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

Abstract

This invention relates to a negative electrode for a sodium secondary battery, the negative electrode comprising: 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, each of the first to fourth negative electrode active material particles comprising hard carbon, and the average particle size (D) of the first negative electrode active material particles is... 50 The average particle size (D) of the second negative electrode active material particles is greater than that of the second negative electrode active material particles. 50 The average particle size (D) of the third negative electrode active material particles 50 The average particle size (D) of the fourth negative electrode active material particles is greater than that of the fourth negative electrode active material particles. 50 The weight of the first negative electrode active material particle is greater than the weight of the second negative electrode active material particle, and the weight of the third negative electrode active material particle is less than the weight of the fourth negative electrode active material particle.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2023-0179484, filed on December 12, 2023, and Korean Patent Application No. 10-2024-0150135, filed on October 29, 2024, the disclosures of which are incorporated herein by reference. Technical Field

[0003] This invention relates to a negative electrode for a sodium secondary battery and a sodium secondary battery comprising the same. Background Technology

[0004] A rechargeable battery is a battery that can be repeatedly used through a discharge process that converts chemical energy into electrical energy and a recharge process that reverses this conversion. In recent years, with the commercialization of mobile electronic products, electric vehicles, and other applications, the demand for rechargeable batteries has increased dramatically. Currently, lithium-ion batteries are the most commercially available type of rechargeable battery. However, lithium reserves, as the primary raw material, are limited and expensive, and insufficient to meet the demand. Therefore, there is a need to develop a new type of rechargeable battery that can replace lithium-ion batteries. Recently, research and development have been actively underway on sodium-ion batteries, which use sodium instead of lithium.

[0005] However, because sodium ions are relatively large in size compared to lithium ions, they have low mobility and poor reactivity. Therefore, when using the negative electrode active material used in conventional lithium secondary batteries, the problem is that sodium secondary batteries cannot exhibit the capacity characteristics previously shown in lithium secondary batteries, or the capacity degrades sharply and its characteristics decline.

[0006] To address the aforementioned issues, a growing trend is to use hard carbon as the negative electrode active material for sodium-ion batteries. This is because hard carbon has a relatively large d-interval (dp) compared to graphite, which is commonly used in lithium-ion batteries. 002 Furthermore, it has a disordered porous structure, making it easy to store sodium ions, which have a larger radius than lithium ions.

[0007] However, when using a single-layer negative electrode active material layer containing hard carbon as the negative electrode active material, problems exist such as: poor output characteristics due to the low specific surface area of ​​hard carbon; and separation of the current collector from the negative electrode active material layer due to the low adhesion of hard carbon.

[0008] Therefore, in order to solve the above problems, it is necessary to develop a negative electrode for sodium secondary batteries, which has excellent adhesion between the current collector and the negative electrode active material layer while further improving the output characteristics. Summary of the Invention

[0009] Technical issues

[0010] One object of the present invention is to solve the above-mentioned problems by providing a negative electrode for sodium secondary batteries, which has excellent output characteristics and low resistance, and also has excellent adhesion between the current collector and the negative electrode active material layer, which can prevent separation between the current collector and the negative electrode active material layer.

[0011] Furthermore, another object of the present invention is to provide a sodium secondary battery comprising the above-described negative electrode for a sodium secondary battery.

[0012] Technical solution

[0013] [1] This invention provides a negative electrode for a sodium secondary battery, the negative electrode comprising: 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 contains first negative electrode active material particles and second negative electrode active material particles. The upper negative electrode active material layer contains third negative electrode active material particles and fourth negative electrode active material particles. The first to fourth negative electrode active material particles each contain hard carbon. The average particle size (D) of the first negative electrode active material particles 50 The particle size of the second negative electrode active material is greater than the average particle size (D). 50 ), The average particle size (D) of the third negative electrode active material particles 50 The average particle size (D) of the fourth negative electrode active material particles is greater than that of the fourth negative electrode active material particles. 50 ), The weight of the first negative electrode active material particles is greater than the weight of the second negative electrode 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.

[0014] [2] In the present invention described in [1] above, the first to fourth negative electrode active material particles may each have a spherical shape.

[0015] [3] In the present invention described in [1] or [2] above, the weight ratio of the first negative electrode active material particles to the second negative electrode active material particles can be from 51:49 to 99:1.

[0016] [4] In the present invention, in at least one of [1] to [3] above, the average particle size (D) of the first negative electrode active material particles and the second negative electrode active material particles is... 50 The ratio can be from 1.1 to 10.

[0017] [5] In the present invention of at least one of [1] to [4] above, the average particle size (D) of the first negative electrode active material particles 50 The particle size can be from 5 μm to 15 μm, and the average particle size (D) of the second negative electrode active material particles is... 50 The diameter can range from 1 μm to 10 μm.

[0018] [6] In the present invention, in at least one of [1] to [5] above, the BET specific surface area of ​​the first negative electrode active material particle can be 1 m². 2 / g to 10 m 2 / g, and the BET specific surface area of ​​the second negative electrode active material particles can be 1.5 m². 2 / g to 12 m 2 / g.

[0019] [7] In any one of the above [1] to [6] of the present invention, the thickness of the lower negative electrode active material layer may be from 30 μm to 200 μm.

[0020] [8] In at least one of [1] to [7] of the present invention, the loading of the lower negative electrode active material layer may be 12.5 mg / 25 cm⁻¹. 2 Up to 250 mg / 25 cm 2 .

[0021] [9] In the present invention of at least one of [1] or [8] above, the weight ratio of the third negative electrode active material particles to the fourth negative electrode active material particles may be from 1:99 to 49:51.

[0022]

[10] In the present invention of at least one of [1] to [9] above, the average particle size (D) of the third negative electrode active material particles and the fourth negative electrode active material particles is... 50 The ratio can be from 1.1 to 10.

[0023]

[11] In the present invention of at least one of [1] to

[10] above, the average particle size (D) of the third negative electrode active material particles 50 The particle size can range from 3 μm to 13 μm, and the average particle size (D) of the fourth negative electrode active material particles is... 50 The diameter can range from 1 μm to 8 μm.

[0024]

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

[11] above, the BET specific surface area of ​​the third negative electrode active material particles can be 1 m². 2 / g to 10 m 2 / g, and the BET specific surface area of ​​the fourth negative electrode active material particles can be 2 m².2 / g to 14 m 2 / g.

[0025]

[13] In the present invention of at least one of [1] to

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

[0026]

[14] In at least one of [1] to

[13] of the present invention, the loading of the upper negative electrode active material layer may be 12.5 mg / 25 cm⁻¹. 2 Up to 250 mg / 25 cm 2 .

[0027]

[15] The present invention provides a sodium secondary battery comprising a negative electrode for a sodium secondary battery of at least one of [1] to

[14] above.

[0028] Beneficial effects

[0029] The negative electrode for sodium secondary batteries according to the present invention is characterized by having a double-layer negative electrode active material layer disposed on the current collector, wherein the lower negative electrode active material layer contains a large number of negative electrode active material particles containing large-diameter hard carbon, and the upper negative electrode active material layer contains a large number of negative electrode active material particles containing small-diameter hard carbon. Furthermore, based on the double-layer structure of the negative electrode active material layer and the weight ratio of large-diameter to small-diameter negative electrode active materials contained in the lower and upper negative electrode active material layers, excellent output characteristics can be achieved, and the adhesion between the current collector and the negative electrode active material layer can also be improved.

[0030] Therefore, when the negative electrode for sodium secondary batteries according to the present invention is applied to a sodium secondary battery, a sodium secondary battery with excellent output characteristics and excellent structural stability can be realized. Attached Figure Description

[0031] The accompanying drawings in this specification illustrate preferred embodiments of the invention and, together with the foregoing description of the invention, serve to better understand the technical concept of the invention. Therefore, the invention should not be construed as being limited to the content described in the drawings. Furthermore, the shape, size, dimensions, proportions, etc., of the elements in the accompanying drawings may be exaggerated to more clearly emphasize the description.

[0032] Figure 1 This is a schematic side view showing the negative electrode according to one embodiment of the present invention. Detailed Implementation

[0033] The preferred embodiments of the present invention will be described below.

[0034] It should be understood that the terms or words used in the specification and claims should not be interpreted as having the meanings defined in common dictionaries, but rather should be further understood as having meanings consistent with their meanings in the context of the relevant field and the technical ideas of the invention, based on the inventor's ability to appropriately define the meanings of the terms or words to best interpret the principles of the invention.

[0035] The terminology used in this specification is for describing embodiments and is not intended to limit the invention. In this specification, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. The terms "comprising" and / or "including" as used in this specification do not exclude the presence or addition of more than one other constituent element.

[0036] In this invention, the average particle size (D) 50 The average particle size (D) refers to the particle size at 50% of the cumulative volume of the particle size distribution of the powder being tested. 50 Laser diffraction can be used to measure the particle size distribution. For example, after dispersing the powder to be tested in a dispersion medium, the dispersion medium is introduced into a commercially available laser diffractometer (e.g., Microtrac MT 3000) and irradiated with ultrasound at approximately 28 kHz at an output of 60 W. This yields a cumulative volume particle size distribution map, from which the particle size corresponding to 50% of the cumulative volume can be calculated to measure the average particle size (D). 50 ).

[0037] In this invention, the "specific surface area" can be measured by the BET method. Preferably, it can be calculated using the BELSORP-mino II from BEL Japan based on the amount of nitrogen absorbed at liquid nitrogen temperature (77 K).

[0038] In this invention, porosity can be calculated using the following mathematical formula A.

[0039] [Mathematical Expression A]

[0040] Porosity (%) = {1 - (actual density / true density)} × 100

[0041] In the above mathematical formula A, the real density is the density of a specific negative electrode active material layer containing pores, and the true density is the density of the solid (constituting the specific negative electrode active material layer) that does not contain 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 containing pores as the real density and the density of the solid (constituting the lower negative electrode active material layer) that does not contain pores as the true density. Similarly, 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 containing pores as the real density and the density of the solid (constituting the upper negative electrode active material layer) that does not contain 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 containing pores as the real density and the density of the solid (constituting the negative electrode active material layer) that does not contain pores as the true density.

[0042] Sodium secondary battery negative electrode

[0043] The negative electrode for a sodium secondary battery according to the present invention will be described below.

[0044] refer to Figure 1 The negative electrode 10 for a sodium secondary battery according to the present invention comprises: 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 comprises first negative electrode active material particles and second negative electrode active material particles, and the upper negative electrode active material layer 220 comprises third negative electrode active material particles and fourth negative electrode active material particles. Each of the first to fourth negative electrode active material particles comprises hard carbon. The average particle size (D) of the first negative electrode active material particles is... 50 The particle size of the second negative electrode active material is greater than the average particle size (D). 50 The average particle size (D) of the third negative electrode active material particles 50 The average particle size (D) of the fourth negative electrode active material particles is greater than that of the fourth negative electrode active material particles. 50 The weight of the first negative electrode active material particle is greater than the weight of the second negative electrode active material particle, and the weight of the third negative electrode active material particle is less than the weight of the fourth negative electrode active material particle.

[0045] In conventional sodium secondary batteries, a negative electrode active material containing hard carbon is used, and a single-layer negative electrode active material layer is applied. In this case, the small specific surface area of ​​hard carbon may lead to poor output characteristics, or the low adhesion of hard carbon may cause the current collector to separate from the negative electrode active material layer, resulting in a decline in the electrochemical characteristics of the battery.

[0046] As a result of continuous research to solve the above problems, the inventors discovered that when the negative electrode active material layer is disposed in a double layer on the current collector, the lower negative electrode active material layer contains a large number of negative electrode active material particles containing large-diameter hard carbon, and the upper negative electrode active material layer contains a large number of negative electrode active material particles containing small-diameter hard carbon. This can achieve excellent output characteristics and also improve the adhesion between the current collector and the negative electrode active material layer. Thus, the inventors completed the present invention.

[0047] For example, the 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.

[0048] (1) Current collector 100

[0049] There are no particular restrictions on the current collector 100, as long as it does not cause chemical changes in the battery and has high conductivity. Preferably, the current collector 100 can be made of: copper, stainless steel, aluminum, nickel, titanium, calcined carbon; copper or stainless steel with surface treatments such as carbon, nickel, titanium, or silver; aluminum-cadmium alloys, etc.

[0050] The thickness of the current collector 100 can typically be 3 to 500 μm.

[0051] The current collector 100 may also have fine irregularities on its surface to enhance the adhesion of the negative electrode active material. For example, the current collector 100 may be used in various forms, including films, sheets, foils, meshes, porous bodies, foams, nonwoven fabrics, etc.

[0052] (2) Lower negative electrode active material layer 210

[0053] In the following description, a preferred embodiment of the lower negative electrode active material layer 210 will be described.

[0054] The lower negative electrode active material layer 210 can be disposed on the current collector 100, preferably on at least one side of the current collector 100, more preferably on one or both sides 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.

[0055] 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 size (D) of the first negative electrode active material particles is... 50 The particle size of the second negative electrode active material is greater than the average particle size (D). 50 The first and second negative electrode active material particles each contain hard carbon.

[0056] The weight of the first negative electrode active material particles is greater than the weight of the second negative electrode active material particles. Preferably, the weight of the first negative electrode active material particles contained in the lower negative electrode active material layer 210 can be greater than the weight of the second negative electrode active material particles contained in the lower negative electrode active material layer 210.

[0057] For example, the lower negative electrode active material layer 210, comprising large-diameter first negative electrode active material particles and small-diameter second negative electrode active material particles, may be a layer in direct contact with the current collector 100. In the case of sodium ion insertion / deintercalation, separation between the negative electrode active material layer and the current collector 100 may occur due to the volume expansion and contraction of the negative electrode active material. To address this issue, compared to the case of the upper negative electrode active material layer 220 described later, the lower negative electrode active material layer 210 according to the invention contains large-diameter first negative electrode active material particles at a weight greater than the weight of the small-diameter second negative electrode active material particles. Because the lower negative electrode active material layer 210 contains large-diameter first negative electrode active material particles with a weight greater than that of the small-diameter second negative electrode active material particles, the adhesive contained in the lower negative electrode active material layer 210 can bond to the current collector 100 better than the combination of the first and second negative electrode active material particles. This results in excellent adhesion between the current collector 100 and the negative electrode active material layer and prevents separation between the negative electrode active material layer and the current collector 100, thereby improving the overall structural stability of the negative electrode.

[0058] On the other hand, in order to achieve high adhesion to the current collector 100, if only large-diameter first negative electrode active material particles 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 may be large. Therefore, the transfer of sodium ions may be poor, resulting in a decrease in output characteristics. Furthermore, because the pore size is increased, the electrode thickness may increase, leading to a decrease in the energy density per unit volume of the battery. Therefore, the negative electrode 10 for sodium secondary batteries according to the present invention is characterized by the fact that because the lower negative electrode active material layer 210 contains small-diameter second negative electrode active material particles at a weight less than the weight of the large-diameter first negative electrode active material particles, the output characteristics are improved. In addition, because the large-diameter first negative electrode active material particles are contained at a weight greater than the weight of the small-diameter second negative electrode active material particles, the adhesion characteristics are maximized.

[0059] Furthermore, since the first and second negative electrode active material particles each contain hard carbon, they can easily store sodium ions with a radius relatively larger than that of lithium ions, thus achieving superior discharge capacity characteristics.

[0060] The first and second negative electrode active material particles can each have a spherical shape. When each particle has a spherical shape, due to the characteristics of the spherical structure, the negative electrode active material particles can be stacked in close contact with each other, thus achieving a high-density negative electrode, thereby improving the energy density per unit volume of the battery.

[0061] Preferably, the sphericity of each of the first and second negative electrode active material particles can be independently from 0.8 to 1, more preferably from 0.85 to 1.0, and even more preferably from 0.9 to 1. When the sphericity falls within the above range, it is preferred because excellent ion conductivity can be achieved while realizing a high-density negative electrode.

[0062] Sphericity can be defined as the value obtained by dividing the circumference of a circle with the same area as the projected image of the negative electrode active material particles by the circumference of the projected image of the negative electrode active material particles. Preferably, sphericity can be defined by the following mathematical formula B.

[0063] [Mathematical Expression B]

[0064] Sphericity = (Circumference of a circle with the same area as the projected image of the negative electrode active material particles) / (Circumference of the projected image of the negative electrode active material particles)

[0065] Sphericity can be measured using a particle analyzer, such as the Sysmex FPIA3000 (manufactured by Mavern). According to the present invention, sphericity can be defined as the average sphericity value of 10 randomly selected particles from the negative electrode active material.

[0066] The weight ratio of the first negative electrode active material particles to the second negative electrode active material particles can be from 51:49 to 99:1, preferably from 55:45 to 95:5, and more preferably from 60:40 to 90:10. When the ratio falls within the above range, the amount of large-diameter negative electrode active material particles contained in the lower negative electrode active material layer 210 can be increased, thereby preventing separation between the current collector 100 and the negative electrode active material layer. Moreover, the content of small-diameter negative electrode active material particles can be appropriate, thereby providing excellent output characteristics.

[0067] The average particle size (D) of the first negative electrode active material particles and the second negative electrode active material particles 50 The ratio can be from 1.1 to 10. Preferably, the average particle size (D) of the first negative electrode active material particles to the second negative electrode active material particles is... 50The ratio can be 1.1 or higher, 1.15 or higher, 1.20 or higher, 1.25 or higher, or 1.30 or higher, and can be 10 or lower, 8 or lower, 7 or lower, 6 or lower, 5 or lower, 4.5 or lower, 4 or lower, 3.5 or lower, 3.0 or lower, or 2.5 or lower. More preferably, the average particle size (D) of the first negative electrode active material particles and the second negative electrode active material particles... 50 The ratio can be from 1.1 to 2.5. When the ratio falls within the above range, it is preferred because the filling rate between the particles of the negative electrode active material can be increased, thereby achieving a high-density electrode.

[0068] The average particle size (D) of the first negative electrode active material particles 50 The particle size can be from 5 μm to 15 μm. Preferably, the average particle size (D) of the first negative electrode active material particles is... 50 The particle size can be 5 μm or larger, 6 μm or larger, 7 μm or larger, or 8 μm or larger, and can be 14 μm or smaller, 13 μm or smaller, 12 μm or smaller, or 11 μm or smaller. More preferably, the average particle size (D) of the first negative electrode active material particles is... 50 The average particle size can be from 8 μm to 11 μm. When the average particle size falls within the above range, the contact area between the adhesive and the current collector 100 at the interface between the current collector 100 and the lower negative electrode active material layer 210 can be increased more than the contact area between the adhesive and the negative electrode active material particles, thereby improving the adhesion characteristics of the electrode.

[0069] The average particle size (D) of the second negative electrode active material particles 50 The particle size can be from 1 μm to 10 μm. Preferably, the average particle size (D) of the second negative electrode active material particles is... 50 The particle size can be 1 μm or larger, 2 μm or larger, 3 μm or larger, 4 μm or larger, or 4.5 μm or larger, and can be less than 10 μm, less than 9 μm, less than 8.5 μm, or less than 8 μm. More preferably, the average particle size (D) of the second negative electrode active material particles is... 50 The particle size can range from 4.5 μm to 8 μm. When the particle size falls within this range, the ionic and electronic conductivity of the negative electrode active material particles can be improved, thereby improving the output characteristics.

[0070] The BET specific surface area of ​​the first negative electrode active material particles can be 1 m². 2 / g to 10 m 2 / g. Preferably, the BET specific surface area of ​​the first negative electrode active material particles can be 1 m². 2 / g or more, 1.5 m 2 / g or more or 2 m 2 / g or more, and can be 10 m2 / g or less, 9 m 2 / g or less, 8 m 2 / g or less, 7 m 2 / g or less, 6 m 2 / g or less, 5 m 2 / g or less or 4 m 2 / g or less. More preferably, the BET specific surface area of ​​the first negative electrode active material particles can be 2 m². 2 / g to 4 m 2 / g. When the BET specific surface area falls within the above range, the side reactions between the negative electrode active material particles and the electrolyte are not excessive, thereby improving the battery's lifespan characteristics, and therefore it is preferred.

[0071] The BET specific surface area of ​​the second negative electrode active material particles can be 1.5 m². 2 / g to 12 m 2 / g. Preferably, the BET specific surface area of ​​the second negative electrode active material particles can be 1.5 m². 2 / g or more, 2 m 2 / g or more, 2.5 m 2 / g or more or 3 m 2 / g or more, and can be 12 m 2 / g or less, 11 m 2 / g or less, 10 m 2 / g or less, 9 m 2 / g or less, 8 m 2 / g or less, 7 m 2 / g or less, 6 m 2 / g or less or 5 m 2 / g or less. More preferably, the BET specific surface area of ​​the second negative electrode active material particles can be 3m². 2 / g to 5 m 2 / g. When the BET specific surface area falls within the above range, the side reactions between the negative electrode active material particles and the electrolyte are not excessive, thereby improving the battery's lifespan characteristics, and therefore it is preferred.

[0072] The thickness of the lower negative electrode active material layer 210 can be from 30 μm to 200 μm. Preferably, the thickness of the lower negative electrode active material layer 210 can be 30 μm or more, 40 μm or more, 50 μm or more, 60 μm or more, or 70 μm or more, and can be less than 200 μm, 180 μm or less, 160 μm or less, 140 μm or less, or 130 μm or less. More preferably, the thickness of the lower negative electrode active material layer 210 can be from 60 μm to 130 μm.

[0073] The loading of the lower negative electrode active material layer 210 can be 12.5 mg / 25 cm⁻¹. 2 Up to 250 mg / 25 cm 2 Preferably, the loading of the lower negative electrode active material layer 210 can be 12.5 mg / 25 cm⁻¹. 2 Above, 17.5 mg / 25 cm 2 Above, 22.5 mg / 25 cm 2 Above, 37.5 mg / 25 cm 2 Above, 42.5 mg / 25 cm 2 Above, 50 mg / 25 cm 2 Above, 55mg / 25 cm 2 Above or 60 mg / 25 cm 2 The above, and can be 250 mg / 25 cm 2 Below, 200 mg / 25 cm 2 Below, 175 mg / 25 cm 2 Below, 150 mg / 25 cm 2 Below or 125 mg / 25 cm 2 More preferably, the loading of the lower negative electrode active material layer 210 can be 60 mg / 25 cm⁻¹. 2 Up to 125 mg / 25 cm 2 .

[0074] The porosity of the lower negative electrode active material layer 210 can be from 20% to 70%. Preferably, the porosity of the lower negative electrode active material layer 210 can be 23% or more, 25% or more, 27% or more, 30% or more, 33% or more, or 35% or more, and can be less than 70%, less than 65%, less than 60%, or less than 55%. More preferably, the porosity of the lower negative electrode active material layer 210 can be from 35% to 55%. The porosity can be calculated using the aforementioned mathematical formula A. When the porosity falls within the above range, the energy density per unit volume of the battery can be increased, and the contact between active material particles can become easier, thereby improving the output characteristics.

[0075] In the lower negative electrode active material layer 210, the content of the first negative electrode active material particles and the second negative electrode active material particles can be from 60% to 99% by weight, preferably from 75% to 95% by weight.

[0076] The lower negative electrode active material layer 210 may also 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.

[0077] The first adhesive can be used for bonding between the first negative electrode active material particles and / or the second negative electrode active material particles, or for adhesion between the lower negative electrode active material layer 210 and the current collector 100. For example, the first adhesive may contain at least one of the following substances: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and materials whose hydrogen is replaced by Li, Na, Ca, etc., or may also contain various copolymers thereof.

[0078] In the lower negative electrode active material layer 210, the content of the first binder can be from 0.5% to 10% by weight, preferably from 1% to 5% by weight.

[0079] There are no particular restrictions on the first conductive material, as long as it does not cause a chemical change in the relevant battery and has conductivity. Examples of materials that can be used include: graphite such as natural or artificial graphite; carbon black materials, including carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, etc.; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders, including aluminum powder, nickel powder, etc.; conductive whiskers, including zinc oxide, potassium titanate, etc.; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0080] In the lower negative electrode active material layer 210, the content of the first conductive material can be from 0.5% to 10% by weight, preferably from 1% to 5% by weight.

[0081] (3) Upper negative electrode active material layer 220

[0082] The upper negative electrode active material layer 220 will be described below.

[0083] An 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 surface of the lower negative electrode active material layer 210 opposite to the surface on which the current collector 100 contacts. More preferably, the upper negative electrode active material layer 220 may exist on the outermost surface of the negative electrode. Furthermore, the upper negative electrode active material layer 220 may exist on the outermost surface of the negative electrode and may face the separator described later.

[0084] Furthermore, when the lower negative electrode active material layer 210 exists on one side and the other side of the current collector 100, the upper negative electrode active material layer 220 can be disposed on the surface of each of the lower negative electrode active material layers 210 disposed on one side and the other side of the current collector 100.

[0085] The upper negative electrode active material layer 220 includes third and fourth negative electrode active material particles. The average particle size (D) of the third negative electrode active material particles is... 50 The average particle size (D) of the fourth negative electrode active material particles is greater than that of the fourth negative electrode active material particles. 50 The third and fourth anode active material particles each contain hard carbon.

[0086] The weight of the third negative electrode active material particles is less than the weight of the fourth negative electrode active material particles. Preferably, the weight of the third negative electrode active material particles contained in the upper negative electrode active material layer 220 can be less than the weight of the fourth negative electrode active material particles contained in the upper negative electrode active material layer 220.

[0087] Preferably, the upper negative electrode active material layer 220, comprising large-diameter third negative electrode active material particles and small-diameter fourth negative electrode active material particles, can exist on the outermost surface of the negative electrode, at a location where sodium ions are directly transferred from the positive electrode. Compared to the aforementioned lower negative electrode active material layer 210, because the upper negative electrode active material layer 220 contains large-diameter third negative electrode active material particles at a weight less than that of the small-diameter fourth negative electrode active material particles, the pores formed by the active material particles in the upper negative electrode active material layer 220 can be minimized, and sodium ions can be uniformly transferred, thus improving the overall output characteristics of the negative electrode.

[0088] On the other hand, in order to achieve excellent output characteristics, if the upper negative electrode active material layer 220 contains only small-diameter fourth negative electrode active material particles, the formation of an SEI film on the surface of the small-diameter negative electrode active material particles with a large specific surface area may accelerate electrolyte decomposition, thereby reducing the electrochemical characteristics of the battery. Furthermore, it may be necessary to increase the amount of binder to prevent electrode delamination, which could lead to increased electrolyte side reactions and a decrease in the energy density per unit volume of the battery. Therefore, the negative electrode 10 for sodium secondary batteries according to the present invention is characterized by improved adhesion characteristics because the upper negative electrode active material layer 220 contains large-diameter third negative electrode active material particles at a weight less than the weight of the small-diameter fourth negative electrode active material particles. Moreover, because it contains small-diameter fourth negative electrode active material particles at a weight greater than the weight of the large-diameter third negative electrode active material particles, the output characteristics can be maximized.

[0089] Furthermore, since the third and fourth negative electrode active material particles each contain hard carbon, they can easily store sodium ions with a radius larger than that of lithium ions, thereby achieving excellent discharge capacity characteristics.

[0090] The third and fourth negative electrode active material particles can each have a spherical shape. When each particle has a spherical shape, due to the characteristics of the spherical structure, the negative electrode active material particles can be stacked in close contact with each other, thus achieving a high-density negative electrode, thereby improving the energy density per unit volume of the battery.

[0091] Specifically, the sphericity of the third and fourth anode active material particles can be independently from 0.8 to 1, preferably from 0.85 to 1.0, and more preferably from 0.9 to 1. When the sphericity falls within the above range, it is preferred because excellent ion conductivity can be achieved while realizing a high-density anode.

[0092] Sphericity can be defined as the value obtained by dividing the circumference of a circle with the same area as the projected image of the negative electrode active material particles by the circumference of the projected image of the negative electrode active material particles. Preferably, sphericity can be defined by the aforementioned mathematical formula B.

[0093] The weight ratio of the third negative electrode active material particles to the fourth negative electrode active material particles can be from 1:99 to 49:51, preferably from 5:95 to 45:55, and more preferably from 10:90 to 40:60. When the ratio falls within the above range, the upper negative electrode active material layer 220 can contain an appropriate amount of large-particle-size negative electrode active material particles, thereby preventing separation between the upper negative electrode active material layer 220 and the lower negative electrode active material layer 210. Furthermore, because it can contain a large amount of small-particle-size negative electrode active material, excellent output characteristics can be achieved.

[0094] The average particle size (D) of the third and fourth negative electrode active material particles is... 50 The ratio can be from 1.1 to 10. Preferably, the average particle size (D) of the third negative electrode active material particles to the fourth negative electrode active material particles is... 50 The ratio can be 1.15 or higher, 1.20 or higher, 1.25 or higher, or 1.30 or higher, and can be 10 or lower, 8 or lower, 7 or lower, 6 or lower, 5 or lower, 4.5 or lower, 4 or lower, 3.5 or lower, 3.0 or lower, or 2.5 or lower. More preferably, the average particle size (D) of the third negative electrode active material particles and the fourth negative electrode active material particles... 50 The ratio can be from 1.1 to 2.5. When the ratio falls within the above range, it is preferred because the filling rate between the particles of the negative electrode active material can be increased, thereby achieving a high-density electrode.

[0095] The average particle size (D) of the third negative electrode active material particles 50 The average particle size (D) of the third negative electrode active material particles can be from 3 μm to 13 μm. Preferably, the average particle size (D) of the third negative electrode active material particles is... 50 The particle size can be 3 μm or larger, 3.5 μm or larger, 4 μm or larger, 4.5 μm or larger, 5.0 μm or larger, or 5.5 μm or larger, and can be less than 13 μm, less than 12 μm, less than 11 μm, less than 10 μm, less than 9 μm, less than 8 μm, or less than 7 μm. More preferably, the average particle size (D) of the third negative electrode active material particles is... 50 The average particle size can be from 5.5 μm to 7 μm. When the average particle size falls within the above range, the adhesion between the negative electrode active material particles can be improved, thereby improving the electrode adhesion characteristics.

[0096] The average particle size (D) of the fourth negative electrode active material particles 50 The average particle size (D) of the fourth negative electrode active material particles can be from 1 μm to 8 μm. Preferably, the average particle size (D) of the fourth negative electrode active material particles is... 50 The particle size can be 1 μm or larger, 1.5 μm or larger, 2 μm or larger, or 2.5 μm or larger, and can be 8 μm or smaller, 7 μm or smaller, 6 μm or smaller, or 5 μm or smaller. More preferably, the average particle size (D) of the fourth negative electrode active material particles is... 50 The average particle size can range from 1 μm to 5 μm. When the average particle size falls within the above range, the ionic and electronic conductivity of the negative electrode active material particles can be improved, thereby improving the output characteristics.

[0097] The BET specific surface area of ​​the third negative electrode active material particles can be 1 m². 2 / g to 10 m 2 / g. Preferably, the BET specific surface area of ​​the third negative electrode active material particles can be 1 m². 2 / g or more, 1.5 m 2 / g or more, 2 m 2 / g or more, 2.5 m 2 / g or more, 3 m 2 / g or more or 3.5 m 2 / g or more, and can be 10 m 2 / g or less, 9 m 2 / g or less, 8 m 2 / g or less, 7 m 2 / g or less, 6 m 2 / g or less or 5 m 2 / g or less. More preferably, the BET specific surface area of ​​the third negative electrode active material particles can be 3m². 2 / g to 5 m 2 / g. When the BET specific surface area falls within the above range, the side reactions between the negative electrode active material particles and the electrolyte are not excessive, thereby improving the battery's lifespan characteristics, and therefore it is preferred.

[0098] The BET specific surface area of ​​the fourth negative electrode active material particles can be 2 m². 2 / g to 14 m 2 / g. Preferably, the BET specific surface area of ​​the fourth negative electrode active material particles can be 2 m². 2 / g or more, 2.5 m 2 / g or more, 3 m 2 / g or more, 3.5 m 2 / g or more or 4 m 2 / g or more, and can be 14 m 2 / g or less, 12 m 2 / g or less, 10 m 2 / g or less, 9 m 2 / g or less, 8 m 2 / g or less, 7 m 2 / g or less, 6 m 2 / g or less or 5.5 m 2 / g or less. More preferably, the BET specific surface area of ​​the fourth negative electrode active material particles can be 4 m². 2 / g to 5.5 m 2 / g. When the BET specific surface area falls within the above range, the side reactions between the negative electrode active material particles and the electrolyte are not excessive, thereby improving the battery's lifespan characteristics, and therefore it is preferred.

[0099] The thickness of the upper negative electrode active material layer 220 can be from 30 μm to 200 μm. Preferably, the thickness of the upper negative electrode active material layer 220 can be 30 μm or more, 40 μm or more, 50 μm or more, 60 μm or more, or 70 μm or more, and can be less than 200 μm, 180 μm or less, 160 μm or less, 140 μm or less, or 130 μm or less. More preferably, the thickness of the upper negative electrode active material layer 220 can be from 60 μm to 130 μm.

[0100] The loading of the upper negative electrode active material layer 220 can be 12.5 mg / 25 cm⁻¹. 2 Up to 250 mg / 25 cm 2 Preferably, the loading of the upper negative electrode active material layer 220 can be 12.5 mg / 25 cm⁻¹. 2 Above, 17.5 mg / 25 cm 2 Above, 22.5 mg / 25 cm2 Above, 37.5 mg / 25 cm 2 Above, 42.5 mg / 25 cm 2 Above, 50 mg / 25 cm 2 Above, 55mg / 25 cm 2 Above or 60 mg / 25 cm 2 The above, and can be 250 mg / 25 cm 2 Below, 200 mg / 25 cm 2 Below, 175 mg / 25 cm 2 Below, 150 mg / 25 cm 2 Below or 125 mg / 25 cm 2 More preferably, the loading of the upper negative electrode active material layer 220 can be 60 mg / 25 cm⁻¹. 2 Up to 125 mg / 25 cm 2 .

[0101] The porosity of the upper negative electrode active material layer 220 can be from 20% to 70%. Preferably, the porosity of the upper negative electrode active material layer 220 can be 23% or more, 25% or more, 27% or more, 30% or more, 33% or more, or 35% or more, and can be less than 70%, less than 65%, less than 60%, or less than 55%. More preferably, the porosity of the upper negative electrode active material layer 220 can be from 35% to 55%. The porosity can be calculated using the aforementioned mathematical formula A. When the porosity falls within the above range, the energy density per unit volume of the battery can be increased, and the contact between active material particles can become easier, thereby improving the output characteristics.

[0102] The thickness ratio of the lower negative electrode active material layer 210 to the upper negative electrode active material layer 220 can be 1:5 to 5:1, preferably 1:4 to 4:1, and more preferably 1:3 to 3:1.

[0103] The sum of the loading amounts of the lower negative electrode active material layer 210 and the upper negative electrode active material layer 220 can be 25 mg / 25 cm⁻¹. 2 Up to 500 mg / 25 cm 2 Preferably 100 mg / 25 cm 2 Up to 350 mg / 25 cm 2 More preferably 120 mg / 25 cm 2 Up to 250 mg / 25 cm 2 When the sum of the loads falls within the above range, the energy density per unit volume of the battery can be increased, and the resistance can be low, resulting in excellent output characteristics.

[0104] In the upper negative electrode active material layer 220, the content of the third negative electrode active material particles and the fourth negative electrode active material particles can be from 60% to 99% by weight, preferably from 75% to 95% by weight.

[0105] The upper negative electrode active material layer 220 may also 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.

[0106] The second adhesive can be used for bonding between the third and / or 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 adhesive may contain at least one of the following substances: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and materials whose hydrogen is replaced by Li, Na, Ca, etc., and may also contain various copolymers thereof.

[0107] In the upper negative electrode active material layer 220, the content of the second binder can be from 0.5% to 10% by weight, preferably from 1% to 5% by weight.

[0108] There are no particular restrictions on the second conductive material, as long as it does not cause a chemical change in the relevant battery and has conductivity. Examples of materials that can be used include: graphite such as natural or artificial graphite; carbon black materials, including carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, etc.; conductive fibers such as carbon fiber or metal fiber; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders, including aluminum powder, nickel powder, etc.; conductive whiskers, including zinc oxide, potassium titanate, etc.; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives, etc.

[0109] In the upper negative electrode active material layer 220, the content of the second conductive material can be from 0.5% to 10% by weight, preferably from 1% to 5% by weight.

[0110] A lower negative electrode active material layer 210 can be prepared by coating a lower layer negative electrode slurry containing first and second negative electrode active material particles, as well as a selective first binder, a first conductive material, and / or a solvent for forming the negative electrode slurry onto the current collector 100 and drying it; alternatively, the lower layer negative electrode slurry can be cast on a separate carrier, and then the film layer separated from the carrier can be stacked onto the current collector 100 to prepare the lower negative electrode active material layer 210. The upper negative electrode active material layer 220 can also be prepared by preparing an upper layer negative electrode slurry in the same way as described above, except that third and fourth negative electrode active material particles and a selective second binder and / or a second conductive material are used.

[0111] In promoting the dispersion of, for example, negative electrode active material particles, binders, and / or conductive materials, the solvent for forming the negative electrode slurry may contain at least one selected from distilled water, N-methyl-2-pyrrolidone (NMP), ethanol, methanol, and isopropanol, preferably distilled water. The solid content of the negative electrode slurry may be from 30% to 80% by weight, preferably from 40% to 70% by weight.

[0112] There are no particular limitations on the preparation of the negative electrode 10 for sodium secondary batteries (wherein the negative electrode active material layer comprises a lower negative electrode active material layer 210 and an upper negative electrode active material layer 220), as long as it can achieve the lower negative electrode active material layer 210 and the upper negative electrode active material layer 220 having the aforementioned characteristics. For example, first negative electrode active material particles and second negative electrode active material particles, as well as a first binder and / or a first conductive material, can be added to a solvent (e.g., distilled water) for forming a negative electrode slurry to prepare a lower layer negative electrode slurry. Third negative electrode active material particles and fourth negative electrode active material particles, as well as a second binder and / or a second conductive material, can be added to a solvent (e.g., distilled water) to prepare an upper layer negative electrode slurry. These slurries can then be coated onto a current collector 100 to prepare the negative electrode 10 for sodium secondary batteries according to the present invention. More preferably, the lower layer negative electrode slurry prepared as described above is coated onto the current collector 100, and then pressed and dried to form a lower negative electrode active material layer 210. The upper layer negative electrode slurry prepared as described above can be coated onto the lower negative electrode active material layer 210, and then pressed and dried to form an upper negative electrode active material layer 220, thereby preparing the negative electrode 10 for a sodium secondary battery according to the present invention. On the other hand, while coating the lower layer negative electrode slurry onto the current collector 100, the upper layer negative electrode slurry is substantially simultaneously coated onto the lower layer negative electrode slurry, and both are simultaneously pressed and dried, thereby preparing the negative electrode 10 for a sodium secondary battery according to the present invention.

[0113] The porosity of the negative electrode 10 for a sodium secondary battery can be from 20% to 70%. Preferably, the porosity of the negative electrode 10 for a sodium secondary battery can be 20% or more, 23% or more, 25% or more, 30% or more, 33% or more, or 35% or more, and can be 70% or less, 65% or less, 60% or less, or 55% or less. More preferably, the porosity of the negative electrode 10 for a sodium secondary battery can be from 35% to 55%. When the porosity falls within the above range, the energy density per unit volume of the battery can be increased, and the resistance can be low, so that the output characteristics can be excellent.

[0114] Sodium secondary battery

[0115] Hereinafter, the sodium secondary battery according to the present invention will be described.

[0116] The sodium secondary battery according to the present invention includes the aforementioned negative electrode 10 for a sodium secondary battery according to the present invention. More preferably, the sodium secondary battery according to the present invention includes the negative electrode 10 for a sodium secondary battery according to the present invention, a positive electrode opposite to the negative electrode 10 for a sodium secondary battery, a separator provided between the negative electrode 10 for a sodium secondary battery and the positive electrode, and an electrolyte. Since the negative electrode 10 for a sodium secondary battery has been described previously, other components will be described hereinafter.

[0117] (Positive electrode)

[0118] The positive electrode can be opposite to the negative electrode for a sodium secondary battery according to the present invention.

[0119] The positive electrode can include a positive electrode current collector and a positive electrode active material layer located on at least one side of the positive electrode current collector.

[0120] There is no particular limitation on the positive electrode current collector as long as it does not cause a chemical change in the battery and has high conductivity. Preferably, the positive electrode current collector can include at least one selected from copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and an aluminum-cadmium alloy. Preferably, it can include aluminum.

[0121] The thickness of the positive electrode current collector can generally be 3 to 500 μm.

[0122] The positive electrode current collector can also have fine concavities and convexities on its surface to enhance the binding force of the positive electrode active material. For example, the positive electrode current collector can be used in various forms including a film, sheet, foil, net, porous body, foam body, non-woven fabric body, etc.

[0123] The positive electrode active material layer can include a positive electrode active material.

[0124] As the positive electrode active material, a compound capable of reversibly inserting and extracting sodium can be used. Specific examples can include: sodium metal oxides (where 0 < x ≤ 1), such as Na xCoO2, 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 V₂O₅; sodium metal phosphates such as Na₃V₂(PO₄)₃, NaFePO₄, NaMn 0.5 Fe 0.5 PO4, Na3V2(PO4)3, Na3Fe2(PO4)3; sodium metal fluorophosphates such as Na2FePO4F or Na3V2(PO4)3; sodium metal fluoride sulfur oxides such as NaFeSO4F; sodium-transition metal complex oxides 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; chalcogenide compounds such as TiS2, ZrS2, VS2, V2S2, TaS2, FeS2, or NiS2, and any one or a mixture of two or more thereof can be used. Among these, Fe-containing compounds can inhibit the dissolution of transition metal ions even when the electrolyte temperature rises within the battery, thus improving the cycle characteristics and discharge capacity retention of sodium secondary batteries. Furthermore, chalcogenide compounds such as TiS2 and ZrS2 can rapidly insert and deintercalate sodium ions, and when used in combination with metallic sodium or alloy-type anode active materials, they can insert and deintercalate sodium ions at a higher level than the anode, thus providing further improved reactivity.

[0125] Considering that the positive electrode active material exhibits sufficient capacity, the content of the positive electrode active material in the positive electrode active material layer can be from 80% to 99% by weight, preferably from 92% to 98.5% by weight.

[0126] The positive electrode active material layer may also include an adhesive and / or conductive material together with the aforementioned positive electrode active material.

[0127] The adhesive is a component that assists in the bonding between active materials and conductive materials, as well as the bonding to current collectors, and may preferably contain at least one selected from the following substances: polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, and may preferably contain polyvinylidene fluoride.

[0128] In order to ensure sufficient bonding between components such as positive electrode active materials, the content of binder in the positive electrode active material layer can be from 1% to 20% by weight, preferably from 1.2% to 10% by weight.

[0129] Conductive materials can be used to assist and improve the conductivity of secondary batteries, and there are no particular limitations, as long as they do not cause chemical changes and are conductive. Preferably, the conductive material may contain at least one selected from the following substances: graphite such as natural graphite or artificial graphite; carbon black materials, including carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, etc.; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders, including aluminum powder, nickel powder, etc.; conductive whiskers, including zinc oxide, potassium titanate, etc.; conductive metal oxides such as titanium oxide; and polyphenylene derivatives, and carbon black may be preferred for improving conductivity.

[0130] To ensure sufficient conductivity, the content of conductive material in the positive electrode active material layer can be from 1% to 20% by weight, preferably from 1.2% to 10% by weight.

[0131] The thickness of the positive electrode active material layer can be from 30 μm to 400 μm, preferably from 50 μm to 110 μm.

[0132] A positive electrode slurry, comprising a positive electrode active material, a selective binder and a conductive material, and a solvent for forming the positive electrode slurry, can be coated onto a positive electrode current collector, and then dried and pressed to prepare the positive electrode.

[0133] The solvent for forming the positive electrode slurry may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and its amount may be such that the positive electrode active material, as well as selective binders and conductive materials, can be contained at a desired viscosity. For example, the solvent for forming the positive electrode slurry may be included in the positive electrode slurry such that the concentration of the solids containing the positive electrode active material, as well as selective binders and conductive materials, is 50% to 95% by weight, preferably 70% to 90% by weight.

[0134] (Septum)

[0135] The separator according to the present invention separates the negative and positive electrodes and provides a path for sodium ions to move. Any separator commonly used in secondary batteries can be used, and there are no particular limitations on the type. For example, as a separator, a porous polymer membrane prepared from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure having two or more layers, can be used. Alternatively, a porous nonwoven fabric formed from high-melting-point glass fiber, polyethylene terephthalate fiber, etc., can also be used. Furthermore, to ensure heat resistance or mechanical strength, a coated separator containing ceramic components or polymer materials can be used, and it can be selectively used in a single-layer or multi-layer structure.

[0136] (electrolytes)

[0137] The electrolyte according to the present invention may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which can be used to prepare sodium secondary batteries, and the types are not limited thereto.

[0138] Preferably, the electrolyte may contain an organic solvent and a sodium salt.

[0139] As an organic solvent, any organic solvent can be used without particular limitation, as long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Preferably, the following organic solvents can be used: ester solvents, including methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone, etc.; ether solvents, including dibutyl ether, tetrahydrofuran, etc.; ketone solvents, including cyclohexanone, etc.; aromatic hydrocarbon solvents, including benzene, fluorobenzene, etc.; carbonate solvents, including dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), etc.; alcohol solvents, including ethanol, isopropanol, etc.; nitriles, including R-CN (where R is a linear, branched, or cyclic C2 to 20 hydrocarbon group, and may contain double bonds, aromatic rings, or ether bonds), etc.; amides, including dimethylformamide, etc.; dioxolane, including 1,3-dioxolane, etc.; or sulfolane.

[0140] For the sodium salt, any compound capable of providing sodium ions used in sodium secondary batteries can be used without particular limitation. Preferably, NaPF6, NaClO4, NaAsF6, NaBF4, NaCF3SO3, NaB(C6H5)4, NaC4F9SO3, NaN(C2F5SO3)2, NaN(C2F5SO2)2, NaN(CF3SO2)2, etc., can be used as the sodium salt. The sodium salt can be used in a concentration range of 0.1 to 2.0 M.

[0141] To improve battery life characteristics, suppress battery capacity decline, and improve battery discharge capacity, the electrolyte may contain at least one additive in addition to the electrolyte components. Examples of additives include: alkylene carbonate halide compounds such as fluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphoric triamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted sulfadiazine ketones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive content can be from 0.1 to 5% by weight, based on the total weight of the electrolyte.

[0142] Sodium secondary batteries containing the aforementioned sodium secondary battery negative electrode according to the present invention can be applied in the fields of mobile devices such as mobile phones, laptops and digital cameras; and in the fields of electric vehicles such as hybrid electric vehicles (HEVs).

[0143] The embodiments of the present invention will be described in detail below to enable those skilled in the art to readily implement the invention. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein.

[0144] Examples and Comparative Examples

[0145] Example 1: Preparation of the negative electrode

[0146] Prepare hard carbon (average particle size (D) 50 10.1 μm, specific surface area: 3.0 m² 2 / g) is used as the first negative electrode active material. Prepare hard carbon (average particle size (D) 50 Micrometer diameter (μm): 7.6 μm, Specific surface area: 3.7 m² 2 / g) is used as the second negative electrode active material. Prepare hard carbon (average particle size (D) 50 Micrometer diameter (μm): 6.0 μm, Specific surface area: 4.1 m² 2 / g) is used as the third negative electrode active material, and Prepare hard carbon (average particle size (D) 50 Micrometer diameter (μm): 4.5 μm, Specific surface area: 4.7 m² 2 / g) is used as the fourth negative electrode active material.

[0147] Prepare carbon black (C65) as a conductive material and SBR-CMC as a binder.

[0148] As a negative electrode slurry for the lower layer, the first negative electrode active material and the second negative electrode active material are mixed to make the weight ratio 90:10. Then, the mixed negative electrode active material, conductive material and binder are mixed in a solvent (water) to make the weight ratio 95:1:4.

[0149] As the negative electrode slurry for the upper layer, the third negative electrode active material and the fourth negative electrode active material are mixed to make the weight ratio 10:90. Then, the mixed negative electrode active material, conductive material and binder are mixed in a solvent (water) to make the weight ratio 95:1:4.

[0150] The prepared lower layer was coated with a negative electrode slurry onto an aluminum foil (thickness: 20 µm) serving 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. Subsequently, the prepared upper layer was coated onto the lower negative electrode active material layer with a negative electrode slurry, and then dried in a vacuum oven at 60°C for 1 hour to form an upper negative electrode active material layer. The prepared upper negative electrode active material layer was then dried in a vacuum oven at 120°C for 12 hours, and then calendered to prepare a negative electrode for a sodium secondary battery. Finally, a negative electrode for a sodium secondary battery was prepared, wherein a current collector, a lower negative electrode active material layer, and an upper negative electrode active material layer are sequentially disposed.

[0151] At this point, the loading of the lower negative electrode active material layer is 90 mg / 25 cm⁻¹. 2 Furthermore, the thickness is 100 μm, and the loading of the upper negative electrode active material layer is 90 mg / 25 cm⁻¹. 2 And its thickness is 100 μm.

[0152] Examples 2 to 3 and Comparative Examples 1 to 5

[0153] The negative electrode was prepared in the same manner as in Example 1, except that the type and weight ratio of the negative electrode active materials contained in the lower negative electrode active material layer and the upper negative electrode active material layer were adjusted as listed in Tables 1 and 2 below.

[0154] [Table 1]

[0155] [Table 2]

[0156] Experimental Example 1: Evaluation of High-Rate Performance

[0157] (Preparation of a coin half-cell)

[0158] A separator (Whatman glass fiber) was placed between the positive electrode (Na metal) and the negative electrode prepared according to Examples 1 to 3 and Comparative Examples 1 to 5, and then an electrolyte was injected to prepare a coin half-cell according to the Examples and Comparative Examples.

[0159] As the electrolyte, a non-aqueous electrolyte solvent containing ethylene carbonate (EC) and propylene carbonate (PC) mixed in a volume ratio of 5:5 is used, wherein fluoroethylene carbonate (FEC) is added in an amount of 2% by weight based on the solvent and 1M of NaPF6 is dissolved.

[0160] (Evaluation of high-rate performance)

[0161] For the coin half-cells of Examples 1 to 3 and Comparative Examples 1 to 5 prepared according to Experimental Example 1, the ratio of 5C discharge capacity to 0.1C discharge capacity was measured, and the high-rate performance was evaluated.

[0162] Specifically, the coin half-cell was charged to 2.0 V at a constant current of 0.3C at 25°C, and then discharged to 0 V at CCCV rates of 0.1C and 5.0C. The change in discharge capacity with rate conditions was measured.

[0163] The measurement results are listed in Table 3 below.

[0164] Experimental Example 2: Evaluation of Adhesion Force

[0165] The adhesion force between the current collector and the negative electrode active material layer was measured for the negative electrodes prepared according to Examples 1 to 3 and Comparative Examples 1 to 5.

[0166] Specifically, the negative electrodes prepared according to Examples 1 to 3 and Comparative Examples 1 to 5 were cut into dimensions of 150 mm in length and 20 mm in width, and the electrode surfaces were attached along the length direction to a glass slide of 75 mm in length and 25 mm in width using double-sided tape.

[0167] That is, the glass slide is pasted onto the area corresponding to half the length of the negative electrode. Then, a 2 kg roller is used to rub the slide 10 times to ensure that the double-sided tape is evenly adhered, and the evaluation sample is prepared accordingly.

[0168] Fix the glass slide portion of the evaluation sample onto the sample stage of the Universal Testing Machine (UTM), and connect the half of the negative electrode that is not attached to the glass slide to the weighing sensor of the UTM machine.

[0169] The load cell was moved upwards by 50 mm at a speed of 100 mm / min, and the weight applied to the load cell was measured. The minimum weight measured between 20 mm and 40 mm of the moving section was then measured as the electrode adhesion force (gf / 20 mm) for each sample. A total of 5 evaluations were performed on each negative electrode, and their average values ​​are listed in Table 3 below.

[0170] [Table 3]

[0171] Referring to Table 2 above, it can be seen that the rate performance and adhesion of Examples 1 to 3 are superior compared to those of Comparative Examples 1 to 5.

[0172] In particular, in the case of Comparative Example 1, it can be seen that the weight ratio of the first negative electrode active material and the second negative electrode active material contained in the lower negative electrode active material layer is the same, and the weight ratio of the third negative electrode active material and the fourth negative electrode active material contained in the upper negative electrode active material layer is the same, so the rate performance and adhesion are both reduced.

[0173] In Comparative Example 2, it can be seen that the amount of large-particle-size negative electrode active material in the lower negative electrode active material layer is less than that of small-particle-size negative electrode active material, thus reducing both rate performance and adhesion.

[0174] In Comparative Example 3, it can be seen that the amount of small-particle-size negative electrode active material in the upper negative electrode active material layer is less than that of large-particle-size negative electrode active material, thus reducing both rate performance and adhesion.

[0175] In Comparative Example 4, it can be seen that the lower negative electrode active material layer contains only large-particle-size negative electrode active material, while the upper negative electrode active material layer contains only small-particle-size negative electrode active material, thus reducing both rate performance and adhesion.

[0176] In Comparative Example 5, it can be seen that the lower negative electrode active material layer contains only small-particle-size negative electrode active material, while the upper negative electrode active material layer contains only large-particle-size negative electrode active material, thus reducing both rate performance and adhesion.

[0177] [Explanation of reference numerals in the attached figures]

[0178] 10: Negative electrode for sodium secondary batteries

[0179] 100: Current collector

[0180] 210: Lower negative electrode active material layer

[0181] 220: Upper negative electrode active material layer

Claims

1. A negative electrode for a sodium secondary battery, the negative electrode comprising: 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 includes third negative electrode active material particles and fourth negative electrode active material particles. Each of the first to the fourth negative electrode active material particles contains hard carbon. The average particle diameter (D 50 ) of the first negative electrode active material particles is greater than the average particle diameter (D 50 ) of the second negative electrode active material particles. The average particle size (D) of the third negative electrode active material particles 50 The particle size of the fourth negative electrode active material particles is greater than the average particle size (D). 50 ), The weight of the first negative electrode active material particle is greater than the weight of the second negative electrode active material particle, and The weight of the third negative electrode active material particle is less than the weight of the fourth negative electrode active material particle.

2. The negative electrode according to claim 1, wherein each of the first negative electrode active material particles to the fourth negative electrode active material particles has a spherical shape.

3. The negative electrode according to claim 1, wherein the weight ratio of the first negative electrode active material particles to the second negative electrode active material particles is 51:49 to 99:

1.

4. The negative electrode according to claim 1, wherein the average particle size (D) of the first negative electrode active material particles and the second negative electrode active material particles is... 50 The ratio is 1.1 to 10.

5. The negative electrode according to claim 1, wherein the average particle size (D) of the first negative electrode active material particles is... 50 The thickness ranges from 5 μm to 15 μm, and The average particle size (D) of the second negative electrode active material particles 50 The range is from 1 μm to 10 μm.

6. The negative electrode according to claim 1, wherein the BET specific surface area of ​​the first negative electrode active material particles is 1 m². 2 / g to 10 m 2 / g, and The BET specific surface area of ​​the second negative electrode active material particles is 1.5 m². 2 / g to 12 m 2 / g.

7. The negative electrode according to claim 1, wherein the thickness of the lower negative electrode active material layer is 30 μm to 200 μm.

8. The negative electrode according to claim 1, wherein the loading of the lower negative electrode active material layer is 12.5 mg / 25cm². 2 Up to 250 mg / 25 cm 2 .

9. The negative electrode according to claim 1, wherein the weight ratio of the third negative electrode active material particles to the fourth negative electrode active material particles is 1:99 to 49:

51.

10. The negative electrode according to claim 1, wherein the average particle size (D) of the third negative electrode active material particles and the fourth negative electrode active material particles is... 50 The ratio is 1.1 to 10.

11. The negative electrode according to claim 1, wherein the average particle size (D) of the third negative electrode active material particles is... 50 The size ranges from 3 μm to 13 μm, and The average particle size (D) of the fourth negative electrode active material particles 50 The thickness ranges from 1 μm to 8 μm.

12. The negative electrode according to claim 1, wherein the BET specific surface area of ​​the third negative electrode active material particles is 1 m². 2 / g to 10 m 2 / g, and The BET specific surface area of ​​the fourth negative electrode active material particles is 2 m². 2 / g to 14 m 2 / g.

13. The negative electrode according to claim 1, wherein the thickness of the upper negative electrode active material layer is 30 μm to 200 μm.

14. The negative electrode according to claim 1, wherein the loading of the upper negative electrode active material layer is 12.5 mg / 25cm². 2 Up to 250 mg / 25 cm 2 .

15. A sodium secondary battery, the sodium secondary battery comprising the negative electrode for a sodium secondary battery as described in claim 1.

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