Cathode for sodium secondary battery, and sodium secondary battery comprising same

Abstract

The present invention relates to a cathode for a sodium secondary battery, and a sodium secondary battery comprising same, and, more specifically, the cathode for a sodium secondary battery has a structure including: a first cathode active material layer including a polyanion-based cathode active material having a structure advantageous for sodium ion diffusion; and a second cathode active material layer including an oxide-based layered cathode active material capable of implementing high discharge capacity and driving voltage, and thus energy density and charge-discharge rate characteristics of the sodium secondary battery can be improved.

Description

Anode for a sodium secondary battery and a sodium secondary battery including the same

[0001] Cross-citation with related applications

[0002] The present application claims the benefit of priority based on Korean Patent Application No. 10-2024-0192182 filed December 20, 2024 and Korean Patent Application No. 10-2025-0206790 filed December 22, 2025, and all contents disclosed in the documents of said Korean patent applications are incorporated into this specification.

[0003] Technology field

[0004] The present invention relates to a positive electrode for a sodium secondary battery and a sodium secondary battery comprising the same.

[0005] As portable electronic devices such as video cameras, mobile phones, and laptop PCs become lighter and more functional, research is being conducted on increasing the capacity and energy density of batteries used as their power sources.

[0006] Lithium-ion batteries are widely used commercially because they have an energy density about three times higher per unit weight compared to conventional lead-acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, and nickel-zinc batteries, and can be rapidly charged.

[0007] However, due to issues such as the limited use of lithium mineral resources, resulting high production costs, and safety concerns caused by the high reactivity of lithium, there is a demand for new battery systems that can replace lithium-ion batteries.

[0008] As one of the new battery systems, sodium secondary batteries are being actively researched for medium-to-large battery applications, such as power storage and electric vehicles, because they are eco-friendly, highly cost-competitive, and have high energy storage characteristics compared to lithium secondary batteries.

[0009] In particular, research on cathode active materials for sodium secondary batteries, which are highly relevant to the realization of high energy density in the aforementioned sodium secondary batteries, is actively underway.

[0010] Typically, the positive active materials used in the above sodium secondary battery include polyanionic positive active materials and O3-type oxide layered positive active materials.

[0011] FIGS. 1A and 1B show cross-sections of a cathode for a sodium secondary battery containing a cathode active material according to the prior art (Fig. 1A: cathode containing a polyanionic cathode active material, FIG. 1B: cathode containing an oxide-based layered cathode active material).

[0012] Referring to FIG. 1a, the polyanionic positive active material (P1) has a three-dimensional structure that serves as a pathway for the movement of sodium ions. Accordingly, due to the three-dimensional structure, the sodium ion diffusion rate in the polyanionic positive active material (P1) is high, so the sodium secondary battery (10) has excellent high-speed charge / discharge characteristics and high structural safety, which has the advantage of improved lifespan characteristics. On the other hand, the sodium secondary battery (10) containing the polyanionic positive active material (P1) has a low discharge capacity, which limits the increase in energy density. In addition, the polyanionic positive active material (P1) has low electrical conductivity, and as the electrical conductivity decreases further as it moves away from the positive current collector (110), it has the disadvantage of being difficult to form a high-loading positive with a thick thickness.

[0013] Referring to FIG. 1b, a sodium secondary battery (10) containing an O3-type oxide-based layered positive electrode active material (P2) has the advantage of achieving high energy density due to high discharge capacity and driving voltage. On the other hand, the sodium secondary battery (10) containing the O3-type oxide-based layered positive electrode active material (P2) has the disadvantage of having inferior rate capability because its structure is unstable, irreversible phase change occurs, and a phenomenon of slow sodium ion kinetics appears. In particular, there is a problem in that this disadvantage becomes more severe as the O3-type oxide-based layered positive electrode active material (P2) moves further away from the separator (300) which acts as a channel for the movement of sodium ions. At this time, the separator (300) may include glass fiber.

[0014] As such, since the cathode active materials for sodium secondary batteries commonly used in conventional technology each have their own advantages and disadvantages, research is continuously being conducted to develop cathodes for sodium secondary batteries that enable the realization of high rate capability and high energy density by improving these disadvantages while further enhancing their advantages.

[0015] [Prior Art Literature]

[0016] (Patent Document 1) Chinese Published Patent No. 116598425

[0017] The objective of the present invention is to provide a cathode for a sodium secondary battery that can improve the energy density and rate capability characteristics of the sodium secondary battery.

[0018] Another objective of the present invention is to provide a sodium secondary battery comprising the anode.

[0019] To achieve the above objective, a first embodiment of the present invention provides a positive electrode for a sodium secondary battery comprising: a positive current collector; and a positive active material layer formed on the positive current collector; wherein the positive active material layer comprises a first positive active material layer formed on the positive current collector; and a second positive active material layer formed on the first positive active material layer; wherein the first positive active material layer comprises a polyanionic positive active material, and the second positive active material layer comprises an oxide-based layered positive active material, and the loading ratio of the first and second positive active material layers is 0.5 to 1.5:1.

[0020] In one embodiment of the present invention, a positive electrode for a sodium secondary battery is provided, wherein the total loading amount of the positive electrode active material layer is 2 to 10 mAh / ㎠.

[0021] In one embodiment of the present invention, a positive electrode for a sodium secondary battery is provided, wherein the polyanionic positive electrode active material comprises a polyanionic compound represented by Formula 1:

[0022] <Chemical Formula 1>

[0023] Na4M3(PO4)2P2O7

[0024] In the above chemical formula 1, M is Fe, Mn, Co, or Ni.

[0025] In one embodiment of the present invention, a positive electrode for a sodium secondary battery is provided, wherein the particle size (D50) of the polyanionic positive electrode active material is 5 μm to 15 μm.

[0026] In one embodiment of the present invention, a cathode for a sodium secondary battery is provided, wherein the polyanionic cathode active material is included in an amount of 90 to 99 weight% based on the total weight of the first cathode active material layer.

[0027] In one embodiment of the present invention, a cathode for a sodium secondary battery is provided, wherein the oxide-based layered cathode active material comprises an oxide-based layered compound represented by the following chemical formula 2:

[0028] <Chemical Formula 2>

[0029] Na x Ni a Fe b Mn c M 1d M 2e O2

[0030] In the above chemical formula 2, M1 and M2 are different and are each selected from the group consisting of Cu, Zn, Co, Al, Mo, Ba, Mg, Ti, Al, Zr, Ca, W, Ce, Ta, Nb, V, Sc, Sr, B, F and P, 0.1 ≤ a ≤ 0.4, 0.1 ≤ b ≤ 0.4, 0.1 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.1, 0 ≤ d ≤ 0.1, and a + b + c + d + e = 1.

[0031] In one embodiment of the present invention, a cathode for a sodium secondary battery is provided, wherein the particle size (D50) of the oxide-based layered cathode active material is 5 μm to 15 μm.

[0032] In one embodiment of the present invention, a cathode for a sodium secondary battery is provided, wherein the oxide-based layered cathode active material is included in an amount of 90 to 99 weight% based on the total weight of the second cathode active material layer.

[0033]

[0034] A second embodiment of the present invention provides a sodium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the same, and an electrolyte according to a first embodiment of the present invention.

[0035] In one embodiment of the present invention, a sodium secondary battery is provided in which the negative electrode comprises hard carbon.

[0036] The sodium secondary battery anode of the present invention has a structure comprising: a first anode active material layer comprising a polyanionic anode active material having a structure favorable for sodium ion diffusion; and a second anode active material layer comprising an oxide-based layered anode active material capable of realizing high discharge capacity and driving voltage, thereby improving the energy density and charge / discharge rate characteristics of the sodium secondary battery.

[0037] FIGS. 1A and 1B show cross-sections of a cathode for a sodium secondary battery containing a cathode active material according to the prior art (Fig. 1A: cathode containing a polyanionic cathode active material, FIG. 1B: cathode containing an oxide-based layered cathode active material).

[0038] FIG. 2 shows a cross-section of a sodium secondary battery according to one embodiment of the present invention.

[0039] Hereinafter, the present invention will be described in more detail to aid in understanding the invention.

[0040] Terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0041]

[0042] cathode for sodium secondary batteries

[0043] The first embodiment of the present invention relates to a cathode for a sodium secondary battery. The cathode for a sodium secondary battery generally uses a polyanionic cathode active material and an oxide-based layered cathode active material, which are used as active materials for a cathode for a sodium secondary battery. By optimizing the position of the cathode active materials within the cathode and the loading amount of the cathode by the cathode active materials, the energy density and charge / discharge rate characteristics of the sodium secondary battery can be improved through the combined use of the cathode active materials.

[0044]

[0045] A positive electrode for a sodium secondary battery according to a first embodiment of the present invention comprises: a positive current collector; and a positive active material layer formed on the positive current collector; wherein the positive active material layer comprises a first positive active material layer formed on the positive current collector; and a second positive active material layer formed on the first positive active material layer; wherein the first positive active material layer comprises a polyanionic positive active material, and the second positive active material layer comprises an oxide-based layered positive active material, and the loading ratio of the first and second positive active material layers may be 0.5 to 1.5:1. Hereinafter, the polyanionic positive active material may be referred to as the first positive active material. Additionally, the oxide-based layered positive active material may be referred to as the second positive active material.

[0046]

[0047] FIG. 2 shows a cross-section of a sodium secondary battery according to one embodiment of the present invention.

[0048] Referring to FIG. 2, the positive electrode (100) for a sodium secondary battery has a structure in which a positive electrode current collector (110), a first positive electrode active material layer (121), and a second positive electrode active material layer (121) are sequentially stacked. The first positive electrode active material layer (121) includes a polyanionic positive electrode active material (P1), and the second positive electrode active material layer (122) includes an oxide-based layered positive electrode active material (P2).

[0049] Due to the polyanionic cathode active material (P1) included in the first cathode active material layer (121), a three-dimensional structure that acts as a sodium ion transport channel is formed, which has the advantage of improving the diffusion rate of sodium ions. Specifically, the anions within the polyanionic cathode active material (P1) can improve the diffusion rate of sodium ions. However, the polyanionic cathode active material (P1) has a low discharge capacity, which limits the increase in energy density, and has a low electrical conductivity, which makes it difficult to implement a thick, high-loading cathode.

[0050] Due to the oxide-based layered positive active material (P2) included in the second positive active material layer (122), the sodium secondary battery has the advantage of achieving high energy density with high discharge capacity and driving voltage. On the other hand, the sodium secondary battery containing the oxide-based layered positive active material has the disadvantage of having inferior rate capability because the structure is unstable, irreversible phase change occurs, and the sodium ion kinetics appear slow.

[0051] By stacking a first positive active material layer (121) and a second positive active material layer (122) each comprising a polyanionic positive active material (P1) and an oxide-based layered positive active material (P2), which have the respective advantages and disadvantages as described above, the respective disadvantages of these are compensated for, thereby improving the energy density and rate capability characteristics of the sodium secondary battery.

[0052] That is, electrical conductivity can be improved by placing a poly anionic positive active material (P1) with low electrical conductivity adjacent to the positive current collector (110). In addition, the mobility of sodium ions can be improved by placing an oxide-based layered positive active material (P2) having a structure that causes the kinetics of sodium ions to appear slowly adjacent to the separator (300), which is a channel for sodium ions.

[0053] In addition, when stacking the first positive active material layer (121) and the second positive active material layer (122), the loading ratio thereof can be optimized to further improve the energy density and rate capability characteristics of the sodium secondary battery.

[0054] The loading ratio (A:B) of the first positive active material layer (121, A) and the second positive active material layer (122, B) may be 0.5 to 1.5:1. If the loading ratio of the first positive active material layer to the second positive active material layer is less than 0.5 (A:B = A / B), the structure is unstable, irreversible phase change occurs, and a phenomenon of slow sodium ion kinetics appears, which may result in inferior rate capability. If the loading ratio of the first positive active material layer to the second positive active material layer exceeds 1.5 (A:B = A / B), there is a limit to improving the energy density of the sodium secondary battery, and it may be difficult to implement a high-loading positive due to low electrical conductivity. Specifically, the loading ratio (A:B=A / B) may be 0.5:1 or higher, 0.55:1 or higher, 0.6:1 or higher, 0.65:1 or higher, 0.7:1 or higher, 0.75:1 or higher, 0.8:1 or higher, 0.85:1 or higher, 0.9:1 or higher, 0.95:1 or higher, 1:1 or higher, and 1.5:1 or lower, 1.4:1 or lower, 1.3:1 or lower, 1.2:1 or lower, or 1.1:1 or lower.

[0055] The loading amount of each of the first and second positive active material layers can be calculated by the following Equation 1:

[0056] <Equation 1>

[0057] Loading amount (mAh / cm²) 2 ) = Capacity of positive active material (mAh / g) × Weight content ratio of positive active material in dry positive film (wt%) × Weight per unit area of ​​dry positive film (g / cm²) 2 ).

[0058] In this case, the loading of each of the first and second positive active material layers may refer to a capacity loading, and accordingly, the loading ratio may refer to a ratio of capacity loadings. The capacity loading is a positive surface area of ​​1 cm² 2 Electricity that can be stored per unit (mAh / cm²) 2 It means ).

[0059] Meanwhile, the above-mentioned capacity loading may also be converted to weight loading. The above-mentioned weight loading is an anode surface area of ​​1 cm² 2 Mass of the coated positive electrode active material (mg / cm²) 2 or g / cm 2 It means ).

[0060] Specifically, the capacity loading of the positive active material layer can be converted into the weight loading of the positive active material layer by the following Equation 2:

[0061] <Equation 2>

[0062] Weight loading of the positive electrode active material layer (mg / cm²) 2 or g / cm 2 )

[0063] = Capacity loading of the positive active material layer (mAh / cm²) 2 ) / Capacity per unit weight of positive active material (mAh / g).

[0064]

[0065] For example, the poly anionic cathode active material included in the first cathode active material layer is Na4Fe3(PO4)2P2O7 (NFPP, capacity per weight of NFPP material: 109 mAh / g), and the capacity loading of the first cathode active material layer is 1 mAh / cm 2 In this case, converting the dose loading to a weight loading according to Equation 1 above yields 9.17 mg / cm² 2 (or 0.00917 g / cm 2 It can be calculated as ).

[0066] In addition, the oxide-based layered compound included in the second positive active material layer is Na x Ni33 Fe 33 Mn 34 O2 (NFM111, capacity per weight of NFM111 material: 140 mAh / g), and the capacity loading of the second positive active material layer is 1 mAh / cm 2 In this case, converting the dose loading to a weight loading according to Equation 1 above yields 7.14 mg / cm² 2 (or 0.00714 g / cm 2 It can be calculated as ).

[0067] In addition, the above weight capacity can be converted into the above capacity loading by utilizing the above Equation 1 in reverse.

[0068]

[0069] In one embodiment of the present invention, the thickness ratio (X:Y) of the thickness (X) of the first positive active material layer and the thickness (Y) of the second positive active material layer may be 0.9 to 3:1. If the thickness ratio (X:Y = X / Y) is less than 0.9, the structure is unstable, irreversible phase change occurs, and a phenomenon of slow sodium ion kinetics appears, which may result in inferior rate capability. If the thickness ratio (X:Y = X / Y) is greater than 3, there is a limit to improving the energy density of the sodium secondary battery, and it may be difficult to implement a high-loading positive electrode due to low electrical conductivity. Specifically, the thickness ratio (X:Y) may be 0.9:1 or greater, 1:1 or greater, or 1.15:1 or greater, and 3:1 or less, 2.9:1 or less, 2.8:1 or less, 2.7:1 or less, 2.6:1 or less, 2.5:1 or less, 2.4:1 or less, 2.3:1 or less, 2.2:1 or less, 2.1:1 or less, 2:1 or less. It may be 2.9:1 or less, 2.8:1 or less, 2.7:1 or less, 2.6:1 or less, 2.5:1 or less, 2.4:1 or less, 2.3:1 or less, 2.2:1 or less, 2.1:1 or less, 2:1 or less, 1.9:1 or less, 1.8:1 or less, 1.7:1 or less, 1.6:1 or less, 1.5:1 or less, 1.4:1 or less, 1.3:1 or less, or 1.2:1 or less.

[0070]

[0071] In one embodiment of the present invention, the total loading amount of the positive active material layer may be 2 to 10 mAh / ㎠. The total loading amount of the positive active material layer is the sum of the loading amount of the first positive active material layer and the loading amount of the second positive active material layer.

[0072] If the total loading amount of the positive active material layer is less than 2 mAh / cm², the energy density of the sodium secondary battery decreases, and it does not correspond to a high-loading electrode. If the total loading amount of the positive active material layer exceeds 10 mAh / cm², in the case of the second positive active material layer, the structure is unstable, causing irreversible phase changes and slow sodium ion kinetics, so the rate capability may be inferior, and in the case of the first positive active material layer, there is a limit to improving the energy density of the sodium secondary battery, and it may be difficult to realize a high-loading positive electrode due to low electrical conductivity.

[0073]

[0074] In one embodiment of the present invention, the polyanionic cathode active material may comprise a polyanionic compound represented by the following chemical formula 1:

[0075] <Chemical Formula 1>

[0076] Na4M3(PO4)2P2O7

[0077] In the above chemical formula 1, M is Fe, Mn, Co, or Ni.

[0078] Preferably, the polyanionic compound may be Na4Fe3(PO4)2P2O7 (NFPP). The NFPP can reduce costs by using Fe.

[0079]

[0080] In one embodiment of the present invention, the particle size (D50) of the polyanionic cathode active material may be 5 μm to 15 μm. When included within the range of the particle size (D50), a three-dimensional structure in the form of a channel through which sodium ions can move may be formed. The particle size (D50) refers to the equivalent spherical diameter when the cumulative volume fraction reaches 50% in the particle size distribution measured by laser diffraction analysis.

[0081] If the particle size (D50) of the above polyanionic cathode active material is less than 5 μm, side reactions with the electrolyte may occur due to an increase in the specific surface area, which may degrade battery performance, and if it exceeds 15 μm, the output characteristics and capacity may be degraded as the sodium ion channel becomes longer due to an increase in particle size. Specifically, the particle size (D50) of the above polyanionic cathode active material may be 5 μm or more, 6 μm or more, 7 μm or more, 8 μm or more, or 9 μm or more, and may be 15 μm or less, 14 μm or less, 13 μm or less, 12 μm or less, or 11 μm or less.

[0082]

[0083] In one embodiment of the present invention, the polyanionic positive electrode active material may be included in an amount of 90 to 99 weight percent based on the total weight of the first positive electrode active material layer.

[0084] If the content of the above-mentioned polyanionic cathode active material is less than 90 weight%, the loading amount of the active material is difficult to correspond to high loading, so the overall energy density of the battery may decrease, and if it exceeds 99 weight%, the content of the binder or conductive material included in the first cathode active material layer is relatively reduced, so the bonding strength or electrical conductivity of the first cathode active material layer may decrease. Specifically, the content of the above-mentioned polyanionic cathode active material may be 90 weight% or more, 91 weight% or more, 92 weight% or more, 93 weight% or more, or 94 weight% or more, and may be 99 weight% or less, 98 weight% or less, 97 weight% or less, 96 weight% or less, or 95 weight% or less.

[0085]

[0086] In one embodiment of the present invention, the oxide-based layered cathode active material may comprise an oxide-based layered compound represented by the following chemical formula 2:

[0087] <Chemical Formula 2>

[0088] Na x Ni a Fe b Mn c M 1d M 2e O2

[0089] In the above chemical formula 2, M1 and M2 are different and are each selected from the group consisting of Cu, Zn, Co, Al, Mo, Ba, Mg, Ti, Al, Zr, Ca, W, Ce, Ta, Nb, V, Sc, Sr, B, F and P, 0.1 ≤ a ≤ 0.4, 0.1 ≤ b ≤ 0.4, 0.1 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.1, 0 ≤ d ≤ 0.1, and a + b + c + d + e = 1.

[0090] Preferably, M1 and M2 may be Ca and Ti, respectively. Additionally, M1 and M2 may be Na x Ni a Fe b Mnc It could be a form doped with O2.

[0091] The oxide-based layered compound represented by the above chemical formula 2 may be a sodium nickel iron manganese oxide (NFM) layered material containing sodium (Na), nickel (Ni), iron (Fe), and manganese (Mn).

[0092] In one embodiment, the oxide-based layered compound may include bimodal particles comprising polycrystalline (PC) particles and single-crystal (SC) particles. The polycrystalline particles are particles in the form of a plurality of single-crystal particles assembled together. The size of the polycrystalline particles may be larger than the size of the single-crystal particles, and the particle size refers to the length of the longest axis of the particle. The oxide-based layered compound can improve the lifespan characteristics of the battery by including bimodal particles of different particle sizes.

[0093]

[0094] In one embodiment of the present invention, the particle size (D50) of the oxide-based layered anode active material may be 5 μm to 15 μm.

[0095] If the particle size (D50) of the oxide-based layered cathode active material is less than 5 μm, side reactions with the electrolyte may occur due to an increase in the specific surface area, thereby degrading battery performance; if it exceeds 15 μm, the particle size increases, causing the layered structure within the active material to lengthen, which may degrade output characteristics and capacity. Specifically, the particle size (D50) of the oxide-based layered cathode active material may be 5 μm or more, 6 μm or more, 7 μm or more, 8 μm or more, or 9 μm or more, and may be 15 μm or less, 14 μm or less, 13 μm or less, 12 μm or less, or 11 μm or less. The particle size (D50) may be based on the Particle Size Distribution (PSD) results.

[0096]

[0097] In one embodiment of the present invention, the oxide-based layered positive electrode active material may be included in an amount of 90 to 99 weight percent based on the total weight of the second positive electrode active material layer.

[0098] If the content of the oxide-based layered positive electrode active material is less than 90 weight%, the loading amount of the active material is difficult to correspond to high loading, so the overall energy density of the battery may decrease, and if it exceeds 99 weight%, the content of the binder or conductive material included in the second positive electrode active material layer is relatively reduced, so the bonding strength or electrical conductivity of the second positive electrode active material layer may decrease. Specifically, the content of the oxide-based layered positive electrode active material may be 90 weight% or more, 91 weight% or more, 92 weight% or more, 93 weight% or more, or 94 weight% or more, and may be 99 weight% or less, 98 weight% or less, 97 weight% or less, 96 weight% or less, or 95 weight% or less.

[0099]

[0100] In one embodiment of the present invention, the first positive active material layer comprises a poly anionic positive active material, a first binder, and a first conductive material.

[0101] The description of the type, content, and physical properties of the above-mentioned poly anionic cathode active material is as described above. The first cathode active material described below refers to the poly anionic cathode active material.

[0102] In addition, the first binder may use any binder known in the industry to maintain the first positive active material in the positive current collector or to organically connect the first positive active materials to further increase the binding force between them.

[0103] The first binder comprises: a fluoropolymer-based binder comprising polyvinylidene fluoride (PVdF) and / or polytetrafluoroethylene (PTFE); a rubber-based binder comprising one or more of styrene butadiene rubber (SBR), acrylonitrile-butidiene rubber, and styrene-isoprene rubber; a cellulose-based binder comprising one or more of carboxyl methyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose; a polyalcohol-based binder; a polyolefin-based binder comprising one or more of polyethylene and polypropylene; a polyimide-based binder; a polyester-based binder; and an acrylic-based binder comprising an acrylic monomer. One or more mixtures or copolymers selected from the group consisting of and silane-based binders may be used. According to one embodiment of the present invention, the binder may be polyvinylidene fluoride (PVdF).

[0104] In addition, the first binder may be included in an amount of 0.01 to 10 weight% based on the total weight of the first positive active material layer. If the content of the first binder is less than 0.01 weight%, the binding strength is reduced, and the first positive active material and the first conductive material may detach; if it exceeds 30 weight%, the ratio of the first positive active material and the first conductive material is relatively reduced, and the battery capacity may decrease. Specifically, the content of the first binder may be 0.01 weight% or more, 1 weight% or more, or 3 weight% or more, and 10 weight% or less, 8 weight% or less, or 6 weight% or less.

[0105]

[0106] In addition, the first conductive material is intended to improve electrical conductivity, and there are no specific limitations as long as it is an electrically conductive material that does not cause chemical changes in the sodium secondary battery.

[0107] The first conductive material may include one or more selected from the group consisting of carbon black, graphite, carbon fiber, carbon nanotube, metal powder, conductive metal oxide, and organic conductive material. The carbon black may include one or more selected from the group consisting of Ketjen black, Super P, Denka black, acetylene black, and furnace black.

[0108] The first conductive material may be included in an amount of 0.01 to 10 weight% based on the total weight of the first positive electrode active material layer. Specifically, the content of the first conductive material may be 0.01 weight% or more, 2 weight% or more, or 4 weight% or more, and 10 weight% or less, 8 weight% or less, or 6 weight% or less. If the content of the first conductive material is less than 0.01 weight%, the conductivity of the positive electrode may be reduced, and if it exceeds 10 weight%, the flexibility of the positive electrode may be reduced.

[0109] In addition, the thickness of the first positive active material layer is not particularly limited and can be set to an appropriate range considering the mechanical strength of the positive, the loading amount, or the capacity of the battery. For example, the thickness of the first positive active material layer can typically be 30㎛ to 300㎛.

[0110]

[0111] In one embodiment of the present invention, the second positive active material layer comprises an oxide-based layered positive active material, a second binder, and a second conductive material.

[0112] The description of the type, content, and physical properties of the oxide-based layered cathode active material is as described above. The second cathode active material described below refers to the oxide-based layered cathode active material.

[0113] In addition, the description of the second binder and the second conductive material may be the same as the type, content, and physical properties of the first binder and the first conductive material described above.

[0114]

[0115] In one embodiment of the present invention, the positive current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery and can be used electrochemically stably at the positive charging voltage. For example, the positive current collector may be one or more selected from the group consisting of copper, aluminum, stainless steel, titanium, silver, palladium, nickel, alloys thereof, and combinations thereof. The stainless steel may be surface-treated with carbon, nickel, titanium, or silver.

[0116] In addition, the form of the anode current collector is not particularly limited, and may be in the form of a film, sheet, foil, net, porous body, foam, or nonwoven fabric. If necessary, fine irregularities may be formed on the surface of the anode current collector, and said irregularities may help improve adhesion with the anode active material layer. The method of forming irregularities on the surface of the anode current collector is not particularly limited, and known methods such as mechanical polishing, electrolytic polishing, or chemical polishing may be applied.

[0117] In addition, the thickness of the positive current collector is not particularly limited and can be set within an appropriate range considering the mechanical strength, productivity, or capacity of the positive electrode. For example, the thickness of the positive current collector can typically be 3㎛ to 500㎛.

[0118]

[0119] Method for manufacturing a positive electrode for a sodium secondary battery

[0120] The present invention also relates to a method for manufacturing a positive electrode for a sodium secondary battery according to a first embodiment of the present invention.

[0121] A method for manufacturing a positive electrode for a sodium secondary battery according to the present invention comprises: (S1) a step of coating a slurry for forming a first positive active material layer on one surface of a positive electrode current collector; (S2) a step of drying the slurry coated on the positive electrode current collector to form a first positive active material layer; (S3) a step of coating a slurry for forming a second positive active material layer on one surface of the first positive active material layer; (S4) a step of drying the slurry coated on the first positive active material layer to form a second positive active material layer; and (S5) a step of rolling a laminate comprising the positive electrode current collector, the first positive active material layer, and the second positive active material layer.

[0122]

[0123] Hereinafter, the method for manufacturing a sodium secondary battery according to the present invention will be described in more detail for each step.

[0124] The type, content, and physical properties of the raw materials used in the manufacturing method of the sodium secondary battery below are as described above.

[0125]

[0126] In one embodiment of the present invention, in step (S1), a slurry for forming a first positive active material layer may be coated on one surface of the positive current collector.

[0127] The above-mentioned slurry for forming the first positive active material layer can be prepared by mixing the first positive active material, the first binder, and the first conductive material, and then adding them to a first organic solvent.

[0128] It is preferable to use a first organic solvent that can uniformly disperse the first positive active material, the first binder, and the first conductive material, and which evaporates easily. Specifically, the first organic solvent may be methylpyrrolidone (N-methyl-2-pyrrolidone, NMP), acetonitrile, methanol, ethanol, tetrahydrofuran, water, isopropyl alcohol, etc. Considering the ease of forming the slurry and its evaporability, the first organic solvent may be methylpyrrolidone.

[0129] In addition, the concentration of the slurry is not specifically limited as long as it allows the coating process to proceed smoothly. For example, the concentration of the slurry may be 50 to 70 weight% based on the solid content. Specifically, the concentration of the slurry may be 50 weight% or more, 51 weight% or more, 52 weight% or more, 53 weight% or more, 54 weight% or more, 55 weight% or more, 56 weight% or more, 57 weight% or more, 58 weight% or more, or 59 weight% or more based on the solid content, and may be 70 weight% or less, 69 weight% or less, 68 weight% or less, 67 weight% or less, 66 weight% or less, 65 weight% or less, 64 weight% or less, 63 weight% or less, 62 weight% or less, or 61 weight% or less. The above solid content refers to the content of the first positive active material, the first binder, and the first conductive material, excluding the first organic solvent, in the slurry for forming the first positive active material layer.

[0130]

[0131] In one embodiment of the present invention, in step (S2), a slurry coated on the positive current collector can be dried to form a first positive active material layer.

[0132] In addition, the drying method is not particularly limited as long as it is a drying method sufficient to remove the organic solvent from the coating layer formed by the coated slurry. For example, the drying may be performed at a temperature of 80 to 130°C.

[0133]

[0134] In one embodiment of the present invention, in step (S3), a slurry for forming a second positive active material layer may be coated on one surface of the first positive active material layer.

[0135] The above slurry for forming the second positive active material layer can be prepared by mixing the second positive active material, the second binder, and the second conductive material, and then adding them to a second organic solvent.

[0136] It is preferable to use a second organic solvent that can uniformly disperse the second positive active material, the second binder, and the second conductive material, and which evaporates easily. Specifically, the second organic solvent may be methylpyrrolidone (N-methyl-2-pyrrolidone, NMP), acetonitrile, methanol, ethanol, tetrahydrofuran, water, isopropyl alcohol, etc. Considering the ease of forming the slurry and its evaporability, the second organic solvent may be methylpyrrolidone.

[0137] In addition, the concentration of the slurry is not specifically limited as long as it allows the coating process to proceed smoothly. For example, the concentration of the slurry may be 50 to 70 weight% based on the solid content. Specifically, the concentration of the slurry may be 50 weight% or more, 51 weight% or more, 52 weight% or more, 53 weight% or more, 54 weight% or more, 55 weight% or more, 56 weight% or more, 57 weight% or more, 58 weight% or more, or 59 weight% or more based on the solid content, and may be 70 weight% or less, 69 weight% or less, 68 weight% or less, 67 weight% or less, 66 weight% or less, 65 weight% or less, 64 weight% or less, 63 weight% or less, 62 weight% or less, or 61 weight% or less. The above solid content refers to the content of the second positive active material, the second binder, and the second conductive material, excluding the second organic solvent, in the slurry for forming the second positive active material layer.

[0138]

[0139] In one embodiment of the present invention, in step (S4), a slurry coated on the first positive active material layer can be dried to form a second positive active material layer.

[0140] In addition, the drying method is not particularly limited as long as it is a drying method sufficient to remove the organic solvent from the coating layer formed by the coated slurry. For example, the drying may be performed at a temperature of 80 to 130°C.

[0141]

[0142] In one embodiment of the present invention, in step (S5), a laminate comprising the positive current collector, the first positive active material layer, and the second positive active material layer can be rolled to obtain a positive electrode for a sodium secondary battery.

[0143] The pressure applied to the laminate during the above rolling can be appropriately adjusted according to the porosity to be formed within the anode. Furthermore, the rolling is not particularly limited as long as it is a means capable of controlling porosity by pressurizing the laminate. For example, the rolling can be performed using a roll press.

[0144]

[0145] Sodium secondary battery

[0146] A second embodiment of the present invention relates to a sodium secondary battery.

[0147] A sodium secondary battery according to a second embodiment of the present invention comprises a positive electrode, a negative electrode, a separator interposed between the two, and an electrolyte.

[0148]

[0149] anode

[0150] In one embodiment of the present invention, the description of the anode is as described above.

[0151]

[0152] cathode

[0153] In one embodiment of the present invention, the cathode may include a cathode current collector and a cathode active material layer formed on the cathode current collector. Alternatively, the cathode may include a cathode active material layer alone without a cathode current collector, in which case the cathode active material layer may include sodium metal.

[0154] The above-mentioned negative current collector is not particularly limited as long as it is conductive without causing chemical changes 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., and aluminum-cadmium alloy may be used. In addition, the above-mentioned negative current collector, like the positive current collector, may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc., having fine irregularities formed on its surface.

[0155] In addition, the above-mentioned cathode active material may include sodium metal, sodium metal-based alloys, sodium intercalating compounds, or carbon-based materials, but is not necessarily limited to these; any material that can be used as a cathode active material in the industry and contains sodium or can absorb / release sodium is acceptable. Here, sodium metal-based alloys may include, for example, alloys of aluminum, tin, indium, calcium, titanium, vanadium, etc. with sodium, but are not limited thereto.

[0156] When using a cathode active material other than sodium metal or sodium alloy, carbon-based materials having a graphene structure may be used. Mixed cathodes of materials such as graphite and carbon graphitized, mixed cathodes of carbon-based materials and metals or alloys, and composite cathodes may be used. Carbon-based materials capable of electrochemically absorbing and releasing sodium ions may be used, such as natural graphite, artificial graphite, mesophase carbon, expanded graphite, carbon fiber, vapor phase growth carbon fiber, pitch-based carbonaceous materials, needle coke, petroleum coke, polyacrylonitrile-based carbon fiber, carbon black, or amorphous carbon materials synthesized by thermal decomposition of cyclic hydrocarbons or cyclic oxygen-containing organic compounds of five or six members.

[0157]

[0158] In addition, the description of the binder and conductive material included in the cathode is omitted as it is the same as the description of the binder and conductive material included in the anode as described above.

[0159] In addition, the above-mentioned negative current collector is not particularly limited as long as it is conductive without causing chemical changes 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., and aluminum-cadmium alloy may be used. In addition, the above-mentioned negative current collector, like the positive current collector, may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc., having fine irregularities formed on its surface.

[0160] In addition, the thickness of the negative electrode current collector is not particularly limited and can be set within an appropriate range considering the mechanical strength of the negative electrode, productivity, or capacity of the battery. For example, the thickness of the negative electrode current collector can typically be 3 μm to 500 μm.

[0161]

[0162] Separator

[0163] In one embodiment of the present invention, the separator can serve as a passage for sodium ions to move, while simultaneously serving as a wall to prevent the anode and cathode from coming into contact.

[0164] The above-mentioned separator is not particularly limited as long as it is used as a separator in the relevant industry. In particular, the above-mentioned separator may have low resistance to sodium ion movement and excellent electrolyte moisture retention capacity.

[0165] For example, the separator may be selected from glass fiber, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or a combination thereof, and may be in the form of a nonwoven or woven fabric. For example, polyolefin-based polymer separators such as polyethylene and polypropylene are mainly used in sodium secondary batteries, and coated separators containing ceramic components or polymer materials may be used to ensure heat resistance or mechanical strength, and may optionally be used in a single-layer or multi-layer structure.

[0166]

[0167] electrolytes

[0168] In one embodiment of the present invention, the electrolyte can serve as a medium that enables the movement of ions between the anode and the cathode.

[0169] The above electrolyte may include a sodium salt and a solvent. In addition, the above electrolyte may further include an additive.

[0170]

[0171] The above sodium salt acts as a channel for moving sodium ions between the positive and negative electrodes in a sodium secondary battery, thereby compensating for the insufficient ionic conductivity of the solvent. Accordingly, the above sodium salt is not particularly limited as long as it is a sodium salt capable of enhancing the movement characteristics of sodium ions. For example, the above sodium salt may include one or more selected from the group consisting of NaPF6, NaFSI, NaClO4, NaBF4, NaTFSI, NaSO3CF3, NaBOB, and NaFOB.

[0172] In addition, the concentration of the sodium salt may be 0.3 to 1.0 M in the electrolyte. The concentration range of the sodium salt may be set considering the degree of improvement in sodium ion characteristics. For example, if the concentration of the sodium salt is less than 0.3 M, the mobility of sodium ions may decrease, and if it exceeds 1 M, the concentration of the sodium salt is excessive and may act as a resistance. Specifically, the concentration of the sodium salt may be 0.3 M or more, 0.4 M or more, or 0.5 M or more, and may be 1.0 M or less, 0.9 M or less, 0.8 M or less, 0.7 M or less, or 0.6 M or less.

[0173]

[0174] In addition, since the above solvent can dissociate and dissolve the sodium salt and the additive to form an electrolyte, the above solvent is not particularly limited as long as it is capable of dissolving the sodium salt and the additive.

[0175] For example, the solvent may include one or more selected from the group consisting of cyclic carbonate solvents, linear carbonate-based solvents, and ether-based solvents.

[0176] The above cyclic carbonate-based solvent may include one or more selected from the group consisting of ethylene carbonate (EC) and propylene carbonate (PC). The above cyclic carbonate-based solvent has a high dielectric constant (EC: 89.6, PC: 64.4), which makes it easy to dissolve sodium salts.

[0177] The above linear carbonate-based solvent may include one or more selected from the group consisting of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). The above linear carbonate-based solvent has a lower viscosity (EC: 1.86, PC: 2.53, DMC: 0.6) compared to cyclic carbonates, which can contribute to improving ion conductivity by lowering the viscosity of the electrolyte.

[0178] The above ether-based solvent may include one or more selected from the group consisting of dimethoxyethane (DME), 1,3-dioxolane, tetraethylene glycol dimethyl ether (TEGDME), and diethylene glycol dimethyl ether (DEGDME). The above ether-based solvent has a high donor number, making it easy to dissolve sodium salts, and due to its low viscosity (DME: 0.46), it can lower the viscosity of the electrolyte and contribute to improving ion conductivity.

[0179]

[0180] In addition, the above additive is a substance added in small amounts to the electrolyte and can play a role in protecting the surface of the electrode so that charging and discharging can be carried out more stably when sodium ions move.

[0181] The above additive may be included in an amount of 0.2 to 10 weight percent based on the total weight of the electrolyte. If the content of the above additive is less than 0.2 weight percent, the functionality of protecting the surface of the electrode during charging and discharging is reduced, which may cause a short circuit, and if it exceeds 10 weight percent, the additive may be included in excess and act as a resistor within the battery. Specifically, the content of the above additive may be 0.2 weight percent or more, 0.5 weight percent or more, 1 weight percent or more, 3 weight percent or more, or 5 weight percent or more, and may be 10 weight percent or less, 8 weight percent or less, or 6 weight percent or less.

[0182]

[0183] These sodium secondary batteries can be used not only as battery cells serving as power sources for small devices, but also as unit cells in medium-to-large battery modules containing multiple battery cells. Examples of such medium-to-large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems; in particular, they can be usefully applied in areas requiring high output, such as hybrid electric vehicles and batteries for renewable energy storage.

[0184] Preferred embodiments are presented below to aid in understanding the present invention; however, the following embodiments are merely illustrative of the invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the invention, and that such changes and modifications fall within the scope of the appended claims.

[0185]

[0186] In the following examples and comparative examples, a cathode for a sodium ion battery was prepared according to the composition as described in Table 1 below.

[0187]

[0188] Anode Active Material Layer First Anode Active Material Layer (A) Second Anode Active Material Layer (B) Loading Ratio (A:B) Example 1: Polyanionic Compound Oxide Layered Compound 1:1 Example 2: Polyanionic Compound Oxide Layered Compound 0.91:1 Comparative Example 1: Polyanionic Compound -1:0 Comparative Example 2: Oxide Layered Compound 0:1 Comparative Example 3: Oxide Layered Compound Polyanionic Compound 1:1 Comparative Example 4: Polyanionic Compound Oxide Layered Compound 3:1 Comparative Example 5: Polyanionic Compound Oxide Layered Compound 1:3

[0189]

[0190] Example 1

[0191] A polyanionic compound Na4Fe3(PO4)2P2O7 (NFPP) as the first positive active material, polyvinylidene fluoride (PVdF) as the first binder, and carbon black as the first conductive material were mixed in a weight ratio of 96:2:2, and then added to an organic solvent, methylpyrrolidone (N-Methyl-2-pyrrolidone, NMP), to prepare a slurry for forming the first positive active material layer. The concentration of the slurry for forming the first positive active material layer was set to 60 wt% based on the weight of the solid content.

[0192] Na, an oxide-based layered compound, as the second positive active material. x Ni 33 Fe 33 Mn 34 O2 (NFM111), polyvinylidene fluoride (PVdF), which is a second binder, and carbon black, which is a second conductive material, were mixed in a weight ratio of 96:2:2, and then added to methylpyrrolidone (N-Methyl-2-pyrrolidone, NMP), which is an organic solvent, to prepare a slurry for forming a second positive active material layer. The concentration of the slurry for forming the second positive active material layer was set to 60 wt% based on the weight of the solid content.

[0193] The slurry for forming the first positive active material layer was coated on one side of an Al foil, which is a positive current collector, and vacuum dried in a vacuum oven at 120°C. Then, the slurry for forming the second positive active material layer was coated on one side of the slurry for forming the first positive active material layer, vacuum dried in a vacuum oven at 120°C, and rolled to manufacture a positive electrode.

[0194] At this time, the anode was manufactured such that the loading ratio (A:B) of the first anode active material layer (A) and the second anode active material layer (B) was 1:1. The loading ratio was controlled using the amount of slurry coated during the slurry coating.

[0195]

[0196] Example 2

[0197] A positive electrode was manufactured in the same manner as in Example 1, except that the loading ratio (A:B) of the first positive electrode active material layer (A) and the second positive electrode active material layer (B) was 0.91:1.

[0198]

[0199] Comparative Example 1

[0200] A positive electrode was prepared in the same manner as in Example 1, except that a second positive electrode active material layer was not formed.

[0201]

[0202] Comparative Example 2

[0203] A positive electrode was prepared in the same manner as in Example 1, except that the first positive electrode active material layer was not formed.

[0204]

[0205] Comparative Example 3

[0206] An anode was prepared in the same manner as in Example 1, except that the first anode active material layer (A) comprises an oxide-based layered compound and the second anode active material layer (B) comprises a polyanionic compound.

[0207]

[0208] Comparative Example 4

[0209] A positive electrode was manufactured in the same manner as in Example 1, except that the loading ratio (A:B) of the first positive electrode active material layer (A) and the second positive electrode active material layer was 3:1.

[0210]

[0211] Comparative Example 5

[0212] A positive electrode was manufactured in the same manner as in Example 1, except that the loading ratio (A:B) of the first positive electrode active material layer (A) and the second positive electrode active material layer was 1:3.

[0213]

[0214] Experimental Example 1: Evaluation of Battery High-Rate Discharge Performance

[0215] High-rate discharge performance evaluation was performed on sodium secondary batteries introduced with the anodes prepared in Examples 1 to 2 and Comparative Examples 1 to 5.

[0216] A coin half-cell was manufactured in the form of a laminate comprising the above-mentioned anode, cathode, and a separator interposed between them. Charge and discharge evaluations were performed using the coin half-cell in the following manner. At this time, sodium metal was used as the cathode, and glass fiber (Cytiva Whatman GF / C) was used as the separator. TM GLASS MICROFIBER FILTERS (100 Circles, Diameter 110 mm) were used.

[0217] To evaluate the performance of the above sodium secondary battery, the voltage is 4.0V (Na / Na) at a current of 0.1C. + After charging until it reaches the range of ), 4.0V (Na / Na + It was charged with a current cut-off of 0.05C while maintaining ). Voltage 2.0V (vs Na / Na +Discharged at a current of 0.1C until it reached ). This process was carried out for 3 cycles, and the discharge capacity of the third cycle was used as the initial discharge capacity of 0.1C. Subsequently, rate capability evaluation was conducted using a protocol to discharge up to a maximum of 1C. Specifically, while maintaining the 0.1C / 0.05C CC / CV (Constant Current / Constant Voltage) charging process, rate capability was observed using a CC discharge protocol of 0.1C / 0.2C / 0.33C / 0 / 1C / 2C / 3C. The 1C capacity retention rate, calculated by dividing the 1C discharge capacity obtained through this evaluation process by the initial discharge capacity of 0.1C, was used as the measurement result of the rate capability, and the results are shown in Table 2 below.

[0218]

[0219] 3C Discharge Capacity Retention Rate (%) Compared to 0.1C Example 191 Example 290 Comparative Example 183 Comparative Example 271 Comparative Example 380 Comparative Example 484 Comparative Example 580

[0220]

[0221] Experimental Example 2: Battery Life Performance Evaluation

[0222] Lifetime performance evaluation was performed on sodium secondary batteries with cathodes introduced in Examples 1 to 2 and Comparative Examples 1 to 5.

[0223] The above life evaluation was conducted in the same manner as Experimental Example 1, except that, unlike the discharge performance evaluation based on changes in the discharge protocol, a protocol of 0.5C / 0.05C CC / CV (Constant Current / Constant Voltage) charging and 0.5 CC discharging was repeated. The 100-cycle capacity retention rate, calculated by dividing the 100-cycle capacity obtained through the evaluation process by the 0.5C first-cycle capacity, was used as the result of the life performance evaluation, and the results are shown in Table 3 below.

[0224]

[0225] 100-cycle capacity retention rate (%) Example 190 Example 285 Comparative Example 184 Comparative Example 270 Comparative Example 381 Comparative Example 484 Comparative Example 581

[0226]

[0227] In Tables 2 and 3 above, the result of Example 1 was found to be the best, in which the loading ratio of the first positive active material layer containing a polyanionic compound, which is the lower layer adjacent to the positive current collector, and the second positive active material layer containing an oxide-based layered compound, which is the upper layer, is 1:1. From this, it was found that the corresponding loading ratio falls within the optimal loading ratio range capable of compensating for the low electrical conductivity of the first positive active material layer containing the polyanionic compound and the low rate characteristics of the second positive active material layer containing the oxide-based layered compound.

[0228] In addition, compared to Example 1, Example 2 has a relatively higher loading and thickness of the second positive active material layer containing an oxide-based layered compound, which is disadvantageous to the ion conduction path during charging and discharging of the second positive active material layer, so the rate capability and lifespan characteristics may be somewhat degraded.

[0229] Comparative Example 1 above is a cathode comprising a single-layer cathode active material layer containing a polyanionic compound. When high loading is applied, the electrical conductivity within the cathode decreases rapidly, and electron movement does not occur smoothly. Consequently, it was found that the capacity retention rate and lifespan performance decreased rapidly compared to Example 1.

[0230] Comparative Example 2 above is a cathode comprising a single-layer cathode active material layer containing an oxide-based layered compound. When high loading is applied, the ion conductivity characteristics within the cathode decrease rapidly, and the rate characteristics decrease. It was found that the capacity retention rate and lifespan performance decrease rapidly compared to Example 1.

[0231] Comparative Example 3 above has a structure in which a positive current collector, a first positive active material layer, and a second positive active material layer are sequentially stacked, wherein the first positive active material layer comprises an oxide-based layered compound and the second positive active material layer comprises a polyanionic compound. It can be seen that the rate capability and lifespan characteristics are degraded because the polyanionic compound with low electrical conductivity is included in the second positive active material layer, which is not adjacent to the positive current collector. Additionally, in Comparative Example 3, the oxide-based layered compound is located in the first positive active material layer adjacent to the current collector, so the rate capability and lifespan characteristics may be degraded due to an increase in ion conduction paths during charging and discharging.

[0232] As shown in Comparative Examples 4 and 5 above, it was confirmed that capacity retention rate and lifespan performance decrease when the loading ratio deviates from the range of 0.5:1 to 1.5:1. From this, it was confirmed that if the proportion of the anode active material layer containing a polyanionic compound is high, the electrical conductivity within the anode decreases, and if the proportion of the anode active material layer containing an oxide-based layered compound is high, the rate capability within the anode decreases.

[0233]

[0234] Experimental Example 3: Measurement of the thickness of the first and second positive electrode active material layers

[0235] In order to determine the appropriate thickness ratio according to the loading ratio of the first and second positive active material layers, the thickness of each of the first and second positive active material layers was measured for Example 1, Comparative Example 4, and Comparative Example 5, in which the structure of the positive electrode is the same but the loading ratio is different. The thickness was measured using a scanning electron microscope (IT-800SHL, JEOL).

[0236]

[0237] Table 4 below shows the results of measuring the thickness of the first and second positive active material layers in Example 1, Comparative Example 4, and Comparative Example 5, in which the structure of the positive electrode is the same but the loading ratio of the first and second positive active material layers is different.

[0238]

[0239] Thickness (㎛) Thickness Ratio (X:Y) First Active Material Layer (X) Second Active Material Layer (Y) Example 1 776 51.18:1 Comparative Example 4 117 32 3.66:1 Comparative Example 5 399 50.41:1

[0240]

[0241] Referring to Table 4 above, Comparative Examples 4 and 5 showed that the ratio of the thickness of the second positive active material layer to the thickness of the first positive active material layer was relatively larger or smaller compared to Example 1. The thickness ratio is attributed to the difference in loading ratio as shown in Table 1 above, and it can be seen that Comparative Examples 4 and 5, in which the difference in thickness between the first and second active material layers is relatively larger compared to Example 1, show a decrease in the high-rate discharge performance and lifespan performance of the battery as shown in Tables 2 and 3 above.

[0242]

[0243] [Explanation of the symbol]

[0244] 10: Sodium secondary battery

[0245] 100: Anode

[0246] 110: Positive current collector

[0247] 120: Positive active material layer

[0248] 121: First positive active material layer, 122: Second positive active material layer

[0249] P1: Polyanionic cathode active material, P2: Oxide-based layered cathode active material

[0250] 200: Cathode

[0251] 300: Separator

Claims

1. A positive current collector; and a positive active material layer formed on the positive current collector; as a positive electrode for a sodium secondary battery, The above positive active material layer comprises: a first positive active material layer formed on the positive current collector; and a second positive active material layer formed on the first positive active material layer. The first positive active material layer comprises a poly anionic positive active material, and The above second positive active material layer comprises an oxide-based layered positive active material, and A cathode for a sodium secondary battery, wherein the loading ratio of the first and second cathode active material layers is 0.5 to 1.5:

1.

2. In Paragraph 1, A positive electrode for a sodium secondary battery, wherein the total loading amount of the positive electrode active material layer is 2 to 10 mAh / ㎠.

3. In Paragraph 1, A cathode for a sodium secondary battery, wherein the above-mentioned polyanionic cathode active material comprises a polyanionic compound represented by Chemical Formula 1: <Chemical Formula 1> Na4M3(PO4)2P2O7 In the above chemical formula 1, M is Fe, Mn, Co, or Ni.

4. In Paragraph 1, A cathode for a sodium secondary battery, wherein the particle size (D50) of the above-mentioned polyanionic cathode active material is 5 μm to 15 μm.

5. In Paragraph 1, A cathode for a sodium secondary battery, wherein the above-mentioned polyanionic cathode active material is included in an amount of 90 to 99 weight% based on the total weight of the first cathode active material layer.

6. In Paragraph 1, A cathode for a sodium secondary battery, wherein the oxide-based layered cathode active material comprises an oxide-based layered compound represented by the following chemical formula 2: <Chemical Formula 2> And x Yes a Fe b Mn c I 1d I 2e O2 In the above chemical formula 2, M1 and M2 are different and are each selected from the group consisting of Cu, Zn, Co, Al, Mo, Ba, Mg, Ti, Al, Zr, Ca, W, Ce, Ta, Nb, V, Sc, Sr, B, F and P, 0.1 ≤ a ≤ 0.4, 0.1 ≤ b ≤ 0.4, 0.1 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.1, 0 ≤ d ≤ 0.1, and a + b + c + d + e = 1.

7. In Paragraph 1, A cathode for a sodium secondary battery, wherein the particle size (D50) of the oxide-based layered cathode active material is 5 μm to 15 μm.

8. In Paragraph 1, A cathode for a sodium secondary battery, wherein the oxide-based layered cathode active material is included in an amount of 90 to 99 weight% based on the total weight of the second cathode active material layer.

9. A sodium secondary battery comprising the positive electrode, the negative electrode, a separator interposed between them, and an electrolyte according to claim 1.

10. In Paragraph 9, A sodium secondary battery in which the above-mentioned negative electrode contains hard carbon.

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