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

Abstract

The present invention relates to an anode for a sodium secondary battery, comprising: a current collector; a lower anode active material layer disposed on the current collector; and an upper anode active material layer disposed on the lower anode active material layer, wherein the lower anode active material layer includes first anode active material particles and second anode active material particles, the upper anode active material layer includes third anode active material particles and fourth anode active material particles, each of the first to fourth anode active material particles includes hard carbon, the average particle diameter (D50) of the first anode active material particles is greater than the average particle diameter (D50) of the second anode active material particles, the average particle diameter (D50) of the third anode active material particles is greater than the average particle diameter (D50) of the fourth anode active material particles, the weight of the first anode active material particles is greater than the weight of the second anode active material particles, and the weight of the third anode active material particles is less than the weight of the fourth anode active material particles.

Description

Anode for sodium secondary battery and sodium secondary battery containing same Cross-citation with related applications This application claims the benefit of priority to Korean Patent Application No. 10-2023-0179484, filed December 12, 2023, and Korean Patent Application No. 10-2024-0150135, filed October 29, 2024, the entire contents of which are incorporated herein by reference. Technical field The present invention relates to a negative electrode for a sodium secondary battery and a sodium secondary battery including the same. Secondary batteries are batteries that can be used repeatedly through the process of discharging and charging in the reverse direction, where chemical energy is converted into electrical energy. Recently, with the commercialization of portable electronic products, electric vehicles, etc., the demand for secondary batteries has been rapidly increasing. Currently, lithium secondary batteries are mainly used as commercial secondary batteries, but the reserves of lithium, the main raw material, are limited, so they are expensive and not enough to meet the demand for secondary batteries. Therefore, the development of a new secondary battery that can replace lithium secondary batteries is required, and recently, research and development on sodium secondary batteries using sodium instead of lithium has been actively attempted. However, since sodium ions are relatively large compared to lithium ions, not only is their movement speed slow, but their reaction activity is also not excellent. Therefore, when using negative active materials used in existing lithium secondary batteries, there is a problem in that the capacity is not expressed compared to the characteristics exhibited by lithium secondary batteries, or rapid capacity degradation and deterioration of characteristics occur. To solve these problems, there is a trend to use hard carbon as an anode active material for sodium secondary batteries. In the case of hard carbon, it has a relatively large d-spacing (interlayer spacing (d)) compared to graphite, which is commonly used in lithium secondary batteries. 002)) and has a disordered porous structure, making it easy to store sodium ions with a larger radius than lithium ions. However, in the case of a negative electrode that uses a negative electrode active material containing hard carbon and applies a single layer of negative electrode active material, there is a problem that the output characteristics are poor due to the low specific surface area of ​​the hard carbon, or that the current collector and the negative electrode active material layer are separated due to the low adhesive strength of the hard carbon. Therefore, to solve these problems, there is a need to develop a negative electrode for a sodium secondary battery that has improved output characteristics and excellent adhesion between the current collector and the negative electrode active material layer. One object of the present invention is to solve the above problems, and to provide a negative electrode for a sodium secondary battery having low resistance and excellent output characteristics, and excellent adhesive strength between a current collector and a negative electrode active material layer, thereby preventing peeling between the current collector and the negative electrode active material layer. In addition, another object of the present invention is to provide a sodium secondary battery including the aforementioned negative electrode for a sodium secondary battery. [1] The present invention comprises a current collector; a lower negative electrode active material layer disposed on the current collector; and an upper negative electrode active material layer disposed on the lower negative electrode active material layer; wherein the lower negative electrode active material layer comprises first negative electrode active material particles and second negative electrode active material particles, and the upper negative electrode active material layer comprises third negative electrode active material particles and fourth negative electrode active material particles, wherein the first negative electrode active material particles to the fourth negative electrode active material particles each comprise hard carbon, and the average particle diameter (D) of the first negative electrode active material particles is 50 ) is the average particle diameter (D) of the second negative electrode active material particles. 50 ) is larger than that of the third negative electrode active material particles, and the average particle diameter (D) of the third negative electrode active material particles is 50 ) is the average particle diameter (D) of the fourth negative electrode active material particles. 50) is larger than the first negative electrode active material particle, the weight of the first negative electrode active material particle is larger than the weight of the second negative electrode active material particle, and the weight of the third negative electrode active material particle is smaller than the weight of the fourth negative electrode active material particle. [2] In the present invention, in the above [1], each of the first negative electrode active material particle to the fourth negative electrode active material particle may be spherical. [3] In the present invention, in the above [1] or [2], the weight ratio of the first negative electrode active material particles and the second negative electrode active material particles may be 51:49 to 99:1. [4] The present invention relates to at least one of the above [1] to [3], wherein the average particle diameter (D) of the first negative electrode active material particles is greater than the average particle diameter (D) of the second negative electrode active material particles. 50 ) The rain can be between 1.1 and 10. [5] The present invention relates to at least one of the above [1] to [4], wherein the average particle diameter (D) of the first negative electrode active material particles 50 ) is 5 μm to 15 μm, and the average particle diameter (D) of the second negative electrode active material particles 50 ) can be 1㎛ to 10㎛. [6] The present invention is characterized in that in at least one of the above [1] to [5], the first negative electrode active material particle has a BET specific surface area of ​​1 m 2 / g to 10m 2 / g, and the second negative electrode active material particles have a BET surface area of ​​1.5 m 2 / g to 12m 2 / g could be. [7] In at least one of the above [1] to [6], the thickness of the lower negative electrode active material layer may be 30 ㎛ to 200 ㎛. [8] The present invention is characterized in that in at least one of the above [1] to [7], the loading amount of the lower negative electrode active material layer is 12.5 mg / 25 cm 2250mg / 25cm 2 It could be. [9] In at least one of the above [1] to [8], the weight ratio of the third negative electrode active material particle and the fourth negative electrode active material particle may be 1:99 to 49:51.

[0010] The present invention relates to at least one of the above [1] to [9], wherein the average particle diameter (D) of the third negative electrode active material particle relative to the fourth negative electrode active material particle 50 ) The rain can be between 1.1 and 10.

[0011] The present invention relates to at least one of the above [1] to

[0010] , wherein the average particle diameter (D) of the third negative electrode active material particles 50 ) is 3 μm to 13 μm, and the average particle diameter (D) of the fourth negative electrode active material particles 50 ) can be 1㎛ to 8㎛.

[0012] The present invention relates to at least one of the above [1] to

[0011] , wherein the third negative active material particle has a BET specific surface area of ​​1 m 2 / g to 10m 2 / g, and the fourth negative electrode active material particle has a BET surface area of ​​2 m 2 / g to 14m 2 / g could be.

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

[0012] , the thickness of the upper negative electrode active material layer may be 30 ㎛ to 200 ㎛.

[0014] The present invention is characterized in that in at least one of the above [1] to

[0013] , the loading amount of the upper negative electrode active material layer is 12.5 mg / 25 cm 2 250mg / 25cm 2 It could be.

[0015] The present invention provides a sodium secondary battery using at least one of the above [1] to

[0014] negative electrodes for a sodium secondary battery. According to the present invention, a negative electrode for a sodium secondary battery comprises a negative electrode active material layer on a current collector as a double layer, wherein the lower negative electrode active material layer comprises a large amount of negative electrode active material particles including large-particle hard carbon, and the upper negative electrode active material layer comprises a large amount of negative electrode active material particles including small-particle hard carbon. According to the double-layer structure of the negative electrode active material layer and the weight ratio of the large-particle negative electrode active material and the small-particle negative electrode active material included in the lower and upper negative electrode active material layers, excellent output characteristics can be realized while improving the adhesive strength between the current collector and the negative electrode active material layer. Therefore, when the negative electrode for a sodium secondary battery according to the present invention is applied to a sodium secondary battery, a sodium secondary battery having excellent output characteristics and excellent structural stability can be realized. The drawings attached to this specification illustrate preferred embodiments of the present invention, and together with the contents of the invention described above, serve to better understand the technical idea of ​​the present invention, so the present invention is not limited to the matters described in such drawings. Meanwhile, the shape, size, scale or ratio of elements in the drawings included in this specification may be exaggerated to emphasize a clearer explanation. FIG. 1 is a schematic side view illustrating a cathode according to one embodiment of the present invention. Hereinafter, the present invention will be described more preferably. The terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as having meanings and concepts that conform to the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term in order to explain his or her own invention in the best manner. The terminology used herein is for the purpose of describing embodiments only and is not intended to limit the invention. In this specification, the singular also includes the plural unless specifically stated otherwise. The terms "comprises" and / or "comprising" as used herein do not exclude the presence or addition of one or more other components other than the mentioned components. In the present invention, the average particle diameter (D 50 ) means the particle size based on 50% of the volume cumulative particle size distribution of the powder of the measurement target. The average particle diameter (D 50 ) can be measured using the laser diffraction method. For example, the target powder is dispersed in a dispersion medium, introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000), and ultrasonic waves of about 28 kHz are irradiated at an output of 60 W, and then a volume-cumulative particle size distribution graph is obtained, and then the particle size corresponding to 50% of the volume-cumulative amount is measured. In the present invention, the “specific surface area” is measured by the BET method, and can be calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mino II of BEL Japan. In the present invention, porosity can be calculated by the following mathematical formula A. [Mathematical Formula A] Porosity (%) = {1-(true density / true density)} × 100 In the above mathematical expression A, the real density is the density of a specific negative electrode active material layer including pores, and the true density is the density of a solid (a solid constituting a specific negative electrode active material layer) that does not include pores. For example, the porosity of the lower negative electrode active material layer can be calculated by defining the density of the lower negative electrode active material layer including pores as the true density, and the density of the solid (the solid constituting the lower negative electrode active material layer) that does not include pores as the true density, and the porosity of the upper negative electrode active material layer can be calculated by defining the density of the upper negative electrode active material layer including pores as the true density, and the density of the solid (the solid constituting the upper negative electrode active material layer) that does not include pores as the true density. Furthermore, as another example, the porosity of the negative electrode can be calculated by defining the density of the negative electrode active material layer including pores as the true density, and the density of the solid (the solid constituting the negative electrode active material layer) that does not include pores as the true density. Cathode for sodium secondary battery Hereinafter, a negative electrode for a sodium secondary battery according to the present invention will be described. Referring to FIG. 1, a negative electrode (10) for a sodium secondary battery according to the present invention includes a current collector (100); a lower negative electrode active material layer (210) disposed on the current collector (100); and an upper negative electrode active material layer (220) disposed on the lower negative electrode active material layer (210). The lower negative electrode active material layer (210) includes first negative electrode active material particles and second negative electrode active material particles, and the upper negative electrode active material layer (220) includes third negative electrode active material particles and fourth negative electrode active material particles. The first to fourth negative electrode active material particles each include hard carbon, and the average particle diameter (D) of the first negative electrode active material particles is 50 ) is the average particle diameter (D) of the second negative electrode active material particles. 50 ) is larger than that of the third negative electrode active material particles, and the average particle diameter (D) of the third negative electrode active material particles is 50 ) is the average particle diameter (D) of the fourth negative electrode active material particles. 50) is larger than the weight of the first negative active material particle, the weight of the first negative active material particle is larger than the weight of the second negative active material particle, and the weight of the third negative active material particle is smaller than the weight of the fourth negative active material particle. In the case of the negative electrode for the conventional sodium secondary battery, a single layer of the negative electrode active material layer was applied while using the negative electrode active material including hard carbon. In this case, the output characteristics are not good due to the low specific surface area of ​​the hard carbon, or the current collector and the negative electrode active material layer are peeled off due to the low adhesive strength of the hard carbon, which causes the problem of the deterioration of the electrochemical characteristics of the battery. As a result of continuous research to solve such problems, the inventors of the present invention have found that when a negative electrode active material layer on a current collector is configured as a double layer, wherein the lower negative electrode active material layer includes a large amount of negative electrode active material particles including large-diameter hard carbon, and the upper negative electrode active material layer includes a large amount of negative electrode active material particles including small-diameter hard carbon, excellent output characteristics can be realized while improving the adhesive strength between the current collector and the negative electrode active material layer, thereby completing the present invention. For example, a negative electrode (10) for a sodium secondary battery according to the present invention includes a current collector (100); a lower negative electrode active material layer (210) disposed on the current collector (100); and an upper negative electrode active material layer (220) disposed on the lower negative electrode active material layer (210). (1) Whole house (100) The current collector (100) is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Preferably, the current collector (100) may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, an aluminum-cadmium alloy, or the like. The above-mentioned collector (100) can typically have a thickness of 3 to 500 μm. The above current collector (100) may form fine irregularities on the surface to strengthen the bonding strength of the negative electrode active material. For example, the current collector (100) may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc. (2) Lower cathode active material layer (210) Next, the lower cathode active material layer (210) will be preferably described. The lower negative electrode active material layer (210) is disposed on the current collector (100), preferably disposed on at least one surface of the current collector (100), and more preferably disposed on one or both surfaces of the current collector (100). Even more preferably, the lower negative electrode active material layer (210) can be in direct contact with the surface of the current collector (100). The lower negative electrode active material layer (210) includes first negative electrode active material particles and second negative electrode active material particles. The average particle diameter (D) of the first negative electrode active material particles 50 ) is the average particle diameter (D) of the second negative electrode active material particles. 50 ) are larger than each other. The first negative electrode active material particles and the second negative electrode active material particles each contain hard carbon. The weight of the first negative electrode active material particle is greater than the weight of the second negative electrode active material particle. Preferably, the weight of the first negative electrode active material particle included in the lower negative electrode active material layer (210) may be greater than the weight of the second negative electrode active material particle included in the lower negative electrode active material layer (210). For example, the lower negative electrode active material layer (210) including the first negative electrode active material particles having a large particle size and the second negative electrode active material particles having a small particle size may be a layer that directly contacts the current collector (100), and is a portion where a peeling phenomenon may occur between the negative electrode active material layer and the current collector (100) due to volume expansion and contraction of the negative electrode active material when sodium ions are inserted / deintercalated. In order to solve this problem, the lower negative electrode active material layer (210) according to the present invention includes the first negative electrode active material particles having a large particle size in a greater weight than the second negative electrode active material particles having a small particle size, compared to the case of the upper negative electrode active material layer (220) described below. Since the lower negative electrode active material layer (210) includes first negative electrode active material particles having a larger particle size than second negative electrode active material particles having a smaller particle size, the binder that may be included in the lower negative electrode active material layer (210) may assist in bonding with the current collector (100) more than in bonding the first and second negative electrode active material particles, thereby realizing excellent adhesive strength between the current collector (100) and the negative electrode active material layer, and preventing peeling between the negative electrode active material layer and the current collector (100), thereby improving the overall structural stability of the negative electrode. Meanwhile, in order to implement high adhesive strength with the current collector (100), if only the first negative electrode active material particles having a large particle size are included in the lower negative electrode active material layer (210), the size of the pores formed by the first negative electrode active material particles is large, so that sodium ions are not transmitted well, which may cause a problem of reduced output characteristics, and as the size of the pores increases, the thickness of the electrode increases, which may cause a problem of reduced energy density of the battery per unit volume. Therefore, the negative electrode (10) for a sodium secondary battery according to the present invention has the characteristics of improving the output characteristics by including the second negative electrode active material particles having a small particle size in the lower negative electrode active material layer (210) at a smaller weight than the first negative electrode active material particles having a large particle size, while maximizing the adhesive strength characteristics by including the first negative electrode active material particles having a larger weight than the second negative electrode active material particles having a small particle size. In addition, since the first negative electrode active material particles and the second negative electrode active material particles each contain hard carbon, sodium ions having a relatively large radius compared to lithium ions can be easily stored, thereby realizing better discharge capacity characteristics. The first negative electrode active material particles and the second negative electrode active material particles may each be spherical. When the particles are each spherical, the negative electrode active material particles can be laminated in close contact with each other due to the structural characteristic of being spherical, thereby realizing high density of the negative electrode and improving the energy density per unit volume of the battery. Preferably, the sphericity of the first negative electrode active material particles and the second negative electrode active material particles may each independently be 0.8 to 1, preferably 0.85 to 1.0, more preferably 0.9 to 1. When the above range is satisfied, it is preferable in that the ion conductivity can be excellent while realizing high density of the negative electrode. The above sphericity can be defined as a value obtained by dividing the circumference of a circle having the same area as the projection image of the negative active material particle by the perimeter of the projection image of the negative active material particle. Preferably, the above sphericity can be defined by the following mathematical formula B. [Mathematical Formula B] Sphericity = (Circumference of a circle with the same area as the projected image of the negative active material particle) / (Perimeter of the projected image of the negative active material particle) The above sphericity can be measured using a particle shape analyzer, for example, sysmex FPIA3000 (manufactured by Mavericks). The sphericity according to the present invention can be defined as an average value of the sphericity of 10 particles randomly selected from the negative active material. The weight ratio of the first negative electrode active material particles and the second negative electrode active material particles may be 51:49 to 99:1, preferably 55:45 to 95:5, and more preferably 60:40 to 90:10. When the above range is satisfied, the number of large-diameter negative electrode active material particles included in the lower negative electrode active material layer (210) increases, thereby preventing a peeling phenomenon between the current collector (100) and the negative electrode active material layer, while including an appropriate amount of small-diameter negative electrode active material, thereby realizing excellent output characteristics. The average particle diameter (D) of the first negative electrode active material particles relative to the second negative electrode active material particles 50 ) may be 1.1 to 10. Preferably, the average particle diameter (D) of the first negative electrode active material particles relative to the second negative electrode active material particles 50 ) may be 1.1 or more, 1.15 or more, 1.20 or more, 1.25 or more, or 1.30 or more, and may be 10 or less, 8 or less, 7 or less, 6 or less, 5 or less, 4.5 or less, 4 or less, 3.5 or less, 3.0 or less, or 2.5 or less. More preferably, the average particle diameter (D) of the first negative electrode active material particles relative to the second negative electrode active material particles 50 ) can be from 1.1 to 2.5. When the above range is satisfied, it is preferable in that the filling ratio between the negative active material particles increases, thereby enabling the implementation of a high-density electrode. The average particle diameter (D) of the first negative electrode active material particles 50 ) may be 5 μm to 15 μm. Preferably, the average particle diameter (D) of the first negative electrode active material particles 50 ) may be 5 ㎛ or more, 6 ㎛ or more, 7 ㎛ or more, or 8 ㎛ or more, and may be 14 ㎛ or less, 13 ㎛ or less, 12 ㎛ or less, or 11 ㎛ or less. More preferably, the average particle diameter (D of the first negative electrode active material particles 50) may be 8 μm to 11 μm. When the above range is satisfied, the contact area between the binder and the current collector (100) is increased more than the contact area between the binder and the negative electrode active material particles at the interface between the current collector (100) and the lower negative electrode active material layer (210), so that the electrode adhesive strength characteristics can be improved. The average particle diameter (D) of the second negative electrode active material particles 50 ) may be 1 μm to 10 μm, preferably the average particle diameter (D) of the second negative electrode active material particles 50 ) may be 1 ㎛ or more, 2 ㎛ or more, 3 ㎛ or more, 4 ㎛ or more, or 4.5 ㎛ or more, and may be 10 ㎛ or less, 9 ㎛ or less, 8.5 ㎛ or less, or 8 ㎛ or less. More preferably, the average particle diameter (D of the second negative electrode active material particles 50 ) can be 4.5 μm to 8 μm. When the above range is satisfied, the ion and electron conductivity within the negative electrode active material particles can be improved, thereby improving the output characteristics. The above first negative electrode active material particles have a BET surface area of ​​1 m 2 / g to 10m 2 / g. Preferably, the first negative electrode active material particle has a BET surface area of ​​1 m 2 / g or more, 1.5m 2 / g or more or 2m 2 / g can be more than 10m 2 / g and below, 9m 2 / g and below, 8m 2 / g and below, 7m 2 / g and below, 6m 2 / g and below, 5m 2 / g or less or 4m 2 / g or less. More preferably, the first negative electrode active material particle has a BET surface area of ​​2 m 2 / g to 4m 2 / g. When the above range is satisfied, it may be desirable in that the life characteristics of the battery may be improved because the side reaction between the negative active material particles and the electrolyte is not excessive. The above second negative electrode active material particles have a BET surface area of ​​1.5 m 2 / g to 12m 2 / g. Preferably, the second negative electrode active material particles have a BET surface area of ​​1.5 m 2 / g or more, 2m 2 / g or more, 2.5m 2 / g or more or 3m 2 / g can be more than 12m 2 / g and below, 11m 2 / g and below, 10m 2 / g and below, 9m 2 / g and below, 8m 2 / g and below, 7m 2 / g and below, 6m 2 / g or less or 5m 2 / g or less. More preferably, the second negative electrode active material particles have a BET specific surface area of ​​3 m 2 / g to 5m 2 / g. When the above range is satisfied, it may be desirable in that the life characteristics of the battery may be improved because the side reaction between the negative active material particles and the electrolyte is not excessive. The thickness of the lower negative electrode active material layer (210) may be 30 ㎛ to 200 ㎛. Preferably, the thickness of the lower negative electrode active material layer (210) may be 30 ㎛ or more, 40 ㎛ or more, 50 ㎛ or more, 60 ㎛ or more, or 70 ㎛ or more, and may be 200 ㎛ or less, 180 ㎛ or less, 160 ㎛ or less, 140 ㎛ or less, or 130 ㎛ or less. More preferably, the thickness of the lower negative electrode active material layer (210) may be 60 ㎛ to 130 ㎛. The loading amount of the lower negative electrode active material layer (210) is 12.5 mg / 25 cm 2 250mg / 25cm 2 It can be. Preferably, the loading amount of the lower negative active material layer (210) is 12.5 mg / 25 cm 2 Above, 17.5mg / 25cm 2 Above, 22.5mg / 25cm 2Above, 37.5mg / 25cm 2 Above, 42.5 mg / 25cm 2 Above, 50 mg / 25cm 2 Above, 55 mg / 25cm 2 or more than 60 mg / 25cm 2 It may be more than 250mg / 25cm 2 Below, 200mg / 25cm 2 Below, 175 mg / 25cm 2 Below, 150 mg / 25cm 2 Less than or equal to 125 mg / 25cm 2 It may be less than or equal to 60 mg / 25 cm. More preferably, the loading amount of the lower negative electrode active material layer (210) is 60 mg / 25 cm. 2 125mg / 25cm 2 It could be. The lower negative electrode active material layer (210) may have a porosity of 20% to 70%. Preferably, the lower negative electrode active material layer (210) may have a porosity of 23% or more, 25% or more, 27% or more, 30% or more, 33% or more, or 35% or more, and 70% or less, 65% or less, 60% or less, or 55% or less. More preferably, the lower negative electrode active material layer (210) may have a porosity of 35% to 55%. The porosity can be calculated by the aforementioned mathematical formula A. When the above range is satisfied, the energy density per unit volume of the battery can be increased, and contact between active material particles can be facilitated, so that the output characteristics can be increased. The first negative electrode active material particles and the second negative electrode active material particles may be included in the lower negative electrode active material layer (210) at 60 wt% to 99 wt%, preferably 75 wt% to 95 wt%. The lower negative electrode active material layer (210) may further include a first binder and / or a first conductive material together with the first negative electrode active material particles and the second negative electrode active material particles. The above first binder can be used for binding between the first negative electrode active material particles and / or the second negative electrode active material or for adhesion between the lower negative electrode active material layer (210) and the current collector (100). For example, the first binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, and materials in which hydrogens of these are substituted with Li, Na or Ca, and may also include various copolymers thereof. The above first binder may be included in the lower negative electrode active material layer (210) at 0.5 wt% to 10 wt%, preferably 1 wt% to 5 wt%. The first conductive material is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, paneth black, lamp black, thermal black, etc.; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; fluorocarbon; metal powder such as aluminum or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. can be used. The above first conductive material may be included in the lower negative electrode active material layer (210) at 0.5 wt% to 10 wt%, preferably 1 wt% to 5 wt%. (3) Upper cathode active material layer (220) Next, the upper cathode active material layer (220) will be described. The upper negative electrode active material layer (220) is disposed on the lower negative electrode active material layer (210). Preferably, the upper negative electrode active material layer (220) may be disposed on the opposite side of the surface of the lower negative electrode active material layer (210) where the lower negative electrode active material layer (210) and the current collector (100) come into contact. More preferably, the upper negative electrode active material layer (220) may be present on the outermost surface of the negative electrode. In addition, the upper negative electrode active material layer (220) may be present on the outermost surface of the negative electrode and may face a separator described below. In addition, when the lower negative electrode active material layer (210) is present on both sides of the current collector (100), the upper negative electrode active material layer (220) can be disposed on each surface of the lower negative electrode active material layer (210) disposed on both sides of the current collector (100). The upper negative electrode active material layer (220) includes third negative electrode active material particles and fourth negative electrode active material particles. The average particle diameter (D) of the third negative electrode active material particles 50 ) is the average particle diameter (D) of the fourth negative electrode active material particles. 50 ) are larger than those of the third negative electrode active material particles. The third negative electrode active material particles and the fourth negative electrode active material particles each contain hard carbon. The weight of the third negative electrode active material particle is smaller than the weight of the fourth negative electrode active material particle. Preferably, the weight of the third negative electrode active material particle included in the lower negative electrode active material layer (210) may be smaller than the weight of the fourth negative electrode active material particle included in the lower negative electrode active material layer (210). Preferably, the upper negative electrode active material layer (220) including the third negative electrode active material particles having a large particle size and the fourth negative electrode active material particles having a small particle size may be present on the outermost surface of the negative electrode and may be present at a position to directly receive sodium ions from the positive electrode. Compared to the lower negative electrode active material layer (210) described above, the upper negative electrode active material layer (220) includes the third negative electrode active material particles having a large particle size in a smaller weight than the fourth negative electrode active material particles having a small particle size, so that the gaps formed by the active material particles in the upper negative electrode active material layer (220) are minimized so that sodium ions can be uniformly transmitted, thereby improving the overall output characteristics of the negative electrode. Meanwhile, if only the fourth negative electrode active material particles having a small particle size are included in the upper negative electrode active material layer (220) in order to implement excellent output characteristics, the formation of an SEI film on the surface of the small negative electrode active material having a large specific surface area may accelerate electrolyte decomposition, thereby decreasing the electrochemical characteristics of the battery, and the required amount of binder to prevent electrode detachment may increase, which may cause problems such as an increase in electrolyte side reactions and a decrease in the energy density per unit volume of the battery. Therefore, the negative electrode (10) for a sodium secondary battery according to the present invention has the characteristics of improving the adhesive characteristics by including the third negative electrode active material particles having a large particle size in the upper negative electrode active material layer (220) at a smaller weight than the fourth negative electrode active material particles having a small particle size, while maximizing the output characteristics by including the fourth negative electrode active material particles having a larger weight than the third negative electrode active material particles having a large particle size. In addition, since the third negative electrode active material particle and the fourth negative electrode active material particle each contain hard carbon, it is easy to store sodium ions having a relatively large radius compared to lithium ions, thereby realizing better discharge capacity characteristics. The third negative electrode active material particle and the fourth negative electrode active material particle may each be spherical. When the particles are each spherical, the negative electrode active material particles can be laminated in close contact with each other due to the structural characteristic of being spherical, thereby realizing high density of the negative electrode and improving the energy density per unit volume of the battery. Specifically, the sphericity of the third negative electrode active material particle and the fourth negative electrode active material particle may each independently be 0.8 to 1, preferably 0.85 to 1.0, more preferably 0.9 to 1. When the above range is satisfied, it is preferable in that the ion conductivity can be excellent while realizing high density of the negative electrode. The above sphericity can be defined as a value obtained by dividing the circumference of a circle having the same area as the projection image of the negative active material particle by the perimeter of the projection image of the negative active material particle. Preferably, the above sphericity can be defined by the mathematical formula B described above. The weight ratio of the third negative electrode active material particles and the fourth negative electrode active material particles may be 1:99 to 49:51, preferably 5:95 to 45:55, and more preferably 10:90 to 40:60. When the above range is satisfied, the upper negative electrode active material layer (220) may contain an appropriate amount of negative electrode active material particles having a large particle size, thereby preventing a peeling phenomenon between the upper and lower negative electrode active material layers (210), while containing a large amount of negative electrode active material having a small particle size, thereby implementing excellent output characteristics. The average particle diameter (D) of the third negative electrode active material particles relative to the fourth negative electrode active material particles 50 ) may be 1.1 to 10. Preferably, the average particle diameter (D) of the third negative electrode active material particles relative to the fourth negative electrode active material particles 50) may be 1.15 or more, 1.20 or more, 1.25 or more, or 1.30 or more, and 10 or less, 8 or less, 7 or less, 6 or less, 5 or less, 4.5 or less, 4 or less, 3.5 or less, 3.0 or less, or 2.5 or less. More preferably, the average particle diameter (D) of the third negative electrode active material particles relative to the fourth negative electrode active material particles 50 ) can be from 1.1 to 2.5. When the above range is satisfied, it is preferable in that the filling ratio between the negative active material particles increases, thereby enabling the implementation of a high-density electrode. The average particle diameter (D) of the third negative electrode active material particles 50 ) may be 3 μm to 13 μm. Preferably, the average particle diameter (D) of the third negative electrode active material particles 50 ) may be 3 ㎛ or more, 3.5 ㎛ or more, 4 ㎛ or more, 4.5 ㎛ or more, 5.0 ㎛ or more, or 5.5 ㎛ or more, and may be 13 ㎛ or less, 12 ㎛ or less, 11 ㎛ or less, 10 ㎛ or less, 9 ㎛ or less, 8 ㎛ or less, or 7 ㎛ or less. More preferably, the average particle diameter (D of the third negative electrode active material particles 50 ) can be 5.5 μm to 7 μm. When the above range is satisfied, the adhesive strength between the negative active material particles can be improved, thereby improving the electrode adhesive strength characteristics. The average particle diameter (D) of the fourth negative electrode active material particles 50 ) may be 1 μm to 8 μm, preferably the average particle diameter (D) of the fourth negative electrode active material particles 50 ) may be 1 ㎛ or more, 1.5 ㎛ or more, 2 ㎛ or more, or 2.5 ㎛ or more, and may be 8 ㎛ or less, 7 ㎛ or less, 6 ㎛ or less, or 5 ㎛ or less. More preferably, the average particle diameter (D of the fourth negative electrode active material particles 50 ) can be 1 μm to 5 μm. When the above range is satisfied, the ion and electron conductivity within the negative electrode active material particles can be improved, thereby improving the output characteristics. The above third negative electrode active material particles have a BET 1m 2 / g to 10m 2 / g. Preferably, the first negative electrode active material particle has a BET surface area of ​​1 m 2 / g or more, 1.5m 2 / g or more, 2m 2 / g or more, 2.5m 2 / g over 3m 2 / g or more or 3.5m 2 / g can be more than 10m 2 / g and below, 9m 2 / g and below, 8m 2 / g and below, 7m 2 / g and below, 6m 2 / g or less or 5m 2 / g or less. More preferably, the first negative electrode active material particle has a BET specific surface area of ​​3 m 2 / g to 5m 2 / g. When the above range is satisfied, it may be desirable in that the life characteristics of the battery may be improved because the side reaction between the negative active material particles and the electrolyte is not excessive. The above fourth negative electrode active material particles have a BET surface area of ​​2 m 2 / g to 14m 2 / g. Preferably, the second negative electrode active material particles have a BET surface area of ​​2 m 2 / g or more, 2.5m 2 / g or more, 3m 2 / g or more, 3.5m 2 / g or more or 4m 2 / g can be more than 14m 2 / g and below, 12m 2 / g and below, 10m 2 / g and below, 9m 2 / g and below, 8m 2 / g and below, 7m 2 / g and below, 6m 2 / g or less or 5.5m 2 / g or less. More preferably, the second negative electrode active material particles have a BET specific surface area of ​​4 m 2 / g to 5.5m 2 / g. When the above range is satisfied, it may be desirable in that the life characteristics of the battery may be improved because the side reaction between the negative active material particles and the electrolyte is not excessive. The thickness of the upper negative electrode active material layer (220) may be 30 ㎛ to 200 ㎛. Preferably, the thickness of the upper negative electrode active material layer (220) may be 30 ㎛ or more, 40 ㎛ or more, 50 ㎛ or more, 60 ㎛ or more, or 70 ㎛ or more, and may be 200 ㎛ or less, 180 ㎛ or less, 160 ㎛ or less, 140 ㎛ or less, or 130 ㎛ or less. More preferably, the thickness of the upper negative electrode active material layer (220) may be 60 ㎛ to 130 ㎛. The loading amount of the upper negative electrode active material layer (220) is 12.5 mg / 25 cm 2 250mg / 25cm 2 It can be. Preferably, the loading amount of the upper negative electrode active material layer (220) is 12.5 mg / 25 cm 2 Above, 17.5mg / 25cm 2 Above, 22.5mg / 25cm 2 Above, 37.5mg / 25cm 2 Above, 42.5 mg / 25cm 2 Above, 50 mg / 25cm 2 Above, 55 mg / 25cm 2 or more than 60 mg / 25cm 2 It may be more than 250mg / 25cm 2 Below, 200mg / 25cm 2 Below, 175 mg / 25cm 2 Below, 150 mg / 25cm 2 Less than or equal to 125 mg / 25cm 2 It may be less than or equal to 60 mg / 25 cm. More preferably, the loading amount of the upper negative electrode active material layer (220) is 60 mg / 25 cm. 2 125mg / 25cm 2It could be. The upper negative electrode active material layer (220) may have a porosity of 20% to 70%. Preferably, the upper negative electrode active material layer (220) may have a porosity of 23% or more, 25% or more, 27% or more, 30% or more, 33% or more, or 35% or more, and 70% or less, 65% or less, 60% or less, or 55% or less. More preferably, the upper negative electrode active material layer (220) may have a porosity of 35% to 55%. The porosity can be calculated by the mathematical formula A described above. When the above range is satisfied, the energy density per unit volume of the battery can be increased, and contact between active material particles can be facilitated, so that the output characteristics can be increased. The thickness ratio of the lower negative electrode active material layer (210) and the upper negative electrode active material layer (220) may be 1:5 to 5:1, preferably 1:4 to 4:1, and more preferably 1:3 to 3:1. The sum of the loading amounts of the lower negative electrode active material layer (210) and the upper negative electrode active material layer (220) is 25 mg / 25 cm 2 500mg / 25cm 2 , preferably 100mg / 25cm 2 350mg / 25cm 2 5, preferably 120mg / 25cm 2 250mg / 25cm 2 If the above range is satisfied, the energy density per unit volume of the battery can be increased, and the resistance can be small, so that the output characteristics of the battery can be excellent. The third negative electrode active material particles and the fourth negative electrode active material particles may be included in the upper negative electrode active material layer (220) at 60 wt% to 99 wt%, preferably 75 wt% to 95 wt%. The upper negative electrode active material layer (220) may further include a second binder and / or a second conductive material together with the third negative electrode active material particles and the fourth negative electrode active material particles. The second binder may be used for bonding between the third negative electrode active material particles and / or the fourth negative electrode active material particles or for adhesion between the upper negative electrode active material layer (220) and the lower negative electrode active material layer (210). For example, the second binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, and materials in which hydrogens of these are substituted with Li, Na or Ca, and may also include various copolymers thereof. The second binder may be included in the upper negative electrode active material layer (220) in an amount of 0.5 wt% to 10 wt%, preferably 1 wt% to 5 wt%. The second conductive material is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, paneth black, lamp black, thermal black, etc.; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; fluorocarbon; metal powder such as aluminum or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. can be used. The second conductive material may be included in the upper negative electrode active material layer (220) at 0.5 wt% to 10 wt%, preferably 1 wt% to 5 wt%. The lower negative electrode active material layer (210) may be manufactured by applying a negative electrode slurry for the lower layer, which includes the first negative electrode active material particles and the second negative electrode active material particles, optionally a first binder, a first conductive agent, and / or a solvent for forming a negative electrode slurry, to a current collector (100) and drying it, or by casting the negative electrode slurry for the lower layer on a separate support, and then laminating the resulting film on the current collector (100) by peeling it off from the support. The upper negative electrode active material layer (220) may also be manufactured by manufacturing the negative electrode slurry for the upper layer in the same manner as above, except that the third and fourth negative electrode active material particles, optionally a second binder, and / or a second conductive agent are used. The solvent for forming the negative electrode slurry may include at least one selected from the group consisting of distilled water, NMP (N-methyl-2-pyrrolidone), ethanol, methanol, and isopropyl alcohol, preferably distilled water, in order to facilitate dispersion of the negative electrode active material particles, binder, and / or conductive agent, for example. The solid content of the negative electrode slurry may be 30 wt% to 80 wt%, preferably 40 wt% to 70 wt%. When the negative electrode active material layer includes the lower negative electrode active material layer (210) and the upper negative electrode active material layer (220), the manufacture of the negative electrode (10) for a sodium secondary battery is not particularly limited as long as the lower negative electrode active material layer (210) and the upper negative electrode active material layer (220) having the above-described characteristics can be implemented. For example, the first and second negative electrode active material particles, the first binder and / or the first conductive agent are added to a solvent for forming a negative electrode slurry (e.g., distilled water) to manufacture a negative electrode slurry for the lower layer, and the third and fourth negative electrode active material particles, the second binder and / or the second conductive agent are added to a solvent (e.g., distilled water) to manufacture a negative electrode slurry for the upper layer, and then these are applied to a current collector (100), thereby manufacturing the negative electrode (10) for a sodium secondary battery according to the present invention. More preferably, the negative electrode slurry for the lower layer manufactured as described above is applied to a current collector (100), rolled, and dried to form a lower negative electrode active material layer (210), and the negative electrode slurry for the upper layer manufactured as described above is applied on the lower negative electrode active material layer (210), rolled, and dried to form an upper negative electrode active material layer (220), thereby manufacturing the negative electrode (10) for a sodium secondary battery according to the present invention. Meanwhile, the negative electrode slurry for the lower layer may be applied to a current collector (100) while substantially simultaneously applying the negative electrode slurry for the upper layer onto the negative electrode slurry for the lower layer, and simultaneously rolling and drying, thereby manufacturing the negative electrode (10) for a sodium secondary battery according to the present invention. The porosity of the negative electrode (10) for the above sodium secondary battery may be 20% to 70%. Preferably, the porosity of the negative electrode (10) for the above sodium secondary battery may be 20% or more, 23% or more, 25% or more, 30% or more, 33% or more, or 35% or more, and may be 70% or less, 65% or less, 60% or less, or 55% or less. More preferably, the porosity of the negative electrode (10) for the above sodium secondary battery may be 35% to 55%. When the above range is satisfied, the energy density per unit volume of the battery may increase, and the resistance may be small, so that the output characteristics of the battery may be excellent. Sodium secondary battery Next, a sodium secondary battery according to the present invention will be described. The sodium secondary battery according to the present invention includes the negative electrode (1) for a sodium secondary battery according to the present invention described above. More preferably, the sodium secondary battery according to the present invention includes the negative electrode (1) for a sodium secondary battery according to the present invention, a positive electrode positioned opposite the negative electrode (1) for a sodium secondary battery, a separator interposed between the negative electrode (1) for a sodium secondary battery and the positive electrode, and an electrolyte. Since the negative electrode (1) for a sodium secondary battery has been described above, only the remaining components will be described below. (anode) The above positive electrode can be opposed to the negative electrode for a sodium secondary battery according to the present invention. The above positive electrode may include a positive electrode current collector; and a positive electrode active material layer positioned on at least one surface of the positive electrode current collector. The positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Preferably, the positive electrode current collector may include at least one selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and an aluminum-cadmium alloy, and preferably may include aluminum. The above positive electrode current collector may typically have a thickness of 3 to 500 μm. The above-mentioned positive electrode current collector may form fine irregularities on the surface to strengthen the bonding strength of the negative electrode active material. For example, the above-mentioned positive electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc. The above positive electrode active material layer may include a positive electrode active material. As the positive electrode active material, a compound capable of reversible intercalation and deintercalation of sodium can be used. A specific example is Na x CoO2, Na x Co 2 / 3 Mn 1 / 3 O2, Na x Fe 1 / 2 Mn 1 / 2 O2, NaCrO2, NaLi 0.2 Ni 0.25 Mn 0.75 O 2.35 , Na 0.44 MnO2, NaMnO2, Na2Fe5Si 12 O 30 , Na 0.7 VO2, or Na 0.33 V2O 5　 Sodium metal oxides such as the following (at this time, 0 <x≤1); Na3V2(PO4)3, NaFePO4, NaMn 0.5 Fe 0.5 Sodium metal phosphates such as PO4, Na3V2(PO4)3, Na3Fe2(PO4)3; Na2FePO4F, Na3V2(PO4) 3　Examples thereof include sodium metal fluorophosphates such as NaFeSO4F; sodium metal fluorosulfoxides such as NaFeO2, NaMnO2, NaNiO2, and NaCoO2; sodium metal fluorides such as Na3FeF6 or Na2MnF6; sodium metal borates such as NaFeBO4 or Na3Fe2(BO4)3; chalcogen compounds such as TiS2, ZrS2, VS2, V2S2, TaS2, FeS2, and NiS2, and one or a mixture of two or more of these may be used. Among these, a compound containing Fe can suppress the dissolution of transition metal ions even when the temperature of the electrolyte in the battery rises, and as a result, can improve the cycle characteristics and discharge capacity retention rate of the sodium secondary battery. In addition, chalcogen compounds such as TiS2, ZrS2, etc. have fast absorption and desorption rates of sodium ions, and when used in combination with metallic sodium or its alloy negative electrode active materials, they can absorb and desorb sodium ions at a higher potential than the negative electrode, thereby exhibiting further increased reactivity. The above-mentioned positive electrode active material may be included in the positive electrode active material layer at 80 to 99 wt%, preferably 92 to 98.5 wt%, taking into account sufficient capacity of the positive electrode active material. The above-described positive electrode active material layer may further include a binder and / or a conductive material together with the above-described positive electrode active material. The above binder is a component that assists in the binding of the active material and the conductive material and the binding to the current collector, and preferably includes at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, and preferably includes polyvinylidene fluoride. The above binder may be included in the positive electrode active material layer at 1 to 20 wt%, preferably 1.2 to 10 wt%, in order to sufficiently secure binding force between components such as the positive electrode active material. The conductive material may be used to assist and improve conductivity in a secondary battery, and is not particularly limited as long as it has conductivity without causing a chemical change. Preferably, the conductive material may include at least one selected from the group consisting of graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, paneth black, lamp black, thermal black; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; fluorocarbon; metal powder such as aluminum or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives, and preferably, the conductive material may include carbon black in terms of improving conductivity. The above-mentioned conductive agent may be included in the positive electrode active material layer at 1 wt% to 20 wt%, preferably 1.2 wt% to 10 wt%, in order to sufficiently secure electrical conductivity. The thickness of the above positive electrode active material layer may be 30 ㎛ to 400 ㎛, preferably 50 ㎛ to 110 ㎛. The above positive electrode can be manufactured by coating a positive electrode slurry including a positive electrode active material and optionally a binder, a conductive material, and a solvent for forming a positive electrode slurry on the positive electrode current collector, and then drying and rolling. The solvent for forming the positive electrode slurry may include an organic solvent such as NMP (N-methyl-2-pyrrolidone), and may be used in an amount that provides a desirable viscosity when including the positive electrode active material, and optionally a binder and a conductive material. For example, the solvent for forming the positive electrode slurry may be included in the positive electrode slurry such that the concentration of the solid content including the positive electrode active material, and optionally a binder and a conductive material is 50 wt% to 95 wt%, preferably 70 wt% to 90 wt%. (Separator) The separator according to the present invention separates the negative electrode and the positive electrode and provides a passage for the movement of sodium ions. Separators generally used in secondary batteries can be used, and the type thereof is not particularly limited. For example, as the separator, a porous polymer film manufactured from a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof, or a porous nonwoven fabric formed from high-melting-point glass fibers, polyethylene terephthalate fibers, or the like can be used. In addition, a coated separator containing a ceramic component or a polymer material to secure heat resistance or mechanical strength can be used, and can optionally be used in a single-layer or multi-layer structure. (electrolyte) The electrolyte according to the present invention may include, but is not limited to, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc. that can be used in the manufacture of a sodium secondary battery. Preferably, the electrolyte may include an organic solvent and a sodium salt. As the organic solvent, any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move may be used without particular limitation. Preferably, the organic solvent includes ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; Examples of solvents that can be used include carbonate solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (wherein R represents a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes. The above sodium salt can be used without any special limitation as long as it is a compound that can provide sodium ions used in a sodium secondary battery. Preferably, the sodium salt is NaPF6, NaClO4, NaAsF6, NaBF4, NaCF3SO3, NaB(C6H5)4, NaC4F9SO3, NaN(C2F5SO3)2, NaN(C2F5SO2)2, NaN(CF3SO2) 2　The concentration of the sodium salt may be used in the range of 0.1 to 2.0 M. In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as fluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxy ethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing battery capacity decrease, and improving the discharge capacity of the battery. In this case, the additives may be contained in an amount of 0.1 to 5 wt% with respect to the total weight of the electrolyte. As described above, a sodium secondary battery including a negative electrode according to the present invention can be applied to portable devices such as mobile phones, laptop computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs). Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein. Examples and Comparative Examples Example 1: Preparation of cathode As the first negative electrode active material, hard carbon (average particle size (D 50 ): 10.1㎛, specific surface area: 3.0m 2 / g) was prepared, and as a second negative electrode active material, hard carbon (average particle size (D 50 ): 7.6㎛, specific surface area: 3.7m 2 / g) was prepared, and as the third negative electrode active material, hard carbon (average particle size (D50 ): 6.0㎛, specific surface area: 4.1m 2 / g) was prepared, and as the fourth negative electrode active material, hard carbon (average particle size (D 50 ): 4.5㎛, specific surface area: 4.7m 2 / g) was prepared. Carbon black (C65) was prepared as a challenge material and SBR-CMC was prepared as a binder. After mixing the first negative electrode active material and the second negative electrode active material in a weight ratio of 90:10 as a negative electrode slurry for the lower layer, the mixed negative electrode active material, the conductive material, and the binder were mixed in a solvent (water) in a weight ratio of 95:1:4. After mixing the third negative electrode active material and the fourth negative electrode active material in a weight ratio of 10:90 as a negative electrode slurry for the upper layer, the mixed negative electrode active material, conductive material, and binder were mixed in a solvent (water) in a weight ratio of 95:1:4. The above-mentioned manufactured lower layer negative electrode slurry was applied to aluminum foil (thickness: 20 μm) as a current collector, and then dried in a vacuum oven at 60°C for 1 hour to form a lower negative electrode active material layer. Thereafter, the above-mentioned manufactured upper layer negative electrode slurry was applied on the lower negative electrode active material layer, and then dried in a vacuum oven at 60°C for 1 hour to form an upper negative electrode active material layer. Thereafter, the resultant was dried in a vacuum oven at 120°C for 12 hours and then rolled (roll pressed) to manufacture a negative electrode for a sodium secondary battery. Finally, a negative electrode for a sodium secondary battery was manufactured in which a current collector, a lower negative electrode active material layer, and an upper negative electrode active material layer were sequentially arranged. At this time, the loading amount of the lower cathode active material layer was 90 mg / 25 cm 2 , the thickness was 100㎛, and the loading amount of the upper negative electrode active material layer was 90mg / 25cm. 2 , and the thickness was 100㎛. Examples 2-3 and Comparative Examples 1-5 A negative electrode was manufactured in the same manner as in Example 1, except that the type and weight ratio of the negative electrode active material included in the lower and upper negative electrode active material layers were controlled as shown in Tables 1 and 2 below. Lower negative electrode active material layer Upper negative electrode active material layer Weight ratio Loading amount (mg / 25cm) 2 )Thickness (㎛)Weight ratioLoading amount (mg / 25cm) 2 ) Thickness (㎛) First negative electrode active material Second negative electrode active material Third negative electrode active material Fourth negative electrode active material Example 1901090100109090100 Example 2703090100307090100 Example 3604090100406090100 Comparative example 1505090100505090100 Comparative example 2307090100505090100 Comparative example 3505090100703090100 Comparative example 4100090100010090100 Lower negative electrode active material layer Upper negative electrode active material layer Weight ratio Loading amount (mg / 25cm) 2 )Thickness (㎛)Weight ratioLoading amount (mg / 25cm) 2 ) Thickness (㎛) 3rd negative electrode active material 4th negative electrode active material 1st negative electrode active material 2nd negative electrode active material Comparative example 50 100 90 100 1000 90 100 Experimental Example 1: High-rate characteristic evaluation (Coin Half Shell Manufacturing) Coin half-cells of the examples and comparative examples were manufactured by inserting a separator (glass fiber, Whatman) between the positive electrode (Na metal) and the negative electrode manufactured in Examples 1 to 3 and Comparative Examples 1 to 5, and then injecting an electrolyte. The above electrolyte was used by mixing ethylene carbonate (EC) and propylene carbonate (PC) in a volume ratio of 5:5 in a non-aqueous electrolyte solvent, adding 2 wt% of fluoroethylene carbonate (FEC) to the solvent, and dissolving 1 M NaPF6. (High-rate characteristic evaluation) For the coin half cells of Examples 1 to 3 and Comparative Examples 1 to 5 manufactured in Experimental Example 1, the high-rate characteristics were evaluated by measuring the ratio of the 5C discharge capacity to the 0.1C discharge capacity. In detail, the coin half-cell was charged to 2.0 V with a constant current of 0.3 C at 25°C, and then discharged to 0 V at CCCV rates of 0.1 C and 5.0 C to measure the discharge capacity according to the rate conditions. The measured results are shown in Table 3 below. Experimental Example 2: Adhesion Evaluation For the negative electrodes manufactured in Examples 1 to 3 and Comparative Examples 1 to 5, the adhesive strength between the current collector and the lower negative electrode active material layer was measured. In detail, the cathodes manufactured in Examples 1 to 3 and Comparative Examples 1 to 5 were cut to a size of 150 mm in length and 20 mm in width, and the electrode surface was attached in the longitudinal direction to a slide glass measuring 75 mm in length and 25 mm in width using double-sided tape. That is, the slide glass was attached to an area corresponding to half of the length of the cathode. Then, a 2 kg roller was rubbed 10 times to ensure that the double-sided tape was evenly attached, thereby producing an evaluation sample. The slide glass portion of the evaluation sample was fixed to the sample stage of a universal testing machine (UTM), and the cathode half without the slide glass attached was connected to the load cell of the UTM equipment. The load cell was moved up to 50 mm at a speed of 100 mm / min and the load applied to the load cell was measured. At this time, the minimum value of the load measured in the 20 mm to 40 mm section of the moving section was measured as the electrode adhesion (gf / 20mm) of each sample. After a total of 5 evaluations for each cathode, the average value is shown in Table 3 below. Experimental Example 1 Experimental Example 25.0C discharge capacity (mAh / g)0.1C discharge capacity (mAh / g)5.0C discharge capacity / 0.1C discharge capacity (%)Adhesive strength (gf / 20mm)Example 116121674.754Example 215920976.250Example 315720576.748Comparative Example 114920672.440Comparative Example 216022172.335Comparative Example 316121674.743Comparative Example 414119472.536Comparative Example 511515474.428 Referring to Table 2 above, it can be seen that both the rate characteristics and the adhesive strength of Examples 1 to 3 are superior to those of Comparative Examples 1 to 5. Specifically, in the case of Comparative Example 1, it can be seen that both the rate characteristics and the adhesive strength are reduced because the weight ratios of the first and second negative electrode active materials included in the lower negative electrode active material layer are the same, and the weight ratios of the third and fourth negative electrode active materials included in the upper negative electrode active material layer are the same. In the case of Comparative Example 2, it can be seen that the amount of large-diameter negative electrode active material contained within the lower negative electrode active material layer is less than that of small-diameter negative electrode active material, resulting in a decrease in both the rate characteristics and the adhesive strength. In the case of Comparative Example 3, it can be seen that the amount of small-sized negative electrode active material contained within the upper negative electrode active material layer is less than that of large-sized negative electrode active material, resulting in a decrease in both the rate characteristics and the adhesive strength. In the case of Comparative Example 4, it can be seen that the lower negative electrode active material layer contains only a negative electrode active material with a large particle size, and the upper negative electrode active material layer contains only a negative electrode active material with a small particle size, resulting in both a decrease in rate characteristics and adhesive strength. In the case of Comparative Example 5, it can be seen that the lower negative electrode active material layer contains only a negative electrode active material with a small particle size, and the upper negative electrode active material layer contains only a negative electrode active material with a large particle size, resulting in a decrease in both the rate characteristics and the adhesive strength. [Explanation of symbols] 10: Cathode for sodium secondary battery 100: Whole house 210: Lower cathode active material layer 220: Upper cathode active material layer

Claims

1. A current collector; a lower negative electrode active material layer disposed on the current collector; and an upper negative electrode active material layer disposed on the lower negative electrode active material layer; The lower negative electrode active material layer comprises first negative electrode active material particles and second negative electrode active material particles, The upper negative electrode active material layer comprises third negative electrode active material particles and fourth negative electrode active material particles, The first negative electrode active material particles to the fourth negative electrode active material particles each contain hard carbon, The average particle diameter (D) of the first negative electrode active material particles 50 ) is the average particle diameter (D) of the second negative electrode active material particles. 50 ) is larger than the The average particle diameter (D) of the third negative electrode active material particles 50 ) is the average particle diameter (D) of the fourth negative electrode active material particles. 50 ) is larger than the The weight of the first negative electrode active material particle is greater than the weight of the second negative electrode active material particle, A negative electrode for a sodium secondary battery, wherein the weight of the third negative electrode active material particle is smaller than the weight of the fourth negative electrode active material particle.

2. In claim 1, A negative electrode for a sodium secondary battery, wherein the first negative active material particle to the fourth negative active material particle are each spherical.

3. In claim 1, A negative electrode for a sodium secondary battery, wherein the weight ratio of the first negative electrode active material particles and the second negative electrode active material particles is 51:49 to 99:

1.

4. In claim 1, The average particle diameter (D) of the first negative electrode active material particles relative to the second negative electrode active material particles 50 ) A negative electrode for a sodium secondary battery having a pH of 1.1 to 10.

5. In claim 1, The average particle diameter (D) of the first negative electrode active material particles50 ) is 5㎛ to 15㎛, The average particle diameter (D) of the second negative electrode active material particles 50 ) is a negative electrode for a sodium secondary battery having a thickness of 1 μm to 10 μm.

6. In claim 1, The above first negative electrode active material particles have a BET surface area of ​​1 m 2 / g to 10m 2 / g and, The above second negative electrode active material particles have a BET surface area of ​​1.5 m 2 / g to 12m 2 / g Anode for sodium secondary battery.

7. In claim 1, A negative electrode for a sodium secondary battery, wherein the thickness of the lower negative electrode active material layer is 30 ㎛ to 200 ㎛.

8. In claim 1, The loading amount of the lower negative electrode active material layer is 12.5 mg / 25 cm 2 250mg / 25cm 2 A negative electrode for a sodium secondary battery.

9. In claim 1, A negative electrode for a sodium secondary battery, wherein the weight ratio of the third negative electrode active material particles and the fourth negative electrode active material particles is 1:99 to 49:

51.

10. In claim 1, The average particle diameter (D) of the third negative electrode active material particles relative to the fourth negative electrode active material particles 50 ) A negative electrode for a sodium secondary battery having a pH of 1.1 to 10.

11. In claim 1, The average particle diameter (D) of the third negative electrode active material particles 50 ) is 3㎛ to 13㎛, The average particle diameter (D) of the fourth negative electrode active material particles 50 ) is a negative electrode for a sodium secondary battery having a thickness of 1 μm to 8 μm.

12. In claim 1, The above third negative electrode active material particles have a BET surface area of ​​1 m 2 / g to 10m 2 / g and, The above fourth negative electrode active material particles have a BET surface area of ​​2 m 2 / g to 14m 2 / g Anode for sodium secondary battery.

13. In claim 1, A negative electrode for a sodium secondary battery, wherein the thickness of the upper negative electrode active material layer is 30 ㎛ to 200 ㎛.

14. In claim 1, The loading amount of the upper negative electrode active material layer is 12.5 mg / 25 cm 2 250mg / 25cm 2 A negative electrode for a sodium secondary battery.

15. A sodium secondary battery comprising the negative electrode of claim 1.

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