Negative electrode active material for sodium secondary battery, method for preparing same, and negative electrode and sodium secondary battery comprising same

Abstract

The present invention relates to a negative electrode active material for a sodium secondary battery, and a negative electrode for a sodium secondary battery and a sodium secondary battery, wherein the negative electrode and the sodium secondary battery comprise the negative electrode active material. More specifically, when a carbonization process is performed using a linear polymer and a thermally decomposable multi-dimensional structure polymer, it is possible to prepare a negative electrode active material for a sodium secondary battery, the negative electrode active material having a pore structure in which sodium ions can be reversibly inserted / detached.

Description

Negative electrode active material for a sodium secondary battery, method for manufacturing the same, negative electrode including the same, and sodium secondary battery

[0001] Cross-citation with related applications

[0002] The present application claims the benefit of priority based on Korean Patent Application No. 10-2024-0163945 filed November 18, 2024 and Korean Patent Application No. 10-2025-0174547 filed November 18, 2025, and all contents disclosed in the documents of said Korean patent applications are incorporated into this specification.

[0003] Technology field

[0004] The present invention relates to a negative electrode active material for a sodium secondary battery, a method for manufacturing the same, a negative electrode including the same, and a sodium secondary battery.

[0005] Rechargeable batteries refer to batteries that can be recharged as well as discharged, allowing for repeated use. Among rechargeable batteries, the representative lithium-ion battery operates on the principle that lithium ions contained in the positive electrode active material move through the electrolyte to the negative electrode and are inserted into the layered structure of the negative electrode active material (charging), and subsequently, the lithium ions that were inserted into the layered structure of the negative electrode return to the positive electrode (discharging). These lithium-ion batteries are currently commercialized and used as small power sources for devices such as mobile phones and laptop computers; however, as they can also be used as large power sources for devices such as hybrid vehicles, demand for them is increasing.

[0006] However, since lithium-ion batteries use rare metal elements such as lithium, there is a concern that they may not be able to meet the increasing demand.

[0007] Accordingly, research is being conducted on sodium secondary batteries that use sodium, which is abundant and inexpensive. Sodium is an alkali metal identical to lithium and has similar physical and chemical properties.

[0008] In sodium secondary batteries, just like in lithium secondary batteries, carbon materials such as graphite are used as negative electrode active materials, and sodium ions are inserted or deinserted between the carbon materials.

[0009] However, sodium ions (Na + The radius (1.03 Å) of ) is lithium ions (Li + Because it is larger than the radius (0.71 Å) of ), Na between carbon domains + To insert it, it is important to ensure an appropriate inter-face spacing.

[0010] Accordingly, hard carbon with a large interplanar spacing is mainly used as the negative electrode active material for sodium secondary batteries.

[0011] In other words, to increase the energy density of sodium secondary batteries, the shape and size of the negative electrode must be consistently controllable, the interplanar spacing must be wide, and Na + It is necessary to design a structure having pores that can be reversibly inserted / extracted.

[0012] Conventional hard carbon is manufactured by carbonizing and grinding a precursor, so its shape and size are inconsistent, which limits the ability to increase energy density when applied as a negative electrode active material for sodium secondary batteries.

[0013] Meanwhile, Korean Registered Patent No. 10-2596196 discloses a process capable of uniformly mass-producing spherical hard carbon microparticles, but Na + Na does not have pores capable of reversibly inserting / extracting + There is a problem with low storage capacity.

[0014] Therefore, as a negative electrode active material for sodium secondary batteries, while maintaining a constant spherical shape and micron-level size, Na + Technology development is required for cathode active materials having pores that can be reversibly inserted / extracted.

[0015] [Prior Art Literature]

[0016] [Patent Literature]

[0017] (Patent Document 1) Korean Registered Patent No. 10-2596196

[0018] As a result of conducting multifaceted research to solve the above problem, the inventors found that when a carbonization process is carried out using polyacrylonitrile (PAN), a carbon precursor, and lignin, a thermally degradable multidimensional structural polymer, as raw materials, Na with a large radius + It was confirmed that a negative electrode active material having a reversibly insertable / extractable pore structure can be manufactured, and that energy density can be improved when the negative electrode active material manufactured in this way is applied to a sodium secondary battery.

[0019] Therefore, the objective of the present invention is Na + The present invention provides a negative electrode active material for a sodium secondary battery having a reversibly insertable / extractable pore structure.

[0020] Another objective of the present invention is Na + The present invention provides a method for manufacturing a negative electrode active material for a sodium secondary battery having a reversibly insertable / extractable pore structure.

[0021] Another objective of the present invention is Na + The present invention provides a negative electrode for a sodium secondary battery comprising a negative electrode active material having a reversibly insertable / extractable pore structure.

[0022] Another objective of the present invention is Na + The present invention provides a sodium secondary battery comprising a negative electrode active material having a reversibly insertable / extractable pore structure.

[0023] To achieve the above objective, the present invention provides a negative electrode active material for a sodium secondary battery having a porous structure comprising a carbon secondary particle formed by assembling a plurality of carbon primary particles, wherein the negative electrode active material has a maximum hysteresis value of a nitrogen isothermal adsorption / desorption curve of 0.8 cm³ / g or less, and the maximum hysteresis value refers to the maximum value of the difference between the amount of N2 adsorbed and the amount of N2 desorbed in the nitrogen isothermal adsorption / desorption curve.

[0024]

[0025] The present invention also comprises the step of (S1) mixing polyacrylonitrile (PAN) and lignin and then dissolving them in a first solvent to obtain a first solution;

[0026] (S2) A step of adding a sacrificial polymer to a second solvent to obtain a second solution;

[0027] (S3) A step of mixing the first solution and the second solution, and then drying to obtain an aggregate; and

[0028] (S4) A step of carbonizing the above aggregate;

[0029] A method for manufacturing a negative electrode active material for a sodium secondary battery is provided, comprising

[0030]

[0031] The present invention also provides a negative electrode for a sodium secondary battery comprising the negative electrode active material.

[0032]

[0033] The present invention also provides a sodium secondary battery comprising the cathode, the anode, a separator interposed between them, and an electrolyte.

[0034] According to the present invention, by using polyacrylonitrile (PAN), a carbon precursor, and lignin, a pyrolytic multidimensional structural polymer, as raw materials and performing a carbonization process, Na with a large radius + It is possible to manufacture a cathode active material having a reversibly insertable / extractable pore structure.

[0035] Also, Na + When a negative electrode active material having this reversibly insertable / extractable pore structure is applied to a sodium secondary battery, the energy density of the battery can be improved.

[0036] FIGS. 1a and 1b are schematic diagrams illustrating the manufacturing process of a negative electrode active material for a sodium secondary battery (1a: negative electrode active material comprising a carbon precursor and a pyrolytic multidimensional structured polymer, 1b: negative electrode active material comprising a carbon precursor and a pyrolytic linear polymer).

[0037] Figures 2a and 2b show scanning electron microscope (SEM) images of the negative electrode active material prepared in the example and comparative example according to the present invention.

[0038] FIGS. 3a and 3b show the nitrogen isothermal adsorption / desorption curves of the cathode active materials prepared in the examples and comparative examples according to the present invention (3a: nitrogen isothermal adsorption / desorption curves of Comparative Example 1, Examples 1-1 and 1-2; 3b: nitrogen isothermal adsorption / desorption curves of Comparative Example 1 and Comparative Examples 2-1 to 2-3).

[0039] FIGS. 4a to 4f are charge-discharge curves for sodium secondary batteries prepared in the examples and comparative examples, 1 st dQ / dV curves in a cycle and 3 rdThis shows the dQ / dV curves in cycles (4a: Example 1-1, 4b: Example 1-2, 4c: Comparative Example 1, 4d: Comparative Example 2-1, 4e: Comparative Example 2-2, 4f: Comparative Example 2-3).

[0040] Hereinafter, the present invention will be described in more detail to aid in understanding the invention.

[0041] Terms or words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, and the inventor describes the invention in more detail below to aid in understanding the invention.

[0042] Terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0043] As used in this specification, the term “pyrolytic multidimensional structure polymer” refers to a polymer in which the molecular chains contained therein have a structure of two or more dimensions. The polymer has the characteristic of being pyrolytic, and pores are formed after pyrolytic decomposition. The pores formed after pyrolytic decomposition form a two-dimensional or three-dimensional structure.

[0044] As used herein, the term “pyrolytic linear polymer” refers to a polymer in which the molecular chains contained therein are linear and traces of the linear polymer remain even after pyrolysis.

[0045] As used in this specification, the term “primary particle” refers to a single particle, and “secondary particle” refers to a multi-particle formed by assembling single particles.

[0046]

[0047] Negative active material for sodium secondary batteries

[0048] The present invention relates to a negative electrode active material for a sodium secondary battery.

[0049] The negative electrode active material for a sodium secondary battery according to the present invention comprises a carbon secondary particle formed by assembling a plurality of carbon primary particles and is a negative electrode active material for a sodium secondary battery having a porous structure, wherein the maximum hysteresis value of the nitrogen isothermal adsorption / desorption curve is 0.8 cm³ / g or less, and the maximum hysteresis value refers to the maximum value of the difference between the amount of N2 adsorbed and the amount of N2 desorbed in the nitrogen isothermal adsorption / desorption curve.

[0050] If the above maximum hysteresis value is 0.8 cm³ / g or less, it means that the difference between the adsorption and desorption amounts of N2 in the negative electrode active material decreases, therefore sodium ions (Na + It can be seen that the reversible insertion / detachment of ) is smoothly carried out. Specifically, the maximum hysteresis value may be 0.8 cm³ / g or less or 0.7 cm³ / g or less. In addition, the lower limit of the maximum hysteresis value is not specifically restricted and may be, for example, 0 cm³ / g, 0.1 cm³ / g or more, 0.2 cm³ / g or more, 0.3 cm³ / g or more, 0.4 cm³ / g or more, 0.5 cm³ / g or more, or 0.6 cm³ / g or more. When the maximum hysteresis value is 0 cm³ / g, it means that N2 is simply adsorbed and desorbed on the surface, and there is no phenomenon of N2 filling or escaping from the internal pores, so it may correspond to a non-porous material. If the above maximum hysteresis value exceeds 8 cm³ / g, it means that open pores are over-distributed, and accordingly, irreversible capacity increases and the density of the cathode may decrease.

[0051]

[0052] FIGS. 1a and 1b are schematic diagrams illustrating the manufacturing process of a negative electrode active material for a sodium secondary battery (1a: negative electrode active material comprising a carbon precursor and a pyrolytic multidimensional structured polymer, 1b: negative electrode active material comprising a carbon precursor and a pyrolytic linear polymer).

[0053] Referring to FIG. 1a, it can be seen that when polyacrylonitrile (PAN) (10), which is a carbon precursor, and lignin (20), which is a pyrolytic multidimensional structural polymer, are mixed and then carbonized, the molecular chains of lignin (20) move to form a negative electrode active material (1) in which pores (P) are formed. The pores (P) are formed by sodium ions (Na + It has a structure that allows for the reversible insertion / extraction of ) to proceed easily. Specifically, the lignin has a multidimensional structure in which phenolic compounds are linked, and upon heat treatment, the cyclic structure of the phenolic compounds in lignin having such a multidimensional structure is rearranged to form a stable cyclic carbon structure. Due to the stable cyclic carbon structure, Na + Pores that facilitate reversible insertion / extraction can be formed.

[0054] Referring to FIG. 1b, it can be seen that when a carbon precursor PAN (10) and a thermally decomposable linear polymer polymethyl methacrylate (Poly(methyl methacrylate), PMMA) (30) are mixed and then carbonized, a negative electrode active material (1) having an internal structure in which PAN (10) and linear PMMA (30) are connected is formed. At this time, due to the morphological characteristics of PAN and PMMA, a pore structure is not formed. The PMMA is a linear acrylic polymer and has a chain structure. The chain structure decomposes into volatile substances upon heat treatment to form a linear structure. The volatile substances may be water, carbon dioxide, and / or low molecular weight MMA (Methyl Methacrylate). Therefore, the PMMA does not form a cyclic carbon structure upon heat treatment, and accordingly, Na + Pores that facilitate reversible insertion / extraction of cannot be formed.

[0055]

[0056] In one embodiment of the present invention, the secondary particle may be a spherical particle.

[0057] The above secondary particles include carbon secondary particles formed by assembling a plurality of carbon primary particles, and when the carbon primary particles are spherical particles, assembly is easy, energy density increases, and Na + It can be advantageous for forming a porous structure in which reversible insertion / extraction of can be smoothly achieved.

[0058]

[0059] In one embodiment of the present invention, the aspect ratio of the spherical particles may be 1 to 1.2.

[0060] If the aspect ratio of the above spherical particles exceeds 1.2, the spherical particles become closer to an ellipse, increasing the tortuosity of the electrode, so Na during charging and discharging +It may hinder the transfer of, and consequently, there may be a limit to the increase in energy density. Specifically, the aspect ratio of the spherical particles may be 1.2 or less, 1.15 or less, 1.1 or less, or 1.05 or less. The closer the aspect ratio of the spherical particles is to 1, the more Na + It can be advantageous to form a tortuosity structure of the electrode that facilitates the smooth movement of electrons. The tortuosity structure refers to a structural feature that indicates how winding the internal pores of the electrode or the pathways for the movement of ions or electrons are. In other words, it is an indicator that shows how much longer the actual path to travel becomes compared to the straight-line distance.

[0061]

[0062] In one embodiment of the present invention, the diameter of the spherical particles may be 0.5 to 25 μm.

[0063] If the diameter of the above spherical particles is less than 0.5 μm, the ratio of volume to surface area increases, reducing the bulk density of the electrode. Consequently, the amount of negative electrode active material that can be contained in the same space decreases, which may lead to a decline in energy density and conductivity. Furthermore, due to the large surface area, excessive reaction with the electrolyte occurs, which may result in the excessive formation of the SEI (Solid Electrolyte Interphase) layer; if it exceeds 25 μm, Na + As the distance of penetration and diffusion into this negative electrode active material increases, Na +The ion conductivity rate may be reduced. Specifically, the diameter of the spherical particles may be 0.5 μm or more, 1 μm or more, 2 μm or more, 3 μm or more, 4 μm or more, 5 μm or more, 6 μm or more, 7 μm or more, 8 μm or more, or 9 μm or more, and may be 25 μm or less, 24 μm or less, 23 μm or less, 22 μm or less, 21 μm or less, 20 μm or less, 19 μm or less, 18 μm or less, 17 μm or less, 16 μm or less, 15 μm or less, 14 μm or less, 13 μm or less, 12 μm or less, or 11 μm or less.

[0064]

[0065] In one embodiment of the present invention, the specific surface area of ​​the negative electrode active material may be 1 to 10 m² / g.

[0066] If the specific surface area of ​​the above negative electrode active material is less than 1 m² / g, Na + The ion transfer rate may be reduced because it cannot sufficiently penetrate into the cathode, and if it exceeds 10 m² / g, the SEI layer may be excessively formed, which may reduce the cycle life. Specifically, the specific surface area of ​​the cathode active material may be 1 m² / g or more, 1.5 m² / g or more, 2 m² / g or more, 2.5 m² / g or more, 3 m² / g or more, 3.5 m² / g or more, 4 m² / g or more, or 4.5 m² / g or more, and may be 10 m² / g or less, 9.5 m² / g or less, 9 m² / g or less, 8.5 m² / g or less, 8 m² / g or less, 7.5 m² / g or less, 7 m² / g or less, 6.5 m² / g or less, 6 m² / g or less, or 5.5 m² / g or less.

[0067]

[0068] In one embodiment of the present invention, the porous structure may include closed pores. Generally, closed pores can serve as storage spaces for reduced Na atoms and facilitate the reversible insertion / extraction of Na+, thereby contributing to capacity enhancement; thus, among porous structures, those containing closed pores may be advantageous as negative electrode active materials for sodium secondary batteries. Accordingly, the performance of the sodium secondary battery can be further improved by forming a porous structure containing closed pores in the negative electrode active material for sodium secondary batteries.

[0069] In addition, the porous structure may also include open pores. Due to the open pores, electrolytes penetrate, facilitating ion diffusion, and Na + It can be stored in an adsorbed form.

[0070] However, if the negative electrode active material for the sodium secondary battery contains only closed pores, the Na diffusion resistance increases, making it difficult to reversibly store Na. In addition, if the negative electrode active material for the sodium secondary battery contains only open pores, the active area of ​​the negative electrode is maximized, leading to the consumption of electrolyte due to side reactions caused by excessive electrolyte reactions, and consequently, a decrease in capacity.

[0071] Therefore, if the negative electrode active material for sodium secondary batteries contains both closed and open pores, these problems are compensated for, allowing the electrolyte to penetrate and facilitate ion diffusion, enabling Na+ ions to be stored in an adsorbed form, and reversible Na + It can have a structure that is advantageous for reducing insertion / extraction and diffusion resistance at high rates.

[0072] For example, the closed pores may be included in an amount of 10 to 50 volume% relative to the total pore volume of the negative electrode active material for the sodium secondary battery. If the closed pores are less than 10 volume%, the ion transport pathway may be restricted, which may degrade battery performance, and if they exceed 50 volume%, structural stability may be reduced. Specifically, the volume of the closed pores may be 10 volume% or more, 15 volume% or more, 20 volume% or more, or 25 volume% or more, and may be 50 volume% or less, 45 volume% or less, 40 volume% or less, 35 volume% or less, or 30 volume% or less.

[0073] In one embodiment, the volume (V) of the closed pore Closed Pore ) can be calculated by the following Equation 1:

[0074] <Equation 1>

[0075]

[0076] In Equation 1 above, ρ true ρ is the true density of the cathode active material, and 2.26 is the theoretical density of graphite with no closed pores. That is, it is the theoretical density of ideal graphite that contains no closed pores. The above “1 / ρ true “ is the total specific volume of the cathode active material calculated from the true density. The “1 / 2.26” is the specific volume of graphite that does not contain any closed pores. Therefore, the closed pore volume of the cathode active material can be calculated from the difference in the specific volumes.

[0077] The above ρ true can be calculated by the following Equation 2 or Equation 3:

[0078] <Equation 2>

[0079]

[0080] In Equation 2 above, m is the mass of the cathode active material, and V is the true volume of the cathode active material. The true volume refers to the actual volume excluding bubbles or gaps; and

[0081] <Equation 3>

[0082]

[0083] In Equation 3 above, d 002 represents the distance between carbon layers. Equation 3 above is ρ true wa d 002 It is a linear regression model representing the relationship between. The above -8.25 and 5.02 are coefficients obtained through regression analysis.

[0084]

[0085] Method for manufacturing a negative electrode active material for a sodium secondary battery

[0086] The present invention also relates to a method for manufacturing a negative electrode active material for a sodium secondary battery.

[0087] A method for manufacturing a negative electrode active material for a sodium secondary battery according to the present invention comprises: (S1) a step of mixing polyacrylonitrile (PAN) and lignin and dissolving them in a first solvent to obtain a first solution; (S2) a step of adding a sacrificial polymer to a second solvent to obtain a second solution; (S3) a step of mixing the first solution and the second solution and then drying them to obtain an aggregate; and (S4) a step of carbonizing the aggregate.

[0088]

[0089] Hereinafter, the method for manufacturing a negative electrode active material for a sodium secondary battery according to the present invention will be explained in more detail step by step.

[0090]

[0091] In step (S1) of the method for manufacturing a negative electrode active material for a sodium secondary battery according to the present invention, polyacrylonitrile (PAN) and lignin are mixed and then dissolved in a first solvent to obtain a first solution.

[0092] The carbon precursor PAN and the pyrolytic multidimensional structural polymer lignin mentioned above may both be carbon precursor polymers that can be converted into carbon materials through heat treatment. Additionally, the carbon precursor polymer undergoes self-assembly (flocculation) in a solution to form primary carbon precursor polymer particles in the form of micelles (hereinafter referred to as 'primary polymer particles'). These primary carbon precursor polymer particles are converted into primary carbon particles by carbonizing them after the first and second solvents are removed through a drying process. Subsequently, a negative electrode active material comprising secondary carbon particles formed by assembling and aggregating the primary carbon particles can be manufactured.

[0093] Hereinafter, the above carbonization precursor polymer may refer to a polymer including PAN and lignin.

[0094]

[0095] In one embodiment of the present invention, the weight ratio of PAN to lignin may be 0.1:1 to 1.5:1.

[0096] If the above weight ratio falls outside the range of 0.1:1 to 1.5:1, Na + A pore structure capable of facilitating smooth reversible insertion / extraction of may not be sufficiently formed. Specifically, the weight ratio of the fibers may be 0.1:1 or greater, 0.2:1 or greater, 0.3:1 or greater, 0.4:1 or greater, 0.5:1 or greater, 0.6:1 or greater, 0.7:1 or greater, 0.8:1 or greater, or 0.9:1 or greater, and may be 1.5:1 or less, 1.4:1 or less, 1.3:1 or less, 1.2:1 or less, or 1.1:1 or less. When a trace amount of lignin is used based on the above weight ratio, Na + A pore structure that facilitates smooth reversible insertion / extraction may not be sufficiently formed, and if an excessive amount of lignin is used, the shape of the cathode active material may not be formed in a spherical shape.

[0097]

[0098] In one embodiment of the present invention, the first solvent may comprise one or more selected from the group consisting of dimethylformamid, diethyl ether, ethanol, methanol, n-propanol, isopropyl alcohol, acetone, n-pentane, ethylene dichloride, methyl acetate, ethyl acetate, acetonitrile, tetrahydrofuran (THF), n-hexane, chlorohexane, chloropentane, carbon tetrachloride, 1,2-dichloroethane, 1,2-dichloroethylene, trichloroethylene, methyl ethyl ketone, or 1,2-dimethoxyethane (DME). The first solvent is not particularly limited as long as it is capable of dissolving the PAN and lignin.

[0099]

[0100] In one embodiment of the present invention, the concentration of the first solution is not particularly limited as long as it is sufficient to dissolve PAN and lignin in the first solvent. For example, the concentration of the first solution may be prepared such that the solid content is 10 to 20 weight%. In this case, the solid content may refer to the weight of PAN and lignin.

[0101]

[0102] In step (S2) of the method for manufacturing a negative electrode active material for a sodium secondary battery according to the present invention, a sacrificial polymer can be added to a second solvent to obtain a second solution.

[0103]

[0104] In one embodiment of the present invention, the sacrificial polymer may comprise one or more selected from the group consisting of styrene-co-acrylonitrile (SAN), polylactic acid (PLA), poly(lactic-co-glycolic acid) [poly(lactic-co-glycolic acid) PLGA], polymethacrylate (PMMA), and poly(vinylidene fluoride) [poly(vinylidene fluoride) PVDF].

[0105] The above sacrificial polymer is an immiscible polymer that does not chemically react with the carbonization precursor polymers, such as PAN and lignin, and causes phase separation when blended with the carbonization precursor polymer, thereby assisting the carbonization precursor polymer in forming micelles in the solution phase. Additionally, the above sacrificial polymer can be removed by drying or easily removed by an organic solvent that dissolves the sacrificial polymer.

[0106]

[0107] In one embodiment of the present invention, the second solvent may comprise one or more selected from the group consisting of dimethylformamid, diethyl ether, ethanol, methanol, n-propanol, isopropyl alcohol, acetone, n-pentane, ethylene dichloride, methyl acetate, ethyl acetate, acetonitrile, tetrahydrofuran (THF), n-hexane, chlorohexane, chloropentane, carbon tetrachloride, 1,2-dichloroethane, 1,2-dichloroethylene, trichloroethylene, methyl ethyl ketone, or 1,2-dimethoxyethane (DME). The second solvent is not particularly limited as long as it is capable of dissolving the sacrificial polymer.

[0108]

[0109] In one embodiment of the present invention, the concentration of the second solution is not particularly limited as long as it is sufficient to dissolve the sacrificial polymer in the second solvent. For example, the concentration of the second solution may be prepared such that the solid content is 20 to 40 weight%. In this case, the solid content may refer to the weight of the sacrificial polymer.

[0110]

[0111] In step (S3) of the method for manufacturing a negative electrode active material for a sodium secondary battery according to the present invention, the first solution and the second solution are mixed and then dried to obtain an aggregate.

[0112] The mixing ratio of the aforementioned carbonization precursor polymer and the sacrificial polymer may be 1:2 to 9 by weight. In this case, spherical porous carbon secondary particles having a diameter of 0.5 to 25 μm can be produced with a high yield.

[0113]

[0114] In one embodiment of the present invention, the first solvent and the second solvent are removed by the drying to obtain an aggregate.

[0115] The above aggregate is a massive aggregate comprising carbonization precursor polymer primary particles and a sacrificial polymer. Specifically, the aggregate is formed by assembling and aggregating carbonization precursor polymer primary particles, and a sacrificial polymer exists between these polymer primary particles.

[0116] The above drying temperature is not particularly limited, and it is appropriate to perform the process at a temperature below the carbonization temperature of the carbonization precursor polymer so that the carbonization precursor polymer is not carbonized. For example, the mixed solution obtained by mixing the first solution and the second solution may be dried at 90°C to 110°C for 22 to 26 hours.

[0117] The first solvent and the second solvent can be removed by the above drying.

[0118]

[0119] In step (S4) of the method for manufacturing a negative electrode active material for a sodium secondary battery according to the present invention, the aggregate can be carbonized.

[0120] By carbonizing the aggregate formed by assembling and aggregating the primary particles of the carbonization precursor polymer, the primary particles of the carbonization precursor polymer are converted into primary carbon particles, and a negative electrode active material comprising secondary carbon particles formed by assembling and aggregating the primary carbon particles can be manufactured.

[0121]

[0122] In one embodiment of the present invention, the carbonization step may be performed by increasing the temperature to 600°C to 1600°C for heat treatment.

[0123] Specifically, the aggregate can be carbonized by heating it to 600°C to 1600°C at a heating rate of 0.5 to 20°C / min under an inert gas atmosphere, and then heat-treating the aggregate for 0.5 to 10 hours. In particular, when the aggregate is carbonized, the carbonization precursor polymer primary particles within the aggregate are converted into carbon primary particles, and at the same time, the sacrificial polymer within the aggregate is thermally decomposed and removed, thereby allowing the carbon primary particles to assemble and aggregated carbon secondary particles to be produced. At this time, the carbon secondary particles may have a spherical porous structure.

[0124]

[0125] In one embodiment of the present invention, the aggregate can be stabilized before the carbonization step.

[0126] Specifically, the temperature can be raised to 200°C to 300°C at a heating rate of 5 to 15°C / min, then the aggregate can be stabilized by heat-treating it for 0.5 to 24 hours in an oxygen-containing atmosphere, and then the stabilized aggregate can be carbonized.

[0127]

[0128] Negative electrode for sodium secondary battery

[0129] The present invention also relates to a negative electrode for a sodium secondary battery comprising the negative electrode active material.

[0130]

[0131] For example, the cathode of the present invention comprises a current collector; and a cathode active material layer described above on at least one surface of the current collector.

[0132] The above current collector is a metal that has high conductivity and allows the slurry of the negative electrode active material to adhere easily, and any metal that is non-reactive within the voltage range of the battery can be used. For example, there are meshes or foils manufactured by aluminum (Al), copper (Cu), gold (Au), nickel (Ni), titanium (Ti), calcined carbon, stainless steel, aluminum alloys (e.g., aluminum-cadmium alloys), copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., or combinations thereof. The thickness of such a current collector is not particularly limited and may be in the range of approximately 3 to 500 μm, which is commonly applied.

[0133] The above-mentioned negative electrode active material layer can be manufactured by coating a negative electrode slurry comprising the aforementioned negative electrode active material, a binder and a solvent, and, if necessary, a conductive material, onto at least one surface of a current collector, drying it, and then rolling it.

[0134] The description of the above-mentioned cathode active material is omitted as it is as previously stated.

[0135] The content of the above-mentioned cathode active material may be 80 to 99 weight percent based on the total amount of the cathode slurry.

[0136]

[0137] In addition, the above binder is used to attach the negative electrode active material particles to each other and to attach the negative electrode active material to the current collector, and is not particularly limited as long as it is commonly used in the industry. For example, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene butylene rubber (SBR), lithium-substituted polyacrylate (Li-PAA), etc., may be used alone or in combination of two or more types.

[0138] The content of such binder is not particularly limited and, for example, may be 0.1 to 20 weight percent based on the total amount of the cathode slurry.

[0139]

[0140] In addition, the conductive material is used to impart conductivity to the electrode and is not particularly limited as long as it is conductive without causing chemical changes to the battery being constructed. Examples include natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon nanotubes, graphene, conductive fibers (e.g., carbon fibers or metal fibers, etc.), metal powders (e.g., fluorocarbons, copper, aluminum, nickel, silver powder, etc.), conductive whiskers (e.g., zinc oxide, potassium titanate, etc.), conductive metal oxides (e.g., titanium oxide, etc.), conductive polymers (e.g., polyphenylene derivatives, etc.), or mixtures thereof. The content of such conductive material may be about 0.1 to 20 weight percent based on the total amount of the negative electrode slurry composition.

[0141]

[0142] In addition, non-limiting examples of the above solvents include dimethyl sulfoxide (DMSO), N-methyl pyrrolidon (NMP), and dimethyl formamide (DMF). The content of these solvents is not particularly limited and can be used in an amount that results in a desirable viscosity of the cathode slurry.

[0143]

[0144] In addition, the coating method of the above-mentioned cathode slurry is not particularly limited as long as it is commonly used in the industry. For example, slot die coating method, gravure coating method, immersion coating method, spray coating method, etc.

[0145]

[0146] sodium secondary battery

[0147] The present invention also relates to a sodium secondary battery comprising the cathode, the anode, a separator interposed between them, and an electrolyte.

[0148] Meanwhile, the present invention provides a secondary battery comprising the aforementioned negative electrode.

[0149] The sodium secondary battery of the present invention comprises the aforementioned negative electrode; a positive electrode, an electrolyte, and a separator.

[0150]

[0151] The above-mentioned anode can be manufactured by mixing an anode active material, a conductive material, a binder, and a solvent to prepare an anode slurry, and then directly coating it onto a metal current collector, or by casting it onto a separate support and laminating a film of the anode active material peeled off from the support onto a metal current collector.

[0152] The positive electrode active material usable in the present invention is not particularly limited as long as it is a positive electrode active material used in the industry for sodium secondary batteries. Non-limiting examples of positive electrode active materials include Na x CoO2(here, 0 <x≤1), Na x Co2 / 3 Mn 1 / 3 O 2　 (here, 0 <x≤1), Na x Fe 1 / 2 Mn 1 / 2 O 2　 (here, 0 <x≤1), NaCrO2, NaLi 0.2 Ni 0.25 Mn 0.75 O 2.35 , Na 0.44 MnO2, NaMnO2, Na 0.7 VO2, Na 0.33 V2O 5,　 Na3V2(PO4)3, NaFePO4, NaMn 0.5 Fe 0.5 PO4, Na3V2(PO4) 3, There are sodium-containing transition metal oxides such as Na2FePO4F, Na3V2(PO4)3, NaFeSO4F, and mixtures thereof.

[0153] The description of the above conductive material, binder, and solvent is omitted because they are the same as those described in the aforementioned cathode section.

[0154]

[0155] The above-mentioned separator is not particularly limited as long as it is used as a separator in the industry, and it is preferable that it be a porous separator. Non-limiting examples include porous polymer films made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer; porous nonwoven fabrics made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc., and these may be used alone or in a laminate of two or more types. In addition, an insulating thin film having ion permeability and mechanical strength may be used.

[0156]

[0157] The above electrolyte may include a non-aqueous solvent and an electrolyte salt, and optionally may further include additives such as an overcharge prevention agent.

[0158] The above-mentioned non-aqueous solvent is not particularly limited as long as it is commonly used as a non-aqueous solvent for non-aqueous electrolytes, and cyclic carbonates, linear carbonates, lactones, ethers, esters, or ketones may be used.

[0159] In addition, examples of the above-mentioned cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BE), fluoroethylene carbonate (FEC), etc., and examples of the above-mentioned linear carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC), and methylpropyl carbonate (MPC). Examples of the above-mentioned lactone include gamma-butyrolactone (GBL), and examples of the above-mentioned ethers include dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, etc. In addition, examples of the above-mentioned esters include n-methyl acetate, n-ethyl acetate, methyl propionate, methyl pivalate, etc., and examples of the above-mentioned ketone include polymethylvinyl ketone. These non-aqueous solvents can be used alone or in a mixture of two or more.

[0160] In addition, the electrolyte salt is not specifically restricted as long as it is an electrolyte salt commonly used for non-aqueous electrolytes. Non-limiting examples of electrolyte salts include A + B - As a salt with a structure similar to that of, A + is Li + , Na + , K + It includes alkali metal cations such as or ions composed of combinations thereof, and B - PF6 - , BF4 - , Cl - , Br - , I - , ClO4 - , AsF6 - , CH3CO2 -, CF3SO3 - , N(CF3SO2)2 - , C(CF2SO2)2 - It is a salt containing anions such as or a combination thereof. These electrolyte salts can be used alone or in a mixture of two or more types. According to one example, the electrolyte salt may be a lithium salt or a sodium salt.

[0161] The aforementioned sodium secondary battery can be manufactured by forming an electrode assembly by placing a separator between a positive electrode and a negative electrode, placing the electrode assembly into a pouch, a cylindrical battery case, or a prismatic battery case, and then injecting an electrolyte. Alternatively, the electrode assembly can be manufactured by stacking the electrode assemblies, impregnating them with an electrolyte, placing the resulting product into a battery case, and sealing it.

[0162]

[0163] These sodium secondary batteries can be used not only as battery cells used as power sources for small devices, but also as unit cells in medium-to-large battery modules containing multiple battery cells. Examples of the medium-to-large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems. They can be particularly useful in areas requiring high power output, such as hybrid electric vehicles and batteries for storing new and renewable energy.

[0164] Preferred embodiments are presented below to aid in understanding the present invention; however, the following embodiments are merely illustrative of the invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the invention, and that such changes and modifications fall within the scope of the appended claims.

[0165]

[0166] In the following examples and comparative examples, a negative electrode active material for a sodium secondary battery was prepared according to the composition as described in Table 1 below.

[0167]

[0168] Raw material for cathode active material Carbon precursor Polymer weight ratio (A:B) (A)(B) Example 1-1 PAN Lignin 75:25 Example 1-2 PAN Lignin 50:50 Comparative Example 1 PAN Lignin - Comparative Example 2-1 PAN PMMA 75:25 Comparative Example 2-2 PAN PMMA 50:50 Comparative Example 2-3 PAN PMMA 25:75

[0169]

[0170] Example 1-1

[0171] (1) Manufacturing of negative electrode active material

[0172] Polyacrylonitrile (PAN), a carbon precursor, and lignin, a pyrolytic multidimensional structural polymer, were prepared.

[0173] A composite precursor was prepared by mixing PAN and lignin in a weight ratio of 75:25. The composite precursor (PAN / Lignin) was mixed with dimethylformaldehyde (DMF), a solvent, to obtain a first solution having a solid content of 15 wt%.

[0174] A second solution having a solid content of 30 wt% was obtained by mixing the sacrificial polymer styrene(co-acrylonitrile), SAN, with the solvent DMF.

[0175] After mixing the first solution and the second solution, the mixture was dried on a hot plate at 100°C for 24 hours. At this time, the weight ratio of the composite precursor (PAN / Lignin) to the sacrificial polymer was mixed to be 2:8 (PAN / Lignin:SAN).

[0176] After that, the mixture of the composite precursor and the sacrificial polymer was carbonized in an electric furnace at a heating rate of 10°C / min up to 1000°C. During the carbonization process, a cathode active material was prepared by undergoing a stabilization process at a heating rate of 5°C / min in the 270°C to 300°C range.

[0177]

[0178] (2)Cathode manufacturing

[0179] A mixture was obtained by mixing the cathode active material prepared in (1) above, carbon black as a conductive material, carboxymethyl cellulose (CMC) as a first binder, and styrene-butadiene rubber (SBR) as a second binder in a weight ratio of 92:5:1.5:1.5. The mixture was mixed with N-methyl-2-pyrrolidone (NMP) solvent to obtain a slurry with a concentration of 15%. The first binder was in the form of a 1% by weight solution of CMC added to distilled water, and an appropriate amount was used to control the concentration. The second binder was in the form of a 40% by weight solution of SBR added to distilled water, and an appropriate amount was used to control the adhesion.

[0180] After that, the above slurry was applied to an aluminum foil serving as a cathode current collector to a thickness of 100 μm, dried for 8 hours under vacuum conditions at 120°C, and then rolled to produce a cathode with a porosity of 30%.

[0181]

[0182] (3) Sodium secondary battery manufacturing

[0183] A sodium secondary battery in the form of a 2030 coin-type half-cell was manufactured by interposing a separator between the negative electrode and the positive electrode and injecting an electrolyte. The negative electrode used was the negative electrode manufactured in (2) above, the counter electrode used was sodium metal, the separator used was glass fiber (KFP / MFV-3(GFC) GLASS MICROFIBER FILTERS), and the electrolyte used was 1.0M NaPF6 (EC:DEC:PC=1:1:1, EC: ethylene carbonate, DEC: diethyl carbonate, PC: propylene carbonate).

[0184]

[0185] Examples 1-2

[0186] A negative electrode active material, a negative electrode, and a sodium secondary battery were prepared in the same manner as in Example 1-1, except that the weight ratio of PAN to lignin was 50:50.

[0187]

[0188] Comparative Example 1

[0189] A negative electrode active material, a negative electrode, and a sodium secondary battery were prepared in the same manner as in Example 1-1, except that lignin, a thermally degradable multidimensional structural polymer, was not used.

[0190]

[0191] Comparative Example 2-1

[0192] A negative electrode active material, a negative electrode, and a sodium secondary battery were prepared in the same manner as in Example 1-1, except that poly(methyl methacrylate) (PMMA) was used instead of lignin, a pyrolytic multidimensional structural polymer.

[0193]

[0194] Comparative Example 2-2

[0195] A negative electrode active material, a negative electrode, and a sodium secondary battery were prepared in the same manner as in Comparative Example 2-1, except that the weight ratio of PAN and PMMA was 50:50.

[0196]

[0197] Comparative Example 2-3

[0198] A negative electrode active material, a negative electrode, and a sodium secondary battery were prepared in the same manner as in Comparative Example 2-1, except that the weight ratio of PAN to PMMA was 25:75.

[0199]

[0200] Experimental Example 1: Confirmation of the form of the cathode active material

[0201] The morphology of the negative electrode active materials prepared in the examples and comparative examples was confirmed using a scanning electron microscope (SEM, HITACHI, S-4700).

[0202]

[0203] Figures 2a and 2b show scanning electron microscope images of the negative electrode active material prepared in the example and comparative example according to the present invention.

[0204] Figure 2a is an SEM image of Comparative Example 1 (PAN 100 wt%), Example 1-1 (PAN:Lignin = 75 wt%: 25 wt%), and Example 1-2 (PAN:Lignin = 50 wt%: 50 wt%), showing that the negative electrode active materials are all in the form of spherical particles.

[0205] In addition, comparing Example 1-1 and Example 1-2, it can be seen that as the content of lignin, a thermally degradable multidimensional structural polymer, increases in the cathode active material, the surface irregularities of the spherical particles increase. When the surface irregularities of the spherical particles increase, the contact area with the electrolyte increases, and Na + Access can be made easier.

[0206] FIG. 2b is an SEM image of Comparative Example 1 (PAN 100 wt%), Comparative Example 2-1 (PAN:PMMA = 75 wt%: 25 wt%), Comparative Example 2-2 (PAN:PMMA = 50 wt%: 50 wt%), and Comparative Example 2-3 (PAN:PMMA = 50 wt%: 50 wt%), showing that the negative electrode active materials are all in the form of spherical particles.

[0207] In addition, it can be seen that in Comparative Examples 2-1 to 2-3, the surface roughness increases as the PMMA content increases.

[0208]

[0209] Experimental Example 2: Physical properties of negative electrode active material and performance analysis of a sodium secondary battery containing the same

[0210] The physical properties of the negative electrode active material prepared in the examples and comparative examples and the performance of the sodium secondary battery containing it were analyzed.

[0211]

[0212] The specific surface area and maximum hysteresis value of the negative electrode active material were measured in the following manner, and the initial charge capacity of the sodium secondary battery containing the negative electrode active material was measured.

[0213]

[0214] (1) Specific surface area

[0215] The specific surface area of ​​the cathode active material was measured according to the BET method. Specifically, after injecting N2 gas according to the BET method, the pressure change of the injected gas was measured, and an adsorption isotherm representing the relationship between relative pressure (the value obtained by dividing the partial pressure of the adsorbed gas (P) by the saturated vapor pressure (P0), P / P0) and the amount of adsorption was obtained through adsorption / desorption data at various pressures. Then, the specific surface area was calculated using Equation 4 below.

[0216]

[0217] <Equation 4>

[0218] S = (Vm·N·A) / V

[0219] N: Avogadro's number (6.022 x 10⁻⁶ 23 )

[0220] A: Unit area of ​​a nitrogen molecule (0.162 nm) 2 )

[0221] V: Volume occupied by 1 mole of nitrogen gas (approx. 22.414 L at standard temperature and pressure)

[0222]

[0223] (2) Maximum hysteresis value

[0224] The maximum hysteresis value was calculated from the nitrogen isothermal insertion / extraction curve.

[0225] FIGS. 3a and 3b show the nitrogen isothermal adsorption / desorption curves of the cathode active materials prepared in the examples and comparative examples according to the present invention (3a: nitrogen isothermal adsorption / desorption curves of Comparative Example 1, Examples 1-1 and 1-2; 3b: nitrogen isothermal adsorption / desorption curves of Comparative Example 1, Comparative Examples 2-1 to 2-3).

[0226] In the nitrogen isotherm adsorption and desorption curves shown in Figures 3a and 3b, when the difference between the adsorption amount and the desorption amount is maximum, this was set as the maximum hysteresis value.

[0227]

[0228] (3) Initial charging capacity

[0229] The initial charge capacity of a sodium secondary battery was measured under the following sodiumation and desodium conditions using a galvanostatic cell tester (WonATech, WBC3000L).

[0230]

[0231] - Sodium conditions

[0232] CC: 0.1C (2 cycles), 0.2C (5 cycles), 0.3C (continues) to 0.01V

[0233] CV to 0.05C @ 0.01V

[0234]

[0235] - Desodium conditions

[0236] CC: 0.1C (2 cycles), 0.2C (5 cycles), 0.3C (5 cycles), 0.5C (5 cycles), 1C (5 cycles), 2C (5 cycles), 5C (5 cycles), 0.3C (continued) to 2V

[0237]

[0238] Table 2 below shows the specific surface area, maximum hysteresis value, and initial charge capacity measured by the method described above.

[0239]

[0240] Specific Surface Area (m² / g) Maximum Hysteresis Value (cm³ / g) Initial Charge Capacity (mAh / g) Example 1-15.526 0.786 148 Example 1-25.164 0.623 151 Comparative Example 13.534 2.329 125 Comparative Example 2-17.220 1.834 138 Comparative Example 2-26.235 1.674 124 Comparative Example 2-39.002 2.327 119

[0241]

[0242] Referring to Figures 3a and 3b and Table 2, in Examples 1-1, 1-2 and Comparative Example 1, Examples 1-1 and 1-2 containing lignin showed an increased specific surface area and an increased initial filling capacity compared to Comparative Example 1 which does not contain lignin.

[0243] In addition, in Comparative Example 1 and Comparative Example 2-1, Comparative Example 2-1 containing PMMA showed an increase in specific surface area and an increase in initial filling capacity compared to Comparative Example 1 not containing PMMA. However, in Comparative Examples 2-1 to 2-3 containing PMMA, it was found that as the PMMA content increased, the specific surface area increased, but the initial filling capacity decreased even compared to Comparative Example 1.

[0244]

[0245] In addition, it can be seen that the initial filling capacity in Comparative Example 2-1 containing PMMA increased compared to Comparative Example 1 not containing PMMA, but the initial filling capacity was significantly smaller compared to Examples 1-1 and 1-2 containing lignin.

[0246] It can be seen that this is due to the difference in capacity resulting from the difference in the maximum hysteresis values ​​in the nitrogen isothermal insertion / extraction curve.

[0247]

[0248] Experimental Example 3: Measurement of Electrochemical Characteristics of a Sodium Secondary Battery

[0249] The electrochemical characteristics of the sodium secondary batteries prepared in the examples and comparative examples were measured by performing charge and discharge under the same conditions as when measuring the initial charge capacity in Experimental Example 2.

[0250]

[0251] FIGS. 4a to 4f are charge-discharge curves for sodium secondary batteries prepared in the examples and comparative examples, 1 st dQ / dV curves in a cycle and 3 rd This shows the dQ / dV curves in cycles (4a: Example 1-1, 4b: Example 1-2, 4c: Comparative Example 1, 4d: Comparative Example 2-1, 4e: Comparative Example 2-2, 4f: Comparative Example 2-3).

[0252] The dQ / dV (Differential capacity curve) is a curve constructed by differentiating the voltage curve resulting from constant current charging and discharging (Q: capacitance, V: voltage). In this case, the occurrence of a peak is interpreted as the presence of a reaction, and the position of the peak represents the reaction voltage. The number of peaks indicates the number of reaction branches, and the area under the peaks represents the capacity at the corresponding reaction / voltage.

[0253]

[0254] Composition cost 1 st Charging capacity (mAh / g) 3 rdCharging Capacity (mAh / g) Example 1-1 PAN:Lignin 75:25 148.44 138.32 Example 1-2 PAN:Lignin 50:50 15 1.04 137.66 Comparative Example 1 PAN 100 125.26 115.36 Comparative Example 2-1 PAN:PMMA 75:25 138.08 119.27 Comparative Example 2-2 PAN:PMMA 50:50 124.88 112.23 Comparative Example 2-3 PAN:PMMA 25:75 119.69 115.87

[0255]

[0256] Referring to FIGS. 4a to 4c and Table 3, Examples 1-1 and 1-2 showed an increase in reversible capacity compared to Comparative Example 1. Furthermore, from the dQ / dV curves, as the lignin content increased in Examples 1-1 and 1-2, Na at around 0.1 to 0.4 V... + It can be seen that insertion proceeds actively. In addition, it can be seen that in the region below 0.1V, the capacity development increases and pore deposition increases.

[0257] Referring to FIGS. 4c to 4f and Table 3, Comparative Example 2-1 showed an increase in reversible capacity compared to Comparative Example 1 due to the addition of PMMA. However, in Comparative Examples 2-1 to 2-3, the reversible capacity was found to decrease as the PMMA content increased. Additionally, from the dQ / dV curves, it was found that in Comparative Examples 2-1 to 2-3, the capacity development decreased in the region of 0.1V or lower due to the addition of PMMA.

[0258]

[0259] From this, it can be seen that when PAN and heteropolymers are used in the manufacture of negative electrode active materials for sodium secondary batteries, the internal structure of the negative electrode active material changes depending on the morphological difference of the heteropolymers. That is, as the heteropolymers, lignin, a thermally degradable multidimensional structure polymer, and PMMA, a thermally degradable linear structure polymer, exhibit different thermal behaviors in the molecular chains contained in the polymers as they undergo a heat treatment process, and accordingly, it can be inferred that the internal structure of the negative electrode active material carbonized by the heat treatment process will be different.

[0260] Specifically, lignin, a multidimensional structural polymer, undergoes heat treatment due to the thermal behavior of the molecular chains contained within it, resulting in Na + It is possible to form an internal structure of a cathode active material having a space that facilitates reversible adsorption and desorption.

[0261] On the other hand, PMMA, a pyrolytic linear polymer, exhibits a form in which its internal molecular chains are linked to PAN, a linear polymer, upon heat treatment, resulting in Na + A space where reversible adsorption and desorption are easy cannot be formed.

[0262]

[0263] Experimental Example 4: Measurement of the volume of closed holes

[0264] The closed pore volume (V) of the cathode active material prepared in the examples and comparative examples using Formula 1 below Closed Pore ) was measured, and the results were recorded in Table 4 below.

[0265]

[0266] <Equation 1>

[0267]

[0268]

[0269] In Equation 1 above, ρ true is the true density of the cathode active material, and 2.26 is the theoretical density of graphite with no closed pores.

[0270]

[0271] The above ρ true is calculated using the following Equation 3:

[0272] <Equation 3>

[0273]

[0274]

[0275] In Equation 3 above, d 002 represents the distance between carbon layers. Equation 3 above is ρ true wa d 002 It is a linear regression model representing the relationship between. The above -8.25 and 5.02 are coefficients obtained through regression analysis.

[0276]

[0277] Composition ratio 2θ(°)d 002 (nm)ρ true (g / cm³)V Closed Pore (cm³) Example 1-1 PAN:Lignin 75:2524.680.3602.040.0477 Example 1-2 PAN:Lignin 50:5024.460.3632.020.0525 Comparative Example 1 PAN 10024.970.3562.090.0360 Comparative Example 2-1 PAN:PMMA 75:2524.860.3582.070.0406 Comparative Example 2-2 PAN:PMMA 50:5025.110.3542.110.0314 Comparative Example 2-3 PAN:PMMA 25:7524.860.3582.070.0406

[0278]

[0279] Referring to Table 4 above, it can be seen that the closed pore volumes of Examples 1-1 and 1-2 are relatively large. Examples 1-1 and 1-2 are negative electrode active materials prepared using lignin, a thermally degradable multidimensional structural polymer, and it was confirmed that the volume of closed pores increases when lignin is used. Since closed pores are advantageous for the reversible insertion / extraction of sodium ions, it was found that negative electrode active materials prepared using lignin are suitable as negative electrode active materials for sodium secondary batteries.

[0280]

[0281] [Explanation of the symbol]

[0282] 1: Cathode active material

[0283] 10: PAN

[0284] 20: Lignin

[0285] 30: PMMA

[0286] P: Qi Gong

Claims

1. A negative electrode active material for a sodium secondary battery having a porous structure and comprising carbon secondary particles formed by assembling multiple carbon primary particles, The above-mentioned cathode active material has a maximum hysteresis value of 0.8 cm³ / g or less on N2 isothermal adsorption / desorption curves, and A negative electrode active material for a sodium secondary battery, wherein the above hysteresis maximum value refers to the maximum value of the difference between the amount of N2 adsorbed and the amount of N2 desorbed in the nitrogen isothermal adsorption / desorption curve.

2. In Paragraph 1, The above secondary particles are spherical particles, a negative electrode active material for a sodium secondary battery.

3. In Paragraph 2, A negative electrode active material for a sodium secondary battery, wherein the aspect ratio of the spherical particles is 1 to 1.

2.

4. In Paragraph 2, A negative electrode active material for a sodium secondary battery, wherein the diameter of the spherical particles is 0.5 to 25 μm.

5. In Paragraph 1, A negative electrode active material for a sodium secondary battery, wherein the specific surface area of ​​the negative electrode active material is 1 to 10 m² / g.

6. In Paragraph 1, A negative electrode active material for a sodium secondary battery, wherein the above porous structure includes closed pores.

7. (S1) A step of mixing polyacrylonitrile (PAN) and lignin and then dissolving them in a first solvent to obtain a first solution; (S2) A step of adding a sacrificial polymer to a second solvent to obtain a second solution; (S3) A step of mixing the first solution and the second solution, and then drying to obtain an aggregate; and (S4) A step of carbonizing the above aggregate; A method for manufacturing a negative electrode active material for a sodium secondary battery, comprising 8. In Paragraph 7, A method for manufacturing a negative electrode active material for a sodium secondary battery, wherein the weight ratio of the above PAN to lignin is 0.1:1 to 1.5:

1.

9. In Paragraph 7, A method for manufacturing a negative electrode active material for a sodium secondary battery, wherein the sacrificial polymer comprises one or more selected from the group consisting of styrene-co-acrylonitrile (SAN), polylactic acid (PLA), poly(lactic-co-glycolic acid) [poly(lactic-co-glycolic acid) PLGA], polymethacrylate (PMMA), and poly(vinylidene fluoride) [poly(vinylidene fluoride) PVDF].

10. In Paragraph 7, A method for manufacturing a negative electrode active material for a sodium secondary battery, wherein the first solvent and the second solvent each comprise one or more selected from the group consisting of dimethylformamid, diethyl ether, ethanol, methanol, n-propanol, isopropyl alcohol, acetone, n-pentane, ethylene dichloride, methyl acetate, ethyl acetate, acetonitrile, tetrahydrofuran (THF), n-hexane, chlorohexane, chloropentane, carbon tetrachloride, 1,2-dichloroethane, 1,2-dichloroethylene, trichloroethylene, methyl ethyl ketone, or 1,2-dimethoxyethane (DME).

11. In Paragraph 7, A method for manufacturing a negative electrode active material for a sodium secondary battery, wherein the carbonization step is heat-treated by raising the temperature to 600 to 1600℃.

12. In Paragraph 7, A method for manufacturing a negative electrode active material for a sodium secondary battery, wherein the material is stabilized by raising the temperature to 200℃ to 300℃, and then carbonized.

13. A negative electrode for a sodium secondary battery comprising the negative electrode active material of claim 1.

14. A sodium secondary battery comprising the negative electrode of claim 13, a positive electrode, a separator interposed between the same, and an electrolyte.

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