; The sodium composite transition metal oxide particles have a relative standard deviation (RSD) of less than 30 for the atomic molar ratio (Co / M) of cobalt to all metals excluding sodium (M) at any four points selected through EDS mapping analysis.

10. 10. The positive electrode active material for a sodium secondary battery according to claim 9, wherein the plurality of sodium composite transition metal oxide particles have a relative standard deviation (RSD) of 2 to 20 in atomic molar ratio (Co / M) of cobalt to total metal (M) excluding sodium at four arbitrary points selected through EDS mapping analysis.

11. 10. The positive electrode active material for a sodium secondary battery according to claim 9, wherein the sodium composite transition metal oxide is a sodium manganese-based oxide containing at least sodium, nickel, and manganese.

12. The positive electrode active material for a sodium secondary battery according to claim 9, wherein the sodium composite transition metal oxide is represented by the following Chemical Formula 1: 【Chemistry 1】 ...Chemical formula 1 In the above 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, and 0.55≦1−x−y−z≦0.

85.

13. The cobalt oxide coating layer is made of sodium cobalt oxide (NaCoO ２ ), cobalt oxide (Co ２ O ３ 10. The positive electrode active material for a sodium secondary battery according to claim 9, comprising:

14. 10. The positive electrode active material for a sodium secondary battery according to claim 9, wherein a content of cobalt (Co) contained in the cobalt oxide coating layer is 0.1 to 10 mol% based on the total metal (M) excluding sodium of the sodium composite transition metal oxide.

15. 10. The positive electrode active material for a sodium secondary battery according to claim 9, wherein the sodium composite transition metal oxide particles having a cobalt oxide coating layer formed on the surface and / or the interior of the particles contain residual sodium of 10,000 ppm or less.

16. A positive electrode for a sodium secondary battery, comprising the positive electrode active material according to claim 9 .

17. A sodium secondary battery comprising the positive electrode, negative electrode, and electrolyte according to claim 16.

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