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

Abstract

The present invention relates to a negative electrode active material for a sodium secondary battery, and a negative electrode and a sodium secondary battery each comprising same. More specifically, the present invention relates to a negative electrode active material for a sodium secondary battery and a method for preparing same, wherein the negative electrode active material has a pore structure capable of reversibly intercalating and de-intercalating sodium ions by form a large amount of closed pores in the active material through performing a carbonization process after modifying the surface of a carbon precursor by alkaline hydrolysis.

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. 10-2025-0016846 filed February 10, 2025 and Korean Patent Application No. 10-2026-0026580 filed February 10, 2026, 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. A representative example of a rechargeable battery is the lithium-ion battery, which operates on the principle that lithium ions contained in the positive electrode active material move through the electrolyte to the negative electrode, where they are inserted into the layered structure of the negative electrode active material (charging), and subsequently return to the positive electrode (discharging). These lithium-ion batteries are currently commercialized and widely used as power sources for small devices such as mobile phones and laptop computers, as well as for large devices like hybrid cars, and demand for them continues to grow.

[0006] However, lithium-ion batteries have limitations in flexibly responding to rapid increases in demand because they use rare metal elements such as lithium. Accordingly, research on sodium-ion batteries, which utilize sodium—which is abundant and inexpensive—is actively underway. Sodium is an alkali metal identical to lithium and has similar physical and chemical properties.

[0007] Like lithium-ion batteries, sodium secondary batteries also use carbon materials such as graphite as the negative electrode active material and operate through the insertion and extraction of sodium ions between the layers of carbon materials. However, sodium ions (Na + The radius (1.03 Å) of ) is lithium ions (Li + Since the radius (0.71 Å) is larger than that of carbon domains, it is important to secure an appropriate interplanar spacing to efficiently insert sodium ions between the carbon domains. For this reason, hard carbon with a wide interplanar spacing is mainly used as the negative electrode active material for sodium secondary batteries.

[0008] Consequently, in order to increase the energy density of sodium secondary batteries, it is necessary to be able to consistently control the shape and size of the negative electrode active material, and it is essential to design a pore structure that allows sodium ions to be reversibly inserted and extracted along with a wide interplanar spacing.

[0009] Conventional hard carbon is manufactured by carbonizing a precursor and then grinding it, resulting in irregular shape and size, which poses an obstacle to improving energy density when applied as a negative electrode active material for sodium secondary batteries. Meanwhile, Korean Registered Patent No. 10-2596196 discloses a process capable of uniformly mass-producing spherical hard carbon microparticles, but it has the problem of low storage capacity because it fails to secure sufficient pores for the reversible insertion and extraction of sodium ions.

[0010] It is known in the industry that reversible insertion and extraction of sodium ions are possible in closed pores, and that high capacity is exhibited in the low-voltage region, particularly when closed pores are formed. Accordingly, there is growing interest in technologies that can efficiently form closed pores within anode active materials for sodium secondary batteries. Therefore, there is an urgent need to develop anode active materials that have a closed pore structure that facilitates the reversible movement of sodium ions while maintaining a spherical shape and a uniform size at the micrometer level.

[0011] [Prior Art Literature]

[0012] (Patent Document 1) Korean Registered Patent No. 10-2596196

[0013] As a result of conducting multifaceted research to solve the above problem, the inventors found that in the process of manufacturing a negative electrode active material by carbonizing polyacrylonitrile (PAN), a carbon precursor, if the polyacrylonitrile is surface-modified by alkaline hydrolysis before carbonization, many closed pores are formed inside the negative electrode active material manufactured after carbonization, and Na + It was confirmed that it has a structure favorable for reversible insertion / extraction.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] To achieve the above objective, a first embodiment of 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 comprises closed pores, the nitrogen content present on the surface of the negative electrode active material is 2 to 4 weight% based on the total weight of the negative electrode active material, and the oxygen content is 4 to 8 weight% based on the total weight of the negative electrode active material.

[0019] In one embodiment of the present invention, a negative electrode active material for a sodium secondary battery is provided, wherein the secondary particles are spherical particles.

[0020] In one embodiment of the present invention, a negative electrode active material for a sodium secondary battery is provided, wherein the aspect ratio of the spherical particles is 1 to 1.2.

[0021] In one embodiment of the present invention, a negative electrode active material for a sodium secondary battery is provided, wherein the diameter of the spherical particles is 0.5 to 25 μm.

[0022] In one embodiment of the present invention, a negative electrode active material for a sodium secondary battery is provided, wherein the specific surface area of ​​the negative electrode active material is 40 to 70 m² / g.

[0023] In one embodiment of the present invention, a negative electrode active material for a sodium secondary battery is provided, wherein the volume of the closed pores is 58 volume% or more based on the total pore volume of the negative electrode active material.

[0024]

[0025] A second embodiment of the present invention provides a method for manufacturing a negative electrode active material for a sodium secondary battery, comprising: (S1) dissolving polyacrylonitrile (PAN) in a first solvent to obtain a first solution; (S2) adding a sacrificial polymer to a second solvent to obtain a second solution; (S3) mixing the first solution and the second solution, and then drying to obtain an aggregate; (S4) hydrolyzing the aggregate with an alkaline solution to surface modify it; and (S5) carbonizing the surface-modified aggregate.

[0026] In one embodiment of the present invention, a method for manufacturing a negative electrode active material for a sodium secondary battery is provided, wherein the sacrificial polymer comprises one or more selected from the group consisting of styrene-co-acrylonitrile (SAN), poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA), polymethacrylate (PMMA), and poly(vinylidene fluoride) (PVDF).

[0027] In one embodiment of the present invention, the first solvent and the second solvent each comprise one or more selected from the group consisting of dimethylformamide, 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, and 1,2-dimethoxyethane (DME), thereby providing a method for manufacturing a negative electrode active material for a sodium secondary battery.

[0028] In one embodiment of the present invention, a method for manufacturing a negative electrode active material for a sodium secondary battery is provided, wherein the alkaline solution comprises one or more selected from the group consisting of sodium hydroxide and potassium hydroxide.

[0029] In one embodiment of the present invention, a method for manufacturing a negative electrode active material for a sodium secondary battery is provided, wherein the carbonization step is heat-treated by raising the temperature to 600 to 1600°C.

[0030] In one embodiment of the present invention, a method for manufacturing a negative electrode active material for a sodium secondary battery is provided, wherein the material is stabilized by raising the temperature to 200°C to 300°C, and then carbonized.

[0031]

[0032] A third embodiment of the present invention provides a negative electrode for a sodium secondary battery comprising the negative electrode active material.

[0033] A fourth embodiment of the present invention 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 modifying the surface of polyacrylonitrile (PAN), a carbon precursor, through alkaline hydrolysis and then performing a carbonization process, a large number of closed pores can be formed within the active material. Through this, sodium ions with a large radius (Na₂C) + A pore structure is secured in which ) can be reversibly inserted and removed, and when the said negative active material is applied to a sodium secondary battery, the energy density of the battery can be significantly improved.

[0035] Figure 1 shows the results of SEM surface analysis of PAN-containing aggregates depending on whether alkaline hydrolysis was performed before the carbonization process.

[0036] Figure 2 shows the FT-IR surface analysis results of PAN-containing aggregates depending on whether alkaline hydrolysis was performed before the carbonization process.

[0037] Figure 3 shows PAN-containing aggregates depending on whether alkaline hydrolysis is performed before the carbonization process. 1 This shows the results of the H-NMR surface analysis.

[0038] Figure 4 is a graph showing the results of the XPS elemental composition analysis of PAN-containing aggregates depending on whether alkaline hydrolysis was performed before the carbonization process.

[0039] Figures 5a to 5c are graphs showing the results of XPS molecular structure analysis of PAN-containing aggregates depending on whether alkaline hydrolysis was performed (5a: C=O molecular structure; 5b: O=CN molecular structure; 5c: CO molecular structure).

[0040] Figures 6a to 6c are graphs showing the results of the thermal characteristics analysis of PAN-containing aggregates depending on whether alkaline hydrolysis was performed (6a: DSC graph; 6b: TGA graph; 6c: DTG graph).

[0041] Figure 7 is a graph of the XRD analysis results of PAN-containing aggregates depending on whether alkaline hydrolysis was performed.

[0042] Figure 8 shows the results of SEM surface analysis of the cathode active material depending on whether alkaline hydrolysis was performed before the carbonization process.

[0043] Figure 9 is a graph showing the results of the XPS elemental composition analysis of the cathode active material depending on whether alkaline hydrolysis was performed before the carbonization process.

[0044] Figure 10 is a graph showing the results of the analysis of the specific surface area of ​​the cathode active material depending on whether alkaline hydrolysis was performed before the carbonization process.

[0045] Figure 11 is a graph showing the results of the pore analysis of the cathode active material depending on whether alkaline hydrolysis is performed before the carbonization process.

[0046] Figure 12 is a graph of the XRD analysis results of the cathode active material depending on whether alkaline hydrolysis was performed before the carbonization process.

[0047] Figure 13 is a graph of the Raman spectroscopic analysis results of the cathode active material depending on whether alkaline hydrolysis was performed before the carbonization process.

[0048] Figure 14 shows the measurement results of charge capacity and Coulomb efficiency for the sodium secondary batteries of Example 1 and Comparative Example 1.

[0049] Figure 15 is a graph showing the pore size distribution for the cathode active materials of Example 1 and Comparative Example 1.

[0050] Hereinafter, the present invention will be described in more detail to aid in understanding the invention.

[0051] 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.

[0052] As used in this specification, the term “carbon precursor” refers to a raw material for forming hard carbon, which is a negative electrode active material, through a carbonization process.

[0053] 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 said single particles.

[0054]

[0055] Negative electrode active material for sodium secondary batteries

[0056] The first embodiment of the present invention relates to a negative electrode active material for a sodium secondary battery.

[0057] 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 has a porous structure, wherein the negative electrode active material comprises a closed pore, the nitrogen content present on the surface of the negative electrode active material is 2 to 4 weight% based on the total weight of the negative electrode active material, and the oxygen content is 4 to 8 weight% based on the total weight of the negative electrode active material.

[0058]

[0059] In one embodiment of the present invention, the porous structure of the negative electrode active material for a sodium secondary battery includes closed pores. Generally, the closed pores can serve as storage spaces for reduced Na atoms, and Na + Since reversible insertion / extraction of pores is smoothly performed and contributes to capacity enhancement, a porous structure containing closed pores can be advantageous as a negative electrode active material for a sodium secondary battery. Therefore, the more a porous structure containing closed pores is formed in the negative electrode active material for a sodium secondary battery, the more the performance of the sodium secondary battery can be further improved.

[0060] In the manufacturing process of a negative electrode active material for a sodium secondary battery as described below, if a heat treatment process is performed after alkaline hydrolysis of a carbon precursor, closed pores can be formed more effectively. As shown in Reaction Scheme 1 below, when polyacrylonitrile (PAN), which is a carbon precursor, is alkaline hydrolyzed, some nitrile (C≡N) groups are substituted with amide (-CONH2) groups and carboxyl (-COOH) groups. During heat treatment, cyclization reactions occur due to the amide (-CONH2) groups and carboxyl (-COOH) groups formed by the alkaline hydrolysis process, preventing the formation of a ladder structure and turbostratic planes due to carbonization, thereby allowing thermal decomposition to proceed and forming a structure favorable for the formation of closed pores.

[0061]

[0062] <Reaction Equation 1>

[0063]

[0064]

[0065] 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.

[0066] 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.

[0067] 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, and Na+ It can be stored in a form where ions are adsorbed, and reversible Na + It can have a structure advantageous for insertion / extraction and reduction of diffusion resistance at high rates.

[0068] For example, the closed pores may be included in an amount of 58 volume% or more relative to the total pore volume of the negative electrode active material for the sodium secondary battery. If the closed pores are less than 58 volume%, the ion transport pathway may be restricted, which may degrade battery performance. Specifically, the volume of the closed pores may be 58 volume% or more, 59 volume% or more, 60 volume% or more, 61 volume% or more, 62 volume% or more, 63 volume% or more, 64 volume% or more, or 65 volume% or more. Although the upper limit of the volume of the closed pores is not specifically limited, it may be 80 weight% or less, 79 weight% or less, 78 weight% or less, 77 weight% or less, 76 weight% or less, 75 weight% or less, 74 weight% or less, 73 weight% or less, 72 weight% or less, or 71 weight% or less. If the volume of the closed pores exceeds 80 weight%, structural stability may be reduced. The volume of the above-mentioned closed pores can be measured through SAXS (Small-Angle X-ray Scattering) analysis and BET (Brunauer-Emmett-Teller) analysis.

[0069]

[0070] In one embodiment of the present invention, the nitrogen content present on the surface of the negative electrode active material is 2 to 4 weight% based on the total weight of the negative electrode active material, and the oxygen content is 4 to 8 weight% based on the total weight of the negative electrode active material.

[0071] Nitrogen atoms have a high electron density and play a role in improving the conductivity of the cathode active material. Therefore, if the nitrogen content is less than 2 wt%, sufficient interaction with sodium ions does not occur, which may lead to a decrease in conductivity. On the other hand, if the nitrogen content exceeds 4 wt%, the change in electron density becomes excessive, which may negatively affect the movement pathway of sodium ions and promote reactions with the electrolyte, causing additional reactions other than reversible reactions, which may reduce the operational stability of the battery.

[0072] Accordingly, the nitrogen content can be controlled within a range of 2 to 4 weight%. More specifically, the nitrogen content may be 2 weight% or more, 2.1 weight% or more, 2.2 weight% or more, 2.3 weight% or more, 2.4 weight% or more, 2.5 weight% or more, 2.6 weight% or more, 2.7 weight% or more, 2.8 weight% or more, 2.9 weight% or more, 3 weight% or more, 3.1 weight% or more, 3.2 weight% or more, 3.3 weight% or more, 3.4 weight% or more, or 3.5 weight% or more, and may be 4 weight% or less, 3.9 weight% or less, 3.8 weight% or less, 3.7 weight% or less, or 3.6 weight% or less.

[0073] In addition, the oxygen content is closely related to the degree of formation of functional groups, such as amide (-CONH₂) groups and carboxyl (-COOH) groups, formed by the alkaline hydrolysis process. If the oxygen content is less than 4 weight%, the formation of the functional groups is insufficient, and closed pores may not be formed to a desired level. On the other hand, if the oxygen content exceeds 8 weight%, alkaline hydrolysis proceeds excessively, which may reduce the structural stability and durability of the cathode active material.

[0074] Accordingly, the oxygen content can be controlled within the range of 4 to 8 weight percent. More specifically, the oxygen content may be 4 wt% or more, 4.1 wt% or more, 4.2 wt% or more, 4.3 wt% or more, 4.4 wt% or more, 4.5 wt% or more, 4.6 wt% or more, 4.7 wt% or more, 4.8 wt% or more, 4.9 wt% or more, 5 wt% or more, 5.1 wt% or more, 5.2 wt% or more, 5.3 wt% or more, 5.4 wt% or more, 5.5 wt% or more, 5.6 wt% or more, 5.7 wt% or more, 5.8 wt% or more, 5.9 wt% or more, 6 wt% or more, 6.1 wt% or more, 6.2 wt% or more, or 6.3 wt% or more, and 8 wt% or less, 7.9 wt% or less, 7.8 wt% or less, 7.7 wt% or less, 7.6 wt% or less, 7.5 wt% or less, 7.4 wt% or less. It may be less than or equal to weight%, 7.3 weight%, 7.2 weight%, 7.1 weight%, 7 weight%, 6.9 weight%, 6.8 weight%, 6.7 weight%, 6.6 weight%, 6.5 weight%, or 6.4 weight%.

[0075]

[0076] In one embodiment of the present invention, the secondary particle may be a spherical particle.

[0077] 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.

[0078]

[0079] In one embodiment of the present invention, the aspect ratio of the spherical particles may be 1 to 1.2.

[0080] If the aspect ratio of the spherical particles exceeds 1.2, the spherical particles become closer to an elliptical shape, increasing the tortuosity of the electrode, which may hinder the transport of Na during charging and discharging, 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 advantageous it may be for forming an electrode structure having low tortuosity that can facilitate the smooth movement of Na.

[0081]

[0082] In one embodiment of the present invention, the diameter of the spherical particles may be 0.5 to 25 μm.

[0083] If the diameter of the spherical particles is less than 0.5 μm, the ratio of volume to surface area increases, which reduces the bulk density of the electrode and decreases the amount of negative electrode active material that can be contained in the same space, thereby potentially lowering energy density and conductivity. Additionally, due to the large surface area, excessive reaction with the electrolyte may occur, leading to the excessive formation of a Solid Electrolyte Interphase (SEI) layer. On the other hand, if the diameter of the spherical particles exceeds 25 μm, the distance over which Na penetrates and diffuses into the negative electrode active material increases, which may reduce the ion conduction rate of Na. 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.

[0084]

[0085] In one embodiment of the present invention, the specific surface area of ​​the negative electrode active material may be 1 to 100 m² / g.

[0086] If the specific surface area of ​​the above-mentioned negative electrode active material is less than 1 m² / g, Na may not penetrate sufficiently into the negative electrode active material, which may reduce the ion transfer rate, and if it exceeds 100 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, 10 m² / g or more, 15 m² / g or more, 20 m² / g or more, 25 m² / g or more, 30 m² / g or more, 35 m² / g or more, 40 m² / g or more, 45 m² / g or more, or 50 m² / g or more, and may be 100 m² / g or less, 95 m² / g or less, 90 m² / g or less, 85 m² / g or less, 80 m² / g or less, 75 m² / g or less, 70 m² / g or less, 65 m² / g or less, 60 m² / g or less, or 55 m² / g or less. Preferably, the specific surface area of ​​the cathode active material may be 40 to 70 m² / g.

[0087]

[0088] In one embodiment of the present invention, the pore volume of the negative electrode active material may be 0.01 to 0.1 cc / g.

[0089] If the pore volume of the above-mentioned negative electrode active material is less than 0.01 cc / g, Na may not penetrate sufficiently into the negative electrode active material, which may reduce the ion transfer rate, and if it exceeds 0.1 cc / g, the SEI layer may be excessively formed, which may reduce the cycle life. Specifically, the pore volume of the cathode active material may be 0.01 cc / g or more, 0.015 cc / g or more, 0.02 cc / g or more, 0.025 cc / g or more, 0.03 cc / g or more, 0.035 cc / g or more, or 0.04 cc / g or more, and may be 0.1 cc / g or less, 0.095 cc / g or less, 0.09 cc / g or less, 0.085 cc / g or less, 0.08 cc / g or less, 0.075 cc / g or less, 0.07 cc / g or less, 0.06 cc / g or less, 0.055 cc / g or less, 0.05 cc / g or less, or 0.045 cc / g or less. Preferably, the pore volume of the negative electrode active material may be 0.02 to 0.06 cc / g.

[0090] As such, since the specific surface area and pore volume of the cathode active material according to the present invention are both relatively large, it can be seen that it has a structure in which a large amount of micropores are formed.

[0091]

[0092] Method for manufacturing a negative electrode active material for a sodium secondary battery

[0093] A second embodiment of the present invention relates to a method for manufacturing a negative electrode active material for a sodium secondary battery.

[0094] A method for manufacturing a negative electrode active material for a sodium secondary battery according to a second embodiment of the present invention comprises: (S1) a step of obtaining a first solution by dissolving polyacrylonitrile (PAN) in a first solvent; (S2) a step of obtaining a second solution by adding a sacrificial polymer to a second solvent; (S3) a step of obtaining an aggregate by mixing the first solution and the second solution, drying, and removing the sacrificial polymer using an organic solvent; (S4) a step of surface-modifying the aggregate by hydrolyzing it with an alkaline solution; and (S5) a step of carbonizing the surface-modified aggregate.

[0095]

[0096] 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.

[0097]

[0098] 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) is dissolved in a first solvent to obtain a first solution.

[0099] The above-mentioned carbon precursor PAN may be a carbonization precursor polymer that can be converted into a carbon material through heat treatment. Additionally, the above-mentioned carbonization precursor polymer may flocculate in a solution to form primary carbonization precursor polymer particles in the form of micelles (hereinafter referred to as “primary polymer particles”). These primary carbonization precursor polymer particles are converted into primary carbon particles by carbonizing after the first solvent is removed through a drying process. Subsequently, the primary carbon particles may be assembled to produce a negative electrode active material containing secondary carbon particles. In the following description, the above-mentioned carbonization precursor polymer may refer to a polymer containing PAN.

[0100]

[0101] In one embodiment of the present invention, the first solvent may comprise one or more selected from the group consisting of dimethylformamide, 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, and 1,2-dimethoxyethane (DME). The first solvent is not particularly limited as long as it is capable of dissolving the PAN.

[0102]

[0103] 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 in the first solvent. For example, 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.

[0104]

[0105] 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 is added to a second solvent to obtain a second solution.

[0106]

[0107] In one embodiment of the present invention, the sacrificial polymer may include one or more selected from the group consisting of styrene-co-acrylonitrile (SAN), poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) [poly(lactic-co-glycolic acid) (PLGA], polymethacrylate (PMMA), and poly(vinylidene fluoride) [poly(vinylidene fluoride) (PVDF].

[0108] The above sacrificial polymer does not chemically react with the above PAN, which is the carbonization precursor polymer, and is an immiscible polymer that causes phase separation when blended with the carbonization precursor polymer, and can act as a matrix in solution to assist the carbonization precursor polymer in forming micelles. In addition, the above sacrificial polymer can be removed by a drying process or can be easily removed by an organic solvent that dissolves the sacrificial polymer.

[0109]

[0110] In one embodiment of the present invention, the second solvent may comprise one or more selected from the group consisting of dimethylformamide, diethyl ether, ethanol, methanol, n-propanol, isopropyl alcohol, 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, and 1,2-dimethoxyethane (DME).

[0111]

[0112] 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 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.

[0113]

[0114] 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, and the sacrificial polymer is removed using an organic solvent to obtain an aggregate.

[0115] The mixing ratio of the aforementioned carbonization precursor polymer and sacrificial polymer may be a weight ratio of 1:2 to 1:9. In this case, spherical porous carbon secondary particles with a diameter of 0.5 to 25 μm can be produced in a high yield.

[0116] In addition, the organic solvent for removing the sacrificial polymer may be acetone, but is not limited thereto as long as it is an organic solvent capable of dissolving and removing the sacrificial polymer.

[0117]

[0118] In one embodiment of the present invention, the first solvent and the second solvent may be removed by the drying process to obtain an aggregate. The aggregate is a bulk particle and comprises a carbonization precursor polymer primary particle and a sacrificial polymer. Specifically, the aggregate has a structure in which the carbonization precursor polymer primary particles are assembled and aggregated, and a sacrificial polymer may exist between the polymer primary particles.

[0119]

[0120] The above drying temperature is not particularly limited, but may be performed at a temperature below the carbonization temperature of the carbonization precursor polymer so that the carbonization precursor polymer is not carbonized. For example, a 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. By doing so, the first solvent and the second solvent may be removed by the drying process.

[0121]

[0122] In step (S4) of the method for manufacturing a negative electrode active material for a sodium secondary battery according to the present invention, the surface of the aggregate can be modified by hydrolyzing it with an alkaline solution. This process may be referred to as 'alkali hydrolysis' below.

[0123]

[0124] The above alkaline solution may be an alkaline aqueous solution. The concentration of the above alkaline aqueous solution is not particularly limited as long as it is sufficient to hydrolyze the nitrile (C≡N) group of PAN, a carbonization precursor polymer contained in the aggregate, into an amide (-CONH₂) group and a carboxyl (-COOH) group. For example, the concentration of the above alkaline solution may be 2N to 10N. If the concentration of the above alkaline solution is less than 2N, the hydrolysis may not occur sufficiently, and if it exceeds 10N, the concentration may be excessive and the PAN may be denatured. Specifically, the concentration of the above alkaline solution may be 2N or more, 3N or more, or 4N or more, and may be 10N or less, 9N or less, 8N or less, 7N or less, or 6N or less.

[0125]

[0126] In addition, the alkaline solution may include one or more bases selected from the group consisting of sodium hydroxide and potassium hydroxide. Considering the alkaline hydrolysis efficiency of the PAN, it may be preferable for the alkaline solution to include sodium hydroxide.

[0127]

[0128] In addition, the mass ratio of the aggregate to the base may be 0.1:1 to 0.5:1, taking into account the efficiency of alkaline hydrolysis. Specifically, the mass ratio may be 0.1:1 or higher, 0.15:1 or higher, or 0.2:1 or higher, and may be 0.5:1 or lower, 0.4:1 or lower, or 0.3:1 or lower.

[0129]

[0130] In step (S5) of the method for manufacturing a negative electrode active material for a sodium secondary battery according to the present invention, the surface-modified aggregate can be carbonized. By carbonizing the aggregate formed by assembling and aggregating the carbonization precursor polymer primary particles, the carbonization precursor polymer primary particles are converted into carbon primary particles, and a negative electrode active material comprising carbon secondary particles formed by assembling and aggregating the carbon primary particles can be manufactured.

[0131]

[0132] In one embodiment of the present invention, the carbonization step may be performed by heating to 600°C to 1600°C. Specifically, the aggregate may be heated to 600°C to 1600°C at a heating rate of 0.5 to 20°C / min under an inert gas atmosphere, and then carbonized by 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 the carbon primary particles are assembled to produce aggregated carbon secondary particles. At this time, the carbon secondary particles may have a spherical porous structure.

[0133]

[0134] The above heating rate may be 0.5 ℃ / min or more, 1 ℃ / min or more, 2 ℃ / min or more, 3 ℃ / min or more, 4 ℃ / min or more, 5 ℃ / min or more, 6 ℃ / min or more, 7 ℃ / min or more, 8 ℃ / min or more, 9 ℃ / min or more, or 10 ℃ / min or more, and may be 20 ℃ / min or less, 19 ℃ / min or less, 18 ℃ / min or less, 17 ℃ / min or less, 16 ℃ / min or less, 15 ℃ / min or less, 14 ℃ / min or less, 13 ℃ / min or less, 12 ℃ / min or less, or 11 ℃ / min or less. In addition, the heat treatment temperature may be 600°C or higher, 650°C or higher, 700°C or higher, 750°C or higher, 800°C or higher, 850°C or higher, 900°C or higher, 950°C or higher, or 1000°C or higher, and may be 1600°C or lower, 1550°C or lower, 1500°C or lower, 1450°C or lower, 1400°C or lower, 1350°C or lower, 1300°C or lower, 1250°C or lower, 1200°C or lower, 1150°C or lower, 1100°C or lower, or 1050°C or lower. In addition, the inert gas is not particularly limited as long as it is an inert gas commonly used in the industry, but may include, for example, one or more selected from the group consisting of nitrogen, argon, and helium.

[0135]

[0136] In one embodiment of the present invention, the aggregate may be stabilized prior to the carbonization step. Specifically, the temperature may be raised to 200°C to 300°C at a heating rate of 1 to 15°C / min, then the aggregate may be stabilized by heat treatment for 0.5 to 24 hours under an oxygen-containing atmosphere, and then the stabilized aggregate may be carbonized. The heating rate may be 1°C / min or more, 2°C / min or more, 3°C / min or more, 4°C / min or more, 5°C / min or more, or 6°C / min or more, and may be 15°C / min or less, 14°C / min or less, 13°C / min or less, 12°C / min or less, 11°C / min or less, or 10°C / min or less. In addition, the above-mentioned temperature may be 200°C or higher, 210°C or higher, 220°C or higher, 230°C or higher, 240°C or higher, or 250°C or higher, and may be 300°C or lower, 290°C or lower, 280°C or lower, 270°C or lower, or 260°C or lower.

[0137]

[0138] Negative electrode for sodium secondary battery

[0139] A third embodiment of the present invention relates to a negative electrode for a sodium secondary battery comprising the negative electrode active material.

[0140]

[0141] For example, the cathode of the present invention may include a current collector; and the aforementioned cathode active material layer formed on at least one surface of the current collector.

[0142] The above current collector is a metal that has high conductivity and allows the negative electrode active material slurry to adhere easily, and is not particularly limited as long as it is non-reactive within the operating voltage range of the battery. For example, aluminum (Al), copper (Cu), gold (Au), nickel (Ni), titanium (Ti), calcined carbon, stainless steel, aluminum alloy (e.g., aluminum-cadmium alloy, etc.), or a mesh or foil made of copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., or a combination thereof, may be used. The thickness of such a current collector is not particularly limited and can typically be in the range of 3 to 500 μm.

[0143]

[0144] The above-mentioned negative electrode active material layer can be manufactured by coating a negative electrode slurry comprising the aforementioned negative electrode active material, binder and solvent, and, if necessary, a conductive material, on at least one surface of a current collector, drying it, and, if necessary, rolling it.

[0145] As the description of the above-mentioned cathode active material is as previously stated, a detailed description thereof is omitted.

[0146] 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.

[0147]

[0148] In addition, the above binder serves to bind the negative electrode active material particles together and to attach the negative electrode active material to the current collector, and is not particularly limited as long as it is one commonly used in the industry. Examples include polyvinylidene fluoride-co-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene-butadiene rubber (SBR), lithium-substituted polyacrylate (Li-PAA), etc., which can be used alone or in combination of two or more. The content of such binder is not particularly limited, but, for example, may be 0.1 to 20 weight percent based on the total amount of the cathode slurry.

[0149]

[0150] 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 chemically reacting with the components of the battery. For example, natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon nanotubes, graphene, conductive fibers (e.g., carbon fibers or metal fibers), metal powders (e.g., copper, aluminum, nickel, silver powders, 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 may be used. The content of such conductive material may be 0.1 to 20 weight% based on the total amount of the cathode slurry composition.

[0151]

[0152] In addition, non-limiting examples of the above solvent include water, dimethyl sulfoxide, N-methyl pyrrolidone, and dimethylformamide. The content of these solvents is not particularly limited, and an appropriate amount may be used to ensure that the cathode slurry has a desirable viscosity. Furthermore, a solvent in which a small amount of alcohol is mixed with water may be used to increase the solubility and dispersibility of the binder. In this case, the alcohol may include one or more selected from the group consisting of ethanol and isopropanol.

[0153]

[0154] In addition, the coating method of the above cathode slurry is not particularly limited as long as it is a method commonly used in the industry, and, for example, slot die coating method, gravure coating method, immersion coating method or spray coating method may be applied.

[0155]

[0156] sodium secondary battery

[0157] A fourth embodiment of the present invention relates to a sodium secondary battery comprising the cathode, the anode, a separator interposed between them, and an electrolyte.

[0158] In addition, the present invention provides a secondary battery comprising the aforementioned negative electrode.

[0159] The sodium secondary battery of the present invention may include the aforementioned negative electrode, positive electrode, electrolyte, and separator.

[0160]

[0161] The above 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. Alternatively, the anode can be manufactured by laminating an anode active material film, which is cast onto a separate support and then peeled off from the support, onto a metal current collector.

[0162] 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 art for sodium secondary batteries. Non-limiting examples of positive electrode active materials include Na x CoO2(here, 0 <x≤1), Na x Co 2 / 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 There are sodium-containing transition metal oxides such as PO4, Na2FePO4F, Na3V2(PO4)3, NaFeSO4F, and mixtures thereof.

[0163] 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.

[0164]

[0165] In addition, the above separator can serve as a channel for sodium ions to move, while simultaneously preventing the anode and cathode from coming into contact with each other.

[0166] The above-mentioned separator is not particularly limited as long as it is used as a separator in the industry, and in particular, it may be one that exhibits low resistance to sodium ion movement and has excellent electrolyte moisture retention capacity.

[0167] For example, the separator may be selected from glass fiber, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or a combination thereof, and may be in the form of a nonwoven or woven fabric. In one embodiment, a polyolefin-based polymer separator such as polyethylene or polypropylene may be mainly used in a sodium secondary battery, and a coated separator containing ceramic components or polymer materials may be used to improve heat resistance or mechanical strength. In addition, the separator may be formed in a single-layer structure or a multi-layer structure.

[0168]

[0169] The above electrolyte may include a non-aqueous solvent and an electrolyte salt, and may optionally further include additives such as an overcharge prevention agent.

[0170]

[0171] The above-mentioned non-aqueous solvent is not particularly limited as long as it is commonly used as a solvent for non-aqueous electrolytes, and cyclic carbonates, linear carbonates, lactones, ethers, esters, or ketones may be used.

[0172]

[0173] Examples of the above-mentioned cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), etc. 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, and 1,2-dimethoxyethane, etc. Additionally, examples of the above-mentioned esters include n-methyl acetate, n-ethyl acetate, methyl propionate, and 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.

[0174] In addition, the electrolyte salt is not particularly limited as long as it is an electrolyte salt commonly used for non-aqueous electrolytes. Non-limiting examples of the above electrolyte salt include A + B - As a salt with a structure like that, A + is Li + , Na + , K + It includes alkali metal cations such as or ions composed of combinations thereof, and B - is 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.

[0175] 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, housing the electrode assembly in a pouch-type, cylindrical, or prismatic battery case, and injecting an electrolyte. Alternatively, it can be manufactured by stacking the electrode assemblies, impregnating them with an electrolyte, housing the resulting stack in a battery case, and sealing it.

[0176]

[0177] These sodium secondary batteries can be applied not only to battery cells used as power sources for small devices, but also to unit cells of 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, and they can be particularly useful for hybrid electric vehicles requiring high output and batteries for storing new and renewable energy.

[0178] Preferred embodiments are presented below to aid in understanding the present invention. However, the following embodiments are merely illustrative of the invention and are not limited to the scope and technical concept of the invention. It is obvious to those skilled in the art that various changes and modifications are possible within the scope and technical concept of the invention, and such changes and modifications should also be interpreted as falling within the scope of the appended claims.

[0179]

[0180] Example 1

[0181] (1) Manufacturing of negative electrode active material

[0182] Polyacrylonitrile (PAN), a carbon precursor, was mixed with dimethylformamide (DMF), a first solvent, to obtain a first solution having a solid content of 15 wt%.

[0183] A sacrificial polymer, styrene(co-acrylonitrile) (SAN), was mixed with a second solvent, DMF, to obtain a second solution having a solid content of 30 wt%.

[0184] After mixing the first and second solutions, the mixture was dried on a hot plate at 80°C for 24 hours. After drying, the sacrificial polymer SAN was selectively removed using a centrifuge with acetone as the solvent to obtain aggregates. The aggregates were formed by the aggregation of polymeric microspheres (PMS) (hereinafter, the aggregates are referred to as “PAN-containing aggregates”).

[0185] Afterward, the above PAN-containing aggregate was immersed in a 5N NaOH aqueous solution and stirred for 12 hours to perform alkaline hydrolysis. At this time, the mass ratio of the PAN-containing aggregate to NaOH was adjusted to 1:4. After stirring, the solution was washed with distilled water until the remaining solution reached a pH of 7.

[0186] Subsequently, the above PAN-containing aggregate was dried in a vacuum oven at a temperature of 120°C for 8 hours to remove moisture, and then carbonized to 1000°C at a heating rate of 10°C / min. During the carbonization process, a stabilization process was performed by heat treating at a heating rate of 5°C / min in the 270°C to 300°C range to produce a cathode active material.

[0187]

[0188] (2)Cathode manufacturing

[0189] A slurry 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 water at a weight ratio of 92:5:1.5:1.5. The first binder was in the form of a solution with a concentration of 1.5 wt% obtained by adding CMC to distilled water, and an appropriate amount was used to control the concentration. The second binder was in the form of a solution with a concentration of 40 wt% obtained by adding SBR to distilled water, and an appropriate amount was used to control the adhesion. In addition, the concentration of the slurry was set to 50 wt% based on the weight of the solid content, and the solid content refers to the weight of the slurry excluding water.

[0190] After that, the above slurry was applied to an aluminum foil, which is a cathode current collector, to a thickness of 800 μm, dried for 8 hours under vacuum conditions at 120°C, and rolled to achieve a porosity of 30% to manufacture a cathode.

[0191]

[0192] (3) Sodium secondary battery manufacturing

[0193] 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, and sodium metal was used as the counter electrode. Glass fiber (KFP / MFV-3(GFC) GLASS MICROFIBER FILTERS) was used as the separator, and as the electrolyte, an electrolyte solution was used in which NaPF6 was dissolved at a concentration of 1M in a mixed solvent containing EC, PC, and DEC (EC:PC:DEC=1:1:1, EC: ethylene carbonate, PC: propylene carbonate, DEC: diethyl carbonate).

[0194]

[0195] Comparative Example 1

[0196] A negative electrode active material, a negative electrode, and a sodium secondary battery were prepared in the same manner as in Example 1, except that the alkaline hydrolysis process was not performed.

[0197]

[0198] Experimental Example 1: Surface Analysis using SEM - PAN-Containing Aggregates

[0199] Surface analysis of the PAN-containing aggregates was performed prior to the carbonization process. Prior to the carbonization process, the aggregates of Example 1 underwent alkaline hydrolysis, while prior to the carbonization process, the aggregates of Comparative Example 1 underwent alkaline hydrolysis.

[0200] The above surface analysis was performed using a scanning electron microscope (SEM, HITACHI, S-4700).

[0201]

[0202] Figure 1 shows the results of SEM surface analysis of PAN-containing aggregates depending on whether alkaline hydrolysis was performed prior to the carbonization process.

[0203] Referring to FIG. 1, the surface roughness of the PAN-containing aggregates of Example 1 and Comparative Example 1 was found to be similar. From this, it can be seen that the alkaline hydrolysis does not have a significant effect on the surface roughness.

[0204]

[0205] Experimental Example 2: Surface Analysis using FT-IR - PAN-Containing Aggregates

[0206] Surface analysis of PAN-containing aggregates was performed prior to the carbonization process. Prior to the carbonization process, the aggregates of Example 1 underwent alkaline hydrolysis, while prior to the carbonization process, the aggregates of Comparative Example 1 underwent alkaline hydrolysis.

[0207] The above surface analysis was performed using an FT-IR spectrometer (Fourier-transform infrared spectrometer, PerkinEmer, Spotlight 400).

[0208]

[0209] Figure 2 shows the FT-IR surface analysis results of PAN-containing aggregates depending on whether alkaline hydrolysis was performed before the carbonization process.

[0210] Referring to FIG. 2, Example 1 has 2240 cm corresponding to C≡N compared to Comparative Example 1. -1 (A) It can be seen that the content of C≡N bonds has decreased as the transmittance at wavelength 1650 cm -1 (B) and 1620 cm -1 (C) It can be seen that the content of C=O and NH₂ / COO groups increased as the peak increased.

[0211] Therefore, in Example 1, it can be confirmed that the C≡N bonds contained in the PAN-containing aggregate were broken down into C=O and NH₂ / COO groups on the surface of the PAN-containing aggregate by the alkaline hydrolysis. In addition, since the C=O and NH₂ / COO groups formed on the surface of the PAN-containing aggregate induce a cyclization reaction during the carbonization process, it is considered to be more favorable for the formation of closed pores.

[0212]

[0213] Experimental Example 3: Surface Analysis using NMR - PAN-Containing Aggregates

[0214] Surface analysis of the PAN-containing aggregates was performed prior to the carbonization process. Prior to the carbonization process, the PAN-containing aggregates of Example 1 underwent alkaline hydrolysis, while prior to the carbonization process, the PAN-containing aggregates of Comparative Example 1 underwent alkaline hydrolysis.

[0215] The above surface analysis was performed using an NMR spectrometer (Nuclear Magnetic Resonance, Bruker BioSpin GmbH, Magnet System 500'54 Ascend).

[0216]

[0217] Figure 3 shows PAN-containing aggregates depending on whether alkaline hydrolysis is performed before the carbonization process. 1 This shows the results of the H-NMR surface analysis.

[0218] Referring to Figure 3, Example 1 showed an increase in peak intensity around 3.45 ppm and a decrease in peak intensity around 3.5 ppm. From this, it can be seen that the nitrile (C≡N) group contained in the PAN-containing aggregate was hydrolyzed into an amide (-CONH2) group and a carboxyl (-COOH) group. It can be inferred that the electron density increased and hydrogen bonding increased due to the hydrolysis, resulting in a shift to a lower ppm value.

[0219]

[0220] Experimental Example 4: Surface Analysis Using XPS - PAN-Containing Aggregates

[0221] Surface analysis of PAN-containing aggregates was performed prior to the carbonization process to confirm their elemental composition and molecular structure. Prior to the carbonization process, the PAN-containing aggregates of Example 1 underwent alkaline hydrolysis, while prior to the carbonization process, the PAN-containing aggregates of Comparative Example 1 underwent alkaline hydrolysis.

[0222] The above surface analysis was performed using an XPS device (X-ray Photoelectron Spectroscopy, Thermo Fisher Scientific, K-Alpha+).

[0223]

[0224] Figure 4 is a graph showing the results of the XPS elemental composition analysis of PAN-containing aggregates depending on whether alkaline hydrolysis was performed before the carbonization process.

[0225] Referring to Fig. 4, it was found that Example 1 had a significantly higher O content compared to Comparative Example 1. From this, it can be seen that in Example 1, the nitrile (C≡N) group contained in the PAN-containing aggregate was hydrolyzed into an amide (-CONH2) group and a carboxyl (-COOH) group, and the O content increased.

[0226]

[0227] Figures 5a to 5c are graphs showing the results of XPS molecular structure analysis of PAN-containing aggregates depending on whether the alkaline hydrolysis was performed (5a: C=O molecular structure; 5b: O=CN molecular structure; 5c: CO molecular structure).

[0228] Referring to Fig. 5a, it was shown that Comparative Example 1 did not have a peak corresponding to the C=O(CO) molecular structure, whereas Example 1 had a peak corresponding to the C=O molecular structure. From this, it can be seen that in Example 1, the nitrile (C≡N) group contained in the PAN-containing aggregate was hydrolyzed into an amide (-CONH2) group and a carboxyl (-COOH) group, thereby forming a C=O molecular structure.

[0229] Referring to Fig. 5b, it was shown that the peak corresponding to the O=CN molecular structure increased in Example 1 compared to Comparative Example 1. From this, it can be seen that in Example 1, an amide (-CONH2) group was formed by hydrolysis.

[0230] Referring to Fig. 5c, it was shown that the peak corresponding to the CO molecular structure increased in Example 1 compared to Comparative Example 1. From this, it can be seen that in Example 1, a carboxyl (-COOH) group was formed by hydrolysis.

[0231]

[0232] Experimental Example 5: Thermal Properties Analysis - PAN-Containing Aggregates

[0233] The thermal characteristics of the PAN-containing aggregates before the carbonization process were analyzed to confirm structural changes depending on whether alkaline hydrolysis was performed. The PAN-containing aggregates of Example 1 before the carbonization process underwent alkaline hydrolysis, while the PAN-containing aggregates of Comparative Example 1 before the carbonization process underwent alkaline hydrolysis.

[0234] The above thermal characteristic analysis was performed using Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and Derivative Thermogravimetry (DTG). A DSC device (PerkinElmer, DSC 4000), a TGA device (PerkinElmer, Spectrum Two FTIR), and a DTG device (PerkinElmer, Spectrum Two FTIR) were used.

[0235]

[0236] Figures 6a to 6c are graphs showing the results of the thermal characteristics analysis of PAN-containing aggregates depending on whether alkaline hydrolysis was performed (6a: DSC graph; 6b: TGA graph; 6c: DTG graph).

[0237] Referring to Figure 6a and Table 1 below, Example 1, which underwent alkaline hydrolysis, showed a slight decrease in the exothermic peak temperature related to the cyclization reaction compared to Comparative Example 1, and the enthalpy (ΔH) also decreased by 4% compared to Comparative Example 1. From these decreases in the exothermic peak temperature and enthalpy related to the cyclization reaction, it can be seen that Example 1 has a change in its chemical structure due to alkaline hydrolysis.

[0238]

[0239] Whether alkaline hydrolysis was performed Exothermic peak temperature related to cyclization reaction (°C) ΔH (J / g) Comparative Example 1 X 3 1 2 4 5 1.53 Example 1 O 3 1 1.8 4 3 3.46

[0240]

[0241] Referring to Fig. 6b, the TGA residual carbon content of Comparative Example 1 and Example 1 was 31.4% and 30.5%, respectively, and it was shown that the TGA residual carbon content of Example 1 was reduced compared to Comparative Example 1 due to alkaline hydrolysis.

[0242] Referring to Fig. 6c, Example 1 showed a reduced melting point compared to Comparative Example 1 due to alkaline hydrolysis.

[0243] Polyacrylic acid (PAA) contains carboxyl (-COOH) groups and initiates thermal decomposition at a lower temperature compared to PAN when heat-treated. Therefore, when carboxyl (-COOH) groups are formed by alkaline hydrolysis of the PAN-containing aggregate as in Example 1, thermal decomposition is initiated at a lower temperature compared to Comparative Example 1, which did not undergo alkaline hydrolysis, and a lower residual carbon content is exhibited.

[0244] From this, it can be seen that the nitrile (C≡N) group contained in the PAN-containing aggregate is hydrolyzed to form amide (-CONH2) group and carboxyl (-COOH) group, and that thermal decomposition behavior occurs due to these groups, and it can be inferred that a structure favorable for closed pores is formed by the hydrolysis.

[0245]

[0246] Experimental Example 6: Microstructure Analysis - PAN-containing aggregates

[0247] Microstructural analysis of PAN-containing aggregates was performed prior to the carbonization process to confirm structural changes depending on whether alkali hydrolysis was performed. Prior to the carbonization process, the aggregates of Example 1 underwent alkali hydrolysis, and prior to the carbonization process, the aggregates of Comparative Example 1 underwent alkali hydrolysis.

[0248] The above microstructure analysis was performed using X-ray diffraction (XRD), and an XRD device (Rigaku, Ultima IV) was used.

[0249]

[0250] Figure 7 is a graph showing the XRD analysis results of PAN-containing aggregates depending on whether the above alkaline hydrolysis was performed.

[0251] Referring to Figure 7 and Table 2 below, it was found that the XRD graphs of Comparative Example 1 and Example 1 are similar, and the measurement results for the crystal structure are similar. From this, it can be seen that the alkali hydrolysis may affect the amorphous region but does not affect the crystalline region. Therefore, since the alkali hydrolysis affects the amorphous region, it can be inferred that if a structure favorable to closed pores is formed by the alkali hydrolysis, it will be formed in the amorphous region.

[0252]

[0253] d 100 (nm)d 110 (nm)FWHM 100 (degree)FWHM 110 (degree) Comparative Example 10.5230.3031.4021.723 Example 10.5220.3031.4381.686d 100 : (100) Interplane distance d corresponding to the crystal plane 110 : (110) Interplanar distance corresponding to the crystal plane FWHM 100 : (100) Full width at half maximum of the diffraction peak 110 : (110) Full width at half maximum of the diffraction peak

[0254]

[0255] Experimental Example 7: Surface Analysis Using SEM - Cathode Active Material

[0256] Surface analysis was performed on the cathode active material manufactured after the above carbonization process. The cathode active material of Example 1 underwent alkaline hydrolysis prior to the above carbonization process, and the cathode active material of Comparative Example 1 did not undergo alkaline hydrolysis prior to the above carbonization process.

[0257] The above surface analysis was performed using a scanning electron microscope (SEM, HITACHI, S-4700).

[0258]

[0259] Figure 8 shows the results of SEM surface analysis of the cathode active material depending on whether alkaline hydrolysis was performed before the carbonization process.

[0260] Referring to FIG. 8, Example 1 was found to have a larger surface roughness compared to Comparative Example 1. From this, it can be seen that the alkaline hydrolysis increases the surface roughness of the cathode active material.

[0261]

[0262] Experimental Example 8: Surface Analysis Using XPS - Cathode Active Material

[0263] The elemental composition was confirmed by performing a surface analysis of the cathode active material produced after the above carbonization process. The cathode active material of Example 1 was subjected to alkaline hydrolysis prior to the above carbonization process, and the cathode active material of Comparative Example 1 was not subjected to alkaline hydrolysis prior to the above carbonization process.

[0264] The above surface analysis was performed using an XPS device (X-ray Photoelectron Spectroscopy, Thermo Fisher Scientific, K-Alpha+).

[0265]

[0266] Figure 9 is a graph showing the elemental composition of the cathode active material as an XPS analysis result depending on whether alkaline hydrolysis was performed before the carbonization process.

[0267] Referring to FIG. 9, it was found that the O content in Example 1 was significantly higher than that in Comparative Example 1. From this, it can be seen that in Example 1, the nitrile (C≡N) group contained in the cathode active material was hydrolyzed into an amide (-CONH2) group and a carboxyl (-COOH) group, and the O content increased.

[0268]

[0269] Experimental Example 9: Specific Surface Area and Pore Analysis - Cathode Active Material

[0270] The specific surface area and pores of the cathode active material produced after the carbonization process were analyzed. The cathode active material of Example 1 underwent alkaline hydrolysis prior to the carbonization process, and the cathode active material of Comparative Example 1 did not undergo alkaline hydrolysis prior to the carbonization process.

[0271] The above analysis of specific surface area and pores was performed using the following method. The Autosorb IQ from Quantachrome was used as the analysis device.

[0272]

[0273] (1) Specific surface area

[0274] 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 1 below.

[0275]

[0276] <Equation 1>

[0277] S = (Vm·N·A) / V

[0278] N: Avogadro's number (6.022 x 10⁻⁶ 23 )

[0279] A: Unit area of ​​a nitrogen molecule (0.162 nm) 2 )

[0280] V: Volume occupied by 1 mole of nitrogen gas (approx. 22.414 L at standard temperature and pressure)

[0281]

[0282] Figure 10 is a graph showing the results of the specific surface area analysis of the cathode active material according to whether alkaline hydrolysis was performed before the carbonization process, and Figure 11 is a graph showing the results of the pore analysis of the cathode active material according to whether alkaline hydrolysis was performed before the carbonization process.

[0283] Referring to FIGS. 10, FIGS. 11 and Table 3 below, it was found that Example 1 had a larger specific surface area and pore volume compared to Comparative Example 1. From this, it can be seen that the specific surface area and pore volume of the cathode active material increase due to the alkaline hydrolysis. It can be inferred that these results are due to the increased formation of micropores caused by the alkaline hydrolysis.

[0284]

[0285] (Carbonization temperature: 1000℃) Specific surface area (㎡ / g) Pore volume (cc / g) Comparative Example 13.5 340.01536 Example 15 3.9 90.04077

[0286]

[0287] Experimental Example 10: Microstructure Analysis - Cathode Active Material

[0288] Microstructure analysis was performed on the cathode active material prepared after the above carbonization process to confirm structural changes depending on whether alkaline hydrolysis was performed. The cathode active material of Example 1 underwent alkaline hydrolysis before the carbonization process, and the cathode active material of Comparative Example 1 underwent alkaline hydrolysis before the carbonization process.

[0289] The above microstructure analysis was performed using X-ray diffraction (XRD) and Raman spectroscopy. Rigaku's Ultima IV was used as the XRD device, and HORIBA's XploRA Plus was used as the Raman spectrometer.

[0290]

[0291] Figure 12 is a graph showing the XRD analysis results of the cathode active material depending on whether alkaline hydrolysis was performed before the carbonization process, and Figure 13 is a graph showing the Raman spectroscopic analysis results of the cathode active material depending on whether alkaline hydrolysis was performed before the carbonization process.

[0292] Referring to FIGS. 12, FIGS. 13 and Table 4 below, it was found that the distance between carbon layers and the degree of disorder of the microstructure in Example 1 increased, and the carbon crystals became smaller. This indicates that the PAN-containing aggregates, whose surfaces were modified by alkaline hydrolysis, underwent cyclization and cross-linking reactions during the carbonization process to form a carbon microstructure favorable for closed pore formation.

[0293]

[0294] (Carbonization temperature: 1000℃)d 002 (nm)L c (nm)L a (nm)I D / I G Comparative Example 13.57 11.13 36.46 0.97 Example 13.6 09.8 2 31.5 20.96d 002 : Distance between carbon layers L c : Vertical size of carbon crystal L a : Horizontal size of carbon crystal I D / I G : Ratio of the intensity of the D band to the G band in a Raman spectrum (a decrease indicates an increase in disorder)

[0295]

[0296] Experimental Example 11: Battery Performance Analysis

[0297] Using a galvanostatic cell tester (WonATech, WBC3000L), the initial charge capacity of the sodium secondary batteries prepared in Example 1 and Comparative Example 1 was measured under the following sodiation and desodiation conditions.

[0298]

[0299] (1) Sodium conditions

[0300] -CC-CV section: CC switches to CV mode when it reaches 0.01V, and CV mode switches to the discharge process when it reaches 0.05C.

[0301] - CC mode conditions per cycle: 0.1C (1~2 cycles), 0.3C (3~500 cycles)

[0302] - CV mode condition per cycle: 0.01V (1~500 cycles)

[0303]

[0304] (2) Desodium conditions

[0305] -CC Section: CC mode switches to the next stage, Rest, when 2V is reached.

[0306] - CC mode conditions per cycle: 0.1C (1~2 cycles), 0.3C (3~500 cycles)

[0307]

[0308] Figure 14 shows the measurement results of charge capacity and Coulomb efficiency for the sodium secondary batteries of Example 1 and Comparative Example 1.

[0309] Referring to FIG. 14, the initial charge capacity of Example 1 and Comparative Example 1 was measured to be 194 mAh / g (Example 1 SC) and 132 mAh / g (Comparative Example 1 SC), respectively, and accordingly, it can be seen that the initial charge capacity of Example 1 is improved by about 47% compared to Comparative Example 1.

[0310] In addition, the initial Coulomb efficiency of Example 1 and Comparative Example 1 is 60% (Example 1 CE) and 48% (Comparative Example 1 CE), respectively, and it can be confirmed that the initial Coulomb efficiency of Example 1 is also superior to that of Comparative Example 1.

[0311]

[0312] From this, it can be confirmed that in Example 1, the surface is modified in a way that facilitates the formation of closed pores by alkaline hydrolysis during the manufacturing process of the negative electrode active material, and as a result, the negative electrode active material with closed pores significantly improves the initial charge capacity and initial Coulomb efficiency of the sodium secondary battery.

[0313]

[0314] Experimental Example 12: Measurement of closed hole volume

[0315] To confirm the correlation between alkaline hydrolysis and closed pores, the volume of closed pores formed in the cathode active materials prepared in Example 1 and Comparative Example 1 was measured.

[0316]

[0317] (1) BET analysis

[0318] The volume of open pores formed in the cathode active material was measured using BET (Brunauer-Emmett-Teller) analysis. Since BET analysis is an analytical method that utilizes nitrogen gas, and nitrogen cannot penetrate into closed pores, the volume of open pores (V) was determined through BET analysis. BET ) was measured.

[0319]

[0320] (2) SAXS analysis

[0321] The total pore volume formed in the cathode active material was measured through SAXS (Small-Angle X-ray Scattering) analysis. The total pore volume is the sum of the volumes of open pores and closed pores. The SAXS analysis uses X-rays to measure the total pore volume (V) including open and closed pores. SAXS ) can be produced.

[0322] Finally, the volume of the closed hole (V closed pore ) is the total pore volume (V) mentioned above. SAXS In ), the volume of the above opening (V BET It can be obtained by subtracting ).

[0323]

[0324] Table 5 below shows the total pore volume (V) obtained through the above BET and SAXS analyses. SAXS ), volume of open pores (V BET ), volume of closed holes (V closed pore This represents the volume ratio (volume%) of closed pores to the total pore volume.

[0325]

[0326] V SAXS V BET V closed pore Volume ratio of closed pores (Volume %) Comparative Example 10.02730.01190.015456% Example 10.03750.01540.022160%

[0327]

[0328] Referring to Table 5 above, it can be seen that the volume ratio of closed pores in Example 1 is relatively higher than that of Comparative Example 1. This result indicates that Example 1 is the result of performing alkaline hydrolysis during the preparation of the cathode active material.

[0329] In addition, FIG. 15 is a graph showing the pore size distribution of the cathode active materials of Example 1 and Comparative Example 1, based on data obtained through SAXS analysis. Referring to FIG. 15, Example 1, which underwent alkaline hydrolysis, exhibited a higher volume fraction in the micropore region of approximately 10 Å or less compared to Comparative Example 1, confirming that alkaline hydrolysis had a significant effect on the formation of micropores. The micropore distribution of 10 Å or less confirmed by the SAXS analysis is a region that is not detected in BET analysis, which is a method of adsorbing nitrogen gas. This implies that the micropores exist in a closed pore state isolated from the outside, and consequently, it can be seen that Example 1 contains more closed pores than Comparative Example 1.

[0330]

[0331] Although the present invention has been described above by limited embodiments and drawings, the present invention is not limited thereto, and it is obvious that various modifications and variations are possible within the scope of the technical spirit of the present invention and the equivalent scope of the claims described below by those skilled in the art to which the present invention belongs.

Claims

A negative electrode active material for a sodium secondary battery having a porous structure and comprising carbon secondary particles formed by assembling a plurality of carbon primary particles, The above-mentioned negative electrode active material includes closed pores, and A negative electrode active material for a sodium secondary battery, wherein the nitrogen content present on the surface of the negative electrode active material is 2 to 4 weight% based on the total weight of the negative electrode active material, and the oxygen content is 4 to 8 weight% based on the total weight of the negative electrode active material. In paragraph 1, The above secondary particles are spherical particles, a negative electrode active material for a sodium secondary battery. 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. 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. In paragraph 1, A negative electrode active material for a sodium secondary battery, wherein the specific surface area of ​​the above negative electrode active material is 40 to 70 m² / g. In paragraph 1, A negative electrode active material for a sodium secondary battery, wherein the pore volume of the negative electrode active material is 0.02 to 0.06 cc / g. In paragraph 1, A negative electrode active material for a sodium secondary battery, wherein the volume of the above closed pores is 58 volume% or more based on the total pore volume of the above negative electrode active material. (S1) A step of obtaining a first solution by dissolving polyacrylonitrile (PAN) in a first solvent; (S2) A step of adding a sacrificial polymer to a second solvent to obtain a second solution; (S3) A step of obtaining an aggregate by mixing the first solution and the second solution, drying, and removing the sacrificial polymer using an organic solvent; (S4) A step of surface modification by hydrolyzing the aggregate with an alkaline solution; and (S5) A step of carbonizing the surface-modified aggregate; A method for manufacturing a negative electrode active material for a sodium secondary battery, comprising In paragraph 8, 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), poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA), polymethacrylate (PMMA), and poly(vinylidene fluoride) (PVDF). In paragraph 8, 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 dimethylformamide, diethyl ether, ethanol, methanol, n-propanol, isopropyl alcohol, acetone, n-pentane, ethylene dichloride, methyl acetate, ethyl acetate, acetonitrile, tetrahydrofuran, n-hexane, chlorohexane, chloropentane, carbon tetrachloride, 1,2-dichloroethane, 1,2-dichloroethylene, trichloroethylene, methyl ethyl ketone, and 1,2-dimethoxyethane. In paragraph 8, A method for manufacturing a negative electrode active material for a sodium secondary battery, wherein the above alkaline solution comprises one or more selected from the group consisting of sodium hydroxide and potassium hydroxide. In paragraph 8, 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℃. In paragraph 8, 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. A negative electrode for a sodium secondary battery comprising the negative electrode active material of claim 1. A sodium secondary battery comprising the negative electrode, the positive electrode, a separator interposed between them, and an electrolyte according to claim 14.

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