Positive electrode for sodium secondary battery and sodium secondary battery comprising the same

Abstract

This invention relates to a positive electrode for a sodium secondary battery. The positive electrode includes 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. The first positive electrode active material layer contains a first positive electrode active material, and the second positive electrode active material layer contains a second positive electrode active material. The first positive electrode active material contains sodium composite transition metal oxide particles having an O3 octahedral crystal structure, and the second positive electrode active material contains sodium composite transition metal oxide particles having a P2 orthorhombic crystal structure. The average particle size (D) between the first and second positive electrode active materials is specified. 50 The ratio of ) is 2:1 to 3:1.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2023-0187456 filed on December 20, 2023 and Korean Patent Application No. 10-2024-0191928 filed on December 19, 2024, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] This invention relates to a positive electrode for a sodium secondary battery and a sodium secondary battery containing a positive electrode, and more specifically, to a positive electrode for a sodium secondary battery and a sodium secondary battery containing a positive electrode, wherein the positive electrode comprises a current collector, a first positive electrode active material layer, and a second positive electrode active material layer. Background Technology

[0004] A rechargeable battery is a battery that can be repeatedly used by converting chemical energy into electrical energy through a discharge process and a recharge process in the reverse direction. Recently, with the commercialization of portable electronic products and electric vehicles, the demand for rechargeable batteries has grown rapidly. Currently, lithium-ion rechargeable batteries are mainly used in commercially available batteries, but lithium reserves, as the main raw material, are limited, making them expensive and insufficient to meet the demand. Therefore, there is a need to develop a new type of rechargeable battery that can replace lithium-ion batteries. Recently, there has been active research and development into sodium-ion rechargeable batteries that use sodium to replace lithium.

[0005] However, because sodium ions are relatively larger than lithium ions, if sodium ions are used as charge carriers, the steric hindrance increases. As a result, when sodium ions are used in secondary batteries, there is a problem of reduced rate performance.

[0006] To address these issues, O3-type and P2-type layered oxides are being developed. However, O3-type layered oxides have small interlayer distances, limiting their high-rate performance and causing a decrease in lifetime performance due to changes in crystal structure during charge and discharge. Furthermore, in P2-type layered oxides, because the distance between the sodium layer and the transition metal layer varies with the location of the sodium sites, there are two types of sodium sites (Na) with different degrees of repulsion to the transition metal. e and Na f And it has a strong repulsive force when passing through Na with transition metals. f When the movement speed decreases, the effect of improving the multiplier performance is insufficient.

[0007] Therefore, in order to solve the above problems, it is necessary to develop a positive electrode for sodium secondary batteries that can achieve better capacity and rate performance. Summary of the Invention

[0008] Technical issues

[0009] To address the aforementioned problems, one aspect of the present invention provides a positive electrode for a sodium secondary battery, the positive electrode comprising: a first positive electrode active material comprising sodium composite transition metal oxide particles having an O3 octahedral crystal structure as a first positive electrode active material layer, and the positive electrode comprising: a second positive electrode active material comprising sodium composite transition metal oxide particles having a P2 orthorhombic crystal structure as a second positive electrode active material layer, wherein the average particle size (D) between the first positive electrode active material and the second positive electrode active material is... 50 The ratio of ) is 2:1 to 3:1.

[0010] Furthermore, another aspect of the present invention provides a sodium secondary battery comprising the above-described positive electrode for a sodium secondary battery.

[0011] Technical solution

[0012] [1] This invention provides a positive electrode for a sodium secondary battery, the positive electrode comprising: Current collector, The first positive electrode active material layer disposed on the current collector, and The second positive electrode active material layer is disposed on the first positive electrode active material layer. The first positive electrode active material layer contains a first positive electrode active material, and The second positive electrode active material layer contains a second positive electrode active material. The first positive electrode active material contains sodium composite transition metal oxide particles with an O3 octahedral crystal structure, and The second positive electrode active material contains sodium composite transition metal oxide particles with a P2 orthorhombic crystal structure. The average particle size (D) between the first positive electrode active material and the second positive electrode active material 50 The ratio of ) is 2:1 to 3:1.

[0013] [2] In this invention, in the above [1], the sodium composite transition metal oxide particles having an O3 octahedral crystal structure may include compounds represented by the following chemical formula 1.

[0014] [Chemical Formula 1]

[0015] Na x1 M 1 O2

[0016] In the above chemical formula 1, 0.7 ≤ x1 ≤ 1, and M 1It may be one or more selected from Fe, Ni, Mn, Co, Mg, Zn, Cu, and Cr.

[0017] [3] In the present invention, in the above [2], M 1 may include Fe, Ni, and Mn.

[0018] [4] In the present invention, in at least one of the above [1] to [3], the sodium composite transition metal oxide particles having a P2 orthorhombic crystal structure may include a compound represented by the following Chemical Formula 2.

[0019] [Chemical Formula 2]

[0020] Na x2 M 2 O2

[0021] In the above Chemical Formula 2, x2 may be 0 < x2 < 0.7, and M 2 may be one or more selected from Fe, Ni, Mn, Co, Mg, Zn, Cu, and Cr.

[0022] [5] In the present invention, in the above [4], M 2 may include Fe, Ni, and Mn.

[0023] [6] In the present invention, in at least one of the above [1] to [5], the average particle diameter (D 50 ) of the first positive electrode active material may be 8 μm to 13 μm.

[0024] [7] In the present invention, in at least one of the above [1] to [6], the average particle diameter (D 50 ) of the second positive electrode active material may be 2 μm to 8 μm.

[0025] [8] In the present invention, in at least one of the above [1] to [7], the weight ratio between the first positive electrode active material and the second positive electrode active material may be 70:30 to 99:1.

[0026] [9] In the present invention, in at least one of the above [1] to [8], the specific surface area of the first positive electrode active material may be 0.2 m 2 / g to 0.7 m 2 / g.

[0027]

[10] In the present invention, in at least one of the above [1] to [9], the specific surface area of the second positive electrode active material may be 0.2 m 2 / g to 0.7 m 2 / g.

[0028]

[11] In this invention, in at least one of [1] to

[10] above, the thickness ratio between the first positive electrode active material layer and the second positive electrode active material layer can be from 1.2:1 to 6:1.

[0029]

[12] In this invention, in at least one of [1] to

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

[0030]

[13] In this invention, in at least one of [1] to

[12] above, the thickness of the second positive electrode active material layer can be from 5 μm to 30 μm.

[0031]

[14] In this invention, in at least one of [1] to

[13] above, the loading ratio between the first positive electrode active material layer and the second positive electrode active material layer can be 2:1 to 7:1.

[0032]

[15] The present invention provides a sodium secondary battery comprising at least one of the above-mentioned [1] to

[13] sodium secondary battery positive electrodes.

[0033] Beneficial effects

[0034] The positive electrode according to the present invention may comprise a current collector, a first positive electrode active material layer and a second positive electrode active material layer stacked sequentially, wherein the first positive electrode active material layer comprises a first positive electrode active material, the first positive electrode active material comprising sodium composite transition metal oxide particles having an O3 octahedral crystal structure, and the second positive electrode active material layer comprises a second positive electrode active material, the second positive electrode active material comprising sodium composite transition metal oxide particles having a P2 orthorhombic crystal structure. Based on the stacking structure of the first positive electrode active material layer and the second positive electrode active material layer and the components contained therein, the positive electrode may possess excellent capacity performance, rate performance and lifetime performance.

[0035] Therefore, if the positive electrode for sodium secondary batteries according to the present invention is applied to sodium secondary batteries, excellent capacity performance and rate performance can be achieved. Attached Figure Description

[0036] The accompanying drawings in this document illustrate preferred embodiments of the invention by way of example and, together with the detailed description of the invention given below, serve to further understand the technical concept of the invention. Therefore, the invention is explained solely based on the content of such drawings. Furthermore, the shapes, dimensions, scales, or ratios of elements in the drawings included in this specification may be exaggerated to emphasize clearer illustrations.

[0037] Figure 1 This is a schematic side view illustrating a positive electrode for a sodium secondary battery according to an embodiment of the present invention. Detailed Implementation

[0038] The present invention will be described in more detail below.

[0039] It should be understood that the terms or words used in this specification and claims should not be construed as having the meanings defined in commonly used dictionaries, but rather as having meanings and concepts consistent with the technical ideas of the invention, based on the inventor's ability to appropriately define the concepts of the terms to best explain the principles of the invention.

[0040] The terminology used herein is for illustrative purposes and is not intended to limit the invention. In this specification, the singular includes the plural unless the context clearly indicates otherwise. As used herein, the terms “comprising” and / or “including” are intended to include the stated constituent elements and do not exclude the possibility of the presence or addition of one or more other constituent elements.

[0041] In this invention, the average particle size (D) 50 The average particle size (D) refers to the particle size of 50% of the volumetric cumulative particle size distribution of the tested powder, such as the first and second positive electrode active materials. 50 The average particle size (D) can be measured using laser diffraction. 50 The volumetric cumulative particle size distribution map can be obtained by dispersing the powder to be tested in a dispersion medium and then introducing the mixture into a commercially available laser diffraction particle size measurement device (such as Microtrac MT 3000) to irradiate it with ultrasonic waves at an output power of 60 W and approximately 28 kHz. The particle size corresponding to 50% of the cumulative volume can then be measured.

[0042] In this invention, the "specific surface area" is measured by the BET method. Specifically, it can be calculated using the Belserp-mino II of BEL Japan Co., Ltd. from the amount of nitrogen adsorbed at liquid nitrogen temperature (77 K).

[0043] Positive electrode for sodium secondary batteries

[0044] The positive electrode for a sodium secondary battery according to the present invention will be described below.

[0045] The positive electrode for a sodium secondary battery according to the present invention comprises: Current collector, The first positive electrode active material layer disposed on the current collector, and The second positive electrode active material layer is disposed on the first positive electrode active material layer. in The first positive electrode active material layer contains the first positive electrode active material, and The second positive electrode active material layer contains a second positive electrode active material. in The first positive electrode active material contains sodium composite transition metal oxide particles with an O3 octahedral crystal structure, and The second positive electrode active material contains sodium composite transition metal oxide particles with a P2 orthorhombic crystal structure. The average particle size (D) between the first positive electrode active material and the second positive electrode active material 50 The ratio of ) is 2:1 to 3:1.

[0046] There are no particular limitations on the current collector according to the present invention, as long as it is conductive and will not cause chemical changes in the battery, and materials such as stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used. Furthermore, the thickness of the positive electrode current collector can typically be from 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to improve the adhesion of the positive electrode active material. For example, the current collector can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0047] The first positive electrode active material layer is disposed on the current collector.

[0048] The first positive electrode active material layer can be disposed on at least one surface of the current collector. Specifically, the first positive electrode active material layer can be disposed on one or both surfaces of the current collector.

[0049] The first positive electrode active material layer contains the first positive electrode active material.

[0050] The first positive electrode active material contains sodium composite transition metal oxide particles with an O3 octahedral crystal structure.

[0051] The O3 octahedral crystal structure refers to the structure of sodium ions located at octahedral sites in the R3-m space group. Specifically, if the molar number of sodium is 0.7 to 1 times the total molar number of transition metals, a sodium composite transition metal oxide with an O3 octahedral crystal structure is achieved.

[0052] Sodium composite transition metal oxide particles with an O3 octahedral crystal structure have a relatively high initial Na content, allowing for the insertion and extraction of more sodium ions and resulting in high capacity, making them suitable for achieving high-energy-density cathodes. Furthermore, the first cathode active material contains sodium composite transition metal oxide particles with an O3 octahedral crystal structure, thus possessing a relatively large contact area with the current collector, allowing for high adhesion between the current collector and the entire active material layer. On the other hand, there is a concern that sodium composite transition metal oxide particles with an O3 octahedral crystal structure can be oxidized upon exposure to atmospheric moisture or CO2, causing atmospherically unstable molecules such as H2O or CO2 to insert Na. + The problem of changes in the structure and composition of the layer or surface can be prevented by the second positive electrode active material layer covering the first positive electrode active material layer.

[0053] More specifically, sodium complex transition metal oxide particles having an O3 octahedral crystal structure may include compounds represented by the following chemical formula 1.

[0054] [Chemical Formula 1]

[0055] Na x1 M 1 O2

[0056] In the above chemical formula 1, x1 can be 0.7 ≤ x1 ≤ 1, and M 1 It can be selected from one or more of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr.

[0057] In the above chemical formula 1, x1 refers to the ratio of the molar content of sodium to the total molar content of transition metals in the compound, and can be 0.7 ≤ x1 ≤ 1, specifically 0.75 ≤ x1 ≤ 1, and more specifically 0.9 ≤ x1 ≤ 1. If the atomic fraction of sodium meets the above range, it can be conducive to the formation of the O3 octahedral crystal structure.

[0058] In the above chemical formula 1, M 1 It can be one or more selected from Fe, Ni, Mn, Co, Mg, Zn, Cu, and Cr, specifically one or more selected from Fe, Ni, and Mn, and more specifically Fe, Ni, and Mn. When using the above elements as M... 1 In the case of sodium complex transition metal oxides, because of the large difference in ionic radii between sodium ions and metal ions, less cation disorder occurs, thus exhibiting structural stability.

[0059] More specifically, the compounds represented by the above chemical formula 1 may include compounds represented by the following chemical formula 1a.

[0060] [Chemical Formula 1a]

[0061] Na x1 Fe y1 Ni y2 Mn y3 O2

[0062] In the above Chemical Formula 1a, x1 can be 0.7 ≤ x1 ≤ 1, specifically 0.75 ≤ x1 ≤ 1, more specifically 0.9 ≤ x1 ≤ 1, and y1, y2, and y3 respectively represent the atomic fractions of Fe, Ni, and Mn in the sodium composite transition metal oxide, and can be 0 < y1 < 1, 0 < y2 < 1, and 0 < y3 < 1, specifically 0.1 < y1 < 0.5, 0.1 < y2 < 0.5, and 0.1 < y3 < 0.5. In this case, it can be that y1 + y2 + y3 = 1.

[0063] The average particle size (D 50 ) of the first positive electrode active material can be 1 μm or more, 5 μm or more, 6 μm or more, 8 μm or more, 9 μm or more, or 10 μm or more, and can be 50 μm or less, 30 μm or less, 17 μm or less, 15 μm or less, 14 μm or less, or 12 μm or less. Specifically, the average particle size (D 50 ) can be 1 μm to 50 μm, more specifically 6 μm to 17 μm, even more specifically 10 μm to 12 μm. If the average particle size (D 50 ) of the first positive electrode active material satisfies the above range, the first positive electrode active material layer has a high electrode density, and thus there are many contact points with the current collector, resulting in an effect beneficial to electron movement.

[0064] The specific surface area of the first positive electrode active material can be 0.05 m 2 / g or more, 0.1 m 2 / g or more, 0.15 m 2 / g or more, 0.2 m 2 / g or more, or 0.4 m 2 / g or more, and can be 20 m 2 / g or less, 10 m 2 / g or less, 1.0 m 2 / g or less, 0.7 m 2 / g or less, or 0.5 m 2 / g or less. Specifically, the specific surface area can be 0.05 m 2 / g to 20 m 2 / g, more specifically 0.2 m 2 / g to 0.7 m 2 / g, or even more specifically 0.4 m 2 / g to 0.5 m 2 / g. If the specific surface area of ​​the first positive electrode active material meets the above range, the adhesion between the current collector and the layer containing sodium composite transition metal oxide particles with an O3 octahedral crystal structure can be improved.

[0065] The thickness of the first positive electrode active material layer can be greater than 1 μm, greater than 10 μm, greater than 20 μm, or greater than 30 μm, and can be less than 500 μm, less than 300 μm, less than 100 μm, less than 70 μm, or less than 50 μm. Specifically, the thickness can be from 1 μm to 500 μm, more specifically from 10 μm to 70 μm, and even more specifically from 30 μm to 50 μm. When the thickness of the first positive electrode active material layer meets the above ranges, it is beneficial for the movement of electrons and ions in the first positive electrode active material layer, thereby exhibiting optimized electrochemical performance.

[0066] The first positive electrode active material layer may include a first binder and / or a first conductive material in addition to the first positive electrode active material.

[0067] The first conductive material is used to impart conductivity to the electrode, and any conductive material can be used without particular restriction, as long as it is electronically conductive and will not cause chemical changes in the battery to be constructed. Specific examples may include: graphite such as natural or artificial graphite; carbon materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, and carbon fibers; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and any one or a mixture of two or more thereof may be used. Based on the total weight of the first positive electrode active material layer, the content of the first conductive material can be from 1% by weight to 30% by weight.

[0068] The first binder is used to improve the bonding between particles of the positive electrode active material and the adhesion between the positive electrode active material and the current collector. Specific examples may include: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one or a mixture of two or more thereof may be used. Based on the total weight of the first positive electrode active material layer, the content of the first binder may be from 1% by weight to 30% by weight.

[0069] The second positive electrode active material layer is provided on the first positive electrode active material layer. Specifically, the second positive electrode active material layer may be provided on the surface of the first positive electrode active material layer, and the surface is opposite to the surface of the first positive electrode active material layer that contacts the current collector.

[0070] In addition, if the first positive electrode active material layer exists on both surfaces of the current collector, the second positive electrode active material layer may be provided on the respective surfaces of the first positive electrode active material layers provided on both surfaces of the current collector.

[0071] The second positive electrode active material layer according to the present invention includes a second positive electrode active material, and the second positive electrode active material includes sodium composite transition metal oxide particles having a P2 orthorhombic crystal structure.

[0072] The P2 orthorhombic crystal structure refers to a structure having a P63 / mmc space group in which sodium ions are located at large trigonal prism sites. It can be understood that specifically, if the molar number of sodium is greater than 0 times to less than 0.7 times the total molar number of transition metals, sodium composite transition metal oxides having a P2 orthorhombic crystal structure are achieved.

[0073] The interlayer distance of the sodium composite transition metal oxide particles having a P2 orthorhombic crystal structure is large, which is beneficial to the insertion and extraction of sodium ions. Thus, it has excellent high-rate performance, and the change in the crystal structure during charge and discharge is small, thus having excellent life performance. However, the density of the P2 orthorhombic crystal structure is low, and thus there is a disadvantage of reduced adhesion to the current collector. Therefore, it is preferable to provide the P2 orthorhombic crystal structure in the second positive electrode active material layer as the upper layer portion. In addition, because the P2 orthorhombic crystal structure is provided in the upper layer portion, covering the O3 octahedral crystal structure that is unstable in the atmosphere, excellent adhesion, rate performance, and life performance can be achieved.

[0074] Specifically, the sodium composite transition metal oxide particles having a P2 orthorhombic crystal structure may include a compound represented by the following Chemical Formula 2.

[0075] [Chemical Formula 2]

[0076] Na x2 M 2 O2

[0077] In the above Chemical Formula 2, x2 may be 0 < x2 < 0.7, and M 2 may be one or more selected from Fe, Ni, Mn, Co, Mg, Zn, Cu, and Cr.

[0078] In the above Chemical Formula 2, x2 represents the ratio of the molar content of sodium to the total molar content of transition metals in the compound, and can be 0 < x2 < 0.7, specifically 0.2 < x2 < 0.7, more specifically 0.4 < x2 < 0.7. If the atomic fraction of sodium satisfies the above range, it can be beneficial to the formation of the P2 orthorhombic crystal structure.

[0079] In the above Chemical Formula 2, M 2 can be one or more selected from Fe, Ni, Mn, Co, Mg, Zn, Cu, and Cr. Specifically, it can be one or more selected from Fe, Ni, and Mn. More specifically, it can be Fe, Ni, and Mn. When using the above elements as M 2 in the case of the sodium composite transition metal oxide, it can exhibit high ionic conductivity and stable structural changes and rate performance during charge and discharge.

[0080] More specifically, the compound represented by the above Chemical Formula 2 may include a compound represented by the following Chemical Formula 2a.

[0081] [Chemical Formula 2a]

[0082] Na x2 Fe z1 Ni z2 Mn z3 O2

[0083] In the above Chemical Formula 2a, x2 can be 0 < x2 < 0.7, specifically 0.2 < x2 < 0.7, more specifically 0.4 < x2 < 0.7, and z1, z2, and z3 respectively represent the atomic fractions of Fe, Ni, and Mn in the sodium composite transition metal oxide, and can be 0 < z1 < 1, 0 < z2 < 1, and 0 < z3 < 1. Specifically, it can be 0.1 < z1 < 0.5, 0.1 < z2 < 0.5, and 0.1 < z3 < 0.5. In this case, it can be that z1 + z2 + z3 = 1.

[0084] The average particle size (D 50 ) of the second positive electrode active material can be 0.5 μm or more, 1 μm or more, 2 μm or more, 3 μm or more, or 4 μm or more, and can be 45 μm or less, 25 μm or less, 10 μm or less, 8 μm or less, or 5 μm or less. Specifically, the average particle size (D 50 ) can be 0.5 μm to 45 μm, more specifically 2 μm to 8 μm, and even more specifically 4 μm to 5 μm. If the average particle size (D 50If the above range is met, the sodium ion solid-phase diffusion distance is shorter than that of the first positive electrode active material layer because the ratio of small particles in the second positive electrode active material layer is high, thereby further achieving a high-rate discharge effect.

[0085] The specific surface area of ​​the second positive electrode active material can be 0.05 m². 2 / g or more, 0.1 m 2 / g or more, 0.2 m 2 / g or more or 0.4 m 2 / g or more, and can be 20 m 2 / g or less, 10 m 2 / g or less, 5 m 2 / g or less, 1 m 2 / g or less, 0.7m 2 / g or less or 0.6 m 2 / g or less. Specifically, the specific surface area can be 0.05 m². 2 / g to 20 m 2 / g, more specifically 0.2m 2 / g to 0.7 m 2 / g, or even more specifically 0.4 m 2 / g to 0.6 m 2 / g. If the specific surface area of ​​the second positive electrode active material meets the above range, the adhesion to the first positive electrode active material layer can be improved.

[0086] The thickness of the second positive electrode active material layer can be greater than 0.5 μm, greater than 2 μm, greater than 4 μm, greater than 5 μm, or greater than 10 μm, and can be less than 300 μm, less than 100 μm, less than 30 μm, less than 20 μm, or less than 15 μm. Specifically, the thickness can be from 0.5 μm to 300 μm, more specifically from 5 μm to 30 μm, and even more specifically from 10 μm to 15 μm. If the thickness of the second positive electrode active material layer meets the above ranges, high-rate discharge performance can be improved by promoting sodium ion diffusion.

[0087] The second positive electrode active material layer may include a second binder and / or a second conductive material in addition to the second positive electrode active material.

[0088] The second conductive material is used to impart conductivity to the electrode, and any conductive material can be used without particular restriction, as long as it is electronically conductive and will not cause chemical changes in the battery to be constructed. Specific examples may include: graphite such as natural or artificial graphite; carbon materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, and carbon fibers; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and any one or a mixture of two or more thereof may be used. Based on the total weight of the second positive electrode active material layer, the content of the second conductive material can be from 1% by weight to 30% by weight.

[0089] The second binder is used to improve the bonding between particles of the positive electrode active material and the adhesion between the positive electrode active material and the current collector. Specific examples may include: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one or a mixture of two or more thereof may be used. Based on the total weight of the second positive electrode active material layer, the content of the second binder may be from 1% by weight to 30% by weight.

[0090] The average particle size (D) between the first positive electrode active material and the second positive electrode active material 50 The ratio of sodium ions to their intercalation / deintercalation diameters can be 2:1 to 3:1, specifically 2.0:1 to 2.7:1, and more specifically 2.1:1 to 2.4:1. If this ratio exceeds the above range, the intercalation / deintercalation reaction of sodium ions slows down, leading to a decrease in the output performance of the sodium secondary battery, resulting in problems with high-rate performance and reduced capacity retention. Therefore, if the average particle size (D...) 50 If the ratio of the two positive electrode active materials meets 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, thereby maintaining stable performance even during high-rate charge and discharge, thus improving high-rate performance and capacity retention.

[0091] Furthermore, the weight ratio between the first positive electrode active material and the second positive electrode active material can be from 50:50 to 99:1, specifically from 60:40 to 99:1, and more specifically from 70:30 to 99:1. If this weight ratio meets the above range, the positive electrode capacity increases, which can improve high-rate performance.

[0092] Furthermore, the thickness ratio between the first positive electrode active material layer and the second positive electrode active material layer can be from 1.2:1 to 30:1, specifically from 1.2:1 to 6:1, and more specifically from 2:1 to 4:1. If this thickness ratio meets the above range, it is beneficial for the movement of electrons and ions between the first positive electrode active material layer and the second positive electrode active material layer, thereby exhibiting high rate performance.

[0093] Furthermore, the loading ratio between the first positive electrode active material layer and the second positive electrode active material layer can be from 1.4:1 to 35:1, specifically from 2:1 to 7:1, and more specifically from 4:1 to 6:1. If this loading ratio meets the above range, the adhesion and capacity of the positive electrode will increase, which can improve high-rate performance.

[0094] The first positive electrode active material layer can be prepared by coating a first positive electrode slurry onto a current collector and drying it. The first positive electrode slurry is prepared by dissolving or dispersing a first positive electrode active material and a selective first binder and / or a first conductive material in a solvent. Alternatively, the first positive electrode active material layer can be prepared by casting a first positive electrode slurry onto a separate support and then pressing the film layer obtained by peeling it from the support onto the current collector. The second positive electrode active material layer can also be achieved by preparing a second positive electrode slurry in the same manner as described above, except that a second positive electrode active material, a second binder, and a second conductive material are used.

[0095] If the positive electrode active material layer comprises a first positive electrode active material layer and a second positive electrode active material layer, there are no particular limitations on the manufacture of the positive electrode for sodium secondary batteries, as long as it can achieve the aforementioned properties of the first and second positive electrode active material layers. For example, the positive electrode for sodium secondary batteries according to the present invention can be manufactured by: preparing a first positive electrode slurry by adding a first positive electrode active material, a selective first binder, and / or a first conductive material to a solvent (e.g., NMP); preparing a second positive electrode slurry by adding a second positive electrode active material, a selective second binder, and / or a second conductive material to a solvent (e.g., NMP); and then coating the first and second positive electrode slurries onto a current collector. More specifically, the positive electrode for sodium secondary batteries according to the present invention can be manufactured by: forming a first positive electrode active material layer by coating the first positive electrode slurry prepared above onto a current collector, calendering, and drying; and forming a second positive electrode active material layer by coating the second positive electrode slurry prepared above onto the first positive electrode active material layer, calendering, and drying. On the other hand, the positive electrode for sodium secondary batteries according to the present invention can also be manufactured by the following operation: while coating a first positive electrode slurry onto a current collector, a second positive electrode slurry is coated onto the coated first positive electrode slurry substantially simultaneously, followed by calendering and drying.

[0096] Sodium secondary battery

[0097] Next, the sodium secondary battery according to the present invention will be described.

[0098] The sodium secondary battery according to the present invention includes the above-described positive electrode for a sodium secondary battery according to the present invention. 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, a negative electrode positioned facing the positive electrode for a sodium secondary battery, a separator between the positive electrode and the negative electrode for a sodium secondary battery, and an electrolyte. Since the positive electrode for a sodium secondary battery has been described above, only the remaining constituent elements will be described below.

[0099] The negative electrode according to the present invention comprises a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0100] There are no particular restrictions on the negative electrode current collector, as long as it has high conductivity and will not cause chemical changes in the battery. Materials used include, for example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, or silver, and aluminum-cadmium alloys. Furthermore, the thickness of the negative electrode current collector can typically range from 3 μm to 500 μm, and, as in the case of the positive electrode current collector, fine irregularities can be formed on its surface to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0101] The negative electrode active material layer selectively includes binders and conductive materials in addition to the negative electrode active material.

[0102] As a negative electrode active material, compounds capable of reversibly inserting and de-inserting sodium can be used. Specific examples include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fibers, hard carbon, soft carbon, and amorphous carbon; metallic compounds that can form alloys with sodium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and metal oxides that can be doped and de-doped with sodium, such as SiO₂. β (0<β<2), SnO2, vanadium oxide and sodium vanadium oxide; or composites containing metal compounds and carbonaceous materials such as Si-C composites or Sn-C composites, and any one of them or a mixture of two or more thereof can be used. Furthermore, metallic sodium thin films can be used as the negative electrode active material.

[0103] Adhesives are used to improve the bonding between particles of the negative electrode active material and the adhesion between the negative electrode active material and the negative electrode current collector. Specific examples may include: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one or mixtures of two or more thereof may be used. Based on the total weight of the negative electrode active material layer, the adhesive content may be from 1% to 30% by weight, specifically from 1% to 20% by weight, and more specifically from 1% to 10% by weight.

[0104] Conductive materials are used to impart conductivity to the negative electrode, and any conductive material can be used without particular restriction, as long as it is electronically conductive and does not cause chemical changes in the battery to be constructed. Specific examples may include: graphite such as natural or artificial graphite; carbon materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, carbon fibers, and carbon nanotubes; metal powders or fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and any one or a mixture of two or more thereof may be used. Based on the total weight of the negative electrode active material layer, the content of conductive material can typically be from 1% to 30% by weight, specifically from 1% to 20% by weight, and more specifically from 1% to 10% by weight.

[0105] As an example, the negative electrode active material layer can be prepared by coating a negative electrode active material layer forming composition onto a negative electrode current collector and then drying it. The negative electrode active material layer forming composition is prepared by dissolving or dispersing the negative electrode active material and selective binders and conductive materials in a solvent. Alternatively, the negative electrode active material layer can be prepared by casting the negative electrode active material layer forming composition onto a separate support and then pressing the film layer peeled off from the support onto the negative electrode current collector.

[0106] The separator according to the present invention separates the negative and positive electrodes and provides a channel for the movement of sodium ions. It can use separators commonly used in secondary batteries, and there are no particular limitations on the type of separator. For example, as a separator, porous polymer membranes made from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers can be used; or stacked structures having two or more layers; or porous nonwoven fabrics formed from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Furthermore, separators coated with ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and can be selectively used in single-layer or multi-layer structures.

[0107] The electrolyte according to the present invention can be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc., which can be used to manufacture sodium secondary batteries, but is not limited thereto.

[0108] Specifically, electrolytes can contain organic solvents and sodium salts.

[0109] As organic solvents, any organic solvent can be used without particular restrictions, as long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as organic solvents, the following can be used: 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; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a linear, branched, or cyclic C2 to C20 hydrocarbon group, and may contain double bonds, aromatic rings, or ether bonds); amides such as dimethylformamide; dioxolane such as 1,3-dioxolane; or sulfolane.

[0110] Sodium salts can be used without particular restrictions, as long as they are compounds that can provide sodium ions used in sodium secondary batteries. Specifically, sodium salts that can be used include NaPF6, NaClO4, NaAsF6, NaBF4, NaCF3SO3, NaB(C6H5)4, NaC4F9SO3, NaN(C2F5SO3)2, NaN(C2F5SO2)2, and NaN(CF3SO2)2. Preferably, sodium salts are used in the concentration range of 0.1M to 2.0M.

[0111] To improve battery life, suppress capacity degradation, and enhance discharge capacity, the electrolyte may contain one or more additives, such as alkylene carbonate halides like fluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphoric triamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted sulfadiazine ketones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. In this case, the additive content can range from 0.1% to 5% by weight, based on the total weight of the electrolyte.

[0112] Sodium secondary batteries comprising the positive electrode for sodium secondary batteries according to the present invention as described above can be applied to: portable devices such as mobile phones, laptops and digital cameras; and electric vehicles such as hybrid electric vehicles (HEVs).

[0113] The present invention will now be described in more detail with reference to specific embodiments. However, the following embodiments are for illustrative purposes only, so as to facilitate understanding of the invention, and do not limit the scope of the invention. It will be apparent to those skilled in the art that various changes and modifications can be made without departing from the scope and spirit of the invention, and it is obvious that such changes and modifications are within the scope of the appended claims.

[0114] Examples and Comparative Examples

[0115] Example 1: Manufacturing of the positive electrode for sodium secondary batteries

[0116] The average particle size (D) of the first positive electrode active material will be used. 50 Its thickness is 10.7 μm and its specific surface area is 0.47 m². 2 / g of NaFe 0.33 Ni 0.33 Mn 0.34 O2, conductive material (carbon black), and binder (PVdF) are mixed in N-methylpyrrolidone (NMP) at a weight ratio of 96:2:2 to prepare a positive electrode slurry for the first positive electrode active material layer.

[0117] The average particle size (D) of the second positive electrode active material will be used. 50 Its thickness is 4.7 μm and its specific surface area is 0.5 m². 2 / g Na 0.67 Fe 0.33 Ni 0.33 Mn 0.34 O2, conductive material (carbon black), and binder (PVdF) are mixed in N-methylpyrrolidone (NMP) at a weight ratio of 96:2:2 to prepare a positive electrode slurry for the second positive electrode active material layer.

[0118] A first positive electrode active material layer is formed by coating a positive electrode slurry onto a 20 μm aluminum foil, which serves as the current collector, and then drying it in a vacuum oven at 120°C for 1 hour. A second positive electrode active material layer is then coated onto the first positive electrode active material layer, and dried in a vacuum oven at 120°C for 1 hour.

[0119] The resulting material was then dried in a vacuum drying oven at 120°C for 12 hours, followed by calendering to manufacture a positive electrode for sodium secondary batteries. Ultimately, a positive electrode for sodium secondary batteries was manufactured, in which a current collector, a first positive electrode active material layer, and a second positive electrode active material layer were sequentially disposed.

[0120] In this case, the thickness of the first positive electrode active material layer is 40 μm and the loading is 1.67 mAh / cm². 2 Furthermore, the thickness of the second positive electrode active material layer is 13 μm and the loading is 0.33 mAh / cm². 2 .

[0121] Example 2: Manufacturing of the positive electrode for sodium secondary batteries

[0122] In addition to using average particle size (D) 50 Its thickness is 12.6 μm and its specific surface area is 0.40 m². 2 / g of NaFe 0.33 Ni 0.33 Mn 0.34 Apart from O2, a positive electrode for a sodium secondary battery was manufactured in the same manner as in Example 1.

[0123] Comparative Example 1: Manufacturing of the positive electrode for sodium secondary batteries

[0124] Except for forming only the first positive electrode active material layer instead of forming the second positive electrode active material layer separately, so as to finally manufacture a positive electrode for a sodium secondary battery in which a current collector and the first positive electrode active material layer are sequentially disposed, the positive electrode for a sodium secondary battery was manufactured in the same manner as in Example 1.

[0125] Comparative Example 2: Manufacturing of the positive electrode for sodium secondary batteries

[0126] A positive electrode slurry was prepared by mixing a positive electrode active material with a conductive material (carbon black) and a binder (PVdF) in N-methylpyrrolidone (NMP) at a weight ratio of 96:2:2. The positive electrode active material was determined by measuring the average particle size (D) of the first positive electrode active material. 50 Its thickness is 10.7 μm and its specific surface area is 0.47 m². 2 / g of NaFe 0.33 Ni 0.33 Mn 0.34O2 and the average particle size (D) of the second positive electrode active material 50 Its thickness is 4.7 μm and its specific surface area is 0.5 m². 2 / g Na 0.67 Fe 0.33 Ni 0.33 Mn 0.34 O2 was prepared by mixing in a 5:1 weight ratio.

[0127] The positive electrode slurry was coated onto a 20 μm thick aluminum foil serving as the current collector and dried in a vacuum drying oven at 120°C for 12 hours, followed by calendering to manufacture a positive electrode for sodium secondary batteries. Ultimately, a positive electrode for sodium secondary batteries was manufactured in which a current collector and a positive electrode active material layer were sequentially disposed.

[0128] In this case, the thickness of the positive electrode active material layer is 50 μm and the loading is 2 mAh / cm². 2 .

[0129] Comparative Example 3: Manufacturing of the positive electrode for sodium secondary batteries

[0130] In addition to using average particle size (D) 50 Its thickness is 4.7 μm and its specific surface area is 0.5 m². 2 / g Na 0.67 Fe 0.33 Ni 0.33 Mn 0.34 O2 is used as the first positive electrode active material and the average particle size (D) is used. 50 Its thickness is 10.7 μm and its specific surface area is 0.47 m². 2 / g of NaFe 0.33 Ni 0.33 Mn 0.34 In addition to using O2 as the second positive electrode active material, a positive electrode for sodium secondary batteries was manufactured in the same manner as in Example 1.

[0131] In this case, the thickness of the first positive electrode active material layer is 13 μm and the loading is 0.33 mAh / cm². 2 Furthermore, the thickness of the second positive electrode active material layer is 40 μm and the loading is 1.67 mAh / cm². 2 .

[0132] Comparative Example 4: Manufacturing of the positive electrode for sodium secondary batteries

[0133] In addition to using average particle size (D) 50 Its thickness is 17.6 μm and its specific surface area is 0.34 m². 2 / g of NaFe 0.33 Ni 0.33 Mn 0.34Apart from O2, a positive electrode for a sodium secondary battery was manufactured in the same manner as in Example 1.

[0134] Comparative Example 5: Manufacturing of the positive electrode for sodium secondary batteries

[0135] In addition to using average particle size (D) 50 Its thickness is 5.2 μm and its specific surface area is 0.58 m². 2 / g of NaFe 0.33 Ni 0.33 Mn 0.34 Apart from O2, a positive electrode for a sodium secondary battery was manufactured in the same manner as in Example 1.

[0136] Experimental Example 1: Evaluation of Rate Capacity Performance

[0137] <Manufacturing of a Coin Half-Battery>

[0138] A separator (glass fiber, Whatman Co., Ltd.) was inserted between the positive and negative electrodes (Na metal) of each sodium secondary battery manufactured in Examples 1 and 2 and Comparative Examples 1 to 5, and an electrolyte was injected therein to manufacture the coin half-cells of each example and comparative example.

[0139] The electrolyte used was prepared by adding 2% by weight of fluoroethylene carbonate (FEC) to a non-aqueous electrolyte solvent in which ethylene carbonate (EC) and propylene carbonate (PC) were mixed in a 5:5 volume ratio, and then dissolving 1 M of LiPF6 therein.

[0140] <Evaluation of High Magnification Performance>

[0141] The high-rate performance of the coin half-cell containing the positive electrode for sodium secondary batteries manufactured in Examples 1 and 2 and Comparative Examples 1 to 5 was evaluated.

[0142] Specifically, the coin half-cell was charged to 4.0 V at a constant current of 0.1C in CC / CV mode at 25°C (with a termination current of 0.05C), and then discharged to 2.0 V at constant currents of 0.1C and 5.0C respectively to measure the discharge capacity. The ratio of the 5C discharge capacity to the 0.1C discharge capacity was then calculated.

[0143] The calculation results are shown in Table 1 below.

[0144] Experimental Example 2: Evaluation of Capacity Retention

[0145] The capacity retention of the coin half-cells containing the positive electrodes for sodium secondary batteries manufactured in Examples 1 and 2 and Comparative Examples 1 to 5 was evaluated.

[0146] Specifically, the coin half-cell is charged to 4.0 V at 25°C in CC / CV mode with a constant current of 1C (with a termination current of 0.1C), and then discharged to 2.0 V with a constant current of 1C. The entire process is set as one cycle, and 100 charge-discharge cycles are performed.

[0147] The capacity retention rate is calculated using the following mathematical formula, and the results are shown in Table 1 below.

[0148] Capacity retention [%] = {(Discharge capacity after 100 cycles / Discharge capacity after 1 cycle)} × 100

[0149] [Table 1]

[0150] Referring to Table 1 above, it can be seen that the coin half-cell containing the sodium secondary battery positive electrode manufactured in Examples 1 and 2 is superior to the coin half-cell containing the sodium secondary battery positive electrode manufactured in Comparative Examples 1 to 5 in terms of rate performance and capacity retention.

[0151] [Figure Labels]

[0152] 10: Positive electrode for sodium secondary batteries

[0153] 100: Current collector

[0154] 110: First positive electrode active material layer

[0155] 120: Second positive electrode active material layer

Claims

1. A positive electrode for a sodium secondary battery, the positive electrode comprising: A current collector; A first positive electrode active material layer provided on the current collector; and A second positive electrode active material layer provided on the first positive electrode active material layer, Wherein: The first positive electrode active material layer contains a first positive electrode active material; and The second positive electrode active material layer contains a second positive electrode active material, Wherein: The first positive electrode active material contains sodium composite transition metal oxide particles having an O3 octahedral crystal structure; and The second positive electrode active material contains sodium composite transition metal oxide particles having a P2 orthorhombic crystal structure, The average particle size (D) between the first positive electrode active material and the second positive electrode active material 50 The ratio of ) is 2:1 to 3:

1.

2. The positive electrode for a sodium secondary battery according to claim 1, wherein the sodium composite transition metal oxide particles having an O3 octahedral crystal structure include a compound represented by the following Chemical Formula 1: [Chemical Formula 1] And x1 I 1 O2 Where in the above Chemical Formula 1, 0.7 ≤ x1 ≤ 1, and M 1 It is selected from one or more of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr.

3. The positive electrode for a sodium secondary battery according to claim 2, wherein M 1 Including Fe, Ni and Mn.

4. The positive electrode for a sodium secondary battery according to claim 1, wherein the sodium composite transition metal oxide particles having a P2 orthorhombic crystal structure include a compound represented by the following Chemical Formula 2: [Chemical Formula 2] And x2 I 2 O2 Where in the above Chemical Formula 2, 0 < x2 < 0.7, and M 2 It is selected from one or more of Fe, Ni, Mn, Co, Mg, Zn, Cu and Cr.

5. The positive electrode for a sodium secondary battery according to claim 4, wherein M 2 Including Fe, Ni and Mn.

6. The positive electrode for a sodium secondary battery according to claim 1, wherein the average particle size (D) of the first positive electrode active material is... 50 The thickness ranges from 6 μm to 17 μm.

7. The positive electrode for a sodium secondary battery according to claim 1, wherein the average particle size (D) of the second positive electrode active material is... 50 The thickness ranges from 2 μm to 8 μm.

8. The positive electrode for a sodium secondary battery according to claim 1, wherein the weight ratio between the first positive electrode active material and the second positive electrode active material is 70:30 to 99:

1.

9. The positive electrode for a sodium secondary battery according to claim 1, wherein the specific surface area of ​​the first positive electrode active material is 0.2 m². 2 / g to 0.7 m 2 / g.

10. The positive electrode for a sodium secondary battery according to claim 1, wherein the specific surface area of ​​the second positive electrode active material is 0.2 m². 2 / g to 0.7 m 2 / g.

11. The positive electrode for a sodium secondary battery according to claim 1, wherein the thickness ratio between the first positive electrode active material layer and the second positive electrode active material layer is 1.2:1 to 6:

1.

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

13. The positive electrode for a sodium secondary battery according to claim 1, wherein the thickness of the second positive electrode active material layer is 5 μm to 30 μm.

14. The positive electrode for a sodium secondary battery according to claim 1, wherein the loading ratio between the first positive electrode active material layer and the second positive electrode active material layer is 2:1 to 7:

1.

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

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