Positive electrode active material for sodium secondary battery, method for preparing 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 increased sharply, and sodium-ion secondary batteries have been attracting attention as an alternative to lithium, an expensive metal. Sodium-ion secondary batteries have an operating principle of insertion / extraction reactions similar to that of lithium-ion secondary batteries, and are therefore one of the next-generation materials with high potential for application to secondary batteries.

[0003] The positive electrode 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 positive electrode active material, the amount of residual sodium by-products present in the form of Na2CO3 and NaOH on the particle surface increases, which leads to gas generation due to electrolyte side reactions during battery operation, a decrease in the capacity and output of the positive electrode material, and a decrease in the lifespan and stability of the battery.

[0004] A washing process is required to remove sodium by-products remaining on the surface of the positive electrode active material. However, after the washing process to remove sodium by-products, there is a problem that the life characteristics are rapidly reduced due to surface defects of the positive electrode active material, so a surface coating is applied to the positive electrode active material particles after washing. However, in the conventional coating process, the coating part is formed in an island shape on the surface of the positive electrode active material particles, making it difficult to form a uniform coating layer over the entire surface of the particles. Meanwhile, an uneven coating layer acts as a resistance layer, which does not improve the capacity before / after coating and deteriorates the electrochemical characteristics of the cell.

[0005] Therefore, a method is required to improve the performance of the positive electrode active material of a sodium ion battery and to improve 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 improving cell performance such as surface stability, capacity, and life characteristics by simultaneously carrying out a water-washing step and a coating step in the manufacture of a positive electrode active material for a sodium secondary battery, and further forming a uniform and capacitive coating layer on the entire surface of the positive electrode active material particles. In addition, the present invention aims to simplify the process and improve process costs by reducing the additional input amount of a basic substance (NaOH) used in the coating layer formation process by using the surface residual sodium of the sodium transition metal composite oxide.

[0008] In addition, 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 and the ratio of residual sodium compounds (NaOH / Na2CO3) on the surface of the sodium transition metal oxide 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 active material for a sodium secondary battery, which can apply different processes depending on the coating amount of the positive active material, thereby forming a uniform coating amount regardless of the change 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] In one embodiment of the present invention, there is provided a method for producing a positive electrode active material for a sodium secondary battery, the method comprising the steps of: a) introducing a sodium composite transition metal oxide and a washing solution into a reactor and stirring the reactants to dissolve residual sodium on a surface of the sodium composite transition metal oxide into the washing solution; b) introducing a cobalt salt into 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 having the cobalt hydroxide formed thereon to form a cobalt coating layer on the particles.

[0012] In step a), the sodium composite transition metal oxide may be produced 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 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 reaction may be increased in pH from 6-8 to 10-12 while stirring the reactor.

[0015] In the 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 substance when a content of the cobalt element contained in the cobalt oxide coating layer is 2 mol% or less based on a total metal (M) excluding sodium of the sodium composite transition metal oxide; and b2) adding a cobalt salt (CS1) and stirring without adding a sodium-containing basic substance when a cumulative content of the cobalt element (Co' / M) added to the reactor is 0 to 2 mol% when a content of the cobalt element contained in the cobalt oxide coating layer is more than 2 mol% based on a total metal (M) excluding sodium of the sodium composite transition metal oxide; and adding a cobalt salt (CS2) and a sodium-containing basic substance together and stirring when a cumulative content of the cobalt element (Co' / M) added to the reactor exceeds 2 mol%.

[0017] In the cobalt hydroxide coprecipitation step b3), after step b1) or step b2), the introduction of the cobalt salt, or the cobalt salt and the sodium-containing basic material is stopped, and the materials introduced into the reactor are further stirred 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 having a cobalt oxide coating layer formed on the surface and / or inside of the particles, wherein the plurality of sodium composite transition metal oxide particles have a relative standard deviation (RSD) of the atomic molar ratio (Co / M) of cobalt to all metals (M) excluding sodium of less than 30 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 in atomic molar ratio (Co / M) of cobalt to total metal (M) excluding sodium at any 4 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, It 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 metals (M) excluding sodium in the sodium composite transition metal oxide.

[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 of 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. Effect 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 are simultaneously performed, thereby simplifying the process and saving production time and process costs.

[0030] Furthermore, according to the present invention, a uniform and capacitive coating layer is formed on the entire surface of the positive electrode active material particles for a sodium secondary battery, thereby improving surface stability and improving battery performance such as capacity and life characteristics. [Brief description of the drawings]

[0031] [Figure 1a] 1 shows the results of low magnification (1.000K) 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 (1.000K) EDS mapping of the positive electrode active material prepared in Comparative Example 1. [Figure 2b] 1 shows the results of high magnification (5,000K) EDS mapping of the positive electrode active material prepared in Comparative Example 1. [Diagram 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 PREFERRED EMBODIMENTS

[0032] The advantages and features of the present invention, and the methods for achieving them, will become 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, and the embodiments are provided to make the disclosure of the present invention complete and to allow those skilled in the art to fully understand the scope of the invention, and the present invention is defined only by the scope of the claims.

[0033] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used as commonly understood by those having ordinary skill in the art to which the present invention belongs. Throughout the specification, when a part is described as "comprising" a certain component, this does not mean that it excludes other components, but that it may further include other components, unless otherwise specified to the contrary. In addition, the singular form includes the plural form unless otherwise specified in the context.

[0034] The present invention provides a method for producing a positive electrode active material for a sodium secondary battery, which comprises simultaneously performing a water washing process and a coating process, and forming a coating layer that is uniformly formed on the surface of particles of the positive electrode active material and has a capacity, the method comprising: a) feeding 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) feeding a cobalt salt into 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 is formed to form a cobalt coating layer on the particles, thereby improving the surface stability of the positive electrode active material and improving electrochemical performance such as capacity and life characteristics.

[0035] Step a) is a step of preferentially using the sodium by-product remaining on the surface of the positive active material without adding an additional basic substance in order to apply the optimal pH for the coprecipitation of cobalt hydroxide, in which a sodium composite transition metal oxide and a washing solution are added to a reactor, and the reactants are stirred to dissolve the sodium by-product (residual sodium) remaining on the surface of the sodium composite transition metal oxide in the washing solution. As a result, the sodium by-product remaining on the surface of the sodium composite transition metal oxide particles can be dissolved in the washing solution to control the pH (11-12) optimized for the coprecipitation of cobalt hydroxide, and there is no need to add an additional basic solution or ammonium solution, which simplifies the process and reduces the process cost.

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

[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, It 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 ratio of Mn in the high-Mn transition metal composition increases, the amount of residual sodium on the surface of the oxide particles after the production of the sodium composite transition metal oxide tends to increase. In the present invention, the amount of the basic substance used in the 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 include 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 of the residual sodium is a strong basic substance and can be used in place of the basic substance (NaOH) added to satisfy the optimum coprecipitation pH of 11-12 during coprecipitation coating, and this process also has the effect of removing residual sodium, but Na2CO3 of the residual sodium is a weak basic substance and requires a large amount of residual sodium to satisfy the optimum coprecipitation pH of 11-12, and if the total amount of residual sodium increases, it becomes difficult to perform uniform coating in the coprecipitation coating process. Therefore, the present invention aims to produce / use the sodium composite transition metal oxide i) within the appropriate total amount of residual sodium range and ii) to have a specific (NaOH / Na2CO3) weight ratio in order to perform the coprecipitation process using NaOH, which can be used as a basic substance in the coprecipitation process among the residual sodium.

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

[0045] In reaction (1), NaOH is a strong base, so Na + and OH - It exists as an ion. 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. In reaction (3), HCO3 - reacts with water again to form OH - It is a weak base that produces

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

[0047] If the (NaOH / Na2CO3) weight ratio exceeds the design range, the proportion of NaOH, which is a relatively strong base, increases, and the pH of the washing solution in the a) washing step may increase excessively (excessively exceeding the optimal coprecipitation pH of 11-12). As a result, even if only cobalt hydroxide is added in the b) coprecipitation step (step b1), the coprecipitation occurs at a pH exceeding the optimal coprecipitation pH of 11-12, making it difficult to achieve uniform cobalt coating. In addition, the total amount of residual sodium increases excessively, and the increased amount of residual sodium makes it difficult to achieve uniform coating. Conversely, if the (NaOH / Na2CO3) weight ratio is below the design range, the proportion of Na2CO3, which is a relatively weak base, increases, and the pH of the washing solution in the a) washing step may decrease excessively (less than the optimal coprecipitation pH of 11-12). b) If only cobalt hydroxide is added in the coprecipitation step (step b1), it becomes difficult to perform cobalt coating because the coprecipitation is performed under conditions that do not reach the optimal pH for coprecipitation, which is 11 to 12. This increases the additional input of basic substances (NaOH) used in the coating layer formation process as in the conventional technology, making it difficult to utilize the residual sodium, and increases the 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. In this way, the sodium composite transition metal oxide can be produced so that i) the total amount of residual sodium is within a suitable range, and ii) the sodium composite transition metal oxide has a specific (NaOH / Na2CO3) weight ratio, in order to carry out the coprecipitation step using NaOH, which can be substantially used as a basic substance in the coprecipitation step, of the residual sodium.

[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 coprecipitation reactor generally used in the preparation of a positive electrode active material, and may be, for example, a batch type reactor, a Couette-Taylor reactor, etc. When the Couette-Taylor reactor is used, the coprecipitation of cobalt hydroxide can be performed more quickly than in a general reactor such as a batch type reactor, thereby reducing the process time.

[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, and 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 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, step b1) of adding the cobalt salt and stirring may be performed without adding a sodium-containing basic substance.

[0053] In addition, in the step b), the step b2) may include a step of adding a cobalt salt (CS1) and stirring without adding a sodium-containing basic substance when the cumulative content of cobalt element (Co' / M) added to the reactor 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 metal (M) excluding sodium of the sodium composite transition metal oxide, and adding a cobalt salt (CS2) and a sodium-containing basic substance together and stirring the mixture when the cumulative content of cobalt element (Co' / M) added to the reactor exceeds 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 an alkaline earth metal, such as NaOH, KOH, or Ca(OH)2, 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 at a temperature of 40° C. to 70° C. in an inert atmosphere such as nitrogen or argon, and the coprecipitation time for coprecipitation of the cobalt hydroxide may be 1 minute to 1 hour, more preferably 5 minutes to 1 hour, and further preferably 10 minutes to 50 minutes. By adjusting the coprecipitation time of the cobalt hydroxide within the above range, a coating layer for improving surface defects and improving battery performance can be effectively formed on the surface of the sodium composite transition metal oxide particles, and the film resistance due to the formation of the coating layer can be minimized. In addition, the coprecipitation process time, which is conventionally long, of about 2 hours, can be shortened, and the process can be simplified, the process cost can be reduced, and a uniform coating layer can be formed.

[0059] According to 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, and thus the washing process for removing sodium by-products remaining on the particle surface and the coating process for improving surface defects and performance can be simultaneously performed, thereby simplifying the process, reducing production time and process costs, and forming a uniform coating layer on the entire particle surface.

[0060] Step c) is for forming a cobalt coating layer 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 on which the cobalt hydroxide is formed.

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

[0062] A coating source may be further mixed during the heat treatment and then heat treated. The coating source may be a coating source used for surface coating of sodium composite transition metal oxide particles, and may contain, 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, and thus not only the surface stability but also the capacity characteristics can be improved.

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

[0065] This allows a coating layer having a uniform capacity to be formed on 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-mentioned range, the non-uniform coating layer acts as a resistance layer, resulting in no capacity improvement effect before / after coating, and further deterioration of the battery characteristics.

[0066] Meanwhile, the relative standard deviation (RSD) is also called the coefficient of variation, and indicates the relative magnitude of the standard deviation with respect to the average value, and can be calculated by dividing the standard deviation by the arithmetic mean (standard deviation / average x 100). In the present invention, as an index for confirming 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 / average x 100) may be calculated by calculating the standard deviation and average of the atomic molar ratio (Co / M) of cobalt to the entire metal (M) calculated at any 4 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 except sodium, and the manganese content among all metals except sodium may be 55 mol% or more, 60 mol% or more, or 65 mol% or more, and 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. The higher the manganese content among all metals except sodium, the higher the capacity can be under a high voltage operating environment, and the concentration of nickel and cobalt can be reduced to improve price competitiveness.

[0068] The sodium composite transition metal oxide may be present 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, 4 to 10 μm, but is not limited thereto. 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, It 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 (M) excluding sodium 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, and if it exceeds 0.8, the position of the sodium ion may change to show an O3 type crystal structure, and the O3 type positive electrode active material has lower air and moisture safety and is more sensitive to synthesis conditions (temperature, atmosphere, etc.) than the P2 type. The Na 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, and thus a uniform and capacitive coating layer is formed on the entire surface of the positive electrode active material particles, thereby improving surface stability and improving 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 sodium composite transition metal oxide particles including the 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 residual sodium (TTS: total sodium, ppm) of 20,000 ppm or less 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 (including) forming the coating layer may generally be measured without performing 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, each including 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 embodiment 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 a chemical change in the battery and has conductivity, and may be, for example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. The positive electrode current collector may usually have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesive force of the positive electrode active material. Such a positive electrode current collector may be provided in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, 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 give conductivity to the electrode, and can be used without any particular limitation as long as it does not cause a chemical change in the positive electrode active material and has conductivity. Non-limiting examples of the conductive material 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 fibers, 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, and conductive polymers such as polyphenylene derivatives. The conductive material may be generally included in an amount of 1% by weight to 30% by weight based on the total weight of the positive electrode active material layer.

[0083] The binder is a material that plays a role in improving the adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material and the current collector. Non-limiting examples of the binder 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, or various copolymers thereof. The binder may be typically 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 method for manufacturing a positive electrode for a sodium secondary battery, except that the positive electrode active material is used. For example, a positive electrode active material layer forming slurry including a positive electrode active material and, optionally, a binder and a conductive material may be applied onto a positive electrode current collector, followed by drying and rolling to manufacture the positive electrode. According to another example, the positive electrode active material layer forming slurry may be cast onto a separate support, and the positive electrode active material layer may be peeled off from the support, and the resulting film may be laminated onto a positive electrode current collector to manufacture the positive electrode.

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

[0086] The 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 positive electrode including the positive electrode active material according to various embodiments of the present invention, there is an advantage that the possibility of a side reaction between the positive electrode active material and the electrolyte is low, thereby improving stability during storage and / or operation and reducing gas generation.

[0089] Hereinafter, the present invention will be described in detail with reference to examples. However, these examples are provided 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] Working 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 equivalent, and then heat-treated at 950℃ 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 5L 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 to 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 input amounts of cobalt sulfate aqueous solution and NaOH aqueous solution were adjusted until the Co / M (=Ni+Mn) atomic molar ratio became 3 mol%, and cobalt hydroxide was coprecipitated for 30 minutes.

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

[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 active material having a cobalt oxide coating layer 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 temperature was kept at 25°C and the stirring speed was 300 rpm for 15 minutes so that the pH was 11 to 12. Next, Na 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 min, the resulting particles were separated and dried in a vacuum oven at 110° C. for 24 h.

[0100] (References) 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 N-methyl-2-pyrrolidone (NMP) to prepare 30 g of positive electrode slurry. The positive electrode slurry was uniformly applied to a 15 μm-thick aluminum thin film and dried in vacuum at 135° C. to prepare a positive electrode for a sodium secondary battery.

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

[0103] Experimental Example Experimental Example 1: Evaluation of Co coating uniformity (Analysis of relative standard deviation RSD) After sampling the sample on the carbon tape, the analysis proceeded with a working distance of 15, an accelerating voltage of 15kV, and KCPS 110-130. During the analysis, one cut was performed at low magnification (x1.000k) in an arbitrary section, and four cuts were performed at high magnification (x5.000k) in an arbitrary section. After that, the atomic% values ​​of Cobalt, Ni, and Mn were calculated, and the Co / (Ni+Mn) atomic ratio (Co mol%) was calculated. After that, the Co mol% RSD (relative standard deviation) of the four cuts at high magnification was calculated. The calculation 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, it was confirmed that the Co-coated positive electrode active material to which the manufacturing method of the present invention was applied had improved coating uniformity, and the 4-point relative standard deviation (RSD) was reduced to 2.1 in the EDS Co mol% analysis. On the other hand, in the case of Comparative Example 1 to which the conventional coating method was applied, the RSD value was 94.5, and it is predicted that the unevenly formed coating layer acts as a resistance layer, making it difficult to obtain the capacity improvement effect before / after coating, and cell characteristics are deteriorated.

[0107] Experimental Example 2: Evaluation of Co coating uniformity by adjusting the total amount of residual Na and compound ratio before coating (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)2 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 positive 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 that can be used in place of the basic substance (NaOH) added to satisfy the optimal coprecipitation pH of 11 to 12 can be used as the residual sodium itself, 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 positive 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) among the residual sodium increases excessively to 185.6 and 268.4, respectively, resulting in a decrease in coating uniformity due to the co-precipitation coating reaction occurring in a state exceeding the optimal co-precipitation pH of 11 to 12.

[0111] In addition, in the case of a positive electrode active material prepared with a Na / M equivalent of 0.61 to 0.67, the (NaOH / Na2CO3) weight ratio is 51 and 61, and the proportion of Na2CO3, which is a relatively weak base, is high. As the pH of the washing solution in the a) washing step decreases, the amount of basic material (NaOH) added to the co-precipitation coating process increases, making it difficult to utilize the residual sodium, and increasing the process complexity and cost.

[0112] Experimental Example 3: Analysis of Unreacted Residual Na Content In order 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, the filtrate was taken and titrated with 0.1 M HCl, and the volume of HCl added until the pH of the filtrate reached 5 was measured to analyze the unreacted sodium by-product remaining on the surface of the particles.

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

[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 large rate compared to Na2CO3, which is analyzed as a reduction in the amount of basic substances used in Co co-precipitation coating by maximizing the use of NaOH among the sodium by-products remaining on the surface.

[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 within a driving voltage range of 2.0 V to 4.6 V at 0.1 C / 0.5 C, and then the 1st charge / discharge capacity was measured. The results are shown in FIG. 3.

[0118] Referring to FIG. 3, it was confirmed that the Co-coated positive electrode active material to which the manufacturing method of the present invention was applied had a uniform and capacitive coating layer formed on the entire surface of the particles, improving surface stability and resulting in improved capacity and life (capacity retention) 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 resistive layer, resulting in no improvement in discharge capacity and a deterioration in capacity retention rate.

[0120] As described above, the present invention has been illustrated and described with reference to specific embodiments. However, it will be apparent 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) charging a sodium composite transition metal oxide and a washing solution into 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 the mixture to coprecipitate 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), 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 calcining the mixture. The method for producing a positive electrode active material for a sodium secondary battery according to claim 1,

3. In the step a), The sodium composite transition metal oxide contains sodium hydroxide (NaOH) and sodium carbonate (Na ２ CO ３ ) combinations, The sodium composite transition metal oxide is a mixture of sodium hydroxide and sodium carbonate, and the weight ratio of the sodium hydroxide to the sodium carbonate is 1:1 (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 the hydrogen atom in the positive electrode active material is 50 to 110.

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

5. 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 has a residual sodium content (TTS: Total Sodium, ppm) of 3,000 to 20,000 ppm.

6. The step b) comprises: b1) 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 material; 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, The cobalt salt (CS1) is added to the reactor so that the cumulative cobalt element 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 a cumulative content (Co' / M) of the cobalt element added to the reactor exceeds 2 mol%.

7. The cobalt hydroxide coprecipitation step (b) is b3) after step b1) or step b2), stopping the introduction of the cobalt salt, or the cobalt salt and the sodium-containing basic material, and further stirring the materials introduced into the reactor for 1 to 10 minutes (min).

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 to exhibit acidity.

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

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

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 the cobalt content (Co) of 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. The sodium composite transition metal oxide particles having a cobalt oxide coating layer formed on the surface and / or inside of the particles contain residual sodium of 10,000 ppm or less. The positive electrode active material for sodium secondary batteries according to claim 9 .

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 according to claim 16, a negative electrode, and an electrolyte.

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