Cathode for sodium secondary battery and sodium secondary battery comprising same

Abstract

The present invention relates to a cathode for a sodium secondary battery, comprising: a current collector; a first cathode active material layer disposed on the current collector; and a second cathode active material layer disposed on the first cathode active material layer, wherein the first cathode active material layer includes a first cathode active material, and the second cathode active material layer includes a second cathode active material. The first cathode active material includes sodium composite transition metal oxide particles having an O3 octahedral crystal structure, the second cathode active material includes Prussian blue analogue particles, and an average particle diameter (D50) ratio of the first cathode active material and the second cathode active material is 2:1 to 10:1.

Description

Anode for sodium secondary battery and sodium secondary battery containing the same Cross-citation with related applications This application claims the benefit of priority to Korean Patent Application No. 10-2023-0187469, filed December 20, 2023, and Korean Patent Application No. 10-2024-0191930, filed December 19, 2024, the entire contents of which are incorporated herein by reference. Technical field The present invention relates to a positive electrode for a sodium secondary battery and a sodium secondary battery comprising the same, and more specifically, to a positive electrode for a sodium secondary battery comprising a current collector, a first positive electrode active material layer, and a second positive electrode active material layer, and a sodium secondary battery comprising the same. Secondary batteries are batteries that can be used repeatedly through the process of discharging and charging in the reverse direction, where chemical energy is converted into electrical energy. Recently, with the commercialization of portable electronic products, electric vehicles, etc., the demand for secondary batteries has been rapidly increasing. Currently, lithium secondary batteries are mainly used as commercial secondary batteries, but the reserves of lithium, the main raw material, are limited, so they are expensive and are not sufficient to meet the demand for secondary batteries. Therefore, the development of a new secondary battery that can replace lithium secondary batteries is required, and recently, research and development on sodium secondary batteries using sodium instead of lithium has been actively attempted. However, since sodium ions are relatively large compared to lithium ions, there is a problem in that when sodium ions are used as charge carriers, the steric hindrance effect increases, which reduces the rate characteristics when applied to secondary batteries. To solve these problems, O3-based layered oxides and Prussian blue analogues are being developed. However, O3-based layered oxides have limitations in high-rate characteristics due to the small interlayer distance, and there is a problem that the cycle characteristics are deteriorated due to changes in the crystal structure during charge and discharge, and Prussian blue analogues are prone to forming vacancies and crystal water, which causes structural instability by forming irregular crystals, and the storage and movement of sodium ions are hindered, which reduces the electrochemical performance. Therefore, in order to solve these problems, there is a need for the development of a positive electrode for sodium secondary batteries that can implement improved capacity characteristics and rate characteristics. One object of the present invention is to solve the above problems, and comprises a first cathode active material including sodium complex transition metal oxide particles having a 03 octahedral crystal structure as a first cathode active material layer, a second cathode active material including Prussian blue analogue particles as a second cathode active material layer, and an average particle diameter (D) of the first cathode active material and the second cathode active material 50 ) provides a cathode for a sodium secondary battery having a ratio of 2:1 to 10:1. In addition, another object of the present invention is to provide a sodium secondary battery including the aforementioned positive electrode for a sodium secondary battery. [1] The present invention comprises a current collector, a first cathode active material layer disposed on the current collector, and a second cathode active material layer disposed on the first cathode active material layer, wherein the first cathode active material layer comprises a first cathode active material, the second cathode active material layer comprises a second cathode active material, the first cathode active material comprises sodium complex transition metal oxide particles having an O3 octahedral crystal structure, the second cathode active material comprises Prussian blue analogue particles, and the average particle diameters (D) of the first cathode active material and the second cathode active material are 50) provides a cathode for a sodium secondary battery having a ratio of 2:1 to 10:1. [2] In the present invention, in the above [1], the sodium complex transition metal oxide particle having the O3 octahedral crystal structure may include a compound represented by the following chemical formula 1. [Chemical Formula 1] Na x1 M 1 O2 In the above chemical formula 1, 0.7≤x1≤1, and M 1 is at least one selected from the group consisting of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr. [3] The present invention, in the above [2], the M 1 may contain Fe, Ni, and Mn. [4] In at least one of the above [1] to [3], the Prussian blue analogue particle may include a compound represented by the following chemical formula 2. [Chemical formula 2] Na x2 M 2 [M 3 (CN)6] In the above chemical formula 2, 1.8≤x2≤2.2, and M 2 is at least one selected from the group consisting of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr, and M 3 may be at least one selected from the group consisting of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr. [5] The present invention relates to at least one of the above [1] to [4], wherein the average particle diameter (D) of the first positive electrode active material 50 ) can be 3㎛ to 19㎛. [6] The present invention relates to at least one of the above [1] to [5], wherein the average particle diameter (D) of the second positive electrode active material 50 ) can be 0.5㎛ to 4.0㎛. [7] In at least one of the above [1] to [6], the weight ratio of the first positive electrode active material and the second positive electrode active material may be 70:30 to 99:1. [8] The present invention is characterized in that in at least one of the above [1] to [7], the specific surface area of ​​the first positive electrode active material is 0.4 m 2 / g to 1.0m 2 / g could be. [9] The present invention is characterized in that in at least one of the above [1] to [8], the specific surface area of ​​the second positive electrode active material is 1 m 2 / g to 5m 2 / g could be.

[0010] In at least one of the above [1] to [9], the thickness ratio of the first positive electrode active material layer and the second positive electrode active material layer may be 1.5:1 to 10:1.

[0011] In at least one of the above [1] to

[0010] , the thickness of the first positive electrode active material layer may be 10 µm to 70 µm.

[0012] In at least one of the above [1] to

[0011] , the thickness of the second positive electrode active material layer may be 10 µm to 40 µm.

[0013] In at least one of the above [1] to

[0012] , the loading ratio of the first positive electrode active material layer and the second positive electrode active material layer may be 2:1 to 10:1.

[0014] The present invention provides a sodium secondary battery comprising at least one positive electrode for a sodium secondary battery among the above [1] to

[0013] . The positive electrode according to the present invention may include a current collector, a first positive electrode active material layer, and a second positive electrode active material layer, which are sequentially laminated, wherein the first positive electrode active material layer includes a first positive electrode active material including sodium composite transition metal oxide particles having an O3 octahedral crystal structure, and the second positive electrode active material layer includes a second positive electrode active material including Prussian blue analogue particles. Depending on the laminated structure of the first positive electrode active material layer and the second positive electrode active material layer and the components included therein, the positive electrode may have excellent capacity characteristics, rate characteristics, and life performance. Therefore, when the positive electrode for a sodium secondary battery according to the present invention is applied to a sodium secondary battery, excellent capacity characteristics and rate characteristics can be implemented. The drawings attached to this specification illustrate preferred embodiments of the present invention, and together with the contents of the invention described above, serve to better understand the technical idea of ​​the present invention, so the present invention is not limited to the matters described in such drawings. Meanwhile, the shape, size, scale or ratio of elements in the drawings included in this specification may be exaggerated to emphasize a clearer explanation. FIG. 1 is a schematic side view illustrating a positive electrode for a sodium secondary battery according to one embodiment of the present invention. Hereinafter, the present invention will be described in more detail. The terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as having meanings and concepts that conform to the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term in order to explain his or her own invention in the best manner. The terminology used herein is for the purpose of describing embodiments only and is not intended to limit the invention. In this specification, the singular also includes the plural unless specifically stated otherwise. The terms "comprises" and / or "comprising" as used herein do not exclude the presence or addition of one or more other components in addition to the components mentioned. In the present invention, the average particle diameter (D 50 ) means the particle size based on 50% of the volume cumulative particle size distribution of the target powder, such as the first positive electrode active material and the second positive electrode active material. The average particle diameter (D 50 ) can be measured using the laser diffraction method. For example, the target powder is dispersed in a dispersion medium, introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000), and ultrasonic waves of about 28 kHz are irradiated at an output of 60 W, and then a volume-cumulative particle size distribution graph is obtained, and then the particle size corresponding to 50% of the volume-cumulative amount is measured. In the present invention, the “specific surface area” is measured by the BET method, and specifically, can be calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mino II of BEL Japan. Cathode for sodium secondary battery Hereinafter, a positive electrode for a sodium secondary battery according to the present invention will be described. A positive electrode for a sodium secondary battery according to the present invention comprises a current collector, a first positive electrode active material layer disposed on the current collector, and a second positive electrode active material layer disposed on the first positive electrode active material layer, wherein the first positive electrode active material layer comprises a first positive electrode active material, the second positive electrode active material layer comprises a second positive electrode active material, the first positive electrode active material comprises sodium composite transition metal oxide particles having an O3 octahedral crystal structure, the second positive electrode active material comprises Prussian blue analogue particles, and the average particle diameters (D) of the first positive electrode active material and the second positive electrode active material are 50 ) is characterized by a ratio of 2:1 to 10:1. The current collector according to the present invention is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and 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. In addition, the positive electrode current collector may typically have a thickness of 3 ㎛ to 500 ㎛, and fine unevenness may be formed on the surface of the positive electrode current collector to increase the adhesive strength of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc. The above first positive electrode active material layer is disposed on the current collector. The first positive electrode active material layer may be disposed on at least one surface of the current collector. Specifically, the first positive electrode active material layer may be disposed on one surface or both surfaces of the current collector. The above first positive electrode active material layer includes a first positive electrode active material. The above first cathode active material includes sodium complex transition metal oxide particles having an O3 octahedral crystal structure. The above O3 octahedral crystal structure refers to the structure of the R3-m space group in which sodium ions are located in octahedral sites. It can be understood that the sodium complex transition metal oxide having the above O3 octahedral crystal structure is specifically implemented when the number of moles of sodium is 0.7 to 1 times the total number of moles of the transition metal. The sodium composite transition metal oxide particles having the O3 octahedral crystal structure have a relatively high initial Na content, so that more sodium ions can be desorbed, and are suitable for implementing a positive electrode having a high capacity and a high energy density. In addition, since the first positive electrode active material includes the sodium composite transition metal oxide particles having the O3 octahedral crystal structure, it has a relatively large contact area with a current collector, so that high adhesion can be achieved between the current collector and the entire active material layer. Meanwhile, the sodium composite transition metal oxide particles having the O3 octahedral crystal structure are oxidized when exposed to moisture or CO2 in the air, and H2O or CO2 molecules are converted to Na + There is a concern that unstable problems may occur in the atmosphere, such as insertion into a layer or changes in surface structure and composition, but since the second positive electrode active material layer described below can cover the first positive electrode active material layer, such problems can be prevented. More specifically, the sodium complex transition metal oxide particle having the O3 octahedral crystal structure may include a compound represented by the following chemical formula 1. [Chemical Formula 1] Na x1 M 1 O2 In the above chemical formula 1, 0.7≤x1≤1, and the above M 1 The metal may be at least one selected from the group consisting of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr. In the chemical formula 1 above, x1 means the ratio of the molar content of sodium to the total molar content of the transition metal in the compound, and may be 0.7≤x1≤1, specifically 0.75≤x1≤1, and more specifically 0.9≤x1≤1. When the atomic fraction of sodium satisfies the above range, the formation of the O3 octahedral crystal structure can occur smoothly. In the above chemical formula 1, the M 1 M may be at least one selected from the group consisting of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr, and specifically may be at least one selected from the group consisting of Fe, Ni, and Mn, and more specifically may be Fe, Ni, and Mn. 1 In the case of sodium complex transition metal oxides using the above elements, structural stability can be expressed because the difference in ionic radius between the sodium ion and the metal ion is large, so the cationic disorder phenomenon is less likely to occur. More specifically, the compound represented by the above chemical formula 1 may include a compound represented by the following chemical formula 1a. [Chemical formula 1a] Na x1 Fe y1 Ni y2 Mn y3 O2 In the chemical formula 1a above, x1 may be 0.7≤x1≤1, specifically 0.75≤x1≤1, more specifically 0.9≤x1≤1, and y1, y2, and y3 represent atomic fractions of Fe, Ni, and Mn in the sodium complex transition metal oxide, respectively, and are 0 <y1<1, 0<y2<1, 0<y3<1, 구체적으로 0.1<y1<0.5, 0.1<y2<0.5, 0.1<y3<0.5일 수 있다. 이때, y1+y2+y3=1일 수 있다. The average particle diameter (D) of the first positive electrode active material 50) may be 1 ㎛ or more, 3 ㎛ or more, 5 ㎛ or more, or 10 ㎛ or more, and may be 50 ㎛ or less, 30 ㎛ or less, 19 ㎛ or less, or 15 ㎛ or less. Specifically, it may be 1 ㎛ to 50 ㎛, more specifically 3 ㎛ to 19 ㎛, and even more specifically 10 ㎛ to 15 ㎛. The average particle diameter (D of the first positive electrode active material 50 ) satisfies the above range, the electrode density of the first positive electrode active material layer is high, so that many contact points with the current collector are created, thereby having the effect of promoting the movement of electrons. The specific surface area of ​​the above first positive electrode active material is 0.05 m 2 / g or more, 0.1m 2 / g or more, 0.4m 2 / g or more or 0.5m 2 / g can be more than 20m 2 / g and below, 10m 2 / g or less, 1.0m 2 / g or less, or 0.6m 2 / g can be less than 0.05m specifically. 2 / g to 20m 2 / g, more specifically 0.4m 2 / g to 1.0m 2 / g, more specifically 0.5m 2 / g to 0.6m 2 / g. When the specific surface area of ​​the first positive electrode active material satisfies the above range, the adhesion between the current collector and the sodium complex transition metal oxide particle composite layer having an O3 octahedral crystal structure can be improved. The thickness of the first positive electrode active material layer may be 1 µm or more, 10 µm or more, 20 µm or more, or 30 µm or more, and may be 500 µm or less, 300 µm or less, 100 µm or less, 70 µm or less, or 50 µm or less. Specifically, it may be 1 µm to 500 µm, more specifically 10 µm to 70 µm, and even more specifically 30 µm to 50 µm. When the thickness of the first positive electrode active material layer satisfies the above range, the first positive electrode active material layer can exhibit optimal electrochemical performance by facilitating electron and ion movement. The above first positive electrode active material layer may further include a first binder and / or a first conductive material together with the first positive electrode active material. The above first conductive material is used to provide conductivity to the electrode, and in the battery to be formed, as long as it does not cause a chemical change and has electronic conductivity, it can be used without special restrictions. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like, and one of these may be used alone or a mixture of two or more may be used. The first conductive material may be included in an amount of 1 wt% to 30 wt% with respect to the total weight of the first positive electrode active material layer. The first binder serves to improve the adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material and the current collector. Specific examples thereof include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof may be used. The first binder may be included in an amount of 1 wt% to 30 wt% with respect to the total weight of the first positive electrode active material layer. The second positive electrode active material layer is disposed on the first positive electrode active material layer. Specifically, the second positive electrode active material layer may be disposed on the opposite side of the surface of the first positive electrode active material layer where the first positive electrode active material layer and the current collector are in contact. In addition, when the first positive electrode active material layer is present on both sides of the current collector, the second positive electrode active material layer can be disposed on each surface of the first positive electrode active material layer disposed on both sides of the current collector. The second positive electrode active material layer according to the present invention comprises a second positive electrode active material, and the second positive electrode active material comprises Prussian blue analogue particles. The above Prussian blue analogue is a type of metal-organic framework, and the lattice structure of the Prussian blue analogue may exhibit a three-dimensional structure of a face-centered cubic structure in which two types of transition metal cations are each coordinated with six cyanide ligands in an octahedral manner, and the cyanide ligands serve as a bridge to connect the two transition metal cations. The above Prussian blue analogue particles have an open framework, which allows the ion diffusion path within the crystal to be wider than that of other crystalline inorganic materials. This enables rapid diffusion of ions, which not only provides high rate characteristics, but also enables reversible insertion and deintercalation of ions with large ionic radii, such as sodium and potassium cations. Therefore, the second cathode active material layer including the Prussian blue analogue particles can be present in the upper portion of the cathode and contribute to improving the rate characteristics of the entire cathode. In addition, Prussian blue analogue particles have a problem in that they have poor adhesion to the current collector due to their low density. However, this problem can be prevented by the present invention by arranging a second positive electrode active material layer on a first positive electrode active material layer. In addition, since the second cathode active material layer is disposed on the first cathode active material layer, stable battery operation can be achieved as a result by covering the first cathode active material, which is unstable in the air. Specifically, the Prussian blue analogue particles may include a compound represented by the following chemical formula 2. [Chemical formula 2] Na x2 M 2 [M 3 (CN)6] In the above chemical formula 2, 1.8≤x2≤2.2, and the above M 2 is at least one selected from the group consisting of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr, and the M 3 may be at least one selected from the group consisting of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr. In the above chemical formula 2, x2 means the ratio of the molar content of sodium to the total molar content of the transition metal in the compound, and may be 1.8≤x2≤2.2, specifically 1.85≤x2≤2.15, and more specifically 1.9≤x2≤2.1. When the atomic fraction of sodium satisfies the above range, it has an ideal form without vacancies due to defects and has a wide space in the structural crystal, so that ion diffusion can occur quickly. In the above chemical formula 2, the M 2 M may be at least one selected from the group consisting of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr, and specifically may be at least one selected from the group consisting of Ni and Mn, and more specifically may be Ni and Mn. 2 When active transition metals such as Fe, Mn or Co are used, Prussian blue analogues containing them can implement high specific capacity by participating in electrochemical reactions and involving two electrons in oxidation and reduction reactions, and M 2 When using inactive transition metals such as Ni, Zn, and Cu, M 3 Since only the transition metal undergoes oxidation and reduction reactions, the specific capacity is low, but excellent cycle characteristics can be realized through structural stability because no structural change occurs during the electrochemical reaction. In addition, M 2 By mixing and synthesizing active and inactive transition metals in a certain ratio, high specific capacity and structural stability can be achieved simultaneously. In the above chemical formula 2, the M 3 M may be at least one selected from the group consisting of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr, and specifically may be at least one selected from the group consisting of Fe and Mn, and more specifically may be Fe. 3For Prussian blue analogues using the above elements, M 3 And the cyanide ligand is connected to enable rapid diffusion of sodium ions and can exhibit high rate characteristics. More specifically, the compound represented by the above chemical formula 2 may include a compound represented by the following chemical formula 2a. [Chemical formula 2a] Na x2 Ni z1 Mn z2 [Fe(CN)6] In the chemical formula 2a, x2 may be 1.8≤x2≤2.2, specifically 1.85≤x2≤2.15, and more specifically 1.9≤x2≤2.1, and z1 and z2 represent atomic fractions of Ni and Mn in the Prussian blue analogue, respectively, and may be 0≤z1≤1, 0≤z2≤1, specifically 0≤z1<0.6, 0≤z2<0.6. In this case, z1+z2=1. The average particle diameter (D) of the second positive electrode active material 50 ) may be 0.1 ㎛ or more, 0.5 ㎛ or more, 1 ㎛ or more, or 1.3 ㎛ or more, and may be 50 ㎛ or less, 30 ㎛ or less, 8 ㎛ or less, 4.0 ㎛ or less, or 2.0 ㎛ or less. Specifically, it may be 0.1 ㎛ to 50 ㎛, more specifically 0.5 ㎛ to 4.0 ㎛, and even more specifically 1.3 ㎛ to 2.0 ㎛. The average particle diameter (D of the second positive electrode active material 50 ) satisfies the above range, the small particle ratio of the second positive electrode active material layer is high, so that the sodium ion solid-phase diffusion distance is short compared to the first positive electrode active material layer, and thus the high-rate discharge effect can be further expressed. The specific surface area of ​​the above second positive electrode active material is 0.1 m 2 / g or more, 0.5m 2 / g or more, 1m 2 / g or more or 3m 2 / g can be more than 100m 2 / g and below, 50m 2 / g and below, 5m2 / g or less or 4m 2 / g can be less than 0.1m specifically. 2 / g to 100m 2 / g, more specifically 1m 2 / g to 5m 2 / g, more specifically 3m 2 / g to 4m 2 / g. When the specific surface area of ​​the second positive electrode active material satisfies the above range, the adhesion with the first positive electrode active material layer can be improved. The thickness of the second positive electrode active material layer may be 1 ㎛ or more, 5 ㎛ or more, 10 ㎛ or more, or 15 ㎛ or more, and may be 200 ㎛ or less, 100 ㎛ or less, 40 ㎛ or less, or 25 ㎛ or less. Specifically, it may be 1 ㎛ to 200 ㎛, more specifically 10 ㎛ to 40 ㎛, and even more specifically 15 ㎛ to 25 ㎛. When the thickness of the second positive electrode active material layer satisfies the above range, high-rate discharge characteristics may be improved through smooth sodium ion diffusion. The second positive electrode active material layer may further include a second binder and / or a second conductive material together with the second positive electrode active material. The second conductive material is used to provide conductivity to the electrode, and can be used without special restrictions as long as it does not cause a chemical change in the battery to be formed and has electronic conductivity. Specific examples thereof include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like. One of these may be used alone or a mixture of two or more may be used. The second conductive material may be included in an amount of 1 wt% to 30 wt% with respect to the total weight of the second positive electrode active material layer. The second binder serves to improve the adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material and the current collector. Specific examples thereof include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof may be used. The second binder may be included in an amount of 1 wt% to 30 wt% with respect to the total weight of the second positive electrode active material layer. The average particle diameter (D) of the first positive electrode active material and the second positive electrode active material 50) can be 1.2:1 to 30:1, specifically 2:1 to 10:1, more specifically 6:1 to 8:1. If it is out of the above range, the insertion / de-insertion reaction of sodium ions is slowed down, which deteriorates the output characteristics of the sodium secondary battery, resulting in a problem of deterioration of the high-rate characteristics and capacity retention rate. Therefore, the average particle diameter (D 50 ) When the ratio satisfies the above range, the adhesion between the first positive electrode active material layer and the second positive electrode active material layer can be improved, and the movement of sodium ions can be optimized to maintain stable performance even during high-rate charge / discharge, thereby improving high-rate characteristics and capacity retention. In addition, the weight ratio of the first positive electrode active material and the second positive electrode active material may be 50:50 to 99:1, specifically 60:40 to 99:1, and more specifically 70:30 to 99:1. When the weight ratio satisfies the above range, the positive electrode capacity may increase, thereby improving the high-rate characteristics. In addition, the thickness ratio of the first positive electrode active material layer and the second positive electrode active material layer may be 1:1 to 20:1, specifically 1.5:1 to 10:1, and more specifically 1.5:1 to 3:1. When the thickness ratio satisfies the above range, the positive electrode adhesive strength and capacity may increase, thereby improving the high-rate characteristics. In addition, the loading ratio of the first positive electrode active material layer and the second positive electrode active material layer may be 1.2:1 to 30:1, specifically 2:1 to 10:1, and more specifically 4:1 to 6:1. When the loading ratio satisfies the above range, the positive electrode adhesion and capacity may increase, and the high-rate characteristics may be improved. The first positive electrode active material layer may be manufactured by applying a first positive electrode slurry prepared by dissolving or dispersing the first positive electrode active material, and optionally the first binder and / or the first conductive material in a solvent, to a current collector and drying the same, or by casting the first positive electrode slurry on a separate support and then laminating the resulting film on a current collector by peeling it off from the support. The second positive electrode active material layer may also be implemented by manufacturing the second positive electrode slurry in the same manner as above, except that the second positive electrode active material, the second binder and the second conductive material are used. When the above-described positive electrode active material layer includes the first positive electrode active material layer and the second positive electrode active material layer, the manufacture of the positive electrode for a sodium secondary battery is not particularly limited as long as the first positive electrode active material layer and the second positive electrode active material layer having the above-described characteristics can be implemented. For example, the first positive electrode active material, optionally a first binder and / or a first conductive material, is added to a solvent (e.g., NMP) to manufacture a first positive electrode slurry, the second positive electrode active material, optionally a second binder and / or a second conductive material, is added to a solvent (e.g., NMP) to manufacture a second positive electrode slurry, and then these are applied to a current collector, thereby manufacturing the positive electrode for a sodium secondary battery according to the present invention. More specifically, the first positive electrode slurry manufactured as described above is applied to a current collector, rolled, and dried to form a first positive electrode active material layer, and the second positive electrode slurry manufactured as described above is applied onto the first positive electrode active material layer, rolled, and dried to form a second positive electrode active material layer, thereby manufacturing a positive electrode for a sodium secondary battery according to the present invention. Meanwhile, the positive electrode for a sodium secondary battery according to the present invention can also be manufactured by applying the first positive electrode slurry to a current collector and, at the same time, substantially simultaneously applying the second positive electrode slurry onto the applied first positive electrode slurry, and simultaneously rolling and drying. Sodium secondary battery Next, a sodium secondary battery according to the present invention will be described. The sodium secondary battery according to the present invention includes the positive electrode for a sodium secondary battery according to the present invention described above. More specifically, the sodium secondary battery according to the present invention includes the positive electrode for a sodium secondary battery according to the present invention, an anode positioned opposite the positive electrode for a sodium secondary battery, a separator interposed between the positive electrode and the anode for a sodium secondary battery, and an electrolyte. Since the positive electrode for a sodium secondary battery has been described above, only the remaining components will be described below. A negative electrode according to the present invention includes a negative electrode current collector and a negative electrode active material layer positioned on the negative electrode current collector. The above negative electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., an aluminum-cadmium alloy, etc. can be used. In addition, the negative electrode current collector can typically have a thickness of 3 ㎛ to 500 ㎛, and, like the positive electrode current collector, fine unevenness can be formed on the surface of the current collector to strengthen the bonding strength of the negative electrode active material. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a non-woven fabric. The above negative electrode active material layer optionally includes a binder and a conductive material together with the negative electrode active material. As the negative active material, a compound capable of reversible intercalation and deintercalation of sodium can be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, hard carbon, soft carbon, and amorphous carbon; metallic compounds capable of alloying with sodium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; SiOβ (0 <β< 2), SnO2, a metal oxide capable of doping and dedoping sodium, such as vanadium oxide, sodium vanadium oxide; or a composite including the above metallic compound and a carbonaceous material, such as a Si-C composite or a Sn-C composite, and any one or a mixture of two or more of these may be used. In addition, a metal sodium thin film may be used as the negative electrode active material. The above binder serves to improve the adhesion between negative active material particles and the adhesive strength between the negative active material and the negative current collector. Specific examples thereof include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one kind alone or a mixture of two or more kinds thereof may be used. The above binder may be included in an amount of 1 wt% to 30 wt%, specifically 1 wt% to 20 wt%, and more specifically 1 wt% to 10 wt%, relative to the total weight of the negative electrode active material layer. The conductive material is used to provide conductivity to the negative electrode, and in the battery to be formed, as long as it does not cause a chemical change and has electronic conductivity, it can be used without special restrictions. Specific examples include graphite such as natural graphite or artificial graphite; carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, carbon fiber, carbon nanotube, etc.; metal powder or metal fiber such as copper, nickel, aluminum, silver, etc.; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, etc., and one of these may be used alone or a mixture of two or more may be used. The conductive material may be typically included in an amount of 1 wt% to 30 wt%, specifically 1 wt% to 20 wt%, and more specifically 1 wt% to 10 wt%, based on the total weight of the negative electrode active material layer. The negative electrode active material layer may be manufactured by, for example, applying a composition for forming a negative electrode active material layer, prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material in a solvent, onto a negative electrode current collector and drying the composition, or by casting the composition for forming a negative electrode active material layer onto a separate support and then laminating the resulting film onto a negative electrode current collector by peeling the film off from the support. The separator according to the present invention separates the negative electrode and the positive electrode and provides a passage for the movement of sodium ions. Separators generally used in secondary batteries can be used, and the type thereof is not particularly limited. For example, as the separator, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof, or a porous nonwoven fabric formed of high-melting-point glass fibers, polyethylene terephthalate fibers, or the like can be used. In addition, a coated separator containing a ceramic component or a polymer material to secure heat resistance or mechanical strength can be used, and can be selectively used in a single-layer or multi-layer structure. The electrolyte according to the present invention may include, but is not limited to, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc. that can be used in the manufacture of a sodium secondary battery. Specifically, the electrolyte may include an organic solvent and a sodium salt. As the organic solvent, any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move may be used without particular limitation. Specifically, the organic solvent may include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; Examples of solvents that can be used include carbonate solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (wherein R represents a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes. The above sodium salt can be used without any special limitation as long as it is a compound that can provide sodium ions used in a sodium secondary battery. Specifically, the sodium salt is NaPF6, NaClO4, NaAsF6, NaBF4, NaCF3SO3, NaB(C6H5)4, NaC4F9SO3, NaN(C2F5SO3)2, NaN(C2F5SO2)2, NaN(CF3SO2) 2　The concentration of the sodium salt may be used in the range of 0.1 to 2.0 M. In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as fluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxy ethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing battery capacity decrease, and improving the discharge capacity of the battery. In this case, the additives may be contained in an amount of 0.1 to 5 wt% with respect to the total weight of the electrolyte. As described above, a sodium secondary battery including a positive electrode for a sodium secondary battery according to the present invention can be applied to portable devices such as mobile phones, laptop computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs). Hereinafter, the present invention will be described in more detail through specific examples. However, the following examples are merely examples to help understand the present invention and do not limit the scope of the present invention. It will be obvious to those skilled in the art that various changes and modifications are possible within the scope and technical idea of ​​the present description, and it is natural that such changes and modifications fall within the scope of the appended patent claims. Examples and Comparative Examples Example 1: Preparation of a cathode for a sodium secondary battery The average particle size (D) of the first cathode active material 50 ) is 11.9㎛ and the specific surface area is 0.51m 2 / g NaFe 0.33 Ni 0.33 Mn 0.34A cathode slurry for the first cathode active material layer was prepared by mixing O2 with a challenge agent (carbon black) and a binder (PVdF) in a weight ratio of 96:2:2 in N-methylpyrrolidone (NMP). The average particle size (D) of the second cathode active material 50 ) is 1.7㎛ and the specific surface area is 3.52m 2 / g Na2Ni 0.5 Mn 0.5 [Fe(CN)6] was mixed with a conductive agent (carbon black) and a binder (PVdF) in a weight ratio of 96:2:2 in N-methylpyrrolidone (NMP) to prepare a cathode slurry for the second cathode active material layer. After applying the positive electrode slurry for the first positive electrode active material layer on a 20 μm aluminum foil as a current collector, the positive electrode slurry was dried in a vacuum oven at 120° C. for 1 hour to form a first positive electrode active material layer. After applying the positive electrode slurry for the second positive electrode active material layer on the first positive electrode active material layer, the positive electrode slurry was dried in a vacuum oven at 120° C. for 1 hour to form a second positive electrode active material layer. Thereafter, the positive electrode for a sodium secondary battery was manufactured by drying in a vacuum oven at 120°C for 12 hours and then rolling. Finally, a positive electrode for a sodium secondary battery was manufactured in which a current collector, a first positive electrode active material layer, and a second positive electrode active material layer were sequentially arranged. At this time, the thickness of the first positive electrode active material layer is 40 μm, and the loading amount is 1.67 mAh / cm 2 The thickness of the second positive electrode active material layer is 19 μm, and the loading amount is 0.33 mAh / cm. 2 It was. Example 2: Preparation of a cathode for a sodium secondary battery The average particle size (D) of the first cathode active material 50 ) is 5.2㎛ and the specific surface area is 0.72m 2 / g NaFe 0.33 Ni 0.33 Mn 0.34 A positive electrode for a sodium secondary battery was manufactured in the same manner as in Example 1, except that O2 was used. Comparative Example 1: Manufacturing of a positive electrode for a sodium secondary battery A positive electrode for a sodium secondary battery was manufactured in the same manner as Example 1, except that only the first positive electrode active material layer was formed without separately forming the second positive electrode active material layer, thereby finally manufacturing a positive electrode for a sodium secondary battery in which a current collector and the first positive electrode active material layer were sequentially arranged. Comparative Example 2: Manufacturing of a positive electrode for a sodium secondary battery A positive electrode for a sodium secondary battery was manufactured in the same manner as Example 1, except that only the second positive electrode active material layer was formed without separately forming the first positive electrode active material layer, thereby finally manufacturing a positive electrode for a sodium secondary battery in which a current collector and a second positive electrode active material layer were sequentially arranged. Comparative Example 3: Manufacturing of a positive electrode for a sodium secondary battery The average particle size (D) of the first cathode active material 50 ) is 11.9㎛ and the specific surface area is 0.51m 2 / g NaFe 0.33 Ni 0.33 Mn 0.34 Average particle size (D) of O2 and second cathode active material 50 ) is 1.7㎛ and the specific surface area is 3.52m 2 / g Na2Ni 0.5 Mn 0.5 A cathode slurry was prepared by mixing a cathode active material containing [Fe(CN)6] in a weight ratio of 5:1 with a conductive agent (carbon black) and a binder (PVdF) in a weight ratio of 96:2:2 in N-methylpyrrolidone (NMP). The above cathode slurry was applied onto a 20 μm thick aluminum foil as a current collector, dried in a vacuum oven at 120° C. for 12 hours, and then rolled to manufacture a cathode for a sodium secondary battery. Finally, a cathode for a sodium secondary battery was manufactured in which a current collector and a cathode active material layer were sequentially arranged. At this time, the thickness of the positive electrode active material layer is 59㎛, and the loading amount is 2mAh / cm 2 It was. Comparative Example 4: Manufacturing of a positive electrode for a sodium secondary battery The average particle size (D) of the first cathode active material 50 ) is 1.7㎛ and the specific surface area is 3.52m 2 / g Na2Ni 0.5 Mn 0.5 [Fe(CN)6] was used, and the average particle size (D) was used as the second cathode active material. 50 ) is 11.9㎛ and the specific surface area is 0.51m 2 / g NaFe 0.33 Ni 0.33 Mn 0.34 A positive electrode for a sodium secondary battery was manufactured in the same manner as in Example 1, except that O2 was used. At this time, the thickness of the first positive electrode active material layer is 19 μm, and the loading amount is 0.33 mAh / cm 2 The thickness of the second positive electrode active material layer is 40 μm, and the loading amount is 1.67 mAh / cm. 2 It was. Comparative Example 5: Manufacturing of a positive electrode for a sodium secondary battery The average particle size (D) of the first cathode active material 50 ) is 20.2㎛ and the specific surface area is 0.40m 2 / g NaFe 0.33 Ni 0.33 Mn 0.34 A positive electrode for a sodium secondary battery was manufactured in the same manner as in Example 1, except that O2 was used. Comparative Example 6: Manufacturing of a positive electrode for a sodium secondary battery The average particle size (D) of the first cathode active material 50 ) is 2.5㎛ and the specific surface area is 0.89m 2 / g NaFe 0.33 Ni 0.33 Mn 0.34 A positive electrode for a sodium secondary battery was manufactured in the same manner as in Example 1, except that O2 was used. Experimental Example 1: High-rate characteristic evaluation <Coin Half-Cell Manufacturing> Coin half cells of the examples and comparative examples were manufactured by interposing a separator (glass fiber, Whatman) between the positive and negative electrodes (Na metal) of the sodium secondary batteries manufactured in the examples 1 to 2 and comparative examples 1 to 6, and then injecting an electrolyte. The above electrolyte was used by mixing ethylene carbonate (EC) and propylene carbonate (PC) in a volume ratio of 5:5 in a non-aqueous electrolyte solvent, adding 2 wt% of fluoroethylene carbonate (FEC) to the solvent, and dissolving 1 M NaPF6. <High-rate characteristic evaluation> High-rate characteristics were evaluated for coin half cells including the positive electrodes for sodium secondary batteries manufactured in Examples 1 to 2 and Comparative Examples 1 to 6 manufactured above. In detail, the coin half-cell was charged to 4.0 V with a constant current of 0.1 C at 25°C in CC / CV mode (end current 0.05 C), then discharged to 2.0 V with constant currents of 0.1 C and 5.0 C to measure the discharge capacity, and then the ratio of the 5 C discharge capacity to the 0.1 C discharge capacity was calculated. The calculation results are shown in Table 1 below. Experimental Example 2: Capacity Retention Rate Evaluation Capacity retention was evaluated for coin half cells including the positive electrodes for sodium secondary batteries manufactured in Examples 1 to 2 and Comparative Examples 1 to 6 manufactured above. In detail, the coin half-cell was charged to 4.0 V (end current 0.1 C) at a constant current of 1 C at 25°C in CC / CV mode, and then discharged to 2.0 V at a constant current of 1 C, which constituted one cycle, and 100 charge-discharge cycles were performed. The capacity retention rate is calculated using the formula below, and the calculation results are shown in Table 1 below. Capacity retention rate [%] = {(discharge capacity after 100 cycles / discharge capacity after 1 cycle)} × 100 Experimental Example 1 Experimental Example 25.0C discharge capacity [mAh / g]0.1C discharge capacity [mAh / g]5.0C discharge capacity / 0.1C discharge capacity [%]Capacity retention rate [%]Example 1131.9147.989.294.0Example 2132.9147.790.093.8Comparative Example 1113.4139.781.288.1Comparative Example 2139.9155.390.182.1Comparative Example 3122.6142.586.087.5Comparative Example 4120.1145.082.888.9Comparative Example 5111.3146.276.192.5Comparative Example 6121.1145.783.190.1 Referring to Table 1 above, it can be seen that the coin half cells including the positive electrodes for sodium secondary batteries manufactured in Examples 1 to 2 have superior rate characteristics and capacity retention rates compared to the coin half cells including the positive electrodes for sodium secondary batteries manufactured in Comparative Examples 1 to 6. [Explanation of symbols] 10: Cathode for sodium secondary battery 100: Whole house 110: First positive electrode active material layer 120: Second positive electrode active material layer

Claims

1. Whole house; A first positive electrode active material layer disposed on the above-mentioned collector; and A second positive electrode active material layer disposed on the first positive electrode active material layer; The first positive electrode active material layer includes a first positive electrode active material, The second positive electrode active material layer includes a second positive electrode active material, The above first cathode active material comprises sodium complex transition metal oxide particles having an O3 octahedral crystal structure, The above second positive electrode active material comprises Prussian blue analogue particles, The average particle diameter (D) of the first positive electrode active material and the second positive electrode active material 50 ) A cathode for a sodium secondary battery having a ratio of 2:1 to 10:

1.

2. In claim 1, The sodium complex transition metal oxide particle having the above O3 octahedral crystal structure is a positive electrode for a sodium secondary battery, comprising a compound represented by the following chemical formula 1: [Chemical Formula 1] And x1 I 1 O2 In the above chemical formula 1, 0.7≤x1≤1, and M 1 is at least one selected from the group consisting of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr.

3. In claim 2, Above M 1 A cathode for a sodium secondary battery containing Fe, Ni, and Mn.

4. In claim 1, The above Prussian blue analogue particles are a positive electrode for a sodium secondary battery comprising a compound represented by the following chemical formula 2: [Chemical formula 2] And x2 I 2 [I 3 (CN)6] In the above chemical formula 2, 1.8≤x2≤2.2, and M 2 is at least one selected from the group consisting of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr, and M 3 is at least one selected from the group consisting of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr.

5. In claim 1, The average particle diameter (D) of the first positive electrode active material 50 ) is a positive electrode for a sodium secondary battery having a diameter of 3 μm to 19 μm.

6. In claim 1, The average particle diameter (D) of the second positive electrode active material 50 ) is a positive electrode for a sodium secondary battery having a thickness of 0.5 μm to 4.0 μm.

7. In claim 1, A positive electrode for a sodium secondary battery, wherein the weight ratio of the first positive electrode active material and the second positive electrode active material is 70:30 to 99:

1.

8. In claim 1, The specific surface area of ​​the above first positive electrode active material is 0.4 m 2 / g to 1.0m 2 / g positive electrode for sodium secondary battery.

9. In claim 1, The specific surface area of ​​the above second positive electrode active material is 1 m 2 / g to 5m 2 / g positive electrode for sodium secondary battery.

10. In claim 1, A positive electrode for a sodium secondary battery, wherein the thickness ratio of the first positive electrode active material layer and the second positive electrode active material layer is 1.5:1 to 10:

1.

11. In claim 1, A positive electrode for a sodium secondary battery, wherein the thickness of the first positive electrode active material layer is 10 ㎛ to 70 ㎛.

12. In claim 1, A positive electrode for a sodium secondary battery, wherein the thickness of the second positive electrode active material layer is 10 ㎛ to 40 ㎛.

13. In claim 1, A positive electrode for a sodium secondary battery, wherein the loading ratio of the first positive electrode active material layer and the second positive electrode active material layer is 2:1 to 10:

1.

14. A sodium secondary battery comprising the positive electrode for a sodium secondary battery of claim 1.

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