Preparation method and application of negative electrode material

Abstract

The invention relates to a preparation method and application of a negative electrode material, and the preparation method comprises the following steps: S1, carrying out carbonization treatment on ZIF-67 in an inert atmosphere to obtain a carbonized product; s2, the carbonized product and selenium powder are subjected to a hydrothermal reaction in the presence of a reducing agent and water, and a hydrothermal product is obtained; and S3, reacting the hydrothermal product with a phenolic monomer, an organic polymer and an organic phosphorus compound in the presence of a solvent, cooling to separate a solid, washing, drying, and then transferring to an inert atmosphere for carbonization treatment to obtain the negative electrode material. The nitrogen-doped porous cobalt diselenide material which is stable in structure and uniform in selenium element distribution is formed through hydrothermal in-situ selenylation, then a coating layer with P uniformly doped with hard carbon is formed on the surface of the nitrogen-doped porous cobalt diselenide material through in-situ polymerization and carbonization, and the prepared negative electrode material has the advantages of high energy density, excellent structural stability and electron / ion transmission. Therefore, the secondary battery containing the negative electrode material has high capacity and excellent rate capability 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] Based on the obvious defects of the current sodium ion battery negative electrode material in energy density, rate performance, cycle stability and the like, the application of the negative electrode material preparation method is provided, and the ZIF-67 carbonization product is used as a template, a nitrogen-doped porous cobalt diselenide material with stable structure and uniform selenium element distribution is formed by in-situ selenization through hydrothermal, then a P-uniformly-doped hard carbon coating layer is formed 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 the secondary battery containing the above negative electrode material has high capacity and excellent rate performance and cycle performance.

[0006] The application provides the following technical scheme:

[0007] The application provides a negative electrode material preparation method, which comprises the following steps:

[0008] S1, carbonize ZIF-67 under an inert atmosphere to obtain a carbonization product;

[0009] S2, hydrothermal reaction of the carbonization product and selenium powder in the presence of a reducing agent and water, separation of a solid to obtain a hydrothermal product;

[0010] S3, reaction of the hydrothermal product and a phenolic monomer, an organic polymer and an organic phosphorus compound in the presence of a solvent, separation of a solid after cooling, and carbonization of the solid under an inert atmosphere after washing and drying to obtain the negative electrode material; the organic polymer is obtained by polymerization of one or more aldehyde monomers.

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

[0012] Further, the ZIF-67 with a dodecahedron structure is synthesized by a precipitation method, specifically as follows: dissolve cobalt nitrate hexahydrate in methanol to obtain solution A; dissolve 2-methylimidazole in methanol to obtain solution B, add solution B to solution A, stir uniformly, and then stand, filter, wash and dry the crystals to obtain the ZIF-67 with a dodecahedron structure.

[0013] Further, in step S1, the carbonization temperature is preferably 500-800 DEG C, and the carbonization time is preferably 1.5-3 h.

[0014] Further, in step S2, first, disperse the carbonization product in water, stir uniformly to form dispersion A, uniformly mix the selenium powder and the reducing agent in water to form dispersion B, then add dispersion B to dispersion A, stir uniformly and perform hydrothermal reaction.

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

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

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

[0018] Further, in step S2, the ratio of the molar amount of the reducing agent to the volume of the water is 30-70 mmol: 1 L, such as 30 mmol: 1 L, 40 mmol: 1 L, 50 mmol: 1 L, 60 mmol: 1 L, 70 mmol: 1 L, and the like.

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

[0020] Further, in step S3, the hydrothermal product is first dispersed in a solvent with a phenolic monomer and an organic polymer, and then subjected to oil bath stirring, and an organic phosphorus compound is added for reaction; preferably, the temperature of the oil bath stirring is 80-110°C, and the time is 1-3 h; the temperature of the reaction is 80-110°C, and the time is 2-5 h.

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

[0022] Further, in step S3, the organic polymer includes one or more of polyformaldehyde, polyacetaldehyde, polybutylaldehyde, polypentylaldehyde, and polymethylpropylaldehyde.

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

[0024] Further, in step S3, the solvent is a mixed solvent of a benzene-based solvent and an alcohol-based solvent, such as a mixed solvent 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, and the like, including but not limited to the mass ratios listed above.

[0026] Further, the molar ratio of the phenolic monomer to the organic polymer is preferably 1:(3-5), such as 1:3, 1:4, 1:5; and the degree of polymerization of the organic polymer is preferably 1000-3500.

[0027] Further, the molar ratio of the phenolic monomer to the organic phosphorus compound is preferably 1:(1-3), such as 1:1, 1:2, 1:3, and the like, including but not limited to the molar ratios listed above.

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

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

[0030] The second aspect of the present application provides a negative electrode material prepared by the preparation method of the first aspect.

[0031] The third aspect of the present application provides a sodium ion battery, which comprises 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 of the second aspect.

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

[0033] Compared with the prior art, the present application has the following beneficial effects:

[0034] The present application uses ZIF-67 carbonization product as a template, and a negative electrode material with stable structure and polyhedral microporous and mesoporous structure can be prepared by simple hydrothermal, in-situ polymerization, and carbonization treatment. The negative electrode material has a core-shell structure, the inner core is a nitrogen-doped porous cobalt diselenide with uniform selenium element distribution, and the shell layer is a coating layer of P-doped hard carbon. Under the synergistic effect of the above-mentioned material and structure, the sodium ion battery constructed by the negative electrode material can have high capacity, excellent rate performance, and cycle stability. Specifically as follows:

[0035] (1) with ZIF-67 carbonization product as a template, so that the negative electrode material has a large number of microporous and mesoporous structures, which is beneficial to the embedding and extraction of sodium ions, improves the structural stability of the material in the charging and discharging process, and is beneficial to the infiltration of the electrolyte, thereby promoting the transmission 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 in the charging and discharging process.

[0036] (2) ZIF-67 carbonization product is a N nitrogen-doped porous substrate with high conductivity, which is beneficial to electron transmission.

[0037] (3) through the in-situ selenization reaction, selenium is uniformly distributed in the core of the negative electrode material, and the metal selenide formed in combination with the hard carbon coating layer ensures that the negative electrode material has a high capacity.

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

[0039] The present application will be further described below in conjunction with specific examples, so that those skilled in the art can better understand the present application and implement it, but the examples are not limiting to the present application.

[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 the present application belongs. The terms used in the specification of the present application herein are only for the purpose of describing specific embodiments and are not intended to limit the present application. The term "and / or" used herein includes any and all combinations of one or more related listed items. The "includes" or "contains" described in the present application can also be replaced by the closed "consists of" or "consists of".

[0041] The existing different types of sodium ion battery negative electrode materials have different degrees of limitations in energy density, rate performance or cycle stability, which limits the promotion of sodium ion batteries in practical applications.

[0042] Based on this, the embodiment part of the present application provides a preparation method of a negative electrode material, comprising the following steps:

[0043] S1, carbonizing ZIF-67 under inert atmosphere to obtain a carbonization product;

[0044] S2, hydrothermal reaction of the carbonization product and selenium powder in the presence of a reducing agent and water, separation of the solid, to obtain a hydrothermal product;

[0045] S3, reacting the hydrothermal product with a phenolic monomer, an organic polymer, and an organic phosphorus compound in the presence of a solvent, after the reaction is complete, separating the solid by cooling, and transferring the solid 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 solve the problem that the existing sodium ion battery negative electrode material is difficult to balance energy density, rate performance and cycle stability, and the serious defects in a certain performance limit the practical application of sodium ion battery. The application provides a preparation of a negative electrode material which can balance high capacity and excellent rate performance and cycle performance, and specifically as follows: a carbonized product (nitrogen-doped porous cobalt carbon material) of ZIF-67 is used as a template, on the one hand, the N-doped ZIF-67 carbonized product has high conductivity, which is beneficial to electron transmission; and the negative electrode material prepared therefrom has a large number of microporous and mesoporous structures, which is beneficial to the embedding and extraction of sodium ions and the immersion of electrolyte, and at the same time provides a buffer space for the volume expansion of the negative electrode material in the charging and discharging process, thereby endowing the negative electrode material with excellent rate performance and cycle stability. Further, the surface of the template is in-situ selenized by a hydrothermal reaction to form a nitrogen-doped porous selenized cobalt carbon material (hydrothermal product), and the formed metal selenide is beneficial to improving the capacity of the negative electrode material. Finally, a coating layer is formed on the surface of the hydrothermal product by in-situ polymerization of a phenolic monomer, an organic polymer and an organic phosphorus compound, and after carbonization treatment, a P-uniformly-doped hard carbon coating layer is obtained, and P-doping is beneficial to increasing the interlayer spacing of the hard carbon and improving the conductivity of the coating layer, thereby being beneficial to accelerating the transmission of sodium ions and electrons, and being beneficial to inhibiting the volume expansion of selenized cobalt in the charging and discharging process, and at the same time, the hard carbon coating layer cooperates with the metal selenide to make the negative electrode material have a higher capacity. The negative electrode material prepared by the above preparation method has a high specific surface area, high conductivity and stable structure, and the sodium ion battery constructed therefrom can balance high capacity and excellent rate performance and cycle performance.

[0047] In the application, the ZIF-67 is preferably a Co-containing MOF material with a dodecahedron structure; in some preferred embodiments, the ZIF-67 is synthesized by a precipitation method, and 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 and stirred uniformly, then crystals are precipitated, and the crystals are filtered, washed and dried to obtain ZIF-67 with a dodecahedron structure; more preferably, the washing solvent is methanol, and the drying temperature is 60-70 DEG C.

[0048] In step S1 of the present application, the carbonization treatment temperature is preferably 500-800 DEG C, for example 500 DEG C, 600 DEG C, 700 DEG C, 800 DEG C, etc., and the time is preferably 1.5-3h, for example 1.5h, 2h, 2.5h, 3h, etc.; more preferably, the temperature is raised to the carbonization treatment temperature at a temperature raising rate of 3-8 DEG C / min.

[0049] In step S2 of the present application, the carbonization product is first dispersed in water, and after stirring to form a dispersion A, the selenium powder and the reducing agent are mixed uniformly in water to form a dispersion B; then the dispersion B is added to the dispersion A, and after stirring, a hydrothermal reaction is performed.

[0050] In step S2 of the present application, the mass ratio of the carbonization product to the selenium powder is preferably 1:(1-3), for example 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, the Co in the carbonization product can be fully reacted with Se to form cobalt selenide, which is beneficial to improve the battery capacity.

[0051] In step S2 of the present application, 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; preferably, the reducing agent is selected from one or more of sodium borohydride, hydrazine hydrate, and ethylenediamine, and the molar amount 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.

[0052] In step S2 of the present application, the hydrothermal reaction temperature is preferably 150-200 DEG C, for example 150 DEG C, 160 DEG C, 170 DEG C, 180 DEG C, 190 DEG C, 200 DEG C, etc., and the time is preferably 10-20h, for example 10h, 12h, 14h, 16h, 18h, 20h, etc.

[0053] In step S3 of the present application, the hydrothermal product is first dispersed in a solvent with a phenolic monomer and an organic polymer, and then oil bath stirring is performed, and an organic phosphorus compound is added for reaction; preferably, the oil bath stirring temperature is 80-110 DEG C, for example 80 DEG C, 90 DEG C, 100 DEG C, 110 DEG C, etc., and the time is 1-3h, for example 1h, 1.5h, 2h, 2.5h, 3h, etc.; the reaction temperature is 80-110 DEG C, for example 80 DEG C, 90 DEG C, 100 DEG C, 110 DEG C, etc., and the time is 2-5h, for example 1h, 1.5h, 2h, 2.5h, 3h, etc.

[0054] The phenolic monomer in step S3 includes one or more of bisphenol A, phenol, cresol, dimethylphenol; the organic polymer includes one or more of polyformaldehyde, polyacetaldehyde, polybutyl aldehyde, polyvaleraldehyde, polymethyl propionaldehyde; the organic phosphorus compound includes one or more of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide, diphenyl phosphine oxide, 3-phosphoryl benzoic acid.

[0055] The solvent in step S3 is a mixed solvent of benzene solvents and alcohol solvents, for example, a mixed solvent of toluene and anhydrous ethanol.

[0056] The mass ratio of the hydrothermal product to the phenolic monomer in step S3 affects the uniformity and thickness of the coating layer. If the amount of the phenolic monomer is too small, the surface of the hydrothermal product cannot be uniformly coated, affecting the structural stability of the negative electrode material. If the amount of the phenolic monomer is too large, the coating layer is too thick, which can seriously reduce the initial efficiency and the specific capacity. Therefore, in order to ensure that the hard carbon precursor can be uniformly coated on the surface of the hydrothermal product and avoid the influence of the too thick coating layer on the initial efficiency and the specific capacity, the mass ratio of the hydrothermal product to the phenolic monomer is preferably controlled in the range of 1:(2-5), for example, 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] The molar ratio of the phenolic monomer to the organic polymer in step S3 is preferably 1:(3-5), for example, 1:3, 1:4, 1:5; in some preferred embodiments of the present application, the degree of polymerization of the organic polymer is preferably 1000-3500, for example, 1000-2000, 2000-3500, etc.; the molar ratio of the phenolic monomer to the organic phosphorus 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] After the reaction is completed in step S3, the temperature is lowered to -4-0℃ to separate the solid, for example, transferred to an ice water bath for cooling treatment.

[0059] In step S3, the temperature of the carbonization treatment is preferably 500-800℃, for example, 500℃, 600℃, 700℃, 800℃, etc., and the time is preferably 1.5-3h, for example, 1.5h, 2h, 2.5h, 3h, etc.; more preferably, the temperature is raised to the carbonization treatment temperature at a temperature raising rate of 3-8℃ / min.

[0060] The embodiment part of the present application also provides a sodium ion battery, which includes a negative electrode sheet, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer arranged on at least one side of the negative electrode current collector along the thickness direction, and the negative electrode active layer contains the above-mentioned negative electrode material; more preferably, the mass proportion of the negative electrode material in the negative electrode active layer is 60%-80%.

[0061] In the present application, the negative current collector is selected from one or more of 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 the present application, the negative active layer further comprises a conductive agent and a binder. Preferably, the mass percentage of the conductive agent in the negative active layer is 10-25%, and the mass percentage of the binder in the negative active layer is 10-15%. In the present application, the types of the conductive agent and the binder are not limited, and any conventional conductive agent and binder can be used. For example, the binder is selected from one or more of polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, sodium carboxymethyl cellulose, polymethacryl, polyacrylic acid, lithium polyacrylate, polyacrylamide, polyamide, polyimide, polyacrylate, butadiene-styrene rubber, sodium alginate, chitosan, polyethylene glycol, and guar gum; and the conductive agent is selected from one or more of conductive carbon black, acetylene black, ketchen black graphite, graphene, and micro-nano wire-shaped conductive material.

[0063] In the present application, the sodium-ion battery further comprises a positive electrode sheet, a separator, and an electrolyte. The types of the positive active material in the positive electrode sheet, the separator, and the electrolyte are not limited, and any conventional material can be used.

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

[0065] ZIF-67 with dodecahedron is synthesized by a precipitation method: 8 mmol (2.33 g) of cobalt nitrate hexahydrate is dissolved in 100 mL of methanol solution to obtain solution A; 30 mmol (2.46 g) of 2-methylimidazole is dissolved in 50 mL of methanol to obtain solution B; solution B is slowly added to solution A, stirred for 15 min, and then left to stand for 12 h; filtration is performed, and washing with methanol is performed for 3 times; drying is performed at 65℃ to obtain ZIF-67 with dodecahedron for use.

[0066] Example 1

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

[0068] (1) ZIF-67 is heated to 600℃ at an Ar atmosphere with a heating rate of 5℃ / min, and carbonization is performed for 2 h to obtain N-doped cobalt-carbon material Co@NC with dodecahedron structure;

[0069] (2) 50 mg of Co@NC was dispersed in 20 mL of deionized water, and after stirring uniformly, solution C was formed; 2 mmol of sodium borohydride powder and 1 mmol of Se powder were slowly added to 20 mL of deionized water, and this solution was used as solution D. Solution D was slowly added to solution C, and after stirring for 30 min, it was transferred to a high-pressure reaction kettle and heated at 160°C for 12 h. After cooling to room temperature, the product was centrifuged, washed and dried to obtain nitrogen-doped porous cobalt selenide carbon material CoSe2@NC;

[0070] (3) 80 mg of CoSe2@NC prepared and 1 mmol of bisphenol A (BPA) and 4 mmol of polyformaldehyde (POM, molecular weight 35000) were dispersed in a mixed solvent of toluene and anhydrous ethanol mixed at a volume ratio of 1:10, and 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. It was heated to 600°C at a heating rate of 5°C / min under Ar atmosphere, and carbonized for 2 h to obtain P, N-doped porous dodecahedral P-CoSe2@NC-1 negative electrode material.

[0071] Preparation of negative electrode sheet: The P-CoSe2@NC-1 negative electrode material prepared in this example, Ketjen black and carboxymethyl cellulose binder were thoroughly ground in a mass ratio of 7:2:1, deionized water was added as a solvent, and after grinding, a uniform slurry was obtained. It was coated on an aluminum foil current collector, vacuum dried at 80°C for 12 h, and cut into a circular electrode sheet with a punching machine.

[0072] Example 2

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

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

[0075] (2) 50 mg of Co@NC was dispersed in 20 mL of deionized water, and after stirring uniformly, solution C was formed; 2 mmol of sodium borohydride powder and 1 mmol of Se powder were slowly added to 20 mL of deionized water, and this solution was used as solution D. Solution D was slowly added to solution C, and after stirring for 30 min, it was transferred to a high-pressure reaction kettle and heated at 160°C for 12 h. After cooling to room temperature, the product was centrifuged, washed and dried to obtain nitrogen-doped porous cobalt selenide carbon material CoSe2@NC;

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

[0077] Preparation of negative electrode sheet: The P-CoSe2@NC-2 negative electrode material prepared in this example, Ketjen black and carboxymethyl cellulose binder were thoroughly ground in a mass ratio of 7:2:1, deionized water was added as a solvent, and after adding it was continued to be ground to obtain a uniform slurry, which was coated on an aluminum foil current collector, vacuum dried at 80°C for 12h, and cut into a circular electrode sheet with a punching machine, ready for use.

[0078] Example 3

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

[0080] (1) ZIF-67 was heated to 750°C at a heating rate of 5°C / min under Ar atmosphere, and carbonized for 2h to obtain N-doped cobalt-carbon material Co@NC with dodecahedral structure;

[0081] (2) 50 mg of Co@NC was dispersed in 20 mL of deionized water, and after stirring uniformly a solution C was formed; 2 mmol of sodium borohydride powder was slowly added in 20 mL of deionized water, and then 1 mmol of Se powder was added, this solution was used as solution D. Solution D was slowly added to solution C, stirred for 30 min, then transferred to a high-pressure reaction kettle and heated at 160°C for 12h, after cooling to room temperature, the product was centrifuged, washed and dried to obtain CoSe2@NC;

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

[0083] Preparation of negative electrode sheet: the P-CoSe2@NC-3 negative electrode material prepared in this example, Ketjen black and carboxymethyl cellulose binder were fully ground in a mass ratio of 7:2:1, deionized water was used as the solvent, and after adding it was continued to be ground, a uniform slurry was obtained, coated on an aluminum foil current collector, vacuum dried at 80°C for 12h, cut into a circular electrode sheet with a punching machine, and used.

[0084] Example 4

[0085] This example relates to the preparation of a negative electrode material and a negative electrode sheet, which is only different from Example 1 in that: in step (3), equimolar amount of phenol is used instead of bisphenol A, and equimolar amount of polymethylacrolein is used instead of polymethylaldehyde, and the rest of the operations are consistent, and P, N doped porous dodecahedron P-CoSe2@NC-4 negative electrode material is prepared.

[0086] Preparation of negative electrode sheet: the P-CoSe2@NC-4 negative electrode material prepared in this example, Ketjen black and carboxymethyl cellulose binder were fully ground in a mass ratio of 7:2:1, deionized water was used as the solvent, and after adding it was continued to be ground, a uniform slurry was obtained, coated on an aluminum foil current collector, vacuum dried at 80°C for 12h, cut into a circular electrode sheet with a punching machine, and used.

[0087] Comparative Example 1

[0088] This comparative example relates to the preparation of a negative electrode material and a negative electrode sheet, which is only different from Example 1 in that: no 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide (DOPO, phosphorus source) is added in step (3), and the rest of the operations are consistent, and N doped porous dodecahedron 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 fully ground in a mass ratio of 7:2:1, deionized water was used as the solvent, and after adding it was continued to be ground, a uniform slurry was obtained, coated on an aluminum foil current collector, vacuum dried at 80°C for 12h, cut into a circular electrode sheet with a punching machine, and used.

[0090] Comparative Example 2

[0091] This comparative example relates to the preparation of a negative electrode material and a negative electrode sheet, which is only different from Example 1 in that: the operation of step (3) is not included, and the rest of the operations are consistent, and nitrogen doped porous cobalt selenide carbon material CoSe2@NC negative electrode material is prepared.

[0092] Preparation of the negative electrode sheet: the CoSe2@NC negative material prepared in the present comparative example, Ketjen black and carboxymethyl cellulose binder were fully ground in a mass ratio of 7:2:1, deionized water was used as the solvent, and after adding, the grinding was continued, to obtain a uniform slurry, which was coated on an aluminum foil current collector, vacuum dried at 80°C for 12h, and cut into a circular electrode sheet by a punching machine, ready for use.

[0093] Comparative Example 3

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

[0095] 100mg of Co@NC was placed at one end of the porcelain boat, 200mg of Se powder was placed at the other end and at the upper air inlet, and selenized at 600°C for 2h in an Ar atmosphere to obtain CoSe2@NC;

[0096] The rest of the operations were consistent, and P-CoSe2@NC negative material with P and N doped porous dodecahedron was prepared.

[0097] Preparation of the negative electrode sheet: the P-CoSe2@NC negative material prepared in the present comparative example, Ketjen black and carboxymethyl cellulose binder were fully ground in a mass ratio of 7:2:1, deionized water was used as the solvent, and after adding, the grinding was continued, to obtain a uniform slurry, which was coated on an aluminum foil current collector, vacuum dried at 80°C for 12h, and cut into a circular electrode sheet by a punching machine, ready for use.

[0098] Application and performance test

[0099] (1) The particle size, specific surface area and porosity of the negative electrode materials prepared in the above examples and comparative examples were tested, wherein the specific surface area of the negative electrode material has the meaning known in the art, and can be measured by the method known in the art. The specific surface area analyzer (Mic Tristar II 3020) was used in the present application to measure the specific surface area and porosity of the negative electrode material by nitrogen adsorption / desorption method. The test results are shown in Table 1 below:

[0100] Table 1

[0101]

[0102]

[0103] As can be seen from Table 1, the particle size of Examples 1-4 and Comparative Example 1 increases to some extent after coating; and compared with the negative electrode material prepared in Example 1, the particle size of the negative electrode material prepared in Example 2, which has a high carbon source content, is larger, but each group is almost in the same level 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 can be seen from Table 2, the sodium ion half-batteries constructed by the negative electrode materials prepared in Examples 1-4 not only have high first coulombic efficiency, but also have high discharge specific capacity and capacity retention rate of no less than 83% after 2000 cycles. The sodium ion half-batteries constructed by the negative electrode material prepared in Example 1 have a capacity retention rate of up to 91.5% after 2000 cycles. This is because the negative electrode material has a N-doped porous cobalt diselenide material with a dodecahedron structure as a core, and a layer of P-doped hard carbon is coated on the surface of the core. The material has a higher specific surface area and porosity, provides more active sites, accelerates the transmission of sodium ions, and the coating of hard carbon inhibits the expansion of cobalt diselenide to some extent, so that the material has stable cycle performance, and the co-doping of N and P elements improves the conductivity of the material. Under the synergistic effect of the above-mentioned material and structure, the sodium ion half-batteries constructed by the negative electrode material exhibit high capacity and excellent cycle stability.

[0112] As can be seen from Examples 1 and 2, the hard carbon layer of P-CoSe2@NC-2 prepared in Example 2 is thicker, and the capacity is lower than that in Example 1. P-CoSe2@NC-3 prepared in Example 3 has a lower specific surface area and porosity, and the material structure stability is slightly worse than that in Example 1. The material structure expands in the later cycle stage, resulting in capacity decay. The specific surface area of the negative electrode material prepared in Example 4 is large, but the porosity is small, and the coating layer is not uniformly coated, resulting in an increase in side reactions in the later cycle stage. Compared with Example 1, the cycle performance is reduced.

[0113] The coating layer of the negative electrode material prepared in Comparative Example 1 is not doped with P elements, the interlayer spacing of the hard carbon layer is smaller than that in Example 1, the sodium storage capacity is slightly worse, and the specific capacity is lower than that in Example 1. The dodecahedron cobalt diselenide in Comparative Example 2 has no coating of carbon layer, and has the worst cycle performance. The material in Comparative Example 3 is selenized at high temperature, and the structure is not so stable. The structure collapses in the later cycle stage, resulting in a decrease in cycle capacity and a low capacity retention rate.

[0114] Specific capacity after 10 cycles at different current densities: under the condition of constant temperature at 25°C, the current was discharged to 0V at a current density of 0.2A / g, 0.5A / g, 1A / g, 2A / g and 5A / g, respectively, and then charged to 3V at the same current density. After 10 cycles at the same current density, the test was ended, and the discharge specific capacity of the last cycle was recorded as the specific capacity after the cycle.

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

[0116] Table 3

[0117]

[0118] As can be seen from Table 3, the P-CoSe2@NC-1 prepared in Example 1 has excellent specific capacity release at a large current density of 5 A / g as the negative electrode material of the sodium ion battery, and the synergistic effect of P and N makes the material have excellent rate performance.

[0119] Compared with Example 1, Comparative Example 1 does not contain P element, and its conductivity is greatly reduced, and the capacity release is obviously lower than that of Example 1. Comparative Example 2 is the precursor of each example, and in the state without coating of hard carbon and without doping of P, the capacity at a large rate is obviously reduced, and CoSe2 has large volume expansion in the charging and discharging process, and the disadvantage is obviously shown without the coating of the carbon layer, and the cycle performance is also the worst. In addition, as can be seen from Example 1 and Comparative Example 3, the fast charging performance of the negative electrode material prepared by high-temperature selenization (Comparative Example 3) is worse than that of the negative electrode material prepared by hydrothermal selenization (Example 1).

[0120] As can be known from the above, the sodium ion battery constructed by the negative electrode material prepared by the application can have high capacity and excellent cycle performance and rate performance.

[0121] The above-described examples are only preferred examples for fully illustrating the application, and the protection scope of the application is not limited thereto. The equivalent substitutions or transformations made by the person skilled in the art on the basis of the application are within the protection scope of the application. The protection scope of the application is subject to 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.

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 hours.

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 10-20h.

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 hours; The reaction is carried out at a temperature of 80-110℃ for 2-5 hours.

5. The preparation method according to claim 1, characterized in that, In step S3, the phenolic monomer includes one or more of bisphenol A, phenol, cresol, and xylenol; The organic polymer includes one or more of polyoxymethylene, polyacetaldehyde, polybutyraldehyde, polypentalaldehyde, and polymethacrylaldehyde; The organophosphorus compound includes one or more of 9,10-dihydro-9-oxo-10-phosphenanthroline-10-oxide, diphenylphosphoxy, and 3-phosphorylbenzoic acid; The solvent is a mixture of benzene-based solvents and alcohol-based solvents.

6. The preparation method according to claim 1 or 5, 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).

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

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

9. 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 8.

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

Images