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 improve the structural stability of O3-type positive electrode active materials to achieve high capacity and excellent lifespan characteristics.SOLUTION: One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, including an O3-type sodium composite transition metal oxide containing at least sodium, a transition metal and a doping metal, where the sodium composite transition metal oxide is a secondary particle in which multiple primary particles are aggregated, and the aspect ratio of the primary particles is in the range between 1:1 and 1:2.5.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 Art

[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 rapidly, and sodium ion secondary batteries have attracted attention in order to replace expensive metal lithium.

[0003] Sodium ion secondary batteries have an operating principle of insertion / desorption reaction similar to that of lithium ion secondary batteries, and thus are one of the next-generation materials with high potential for application to secondary batteries. However, they exhibit low performance in terms of capacity, life characteristics, rate characteristics, etc. compared to lithium ion secondary batteries, and there are difficulties in commercialization. In order to commercialize sodium ion secondary batteries, it is essential to develop a positive electrode active material with high performance.

[0004] As the positive electrode active material of a sodium ion secondary battery, typically, a layered structure transition metal oxide having an excellent electrochemical performance while having a simple structure and being easy to synthesize is used. The layered structure transition metal oxide is typically classified into O3-type and P2-type according to the crystal structure. The positive electrode active material based on the O3-type structure shows a composition such as Na x (TM)O2(2 / 3 < x ≦ 1), and the positive electrode active material based on the P2-type structure has a composition of Na x (TM)O2(x ≦ 2 / 3).

[0005] Generally, O3-type layered oxides have a higher energy density than P2-type layered oxide particles, but have the disadvantage of undergoing greater structural changes during the charge / discharge process, resulting in reduced cycle stability. P2-type layered oxides have relatively excellent cycle stability, but are difficult to commercialize due to their low sodium content and relatively low energy density.

[0006] However, O3-type oxide particles have problems such as reduced battery life and stability due to sodium by-products present on the particle surface in the form of Na2CO3 and NaOH, which cause gas generation due to electrolyte side reactions during battery operation and reduce the capacity and output of the positive electrode active material.O3-type oxide particles also have the problem of losing their structure when washed with water to remove residual sodium, as all the internal sodium escapes.

[0007] The present invention aims to realize high capacity and excellent life characteristics by improving the structural stability of the O3-type positive electrode active material. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] China Patent Publication CN 115732674 A [Patent Document 2] Korean Patent Publication KR10-2022-0048915 [Patent Document 3] Korean Patent Registration KR 2228659 B1 Summary of the Invention [Problem to be solved by the invention]

[0009] Another object of the present invention is to provide a method for manufacturing a positive electrode active material, which can improve battery performance such as structural stability, capacity characteristics, and life characteristics by performing a roasting process and a Cu doping process during the manufacturing of the positive electrode active material. Another objective is to increase the average particle size of primary particles in O3-type layered oxides, which are secondary particles formed by agglomeration of multiple primary particles, synthesize Cu-doped transition metal oxides, improve air stability and water stability, and alleviate the problems of secondary particle cracking and structural collapse after water washing. [Means for solving the problem]

[0010] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, comprising an O3-type sodium composite transition metal oxide including at least sodium, a transition metal, and a doping metal, wherein the sodium composite transition metal oxide is a secondary particle formed by agglomeration of a plurality of primary particles, and the aspect ratio of the primary particles is 1:1 to 1:2.5.

[0011] The ratio (D2 / D1) of the average particle size (D2) of the secondary particles to the average particle size (D1) of the primary particles may be 2.5 to 10.

[0012] The primary particles may have an average particle size D1 of 0.8 to 2.5 μm, and the secondary particles may have an average particle size D2 of 6 to 12 μm.

[0013] The doping metal may be copper (Cu).

[0014] The sodium composite transition metal oxide may be represented by the following Chemical Formula 1.

[0015] [ka]

[0016] In the above formula 1, TM is at least one selected from Co, Ni, Mn, and Fe, and M is at least one selected from P, Sr, Ba, Ti, Zr, W, Co, Mg, Al, Zn, Ce, Hf, Ta, F, Cr, V, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, and 0.8 ≦ a ≦1.0, 0.01≦x≦0.1, 0≦y≦0.1, 0.8≦1−xy≦0.99.

[0017] The positive electrode active material may have a (003) peak full width at half maximum (FWHM(003)) of 0.1599 to 0.3399 at 2θ of 15° to 17.5° in XRD analysis.

[0018] The positive electrode active material may have a residual sodium content (TTS, total sodium) of 100 to 3,000 ppm.

[0019] Another embodiment of the present invention provides a method for preparing a positive electrode active material for a sodium secondary battery, the method comprising: roasting a transition metal hydroxide precursor; dry-mixing the roasted precursor prepared in the roasting process with a doping metal compound; and mixing the metal-doped roasted precursor prepared in the dry-mixing process with a sodium compound in an equivalent ratio of Na / M (total metals excluding Na) greater than 0.8 and less than 1, followed by calcination.

[0020] The roasting step may be carried out at a temperature of 700 to 1,100°C in an oxidizing atmosphere.

[0021] In the dry mixing step, the doping metal compound may be copper (Cu) acetate, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide or a combination thereof.

[0022] The firing step may be carried out at a temperature of 800 to 1,100°C.

[0023] The method may further include a step of washing the sodium transition metal oxide produced in the calcination step with water.

[0024] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery including the positive electrode active material, and a sodium secondary battery including the positive electrode and a negative electrode. [Effects of the Invention]

[0025] In the present invention, cracking of secondary particles can be minimized even after washing with water. In relation to this, the full width at half maximum (FWHM(003)) of the (003) peak, which is the main peak of O3-type, is maintained at the same level as before washing with water, so that the O3-type crystal structure is maintained and residual Na on the particle surface can be removed to a low level. [Brief explanation of the drawings]

[0026] [Figure 1a] 1 shows the results of SEM-EDS mapping analysis of the surface of positive electrode active material (secondary) particles prepared in Example 1. [Figure 1b] 1 shows the results of cross-sectional SEM-EDS mapping analysis of positive electrode active material (secondary) particles prepared in Example 1 and Comparative Example 1. [Figure 2a] 1 shows the results of comparative SEM analysis of positive electrode active material particles produced in Example 1 and Comparative Examples 1 to 3 before and after washing with water. [Figure 2b] 1 shows the results of comparative SEM analysis of positive electrode active material particles produced in Example 1 and Comparative Examples 1 to 3 before and after washing with water. [Figure 3a] 1 shows the results of comparative XRD analysis of positive electrode active material particles produced in Example 1 and Comparative Examples 1 to 3 before and after washing with water. [Figure 3b] 1 shows the results of comparative XRD analysis of positive electrode active material particles produced in Example 1 and Comparative Examples 1 to 3 before and after washing with water. [Figure 4] 1 is a graph showing the residual Na content (TTS, total sodium) on the surface of the positive electrode active material after washing with water in Example 1 and Comparative Examples 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0027] 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.

[0028] 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.

[0029] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, comprising an O3-type sodium composite transition metal oxide containing at least sodium, a transition metal, and a doping metal, the sodium composite transition metal oxide being secondary particles formed by agglomeration of a plurality of primary particles, the primary particles having an aspect ratio of 1:1 to 1:2.5.

[0030] It is generally known that O3 oxide anode materials have poor air stability. Specifically, when exposed to air or in contact with water, the oxidation reaction of water and H + / Na +Exchange occurs, generating residual Na on the surface, such as NaOH and Na2CO3. The generated CO3 is embedded in the transition metal layer to form CO4 tetrahedra, which slows down Na+ diffusion and deteriorates electrochemical properties. Therefore, the weakened bonding between TM-O can be effectively regulated by introducing doping metals to adjust the charge transfer between Na and O, building stronger Na2O bond energy. It has structural stability and excellent air stability.

[0031] In addition, in the present invention, a roasted precursor is prepared by roasting at a high temperature to realize the aspect ratio of the primary particles, and then a doping metal is dry-mixed with the roasted precursor to uniformly disperse the doping metal on the surface of the roasted precursor particles.

[0032] In the present invention, the use of a roasted precursor that has been oxidatively roasted at high temperatures increases the size of primary particles, reduces the specific surface area, and potentially increases the density of primary particles aggregated into secondary particles. This increases (1) the bonding strength between primary particles, thereby reducing particle cracking even after water washing, and improves the cohesion between primary particles, preventing sodium detachment from the crystal structure due to damage caused by water washing. Furthermore, (2) due to the characteristics of the doping metal used in small amounts, it has the effect of relatively increasing the doping dispersion on the surfaces of primary particles with reduced specific surface area and secondary particles. Therefore, compared to the objective or effect of conventional particle surface doping, which primarily aims to improve surface properties, the present invention allows the doping metal to be uniformly positioned inside secondary particles and at the grain boundaries of primary particles, further improving the structural stability of the O3-type sodium composite transition metal oxide.

[0033] Specifically, the aspect ratio of the primary particles may be 1:1 to 1:2.5, e.g., 1:1 to 1:2.4, 1:1 to 1:2.3, 1:1 to 1:2.2, 1:1 to 1:2.1, 1:1 to 1:2, 1:1 to 1:1.9, 1:1 to 1:1.8, 1:1 to 1:1.7, or 1:1 to 1:1.6, preferably 1:1 to 1:1.5. In the present invention, the oxidative roasting process of the cathode active material precursor may be performed at a high temperature to achieve the range of aspect ratios of the primary particles. Furthermore, by performing roasting and doping stepwise under specific conditions, the size of the primary particles constituting the secondary particles in the cathode active material can be increased and the aspect ratio can be reduced, thereby providing a cathode active material with improved energy density, high voltage stability, life characteristics, and high-rate characteristics.

[0034] The ratio (D2 / D1) of the average particle size (D2) of the secondary particles to the average particle size (D1) of the primary particles may be 2.5 to 10, for example, 2.5 to 8, 2.5 to 6, or 2.5 to 5. If the size ratio (D2 / D1) of the primary particles exceeds 10, the primary particles may be too small, resulting in the formation of no primary particles. As a result, the O3 structure may not be formed, and a large amount of impurities may be generated. This may be due to the roasting reaction not proceeding smoothly or the O3 structure not being crystallized, for example, due to roasting at a low temperature, roasting for a short time, and / or non-uniform roasting. Meanwhile, the size of the primary particles may be the length of the major axis.

[0035] The average particle size D1 of the primary particles may be 0.8 to 2.5 μm, for example, 0.8 to 2.3 μm, 0.8 to 2 μm, 0.8 to 1.7 μm, or 1 to 1.5 μm. The average particle size D2 of the secondary particles may be 6 to 12 μm, for example, 6 to 10 μm or 6 to 8 μm. When the primary particles and secondary particles contained in the positive electrode active material satisfy at least the above conditions, the particle density within the positive electrode active material can be improved. This can improve the electrochemical properties of the positive electrode active material.

[0036] The term "aspect ratio" as used herein refers to the ratio (length / width) of the major axis (length) to the minor axis (width) of the primary particle. When the major axis indicates the direction of the relatively long region of the primary particle, the minor axis indicates the length of the relatively short region located on the same plane as the major axis. In this case, the primary particle may have a plate shape, and the length in the thickness direction of the primary particle is significantly shorter than the length in the plane directions (major axis and minor axis) of the primary particle. Meanwhile, the minor axis may be in a direction perpendicular to the major axis, and the "aspect ratio" of the primary particle can be calculated as the ratio of the major axis to the minor axis of the primary particle measured from the surface of the primary particle.

[0037] Meanwhile, in the present invention, 50% or more, for example 60% or 70% or more of the total number of primary particles constituting the secondary particles may have the aspect ratio, primary particle size, or ratio of secondary particle size to primary particle size (D2 / D1) within the above-mentioned ranges, or at least 10 or at least 20 of the primary particles constituting the secondary particles may have the aspect ratio, primary particle size, or ratio of secondary particle size to primary particle size (D2 / D1) within the above-mentioned ranges.

[0038] The doping metal may be copper (Cu). The Cu doping can partially replace Fe and Ni among the transition metals, thereby reducing the amount of Fe and Ni that migrate. In addition, the Cu doping can improve the average valence state of Mn through charge compensation, thereby reducing the amount of Mn. 3+The Jahn-Teller effect can be reduced. Also, when compared with the case where Co is uniformly doped, Cu tends to be doped non-uniformly due to its own characteristics. However, in the present invention, there is an effect that the relative doping dispersion degree increases on the surfaces of the primary particles and the secondary particles with a reduced specific surface area. Therefore, the non-uniformity of Cu doping can be significantly improved. As a result, although the O3-type oxide particles have a problem that when water washing is applied to remove residual Na, all the internal Na escapes and the structure cannot be maintained, air stability and water stability can be improved, and the problems of cracking and structural collapse of the secondary particles after water washing can be improved.

[0039] Specifically, the sodium composite transition metal oxide may be represented by the following Chemical Formula 1.

[0040]

Chemical Formula

[0041] In Chemical Formula 1, TM is at least one selected from Co, Ni, Mn, and Fe, M is at least one selected from P, Sr, Ba, Ti, Zr, W, Co, Mg, Al, Zn, Ce, Hf, Ta, F, Cr, V, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, and 0.8 ≦ a ≦ 1, 0.01 ≦ x ≦ 0.1, 0 ≦ y ≦ 0.1, 0.8 ≦ 1 - x - y ≦ 0.99.

[0042] When the Na equivalent of the O3-type layered oxide is less than 0.80 (a < 0.80), the electrochemical properties may deteriorate due to the P3-type structure of the oxide and the arrangement of the lattice units. Conversely, when the Na equivalent exceeds 1.0 (0.1 < a), there are disadvantages such as a decrease in air and moisture stability and sensitivity to synthesis conditions such as temperature and atmosphere.

[0043] The positive electrode active material of the present invention may have a (003) peak full width at half maximum (FWHM(003)) of 0.1599 to 0.3399 at 2θ angles of 15° to 17.5° in XRD analysis. The XRD analysis results may be obtained after washing the prepared positive electrode active material with water to remove residual surface sodium. As a result, the positive electrode active material may well maintain its O3-type crystal structure without particle cracking or structural collapse even after removing residual surface sodium by washing with water.

[0044] The positive electrode active material of the present invention may have a residual sodium content (TTS, total sodium) reduced to 100 to 3,000 ppm, specifically 500 to 3,000 ppm, which can suppress gas generation caused by residual sodium and improve battery life characteristics.

[0045] On the other hand, the content of residual Na (TTS, Total Sodium) may be a value (TTS, Total Sodium) obtained by separately calculating the total amount of Na alone among compounds containing residual Na (e.g., NaOH or Na2CO3).

[0046] Another embodiment of the present invention provides a method for preparing a positive electrode active material for a sodium secondary battery.

[0047] The preparation method includes a step of roasting a transition metal hydroxide precursor, a step of dry-mixing the roasted precursor prepared in the roasting step with a doping metal compound, and a step of mixing the metal-doped roasted precursor prepared in the dry-mixing step with a sodium compound in an equivalent ratio of Na / M (total metals excluding Na) of more than 0.8 but less than 1, followed by calcination.

[0048] The roasting process involves heat-treating a transition metal hydroxide precursor at a high temperature. When a hydroxide precursor is doped with a transition metal using a dry method without roasting, particle cracking can occur in the secondary or primary oxide particles after subsequent heat treatment for sodium insertion. This phenomenon is believed to be caused by organic elements, such as acetate, sulfide, nitride, and phosphide, or oxygen elements, such as oxide, oxyhydroxide, and hydroxide, among the anion groups contained in the doping compound. Furthermore, when a transition metal is doped using a wet method, the selection of doping compounds is limited and the process becomes more complicated, resulting in increased costs.

[0049] The roasting process may be performed in an oxidizing atmosphere at a temperature of 750 to 1050°C, preferably 800 to 1050°C, 800 to 950°C, 850 to 1000°C, or 850 to 950°C. If the roasting temperature is higher than this range, the primary particles of the synthesized roasted precursor may become too large, limiting Na ion diffusion on the particle surface. Conversely, if the roasting temperature is lower than this range, the primary particles may not grow sufficiently, making it difficult to achieve the desired primary particle aspect ratio. The roasting time is not particularly limited, but may be preferably 6 to 15 hours, 8 to 15 hours, or 8 to 13 hours.

[0050] When the roasting process is performed, the specific surface area and porosity of the roasted precursor decrease, and the particle size of the primary particles in the secondary particles increases, resulting in a higher degree of aggregation. This increases the tap density of the roasted precursor, preventing the problem of particle cracking that occurs when dry doping is performed without roasting.

[0051] The transition metal hydroxide precursor may be represented by the following Chemical Formula 3, and the roasted precursor may be represented by the following Chemical Formula 4.

[0052] [ka] (TM is at least one selected from Co, Ni, Mn, and Fe)

[0053] [ka]

[0054] (TM is at least one selected from Co, Ni, Mn, and Fe)

[0055] The dry mixing process involves dry mixing the roasted precursor prepared in the roasting process with a doping metal compound. When dry doping is performed after the roasting process, the primary particles have a dense shape through roasting, and the bonding strength between the primary particles increases, improving the metal doping effect (cohesion between primary particles). Furthermore, when Cu doping is performed without roasting, some of the structure after water washing is maintained, but cracks are likely to occur between the primary particles.

[0056] By applying the dry method, Cu can be uniformly doped on the surface and inside of the roasted precursor particles. On the other hand, when doping with a transition metal using a wet method, the choice of doping compound is limited and the process becomes complicated, resulting in increased costs, which is undesirable.

[0057] The copper (Cu) compound may be a copper (Cu) acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide, or a combination thereof.

[0058] The calcination process is a step of mixing the copper (Cu)-doped roasted precursor and a sodium compound in an equivalent amount of Na / M (total metals excluding Na) of more than 0.8 and less than 1 to produce an O3-type sodium transition metal oxide, and then calcining the mixture.

[0059] The copper (Cu)-doped roasted precursor may be mixed with a sodium compound at an equivalent ratio of Na / M (total metal excluding Na) of more than 0.8 and less than 1, or more than 0.8 and less than 0.95. When the amount of sodium compound is within this range, the resulting cathode active material may have an O3-type layered crystalline structure, which provides higher energy density, high air and moisture stability, and low sensitivity to synthesis conditions (such as temperature and atmosphere). Furthermore, within this sodium content range, battery discharge capacity can be improved and residual unreacted sodium can be minimized.

[0060] The calcination may be carried out at a temperature of 700°C to 1,100°C. When the calcination temperature is within this range, the reaction between the raw materials occurs sufficiently, allowing particles to grow uniformly. The calcination may more preferably be carried out at a temperature of 750 to 1,050°C, 850 to 1,050°C, or 900 to 1,000°C. The calcination may be carried out for 5 to 40 hours. When the calcination time is within this range, a highly crystalline positive electrode active material is obtained, the particle size is appropriate, and production efficiency can be improved. The calcination may more preferably be carried out for 5 to 20 hours, 5 to 18 hours, 8 to 15 hours, or 10 to 14 hours.

[0061] The sodium compound may be at least one selected from the group consisting of Na2CO3, NaOH, NaNO3, CH3COONa and Na2(COO)2, and preferably Na2CO3, NaOH or a combination thereof.

[0062] 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.

[0063] The positive electrode includes a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector, and the positive electrode active material according to one aspect of the present invention is present in the positive electrode active material layer.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] According to yet another aspect of the present invention, there is provided an electrochemical device including the above-described anode. The electrochemical device may be, for example, a battery or a capacitor, and more specifically, a sodium secondary battery.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] Example Example 1 Ni 0.33 Fe 0.33 Mn 0.33 The (OH)2NFM11 precursor was placed in an alumina crucible and then oxidized and roasted at 950°C for 6 hours in an air atmosphere, and then cooled to room temperature to produce the roasted precursor (Ni-Fe-Mn)O4.

[0075] Dry doping was performed by mixing the prepared roasted precursor and Cu(OH)2 at Cu / M (M = Ni + Fe + Mn + Cu)2 at mol% using a hand mixer.

[0076] The prepared Cu 2 at mol% doped roasted precursor was mixed with Na2CO3 at Na / (Ni+Fe+Mn+Cu) = 0.85 equivalents to obtain a mixture. The prepared mixture was placed in an alumina crucible and fired at 950°C for 6 hours in an O2 atmosphere, and then cooled to room temperature to obtain O3-type Na 0.85 Ni 0.33 Fe 0.31 Mn 0.33 Cu 0.02 O2 cathode active material was produced.

[0077] The prepared positive electrode active material was placed in a reactor containing distilled water, washed with water at a temperature of 5 to 50°C and a stirring speed of 350 rpm for 1 hour, and then dried under vacuum at a temperature of 120°C for 12 hours.

[0078] 85 wt% of the prepared positive electrode active material, 10 wt% of carbon black, and 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.

[0079] A sodium secondary battery (coin cell) was fabricated using a sodium metal plate as a counter electrode to the anode, porous glass fiber (thickness: 200 μm) as a separator, and an electrolyte solution containing 1.0 M NaPF6 in a solvent mixed with propylene carbonate and fluoroethylene carbonate in a volume ratio of 98:2.

[0080] (Comparative Example 1) A positive electrode active material and a sodium secondary battery were manufactured in the same manner as in Example 1, except that the oxidation roasting process was not performed.

[0081] (Comparative Example 2) Those that have not undergone the oxidative roasting process,

[0082] A positive electrode active material and a sodium secondary battery were manufactured in the same manner as in Example 1, except that the NFM11 precursor, Cu(OH)2 at 2 at.mol% Cu, and Na2CO3 at Na / (Ni+Fe+Mn+Cu) = 0.85 equivalents were simultaneously mixed, and then placed in an alumina crucible and fired at 950°C for 6 hours in an O2 atmosphere.

[0083] (Comparative Example 3) A positive electrode active material and a sodium secondary battery were manufactured in the same manner as in Example 1, except that the oxidation roasting process and the Cu dry doping process were not performed.

[0084] Comparative Example 4 A positive electrode active material and a sodium secondary battery were manufactured in the same manner as in Example 1, except that Co(OH) was used instead of Cu(OH).

[0085] Experimental example Experimental Example 1: SEM-EDS mapping analysis of positive electrode active material particles The surface of the positive electrode active material particles prepared in Example 1 was subjected to SEM-EDS mapping analysis, and the results are shown in FIG. 1a.

[0086] The positive electrode active material particles prepared in Example 1 and Comparative Example 1 were subjected to cross-sectional SEM-EDS mapping analysis, and the results are shown in FIG. 1b.

[0087] In FIG. 1a, it can be seen that Cu partially aggregates on the surface of the secondary particles of the positive electrode active material of Example 1, but that Cu is doped relatively uniformly over the entire surface of the secondary particles.

[0088] In FIG. 1b, it can be seen that Cu is unevenly aggregated inside the secondary particles and on the surface of the primary particles. However, in Example 1, as compared to Comparative Example 1, the size of the primary particles increases (see FIG. 2b), and therefore Cu is dispersed on the surface of the primary particles with increased size, and it was confirmed that the overall Cu doping uniformity increases based on the cross section of the secondary particles.

[0089] As a result, Cu was doped unevenly, but subsequent experiments confirmed that the O3-type crystal structure was well maintained and that the outflow (desorption) of Na from the lattice structure was prevented after washing the positive electrode active material with water.

[0090] Experimental Example 2: Comparison of positive electrode active material particles before and after washing with water The positive electrode active material particles prepared in Example 1 and Comparative Examples 1 to 3 were subjected to comparative SEM and XRD analysis before and after washing with water, and the results are shown in FIGS. 2a, 2b, and 3.

[0091] 2a and 2b, when observing the surface shape using an SEM, cracks in some secondary particles were observed after washing with water. It was confirmed that Example 1 had the least particle cracks, while Comparative Example 3 had most of the secondary particles cracked.

[0092] 2b, it can be seen that in Example 1, by performing the oxidizing roasting process, the average particle diameters (D1) (D50) of the multiple primary particles constituting the secondary particles were in the range of 800 nm to 2.5 μm, the average particle diameters (D2) (D50) of the secondary particles were in the range of 6 to 12 μm, the aspect ratio of the primary particles was in the range of 1:1 to 1:2.5, and the average particle diameter ratio (D2 / D1) was in the range of 2.5 to 10. On the other hand, in Comparative Examples 1 to 3, the average particle diameter of the primary particles was smaller and the aspect ratio of the primary particles was higher than in Example 1, so they exhibited needle or rod shapes and had a relatively high average particle diameter ratio of more than 10.

[0093] [Table 1]

[0094] 3 and Table 1, the XRD titration results showed that the (003) peak, which is the main peak of O3-type, appeared in the 2θ range of 15° to 17.5°, confirming that the O3-type crystal structure was maintained after washing with water in Example 1. Specifically, the full width at half maximum (FWHM(003)) of the (003) peak was measured to be 0.1847.

[0095] On the other hand, in Comparative Examples 1 to 3, the FWHM(003) increased to 0.27 to 0.66, and it was confirmed that the O3-type crystal structure collapsed after washing with water, and most of the particles cracked in Comparative Example 3, which had the highest FWHM(003).

[0096] In addition, in Comparative Example 4, in which Co was doped instead of Cu, it was confirmed that most of the positive electrode active material particles were cracked after washing with water.

[0097] Experimental Example 3: Measurement of residual sodium content (TTS) In Example 1 and Comparative Examples 1 to 3, the residual Na content on the surface of the positive electrode active material after washing with water was measured, and the results are shown in FIG.

[0098] The residual sodium content was determined by measuring each compound containing residual Na (e.g., NaOH or Na2CO3) using potentiometric neutralization titration, and then separately calculating the total amount of Na alone (TTS, Total Sodium).

[0099] The calculation method is as shown in the following formula 1. Formula 1

[0100] TTS (Total Na) = NaOH analysis value (%) × Na / NaOH + Na2CO3 analysis value (%) × 2Na / Na2CO3

[0101] Referring to FIG. 4, it was confirmed that in Example 1 and Comparative Examples 1 to 3, the residual Na content (TTS) after washing with water was low, at around 3,000 ppm.

[0102] Experimental Example 4: Evaluation of battery performance The sodium secondary batteries manufactured in Example 1 and Comparative Examples 1 to 3 were subjected to charge-discharge experiments at 25°C, voltage range of 2.0V to 4.6V, and discharge rate of 0.1C to 2.0C using an electrochemical analyzer (Toyo, Toscat-3100) to measure the initial charge capacity, initial discharge capacity, initial reversible efficiency, and rate capability (C-rate).

[0103] In addition, the same sodium secondary battery was charged / discharged 50 times at 25°C and 1C / 1C within a driving voltage range of 2.5V to 4.3V, and then the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention) was measured.

[0104] The measurement results are shown in Table 2 below.

[0105] [Table 2]

[0106] Referring to Table 2, it was confirmed that Example 1 had the best electrochemical properties.

[0107] 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. An O3-type sodium complex transition metal oxide containing at least sodium, a transition metal, and a doping metal, the sodium composite transition metal oxide is a secondary particle formed by agglomeration of a plurality of primary particles, and the aspect ratio of the primary particles is 1:1 to 1:2.5; The positive electrode active material for a sodium secondary battery, wherein the doping metal is copper (Cu).

2. 2. The positive electrode active material for a sodium secondary battery according to claim 1, wherein a ratio (D2 / D1) of an average particle size (D2) of the secondary particles to an average particle size (D1) of the primary particles is 2.5 to 10.

3. 2. The positive electrode active material for a sodium secondary battery according to claim 1, wherein the primary particles have an average particle size D1 of 0.8 to 2.5 μm, and the secondary particles have an average particle size D2 of 6 to 12 μm.

4. The positive electrode active material for a sodium secondary battery according to claim 1 , wherein the sodium composite transition metal oxide is represented by the following Chemical Formula 1: 【Chemistry 1】 In the above Chemical Formula 1, TM is at least one selected from Co, Ni, Mn, and Fe, M is at least one selected from P, Sr, Ba, Ti, Zr, W, Co, Mg, Al, Zn, Ce, Hf, Ta, F, Cr, V, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd; 0.8≦a≦1.0, 0.01≦x≦0.1, 0≦y≦0.1, and 0.8≦1−x−y≦0.

99.

5. The positive electrode active material has a (003) peak at 2θ of 15° to 17.5° in XRD analysis.

2. The positive electrode active material for a sodium secondary battery according to claim 1, wherein the full width at half maximum (FWHM(003)) is 0.1599 to 0.3399.

6. 10. The positive electrode active material for a sodium secondary battery according to claim 1, wherein the positive electrode active material has a residual Na content (TTS, Total Sodium) of 100 to 3,000 ppm.

7. roasting the transition metal hydroxide precursor; dry-mixing the roasted precursor prepared in the roasting process with a doping metal compound; mixing the metal-doped roasted precursor prepared in the dry mixing process with a sodium compound in an equivalent amount of Na / M (total metals excluding Na) of more than 0.8 but less than 1, and then calcining the mixture.

8. 8. The method for producing a positive electrode active material for a sodium secondary battery according to claim 7, wherein the roasting process is performed in an oxidizing atmosphere at a temperature of 700 to 1,100°C.

9. In the dry mixing step, 8. The method for producing a positive electrode active material for a sodium secondary battery according to claim 7, wherein the doping metal compound is a copper (Cu) acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide, or a combination thereof.

10. The method for producing a positive electrode active material for a sodium secondary battery according to claim 7, wherein the calcination step is carried out at a temperature of 800 to 1,100°C.

11. 8. The method for producing a positive electrode active material for a sodium secondary battery according to claim 7, further comprising a step of washing the sodium transition metal oxide produced in the calcination step with water.

12. A positive electrode for a sodium secondary battery, comprising the positive electrode active material according to claim 1.

13. A sodium secondary battery comprising the positive electrode and negative electrode according to claim 12.

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