Preparation method of negative electrode material and application thereof

Abstract

The application relates to a preparation method of a negative electrode material and application thereof, and comprises the following steps: S1, carbonization treatment of ZIF-67 under an inert atmosphere to obtain a carbonization product; S2, hydrothermal reaction of the carbonization product and selenium powder in the presence of a reducing agent and water to obtain a hydrothermal product; S3, reaction of the hydrothermal product with a phenolic monomer, an organic polymer and an organic phosphorus compound in the presence of a solvent, separation of a solid after cooling, and transfer of the solid to an inert atmosphere after washing and drying for carbonization treatment to obtain the negative electrode material. The application forms a nitrogen-doped porous cobalt diselenide material with stable structure and uniform selenium element distribution through in-situ seleniumization of hydrothermal, then forms a coating layer of P-uniformly-doped hard carbon on the surface through in-situ polymerization and carbonization, the prepared negative electrode material has high energy density, excellent structural stability and electron / ion transmission, so that a secondary battery containing the negative electrode material has high capacity and excellent rate performance and cycle performance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and specifically to a method for preparing a negative electrode material and its application. Background Technology

[0002] With the rapid development of renewable energy storage and electric vehicles, the shortage and rising cost of lithium resources are becoming increasingly prominent issues. Sodium-ion batteries, due to their abundant sodium resources, low cost, and similar electrochemical mechanism to lithium-ion batteries, have become an important alternative technology in the field of large-scale energy storage. However, the relatively large radius of sodium ions (…) vs Lithium-ion This results in slow diffusion kinetics and significant volume expansion in traditional anode materials, severely restricting the cycle stability and rate performance of the battery.

[0003] Existing anode materials for sodium-ion batteries are mainly divided into metal oxides, metal alloys, and carbon. Metal oxides have a stable crystal structure, relatively small volume change during charge-discharge cycles, and long cycle life, but poor conductivity and low capacity, making them unsuitable for batteries with high energy density requirements. Metal alloys have a theoretically high capacity, meeting high energy density requirements, but significant volume expansion during the reaction process, and separation of the active material from the electrical contacts, leads to poor cycle performance. For cost considerations, the most common anode material used in batteries is carbon-based, which mostly has a porous structure, making it more suitable for the insertion and extraction of sodium ions. Currently, carbon-based materials for sodium-ion batteries are mainly divided into soft carbon and hard carbon. Soft carbon has low cost and high conductivity, but its small interlayer spacing makes sodium ion insertion / extraction difficult, resulting in low sodium storage capacity. Hard carbon has a larger interlayer spacing and nanopores, providing more sodium storage sites, and its sodium storage capacity is significantly higher than soft carbon. It is currently the closest to commercialization among anode materials, but sodium-ion batteries constructed with it suffer from low initial coulombic efficiency, poor rate performance, and insufficient long-term cycle stability.

[0004] Given the shortcomings of current sodium-ion battery anode materials in terms of energy density, rate performance, and cycle stability, there is an urgent need for a sodium-ion battery anode material that combines high energy density, excellent rate performance, and cycle stability in order to promote the practical application of sodium-ion batteries. Summary of the Invention

[0005] Given the significant shortcomings of current sodium-ion battery anode materials in terms of energy density, rate performance, and cycle stability, which hinder the practical application of sodium-ion batteries, this invention provides a method for preparing an anode material and its application. Using ZIF-67 carbonization products as a template, a nitrogen-doped porous cobalt diselenide material with a stable structure and uniform selenium distribution is formed through hydrothermal in-situ selenization. Then, a uniformly doped hard carbon coating layer is formed on its surface through in-situ polymerization and carbonization. The prepared anode material has high energy density, excellent structural stability, and electron / ion transport, thus enabling secondary batteries containing the above-mentioned anode material to have both high capacity and excellent rate and cycle performance.

[0006] This invention provides the following technical solutions:

[0007] The first aspect of this invention provides a method for preparing a negative electrode material, comprising the following steps:

[0008] S1. ZIF-67 is carbonized under an inert atmosphere to obtain carbonized products;

[0009] S2. The carbonized product and selenium powder are subjected to a hydrothermal reaction in the presence of a reducing agent and water, and the solid is separated to obtain the hydrothermal product.

[0010] S3. The hydrothermal product is reacted with phenolic monomers, organic polymers, and organophosphorus compounds in the presence of a solvent. After the reaction is complete, the solid is separated by cooling. The solid is washed, dried, and then transferred to an inert atmosphere for carbonization treatment to obtain the negative electrode material. The organic polymer is obtained by polymerization of one or more aldehyde monomers.

[0011] Furthermore, in step S1, the ZIF-67 has a dodecahedral structure and is a Co-containing MOF material.

[0012] Furthermore, the ZIF-67 with a dodecahedral structure is synthesized by precipitation method, specifically as follows: cobalt nitrate hexahydrate is dissolved in methanol to obtain solution A; 2-methylimidazole is dissolved in methanol to obtain solution B; solution B is added to solution A, stirred evenly, and allowed to stand. After crystal precipitation, the solution is filtered, washed, and dried to obtain the ZIF-67 with a dodecahedral structure.

[0013] Furthermore, in step S1, the carbonization treatment temperature is preferably 500-800℃, and the time is preferably 1.5-3h.

[0014] Further, in step S2, the carbonized product is first dispersed in water and stirred evenly to form dispersion A. The selenium powder and the reducing agent are then mixed evenly in water to form dispersion B. Dispersion B is then added to dispersion A and stirred evenly to carry out a hydrothermal reaction.

[0015] Further, in step S2, the mass ratio of the carbonized product to the selenium powder is preferably 1:(1-3), such as 1:1, 2:3, 1:2, 2:5, 1:3, etc., including but not limited to the mass ratios listed above.

[0016] Further, in step S2, the molar ratio of the selenium powder to the reducing agent is 2:(3-5), for example 2:3, 2:4, 2:5, etc., including but not limited to the mass ratios listed above.

[0017] Furthermore, in step S2, the reducing agent may be selected from one or more of sodium borohydride, hydrazine hydrate, and ethylenediamine.

[0018] Further, in step S2, the molar ratio of the reducing agent to the volume of water is 30-70 mmol:1L, for example, 30 mmol:1L, 40 mmol:1L, 50 mmol:1L, 60 mmol:1L, 70 mmol:1L, etc.

[0019] Furthermore, in step S2, the temperature of the hydrothermal reaction is preferably 150-200℃, and the time is preferably 10-20h.

[0020] Further, in step S3, the hydrothermal product, phenolic monomer, and organic polymer are first dispersed in a solvent and stirred in an oil bath, and then an organophosphorus compound is added to react; preferably, the oil bath stirring temperature is 80-110℃ and the time is 1-3h; the reaction temperature is 80-110℃ and the time is 2-5h.

[0021] Furthermore, in step S3, the phenolic monomer includes one or more of bisphenol A, phenol, cresol, and xylenol.

[0022] Further, in step S3, the organic polymer includes one or more of polyoxymethylene, polyacetaldehyde, polybutyraldehyde, polypentalaldehyde, and polymethacrylaldehyde.

[0023] Further, in step S3, the organophosphorus compound includes one or more of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide, diphenylphosphine oxide, and 3-phosphorylbenzoic acid.

[0024] Furthermore, in step S3, the solvent is a mixture of benzene-based solvents and alcohol-based solvents, such as a mixture of toluene and anhydrous ethanol.

[0025] Further, in step S3, the mass ratio of the hydrothermal product to the phenolic monomer is preferably 1:(2-5), such as 1:2, 2:5, 1:3, 2:7, 1:4, 2:9, 1:5, etc., including but not limited to the mass ratios listed above.

[0026] Furthermore, the molar ratio of the phenolic monomer to the organic polymer is preferably 1:(3-5), for example 1:3, 1:4, or 1:5; the degree of polymerization of the organic polymer is preferably 1000-3500.

[0027] Furthermore, the molar ratio of the phenolic monomer to the organophosphorus compound is preferably 1:(1-3), such as 1:1, 1:2, 1:3, etc., including but not limited to the molar ratios listed above.

[0028] Furthermore, in step S3, after the reaction is complete, the temperature is lowered to -4 to 0°C to separate the solid.

[0029] Furthermore, in step S3, the carbonization treatment temperature is preferably 500-800℃, and the time is preferably 1.5-3h.

[0030] A second aspect of the present invention provides a negative electrode material prepared by the preparation method described in the first aspect.

[0031] A third aspect of the present invention provides a sodium-ion battery, the sodium-ion battery comprising a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active layer disposed on at least one side of the negative electrode current collector along the thickness direction, the negative electrode active layer comprising the negative electrode material described in the second aspect.

[0032] Furthermore, the mass percentage of the negative electrode material in the negative electrode active layer is 60%-80%.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] This invention uses ZIF-67 carbonization products as a template to prepare a stable anode material with polyhedral microporous and mesoporous structures through simple hydrothermal, in-situ polymerization, and carbonization processes. This anode material has a core-shell structure, with a core of nitrogen-doped porous cobalt diselenide with uniformly distributed selenium, and a shell of uniformly p-doped hard carbon. The synergistic effect of these materials and structures allows sodium-ion batteries constructed from this anode material to possess high capacity, excellent rate performance, and cycle stability. Specifically:

[0035] (1) Using ZIF-67 carbonization products as templates, the negative electrode material has a large number of micropores and mesopores. On the one hand, it is beneficial to the insertion and extraction of sodium ions, which improves the structural stability of the material during the charging and discharging process. On the other hand, it is beneficial to the wetting of the electrolyte, which promotes the transport of sodium ions. In addition, it can provide a certain buffer space for the volume expansion of cobalt selenide, thereby further improving the structural stability of the material during the charging and discharging process.

[0036] (2) ZIF-67 carbide products are nitrogen-doped porous substrates with high conductivity, which is beneficial for electron transport.

[0037] (3) Selenium is evenly distributed in the core of the negative electrode material through hydrothermal in-situ selenization reaction. The metal selenide formed combines with the hard carbon of the coating layer, thus ensuring that the negative electrode material has a high capacity.

[0038] (4) The P-doped hard carbon coating layer formed by in-situ polymerization and carbonization has a large interlayer spacing, which is not only conducive to accelerating the transport of sodium ions and electrons, but also conducive to improving the capacity of the material, and to a certain extent suppressing the expansion of cobalt diselenide. Detailed Implementation

[0039] The present invention will be further described below with reference to specific embodiments, so that those skilled in the art can better understand and implement the present invention, but the embodiments are not intended to limit the present invention.

[0040] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term “and / or” as used herein includes any and all combinations of one or more of the associated listed items. The terms “comprising” or “including” as used herein may also be replaced with the closed form “is” or “consisting of”.

[0041] Existing sodium-ion battery anode materials have varying degrees of limitations in terms of energy density, rate performance, or cycle stability, which restricts the advancement of sodium-ion batteries in practical applications.

[0042] Based on this, an embodiment of the present invention provides a method for preparing a negative electrode material, comprising the following steps:

[0043] S1. ZIF-67 is carbonized under an inert atmosphere to obtain carbonized products;

[0044] S2. The carbonized product and selenium powder are subjected to a hydrothermal reaction in the presence of a reducing agent and water, and the solid is separated to obtain the hydrothermal product.

[0045] S3. The hydrothermal product is reacted with phenolic monomers, organic polymers, and organophosphorus compounds in the presence of a solvent. After the reaction is complete, the solid is separated by cooling. The solid is washed, dried, and then transferred to an inert atmosphere for carbonization treatment to obtain the negative electrode material. The organic polymer is obtained by polymerization of one or more aldehyde monomers.

[0046] To address the challenge of existing sodium-ion battery anode materials failing to simultaneously achieve high energy density, rate performance, and cycle stability, with significant shortcomings in certain performance aspects limiting the practical application of sodium-ion batteries, this invention provides a method for preparing an anode material that combines high capacity with excellent rate and cycle performance. Specifically, using a carbonized product of ZIF-67 (nitrogen-doped porous cobalt-carbon material) as a template, the nitrogen-doped ZIF-67 carbonized product exhibits high conductivity, facilitating electron transport. Furthermore, the anode material prepared from it possesses numerous micropores and mesopores, which are beneficial for sodium ion insertion and extraction, as well as electrolyte infiltration. Simultaneously, it provides a buffer space for volume expansion during charge and discharge, thereby endowing the anode material with excellent rate performance and cycle stability. Further, a nitrogen-doped porous cobalt-carbon selenide material (hydrothermal product) is formed on the template surface through in-situ selenization via a hydrothermal reaction. The resulting metal selenide is beneficial for improving the capacity of the anode material. Finally, a coating layer is formed on the surface of the hydrothermal products through in-situ polymerization using phenolic monomers, organic polymers, and organophosphorus compounds. After carbonization, a uniformly P-doped hard carbon coating layer is obtained. P doping helps increase the interlayer spacing of the hard carbon and improve the conductivity of the coating layer, thereby accelerating the transport of sodium ions and electrons and suppressing the volume expansion of cobalt selenide during charge and discharge. Simultaneously, the hard carbon coating layer, in conjunction with the metal selenide, enables the anode material to have a high capacity. The anode material prepared by the above method has a high specific surface area, high conductivity, and a stable structure. Sodium-ion batteries constructed from it can exhibit both high capacity and excellent rate and cycle performance.

[0047] In this invention, ZIF-67 is preferably a Co-containing MOF material with a dodecahedral structure. In some preferred embodiments, ZIF-67 is synthesized by precipitation method, specifically as follows: cobalt nitrate hexahydrate is dissolved in methanol to obtain solution A; 2-methylimidazole is dissolved in methanol to obtain solution B; solution B is added to solution A, stirred evenly, and allowed to stand. After crystal precipitation, the mixture is filtered, washed, and dried to obtain ZIF-67 with a dodecahedral structure. More preferably, the washing solvent is methanol, and the drying temperature is 60-70℃.

[0048] In step S1 of the present invention, the carbonization temperature is preferably 500-800℃, such as 500℃, 600℃, 700℃, 800℃, etc., and the time is preferably 1.5-3h, such as 1.5h, 2h, 2.5h, 3h, etc.; more preferably, the temperature is increased to the carbonization temperature at a heating rate of 3-8℃ / min.

[0049] In step S2 of this invention, the carbonization product is first dispersed in water and stirred evenly to form dispersion A. Selenium powder and reducing agent are mixed evenly in water to form dispersion B. Then, dispersion B is added to dispersion A and stirred evenly to carry out a hydrothermal reaction.

[0050] In step S2 of this invention, the mass ratio of carbonized product to selenium powder is preferably 1:(1-3), such as 1:1, 2:3, 1:2, 2:5, 1:3, etc., including but not limited to the mass ratios listed above. By adding an appropriate amount of selenium powder, while avoiding excessive waste of selenium powder, the Co in the carbonized product can fully react with Se to form cobalt selenide, which is beneficial to improving battery capacity.

[0051] In step S2 of this invention, the molar ratio of selenium powder to reducing agent is 2:(3-5), such as 2:3, 2:4, 2:5, etc., including but not limited to the mass ratios listed above; preferably, the reducing agent is selected from one or more of sodium borohydride, hydrazine hydrate, and ethylenediamine, and the molar amount of reducing agent to the volume ratio of water is 30-70 mmol:1L, such as 30 mmol:1L, 40 mmol:1L, 50 mmol:1L, 60 mmol:1L, 70 mmol:1L, etc.

[0052] In step S2 of the present invention, the temperature of the hydrothermal reaction is preferably 150-200℃, such as 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, etc., and the time is preferably 10-20h, such as 10h, 12h, 14h, 16h, 18h, 20h, etc.

[0053] In step S3 of this invention, the hydrothermal product, phenolic monomer, and organic polymer are first dispersed in a solvent and stirred in an oil bath. Then, an organophosphorus compound is added to carry out the reaction. Preferably, the oil bath stirring temperature is 80-110℃, such as 80℃, 90℃, 100℃, 110℃, etc., and the time is 1-3h, such as 1h, 1.5h, 2h, 2.5h, 3h, etc. The reaction temperature is 80-110℃, such as 80℃, 90℃, 100℃, 110℃, etc., and the time is 2-5h, such as 1h, 1.5h, 2h, 2.5h, 3h, etc.

[0054] In step S3 of this invention, the phenolic monomers include one or more of bisphenol A, phenol, cresol, and xylenol; the organic polymers include one or more of polyoxymethylene, polyacetaldehyde, polybutyraldehyde, polypentanaldehyde, and polymethacrylaldehyde; and the organophosphorus compounds include one or more of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide, diphenylphosphine, and 3-phosphorylbenzoic acid.

[0055] In step S3 of this invention, the solvent is a mixture of benzene-based solvents and alcohol-based solvents, such as a mixture of toluene and anhydrous ethanol.

[0056] In step S3 of this invention, the feeding ratio of hydrothermal products to phenolic monomers affects the uniformity and thickness of the coating layer. If the amount of phenolic monomers added is too small, it will lead to uneven coating on the surface of the hydrothermal products, affecting the structural stability of the negative electrode material. If the amount of phenolic monomers added is too large, the coating layer will be too thick, which will seriously reduce the first efficiency and specific capacity. Therefore, in order to ensure that the hard carbon precursor can be uniformly coated on the surface of the hydrothermal products and to avoid the coating layer being too thick and affecting the first efficiency and specific capacity, it is preferable to control the mass ratio of hydrothermal products to phenolic monomers within the range of 1:(2-5), such as 1:2, 2:5, 1:3, 2:7, 1:4, 2:9, 1:5, etc., including but not limited to the mass ratios listed above.

[0057] In step S3 of this invention, the molar ratio of phenolic monomer to organic polymer is preferably 1:(3-5), for example 1:3, 1:4, 1:5; in some preferred embodiments of this invention, the degree of polymerization of organic polymer is preferably 1000-3500, for example 1000-2000, 2000-3500, etc.; the molar ratio of phenolic monomer to organophosphorus compound is preferably 1:(1-3), for example 1:1, 1:2, 1:3, etc., including but not limited to the molar ratios listed above.

[0058] In step S3 of this invention, after the reaction is complete, the temperature is lowered to -4 to 0°C to separate the solid, for example, by transferring it to an ice-water bath for cooling.

[0059] In step S3 of the present invention, the carbonization temperature is preferably 500-800℃, such as 500℃, 600℃, 700℃, 800℃, etc., and the time is preferably 1.5-3h, such as 1.5h, 2h, 2.5h, 3h, etc.; more preferably, the temperature is increased to the carbonization temperature at a heating rate of 3-8℃ / min.

[0060] The present invention also provides a sodium-ion battery, including a negative electrode sheet, the negative electrode sheet including a negative electrode current collector and a negative electrode active layer disposed on at least one side of the negative electrode current collector along the thickness direction, the negative electrode active layer comprising the aforementioned negative electrode material; more preferably, the mass percentage of the negative electrode material in the negative electrode active layer is 60%-80%.

[0061] In this invention, the negative electrode current collector is selected from one or more of the following: aluminum foil, carbon-coated aluminum foil, copper foil, porous copper foil, foamed nickel / copper foil, zinc-plated copper foil, nickel-plated copper foil, carbon-coated copper foil, nickel foil, titanium foil, and carbon-containing porous copper foil.

[0062] In this invention, the negative electrode active layer further comprises a conductive agent and a binder. Preferably, the conductive agent accounts for 10%-25% of the mass of the negative electrode active layer, and the binder accounts for 10%-15% of the mass of the negative electrode active layer. In this invention, the types of conductive agents and binders are not limited, and any conductive agents and binders conventionally used in the art are acceptable. For example, the binder is selected from one or more of polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, sodium carboxymethyl cellulose, polymethacrylamide, polyacrylic acid, lithium polyacrylate, polyacrylamide, polyamide, polyimide, polyacrylate, styrene-butadiene rubber, sodium alginate, chitosan, polyethylene glycol, and guar gum. The conductive agent is selected from one or more of conductive carbon black, acetylene black, Ketjen black graphite, graphene, micro / nano-wire conductive materials, and micro / nano-tubular conductive materials.

[0063] In this invention, the sodium-ion battery also includes a positive electrode, a separator, and an electrolyte. This invention does not limit the type of positive active material in the positive electrode, the type of separator, or the type of electrolyte; any material conventionally used in the art is acceptable.

[0064] The synthesis method of ZIF-67 with dodecahedrons used in the following examples and comparative examples is as follows:

[0065] ZIF-67 with dodecahedrons was synthesized by precipitation method: 8 mmol (2.33 g) cobalt nitrate hexahydrate was dissolved in 100 mL of methanol solution, which is solution A; 30 mmol (2.46 g) 2-methylimidazole was dissolved in 50 mL of methanol, which is solution B; solution B was slowly added to solution A and stirred for 15 min, then allowed to stand for 12 h, filtered, washed 3 times with methanol, and dried at 65 °C to obtain ZIF-67 with dodecahedrons for use.

[0066] Example 1

[0067] This embodiment relates to the preparation of a negative electrode material and a negative electrode sheet, the specific process of which is as follows:

[0068] (1) ZIF-67 was heated to 600℃ in Ar atmosphere at a heating rate of 5℃ / min and held for 2h to obtain N-doped cobalt carbon material Co@NC with dodecahedral structure;

[0069] (2) Disperse 50 mg of Co@NC in 20 mL of deionized water and stir until homogeneous to form solution C; slowly add 2 mmol of sodium borohydride powder to 20 mL of deionized water, and then add 1 mmol of Se powder. This solution is used as solution D. Slowly add solution D to solution C, stir for 30 min, and then transfer to a high-pressure reactor. Heat at 160 °C for 12 h, cool to room temperature, centrifuge the product, wash and dry it to obtain nitrogen-doped porous cobalt selenide carbon material CoSe2@NC;

[0070] (3) 80 mg of the prepared CoSe2@NC and 1 mmol of bisphenol A (BPA) and 4 mmol of polyoxymethylene (POM, molecular weight 35000) were dispersed in a mixed solvent of toluene and anhydrous ethanol at a volume ratio of 1:10. The mixture was stirred in an oil bath at 95 °C for 2 h. Then, 1.5 mmol of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide (DOPO, phosphorus source) was added and reacted for 3 h. After the product was completely precipitated in ice water, it was washed and dried. The product was then heated to 600 °C at a heating rate of 5 °C / min under Ar atmosphere and held for 2 h to obtain P-CoSe2@NC-1 anode material with P and N doped porous dodecahedrons.

[0071] Preparation of negative electrode sheet: The P-CoSe2@NC-1 negative electrode material prepared in this embodiment, Ketjen black and carboxymethyl cellulose binder are ground thoroughly in a mass ratio of 7:2:1. Deionized water is added as a solvent and grinding is continued to obtain a uniform slurry. The slurry is coated on an aluminum foil current collector and dried under vacuum at 80°C for 12 hours. The slurry is then cut into circular electrode sheets using a punching machine and set aside for use.

[0072] Example 2

[0073] This embodiment relates to the preparation of a negative electrode material and a negative electrode sheet, the specific process of which is as follows:

[0074] (1) ZIF-67 was heated to 600℃ in Ar atmosphere at a heating rate of 5℃ / min and held for 2h to obtain N-doped cobalt carbon material Co@NC with dodecahedral structure;

[0075] (2) Disperse 50 mg of Co@NC in 20 mL of deionized water and stir until homogeneous to form solution C; slowly add 2 mmol of sodium borohydride powder to 20 mL of deionized water, and then add 1 mmol of Se powder. This solution is used as solution D. Slowly add solution D to solution C, stir for 30 min, and then transfer to a high-pressure reactor. Heat at 160 °C for 12 h, cool to room temperature, centrifuge the product, wash and dry it to obtain nitrogen-doped porous cobalt selenide carbon material CoSe2@NC;

[0076] (3) 80 mg of CoSe2@NC, 1.5 mmol of bisphenol A (BPA), and 6 mmol of polyoxymethylene (POM, molecular weight 35000) were dispersed in a mixed solvent of toluene and anhydrous ethanol at a volume ratio of 1:10. The mixture was stirred in an oil bath at 95 °C for 2 h. Then, 2 mmol of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide (DOPO, phosphorus source) was added and reacted for 3 h. After the product was completely precipitated in ice water, it was washed and dried. The product was then heated to 600 °C at a heating rate of 5 °C / min under an Ar atmosphere and held for 2 h to carbonize and obtain P-CoSe2@NC-2 anode material with P and N doped porous dodecahedrons.

[0077] Preparation of negative electrode sheet: The P-CoSe2@NC-2 negative electrode material prepared in this embodiment, Ketjen black and carboxymethyl cellulose binder are ground thoroughly in a mass ratio of 7:2:1. Deionized water is added as a solvent and grinding is continued to obtain a uniform slurry. The slurry is coated on an aluminum foil current collector and dried under vacuum at 80°C for 12 hours. The slurry is then cut into circular electrode sheets using a punching machine and set aside for use.

[0078] Example 3

[0079] This embodiment relates to the preparation of a negative electrode material and a negative electrode sheet, the specific process of which is as follows:

[0080] (1) ZIF-67 was heated to 750℃ in Ar atmosphere at a heating rate of 5℃ / min and held for 2h to obtain N-doped cobalt carbon material Co@NC with a dodecahedral structure.

[0081] (2) Disperse 50 mg of Co@NC in 20 mL of deionized water and stir until homogeneous to form solution C; slowly add 2 mmol of sodium borohydride powder to 20 mL of deionized water, then add 1 mmol of Se powder, and this solution is used as solution D. Slowly add solution D to solution C, stir for 30 min, transfer to a high-pressure reactor, heat at 160 °C for 12 h, cool to room temperature, centrifuge the product, wash and dry to obtain CoSe2@NC;

[0082] (3) 80 mg of CoSe2@NC, 1 mmol of bisphenol A (BPA), and 4 mmol of polyoxymethylene (POM, molecular weight 35000) were dispersed in a mixed solvent of toluene and anhydrous ethanol at a volume ratio of 1:10. The mixture was stirred in an oil bath at 95 °C for 2 h. Then, 1.5 mmol of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide (DOPO, phosphorus source) was added and reacted for 3 h. After the product was completely precipitated in ice water, it was washed and dried. The product was then heated to 750 °C at a heating rate of 5 °C / min under an Ar atmosphere and held for 2 h to obtain P-CoSe2@NC-3 material with P and N doped porous dodecahedrons.

[0083] Preparation of negative electrode sheet: The P-CoSe2@NC-3 negative electrode material prepared in this embodiment, Ketjen black and carboxymethyl cellulose binder are ground thoroughly in a mass ratio of 7:2:1. Deionized water is added as a solvent and grinding is continued to obtain a uniform slurry. The slurry is coated on an aluminum foil current collector and dried under vacuum at 80°C for 12 hours. The slurry is then cut into circular electrode sheets using a punching machine and set aside for use.

[0084] Example 4

[0085] This embodiment relates to the preparation of a negative electrode material and a negative electrode sheet. The only difference from Example 1 is that in step (3), equimolar amounts of phenol are used to replace bisphenol A, and equimolar amounts of polymethyl acrolein are used to replace polyoxymethylene. All other operations are the same, and P-CoSe2@NC-4 negative electrode material with P and N doped porous dodecahedrons is prepared.

[0086] Preparation of negative electrode sheet: The P-CoSe2@NC-4 negative electrode material prepared in this embodiment, Ketjen black and carboxymethyl cellulose binder are ground thoroughly in a mass ratio of 7:2:1. Deionized water is added as a solvent and grinding is continued to obtain a uniform slurry. The slurry is coated on an aluminum foil current collector and dried under vacuum at 80°C for 12 hours. The slurry is then cut into circular electrode sheets using a punching machine and set aside for use.

[0087] Comparative Example 1

[0088] This comparative example relates to the preparation of a negative electrode material and a negative electrode sheet. The only difference from Example 1 is that 9,10-dihydro-9-oxy-10-phosphaphenanthrene-10-oxide (DOPO, phosphorus source) was not added in step (3). All other operations are the same, and N-doped porous dodecahedral CoSe2@NC-1 negative electrode material is prepared.

[0089] Preparation of negative electrode sheet: The CoSe2@NC-1 negative electrode material prepared in this comparative example, Ketjen black and carboxymethyl cellulose binder were ground thoroughly in a mass ratio of 7:2:1. Deionized water was added as a solvent and grinding was continued to obtain a uniform slurry. The slurry was coated on an aluminum foil current collector and dried under vacuum at 80°C for 12 hours. The slurry was then cut into circular electrode sheets using a punching machine and set aside for use.

[0090] Comparative Example 2

[0091] This comparative example relates to the preparation of a negative electrode material and a negative electrode sheet. The only difference from Example 1 is that step (3) is not included. All other operations are the same, and nitrogen-doped porous cobalt selenide carbon material CoSe2@NC negative electrode material is prepared.

[0092] Preparation of negative electrode sheet: The CoSe2@NC negative electrode material prepared in this comparative example, Ketjen black and carboxymethyl cellulose binder were ground thoroughly in a mass ratio of 7:2:1. Deionized water was added as a solvent and grinding was continued to obtain a uniform slurry. The slurry was coated on an aluminum foil current collector and dried under vacuum at 80°C for 12 hours. The slurry was then cut into circular electrode sheets using a punching machine and set aside for use.

[0093] Comparative Example 3

[0094] This comparative example relates to the preparation of a negative electrode material and a negative electrode sheet. The only difference from Example 1 is that in step (2), high-temperature selenization is used instead of hydrothermal selenization reaction, as follows:

[0095] 100 mg of Co@NC was placed at one end of a ceramic boat, and 200 mg of Se powder was placed at the other end and placed upwind. The boat was selenized at 600 °C for 2 h in an Ar atmosphere to obtain CoSe2@NC.

[0096] The remaining operations were the same, and P-CoSe2@NC anode material with P and N doped porous dodecahedrons was prepared.

[0097] Preparation of negative electrode sheet: The P-CoSe2@NC negative electrode material prepared in this comparative example, Ketjen black and carboxymethyl cellulose binder were ground thoroughly in a mass ratio of 7:2:1. Deionized water was added as a solvent and grinding was continued to obtain a uniform slurry. The slurry was coated on an aluminum foil current collector and dried under vacuum at 80°C for 12 hours. The slurry was then cut into circular electrode sheets using a punching machine and set aside for use.

[0098] Application and performance testing

[0099] (1) The particle size, specific surface area, and porosity of the negative electrode materials prepared in the above embodiments and comparative examples were tested. The specific surface area of ​​the negative electrode material has a meaning known in the art and can be determined using methods known in the art. In this invention, a specific surface area analyzer (McTristar II 3020) was used to measure the specific surface area and porosity of the negative electrode material using the nitrogen adsorption / desorption method. The test results are shown in Table 1 below:

[0100] Table 1

[0101]

[0102]

[0103] As shown in Table 1, the particle size of Examples 1-4 and Comparative Example 1 increased to some extent after coating compared with Comparative Example 2. Moreover, compared with the negative electrode material prepared in Example 1, the particle size of the negative electrode material prepared in Example 2 with high carbon source content was larger, but the particle size of each group was almost in the same range.

[0104] The uncoated porous dodecahedral cobalt diselenide carbon material prepared in Comparative Example 2 has a specific surface area of ​​94.6 m². 2 The specific surface area and porosity of the anode material prepared in Example 1 were significantly improved after hard carbon coating, with a porosity of 21.6% per gram. Among the anode materials prepared above, the anode material prepared in Example 2 with a high carbon source content had the highest specific surface area and porosity. However, after increasing the heat treatment temperature, the specific surface area and porosity of the anode material in Example 3 decreased significantly compared to Example 1. The specific surface area of ​​the anode material prepared in Example 4 with different carbon sources was further improved, but the porosity decreased, presumably due to uneven coating. In addition, the anode material prepared by high-temperature selenization in Comparative Example 3 had a significantly lower specific surface area and porosity compared to the anode material prepared by hydrothermal selenization.

[0105] (2) The negative electrode sheets prepared in the above examples and comparative examples were used to assemble sodium-ion batteries, and the electrochemical performance of different sodium-ion batteries was tested. The specific operation is as follows:

[0106] The sodium-ion half-cell uses a CR-2032 battery casing, with a sodium plate as the counter electrode. The electrolyte is a 1 mol / L solution of ethane carbonate and diethyl carbonate in a 1:1 volume ratio (EC:DEC, 1:1). -1 The electrolyte was sodium perchlorate (NaClO4), and the separator was a GF / C glass fiber membrane. Half-cell assembly was performed in an argon-filled glove box. Electrochemical tests were conducted after a 16-hour rest period.

[0107] Specific capacity and coulombic efficiency tests after cycling at a current density of 0.2 A / g: Using the Land battery testing system, under constant temperature conditions of 25℃, the battery was discharged to 0V at a constant current density of 0.2 A / g, and then charged to 3V at a constant current density of 0.2 A / g. This constitutes one complete charge-discharge cycle, recorded as one cycle. This process was repeated until the number of cycles equaled 2000. The test was then terminated, and the discharge specific capacity of the 2000th cycle was recorded as the specific capacity after 2000 cycles. The coulombic efficiency of the 2000th cycle = discharge specific capacity of the 2000th cycle / charge specific capacity of the 2000th cycle. The capacity retention rate after 2000 cycles = discharge specific capacity of the 2000th cycle / discharge specific capacity of the 3rd cycle.

[0108] The test results are shown in Table 2 below:

[0109] Table 2

[0110]

[0111] As shown in Table 2, the sodium-ion half-cells constructed with the anode materials prepared in Examples 1-4 not only exhibit high initial coulombic efficiency but also maintain high discharge specific capacity after 2000 charge-discharge cycles, with a capacity retention rate of no less than 83%. Among them, the sodium-ion half-cell constructed with the anode material prepared in Example 1 has a capacity retention rate as high as 91.5% after 2000 cycles. This is because these anode materials use N-doped porous cobalt diselenide with a dodecahedral structure as the core, and coat it with a layer of P-doped hard carbon. This not only gives the material a higher specific surface area and porosity, providing more active sites and accelerating sodium ion transport, but the hard carbon coating also inhibits the expansion of cobalt diselenide to a certain extent, resulting in stable cycle performance. Furthermore, the co-doping of N and P elements improves the conductivity of the material. Under the synergistic effect of the above materials and structures, the sodium-ion half-cells constructed with these anode materials exhibit high capacity and excellent cycle stability.

[0112] As shown in Examples 1 and 2, the P-CoSe2@NC-2 prepared in Example 2 has a thicker hard carbon layer, resulting in a lower capacity utilization compared to that in Example 1. The P-CoSe2@NC-3 (Example 3), prepared at a higher carbonization temperature, has a lower specific surface area and porosity, and its structural stability is slightly worse than that of Example 1. In the later stages of cycling, the material's structural expansion leads to capacity decay. The negative electrode material prepared in Example 4 has a large specific surface area but low porosity, and its coating layer is not uniformly coated, resulting in increased side reactions in the later stages of cycling. Compared to Example 1, its cycling performance is somewhat reduced.

[0113] The negative electrode material prepared in Comparative Example 1 did not have phosphorus doping in its coating layer, and its hard carbon layer had a smaller interlayer spacing than that in Example 1, resulting in slightly lower sodium storage capacity and a lower specific capacity. The dodecahedral cobalt diselenide in Comparative Example 2 lacked a carbon layer coating, exhibiting the worst cycle performance. The material in Comparative Example 3 underwent high-temperature selenization, resulting in a less stable structure. Structural collapse occurred in the later stages of cycling, leading to decreased cycle capacity and low capacity retention.

[0114] Specific capacity after 10 cycles at different current densities: Under constant temperature conditions of 25℃, the cells were discharged to 0V at constant current densities of 0.2A / g, 0.5A / g, 1A / g, 2A / g, and 5A / g, respectively. Then, they were charged to 3V at the same constant current density. The test was completed after 10 cycles at the same current density. The discharge specific capacity of the last cycle was recorded as the specific capacity after cycling.

[0115] The test results are shown in Table 3 below:

[0116] Table 3

[0117]

[0118] As shown in Table 3, the P-CoSe2@NC-1 prepared in Example 1, as the negative electrode material of sodium-ion battery, exhibits excellent specific capacity at a high current density of 5A / g under the synergistic effect of P and N. At the same time, the outer hard carbon layer contributes to sodium storage, giving the material excellent rate performance.

[0119] Compared to Example 1, Comparative Example 1, lacking P doping, exhibits significantly reduced conductivity and a much lower capacity than Example 1. Comparative Example 2, serving as the precursor for each example, shows a marked decrease in capacity at high rates without hard carbon coating and P doping. CoSe2 exhibits substantial volume expansion during charge and discharge, and the lack of a carbon layer clearly demonstrates its disadvantages. Combined with Table 2, its cycle performance is also the worst. Furthermore, as shown in Example 1 and Comparative Example 3, the fast-charging performance of the anode material prepared by high-temperature selenization (Comparative Example 3) is inferior to that of the anode material prepared by hydrothermal selenization (Example 1).

[0120] In summary, sodium-ion batteries constructed from the negative electrode material prepared by this invention can possess both high capacity and excellent cycle performance and rate performance.

[0121] The embodiments described above are merely preferred examples to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the claims.

Claims

1. A method for preparing a negative electrode material, characterized in that, Includes the following steps: S1. ZIF-67 is carbonized under an inert atmosphere to obtain carbonized products; S2. The carbonized product and selenium powder are subjected to a hydrothermal reaction in the presence of a reducing agent and water, and the solid is separated to obtain the hydrothermal product. S3. The hydrothermal product is reacted with phenolic monomers, organic polymers, and organophosphorus compounds in the presence of a solvent. After the reaction is complete, the solid is separated by cooling. The solid is washed, dried, and then transferred to an inert atmosphere for carbonization treatment to obtain the negative electrode material. The organic polymer is obtained by polymerization of one or more aldehyde monomers. The phenolic monomers include one or more of bisphenol A, phenol, cresol, and xylenol. The organic polymer includes one or more of polyoxymethylene, polyacetaldehyde, polybutyraldehyde, polypentanaldehyde, and polymethacrylaldehyde. The organophosphorus compounds include one or more of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide, diphenylphosphine, and 3-phosphorylbenzoic acid. The solvent is a mixed solvent of benzene solvents and alcohol solvents.

2. The preparation method according to claim 1, characterized in that, In step S1, the ZIF-67 has a dodecahedral structure; The carbonization process is carried out at a temperature of 500-800 ℃ for 1.5-3 h.

3. The preparation method according to claim 1, characterized in that, Step S2 includes at least one of the following features: (1) First, disperse the carbonized product in water and stir it evenly to form dispersion A. Then, mix the selenium powder and the reducing agent evenly in water to form dispersion B. Then, add dispersion B to dispersion A and stir it evenly to carry out hydrothermal reaction. (2) The mass ratio of the carbonized product to the selenium powder is 1:(1-3); (3) The molar ratio of the selenium powder to the reducing agent is 2:(3-5); (4) The reducing agent is selected from one or more of sodium borohydride, hydrazine hydrate, and ethylenediamine; (5) The molar ratio of the reducing agent to the volume of the water is 30-70 mmol: 1 L; (6) The temperature of the hydrothermal reaction is 150-200 ℃ and the time is 10-20 h.

4. The preparation method according to claim 1, characterized in that, In step S3, the hydrothermal product, phenolic monomer, and organic polymer are first dispersed in a solvent and stirred in an oil bath, and then an organophosphorus compound is added to carry out the reaction. The oil bath stirring temperature is 80-110 ℃, and the time is 1-3 h; The reaction is carried out at a temperature of 80-110 °C for 2-5 h.

5. The preparation method according to claim 1, characterized in that, Step S3 includes at least one of the following features: (1) The mass ratio of the hydrothermal product to the phenolic monomer is 1:(2-5); (2) The molar ratio of the phenolic monomer to the organic polymer is 1:(3-5), and the degree of polymerization of the organic polymer is 1000-3500; (3) The molar ratio of the phenolic monomer to the organophosphorus compound is 1:(1-3).

6. The preparation method according to claim 1, characterized in that, In step S3, after the reaction is complete, the temperature is lowered to -4~0 ℃ to separate the solid. The carbonization process is carried out at a temperature of 500-800 ℃ for 1.5-3 h.

7. A negative electrode material, characterized in that, It is prepared by the preparation method described in any one of claims 1-6.

8. A sodium-ion battery, characterized in that, The sodium-ion battery includes a negative electrode sheet, the negative electrode sheet includes a negative current collector and a negative active layer disposed on at least one side of the negative current collector along the thickness direction, the negative active layer comprising the negative electrode material of claim 7.

9. The sodium-ion battery according to claim 8, characterized in that, The negative electrode material accounts for 60%-80% of the mass of the negative electrode active layer.

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