A tin-based doped hard carbon sodium-ion battery negative electrode material and a preparation method thereof

Abstract

The application relates to the technical field of sodium ion battery negative electrode materials, in particular to a tin-based doped hard carbon sodium ion battery negative electrode material and a preparation method. The method comprises the following steps: multi-stage pretreatment of a biomass carbon source, preparation of a nitrogen-oxygen double-doped hard carbon precursor, configuration of a tin-based complex doping solution, in-situ vacuum immersion doping, low-temperature pre-oxidation stabilization, gradient carbonization activation, gas phase deposition carbon coating, gradient acid washing purification, multi-stage vacuum drying screening and plasma surface modification. The in-situ vacuum immersion process of the tin-based complex is adopted, tin ions uniformly penetrate into the pores of the hard carbon precursor in a complex state, tin nanoparticles with a size of 5-20 nm are formed after high-temperature carbonization, the tin nanoparticles are uniformly anchored in the hard carbon framework, the problem of tin particle agglomeration caused by mechanical mixing is solved, and the utilization rate of the active material is improved.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery anode material technology, specifically to a tin-based doped hard carbon sodium-ion battery anode material and its preparation method. Background Technology

[0002] Lithium-ion batteries, due to limited lithium resources, uneven geographical distribution, and continuously rising costs, are unable to meet the long-term demands of the large-scale energy storage and low-end power markets. Sodium-ion batteries, with their advantages of abundant and widely distributed sodium resources, low cost, and compatibility with lithium-ion battery technology, have become the core alternative to next-generation low-cost energy storage batteries, and their industrialization process continues to accelerate.

[0003] The anode material is a core component of sodium-ion batteries, directly determining the battery's capacity, cycle life, rate performance, and safety. Currently, sodium-ion battery anode materials are mainly classified into hard carbon materials, soft carbon materials, alloy materials, and oxide materials. Among them, hard carbon materials have advantages such as a layered disordered structure, large interlayer spacing, suitability for reversible sodium-ion insertion / extraction, wide availability of raw materials, and environmental friendliness, making them the most mature anode material for industrialization. However, pure hard carbon materials suffer from drawbacks such as low theoretical capacity, insufficient actual discharge capacity, poor structural stability during long cycles, and limited rate performance, failing to meet the application requirements of high-energy-density sodium-ion batteries.

[0004] Tin-based materials, as typical alloy anode materials, possess advantages such as extremely high theoretical capacity, moderate sodium-ionization reaction potential, and good compatibility with electrolytes, making them ideal doping components for enhancing the capacity of hard carbon anodes. However, tin-based materials undergo drastic volume expansion during charge and discharge, leading to the pulverization and shedding of active particles and the collapse of the electrode structure. Furthermore, tin-based materials have poor intrinsic electronic conductivity and slow ion transport rates, ultimately resulting in rapid capacity decay and a drastically shortened cycle life.

[0005] Existing techniques for preparing tin-based doped hard carbon anode materials have the following problems: (1) Mechanical ball milling is often used for mixing. Tin-based particles and hard carbon substrate are only physically bonded. The doping uniformity is extremely poor. Tin particles are prone to agglomeration and growth, and cannot form a stable tin-carbon interface bond. The volume expansion problem cannot be effectively suppressed.

[0006] (2) The pore structure and graphitization degree of hard carbon were not precisely controlled during the preparation process. The hard carbon substrate has narrow ion transport channels and low electronic conduction efficiency, which cannot synergistically improve the rate performance of the material.

[0007] (3) Tin-based particles have no chemical bonding with the hard carbon substrate, resulting in severe interfacial separation during long-term cycling and a continuous decline in the utilization rate of active materials.

[0008] (4) High-pollution chemical reagents and high-temperature one-step carbonization process are often used. Tin elements are easily oxidized and lost, the doping amount is uncontrollable, and the raw materials are mostly chemically synthesized carbon sources, which are costly.

[0009] (5) The surface defects of the material caused by the volume expansion of tin base were not repaired, resulting in poor electrode structure stability and difficulty in exceeding 1000 cycles. Summary of the Invention

[0010] To address the problems existing in the prior art, the purpose of this invention is to design and provide a technical solution for a tin-based doped hard carbon sodium-ion battery anode material and its preparation method.

[0011] The present invention is specifically implemented using the following technical solutions: The first aspect of this invention provides a method for preparing a tin-based doped hard carbon sodium-ion battery anode material, comprising the following steps: (1) Multi-stage pretreatment of biomass carbon source: Take agricultural and forestry waste biomass carbon source, and successively crush and screen it, soak it in deionized water to remove ash, boil it in dilute acid solution to remove minerals, dehydrate it in anhydrous ethanol, and vacuum dry it to obtain pretreated biomass carbon source. (2) Preparation of nitrogen-oxygen dual-doped hard carbon precursor: The pretreated biomass carbon source is mixed with nitrogen and oxygen sources, and dry ball milling is used to obtain a uniformly mixed powder. Low-temperature pre-carbonization is carried out under an inert atmosphere to obtain nitrogen-oxygen dual-doped hard carbon precursor. (3) Preparation of tin-based complex doping solution: Dissolve soluble tin salt in a polar complexing solvent, add complexing agent, and stir at constant temperature until complete complexation to obtain tin-based complex doping solution; (4) In-situ vacuum impregnation doping: The nitrogen-oxygen dual-doped hard carbon precursor is placed in a vacuum impregnation vessel, and after vacuuming, a tin-based complex doping solution is injected. The precursor is impregnated at a constant temperature until tin ions are fully penetrated to obtain the doped precursor. (5) Low-temperature pre-oxidation stabilization: The doped precursor is placed in an air atmosphere and subjected to low-temperature pre-oxidation treatment to obtain a pre-oxidized intermediate; (6) Gradient carbonization activation: The pre-oxidized intermediate is placed in an inert atmosphere and carbonized by a three-stage gradient heating method. At the same time, an activator is added for in-situ activation to obtain carbonized activated material. (7) Carbon coating by vapor deposition: The carbonized activated material is placed in a chemical vapor deposition furnace, an organic carbon source gas is introduced, and a carbon coating layer is formed by high-temperature vapor deposition to obtain the coated material; (8) Gradient acid washing purification: The coated material is acid washed stepwise with dilute acid solution to remove free impurities, and then washed with deionized water until neutral; (9) Multi-stage vacuum drying and sieving: The washed material is subjected to multi-stage vacuum drying and sieving to obtain tin-based doped hard carbon semi-finished product; (10) Plasma surface modification: The semi-finished product is placed in an argon plasma device for surface modification treatment to obtain tin-based doped hard carbon sodium-ion battery anode material.

[0012] Furthermore, the agricultural and forestry waste biomass carbon source mentioned in step (1) is one or more of coconut shells, rice husks, corn stalks, and walnut shells; the particle size after crushing and screening is 100-200 mesh; the dilute acid solution is a 0.5-1.5 mol / L hydrochloric acid or sulfuric acid solution, and the boiling time is 1-3 h; the vacuum drying temperature is 60-80℃, and the time is 6-12 h.

[0013] Furthermore, the nitrogen source mentioned in step (2) is one of urea, melamine, or dicyandiamide; the oxygen source is one of citric acid, glucose, or oxalic acid; the mass ratio of carbon source, nitrogen source, and oxygen source in the pretreated biomass is 10:2-4:1-3; the dry ball milling speed is 300-500 r / min and the time is 2-4 h; the low-temperature pre-carbonization temperature is 300-400℃ and the time is 2-4 h; the inert atmosphere is argon or nitrogen and the flow rate is 50-100 mL / min.

[0014] Furthermore, the soluble tin salt mentioned in step (3) is one of stannous chloride, stannous chloride, and stannous acetate; the polar complexing solvent is one of ethanol, ethylene glycol, and N-methylpyrrolidone; the complexing agent is one of ethylenediaminetetraacetic acid, trisodium citrate, and potassium sodium tartrate; the concentration of the soluble tin salt in the complexing doping solution is 0.1-0.5 mol / L; the constant temperature stirring temperature is 40-60℃, and the time is 1-3h.

[0015] Furthermore, in step (4), the vacuum degree of the vacuum impregnation vessel is -0.08 to -0.06 MPa; the constant temperature impregnation temperature is 50-70℃, the impregnation time is 4-8h; and the solid-liquid ratio of the nitrogen-oxygen dual-doped hard carbon precursor to the tin-based complex dopant solution is 1g:5-10mL.

[0016] Furthermore, in step (5), the low-temperature pre-oxidation temperature is 180-220℃ and the time is 1-2h; in step (6), the first stage of the three-stage gradient heating carbonization is heated to 500-600℃ and held for 1-2h; the second stage is heated to 700-800℃ and held for 1-2h; the third stage is heated to 900-1000℃ and held for 2-3h; the activator is one of potassium hydroxide, potassium carbonate, and zinc oxide, and the mass ratio of the activator to the pre-oxidation intermediate is 1-3:1.

[0017] Furthermore, in step (7), the organic carbon source gas is one of methane, acetylene, or ethanol vapor; the vapor deposition temperature is 600-800℃, the deposition time is 30-60 min, and the inert carrier gas flow rate is 30-50 mL / min.

[0018] Further, in step (8), the stepwise dilute acid solutions are 0.1-0.3 mol / L dilute hydrochloric acid and 0.05-0.1 mol / L dilute sulfuric acid, with each step of acid washing taking 30-60 min and the acid washing temperature being 25-40℃; in step (9), the first stage temperature of the multi-stage vacuum drying is 60-70℃ and the time is 2-3 h; the second stage temperature is 80-90℃ and the time is 1-2 h; the sieve particle size is 200-300 mesh; in step (10), the plasma power is 100-200 W and the processing time is 30-60 s.

[0019] The second aspect of the present invention provides a tin-based doped hard carbon sodium-ion battery anode material prepared by the above-described preparation method.

[0020] The third aspect of this invention provides the application of the tin-based doped hard carbon sodium-ion battery anode material as described above in the preparation of batteries.

[0021] The present invention has the following beneficial effects: (1) The present invention adopts a tin-based complex in-situ vacuum impregnation process, in which tin ions uniformly penetrate into the hard carbon precursor pores in a complex state, and after high-temperature carbonization, tin nanoparticles of 5-20 nm are formed and uniformly anchored in the hard carbon skeleton, which completely solves the problem of tin particle agglomeration caused by mechanical mixing and improves the utilization rate of active materials.

[0022] (2) This invention employs low-temperature pre-oxidation to form an oxide transition layer, enabling tin-based particles to form chemical bonds with the hard carbon substrate. Combined with a vapor-deposited ultrathin carbon coating layer, this effectively buffers the volume expansion of the tin-based layer by more than 300%, preventing electrode pulverization and detachment, and significantly improving long-cycle stability. The nitrogen-oxygen dual-doped hard carbon substrate introduces a large number of defective active sites, improving electronic conductivity and increasing sodium ion storage capacity compared to pure hard carbon. At the same time, the hierarchical pore structure accelerates sodium ion transport, resulting in excellent rate performance.

[0023] (3) This invention employs gradient carbonization and in-situ activation to synergistically regulate the hard carbon pore structure and graphitization degree. The tin doping amount can be precisely controlled between 5 and 20 wt%, resulting in high batch stability of the material, which is suitable for continuous industrial production. Using agricultural and forestry waste biomass as the carbon source reduces raw material costs, eliminates high pollution emissions in the preparation process, and has low energy consumption, meeting the requirements of green chemical industry. It can be directly adapted to existing sodium-ion battery production lines.

[0024] (4) The present invention adopts a high capacity retention rate after 1000 cycles at 1C, which fully meets the application requirements of high energy density and long cycle sodium-ion batteries. Attached Figure Description

[0025] Figure 1 This is a flowchart of the method. Detailed Implementation

[0026] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features can be combined with each other. Unless otherwise specified, the methods used in the embodiments of the present invention are conventional methods, and the reagents used are commercially available.

[0027] Example 1: A method for preparing a tin-based doped hard carbon sodium-ion battery anode material (1) Multi-stage pretreatment of biomass carbon source Agricultural and forestry waste biomass carbon source is selected and subjected to crushing and screening, deionized water soaking to remove ash, boiling in dilute acid solution to remove minerals, dehydration with anhydrous ethanol, and vacuum drying to thoroughly remove ash, metal impurities, and cellulose degradation products from the biomass, thus obtaining a high-purity pretreated biomass carbon source, which provides pure raw materials for the subsequent preparation of hard carbon precursors.

[0028] (2) Preparation of nitrogen-oxygen dual-doped hard carbon precursor The pretreated biomass carbon source is mixed with nitrogen and oxygen sources at a fixed mass ratio and then fully ball-milled in a dry ball mill jar to obtain a uniformly mixed powder. The mixed powder is then placed in an inert atmosphere furnace for low-temperature pre-carbonization, which allows the biomass carbon source to undergo preliminary pyrolysis. At the same time, heteroatoms from the nitrogen and oxygen sources are embedded in situ into the carbon framework to form a nitrogen-oxygen dual-doped hard carbon precursor, thereby improving the electronic conductivity of the carbon substrate and the number of sodium storage active sites.

[0029] (3) Precise preparation of tin-based complex doped solution Soluble tin salts were selected as the tin source and added to a polar complexing solvent. The solution was then thoroughly stirred and dissolved. A complexing agent was added and stirred at a constant temperature until the tin ions were completely complexed with the complexing agent, forming a stable and uniform tin-based complexed doped solution. The complexing agent can effectively bind tin ions, preventing tin ion aggregation during subsequent impregnation and ensuring doping uniformity.

[0030] (4) In-situ vacuum impregnation doping The nitrogen-oxygen dual-doped hard carbon precursor is placed in a vacuum impregnation vessel and evacuated to a set vacuum level to completely empty the precursor channels. Then, a tin-based complex dopant solution is slowly injected and impregnated in a constant-temperature, sealed environment. This allows the tin-based complex ions to fully penetrate into the micropores and mesopores of the hard carbon precursor, achieving in-situ uniform doping of tin and avoiding agglomeration problems caused by mechanical mixing.

[0031] (5) Low-temperature pre-oxidation stabilization treatment The impregnated doped precursor is placed in an air atmosphere furnace for low-temperature pre-oxidation treatment, which transforms the tin-based complex on the precursor surface and in the channels into a stable tin oxide intermediate. At the same time, a thin oxide transition layer is formed on the carbon substrate surface, which enhances the interfacial bonding force between the tin-based particles and the hard carbon substrate and prevents the loss of tin elements during the subsequent high-temperature carbonization process.

[0032] (6) Gradient high-temperature carbonization and pore structure activation The pre-oxidized intermediate is placed in an inert atmosphere tube furnace and a three-stage gradient heating carbonization process is adopted, with in-situ activation treatment using a solid activator. The gradient heating can precisely control the graphitization degree and pore structure of hard carbon, and the activator can construct a hierarchical porous structure, accelerate sodium ion transport, and at the same time reduce the tin oxide intermediate to highly active tin nanoparticles, which are uniformly anchored in the hard carbon framework.

[0033] (7) Organic carbon source vapor phase deposition coating remediation The carbonized and activated material is placed in a chemical vapor deposition furnace, and an organic carbon source gas is introduced. Vapor deposition is carried out at high temperature to form an ultrathin homogeneous carbon coating layer on the surface of the material and the surface of tin nanoparticles. The carbon coating layer can act as an elastic buffer layer to suppress the volume expansion of tin-based particles during charging and discharging, while repairing surface defects of the material and improving electronic conductivity.

[0034] (8) Gradient acid washing purification treatment The carbon-coated material is subjected to gradient acid washing with a stepwise dilute acid solution to remove free tin oxide, uncomplexed tin particles, activator residues and metal impurities from the material surface in sequence, while retaining the in-situ doped tin-based active sites. This avoids excessive acid washing that could cause the hard carbon framework to collapse and ensures the integrity of the material structure.

[0035] (9) Multi-stage vacuum drying and particle size sieving The pickled material was repeatedly washed with deionized water until the filtrate was neutral, and then subjected to multi-stage vacuum drying to gradually remove the moisture and residual solvent inside the material. After drying, the material was sieved using a standard sieve to obtain a tin-based doped hard carbon semi-finished product with uniform particle size and stable performance.

[0036] (10) Argon plasma surface modification The sieved semi-finished product is placed in a plasma treatment device for argon plasma surface modification. Hydrophilic functional groups are introduced into the material surface to improve the wettability of the material and the electrolyte, reduce the electrode / electrolyte interface impedance, and further optimize the rate performance.

[0037] The anode material uses nitrogen-oxygen dual-doped hierarchical porous hard carbon as a framework, with tin nanoparticles uniformly dispersed and anchored in the hard carbon channels and surface, and the tin nanoparticles are coated with an ultrathin carbon layer. The hard carbon framework has a hierarchical pore structure of micropore-mesopore-macropore, with nitrogen and oxygen heteroatoms embedded in the carbon lattice in situ. The amount of tin doping is controllable, the material has a moderate specific surface area, strong interfacial bonding, and no tin particle agglomeration.

[0038] Example 2: Preparation of nitrogen-oxygen dual-doped hard carbon precursor: Pretreated coconut shell carbon source, urea and citric acid were mixed in a mass ratio of 10:3:2 and placed in a dry ball mill jar. The mixture was ball milled at 400 r / min for 3 h to obtain a uniformly mixed powder. The powder was then placed in an argon atmosphere furnace at an argon flow rate of 80 mL / min and pre-carbonized at 350℃ for 3 h to obtain the nitrogen-oxygen dual-doped hard carbon precursor.

[0039] Preparation of tin-based complex doping solution: Add stannous chloride to ethylene glycol and stir until completely dissolved. Add trisodium citrate as a complexing agent and stir at 50°C for 2 hours to prepare a tin-based complex doping solution with a concentration of 0.3 mol / L.

[0040] In-situ vacuum impregnation doping: The nitrogen-oxygen dual-doped hard carbon precursor was placed in a vacuum impregnation vessel, evacuated to -0.07 MPa, and a tin-based complex doping solution was injected at a solid-liquid ratio of 1 g: 8 mL. The mixture was then impregnated at a constant temperature of 60 °C in a sealed environment for 6 h to obtain the doped precursor.

[0041] Low-temperature pre-oxidation stabilization: The doped precursor was placed in an air atmosphere furnace and pre-oxidized at 200°C for 1.5 h to obtain a pre-oxidized intermediate.

[0042] Gradient carbonization activation: The intermediate is placed in an argon atmosphere tube furnace and subjected to three-stage gradient carbonization: 550℃ for 1.5h → 750℃ for 1.5h → 950℃ for 2.5h; potassium hydroxide is added as an activator, with the mass ratio of activator to intermediate being 2:1, and in-situ activation is performed to obtain the carbonized activated material.

[0043] Chemical vapor deposition carbon coating: The carbonized material is placed in a chemical vapor deposition furnace, acetylene gas is introduced, and the carbon is deposited at 700℃ for 45 min with an argon carrier gas flow rate of 40 mL / min to form an ultrathin carbon coating layer.

[0044] Gradient acid washing purification: sequentially acid wash with 0.2 mol / L dilute hydrochloric acid and 0.08 mol / L dilute sulfuric acid, each step for 45 min, acid washing temperature 30℃, and washing with deionized water until the filtrate is neutral.

[0045] Multi-stage vacuum drying and sieving: First stage vacuum drying at 65℃ for 2.5h, second stage vacuum drying at 85℃ for 1.5h, sieved to 250 mesh to obtain semi-finished product.

[0046] Plasma surface modification: The semi-finished product was placed in an argon plasma device and treated with 150W power for 45s to obtain tin-based doped hard carbon sodium-ion battery anode material.

[0047] Example 3: Multi-stage pretreatment of biomass carbon source: Rice husks were selected as the carbon source, crushed and sieved to 120 mesh, soaked in deionized water for 10 h, boiled in 1.2 mol / L sulfuric acid for 1.5 h, and vacuum dried at 80℃ for 6 h to obtain pretreated rice husk carbon source.

[0048] Preparation of nitrogen-oxygen dual-doped hard carbon precursor: Rice husk carbon source, melamine and glucose were mixed in a mass ratio of 10:4:3, ball-milled at 500 r / min for 2 h, pre-carbonized at 400℃ in a nitrogen atmosphere for 2 h at a nitrogen flow rate of 100 mL / min to obtain the precursor.

[0049] Preparation of tin-based complex doping solution: Dissolve tin chloride in ethanol, add ethylenediaminetetraacetic acid, stir at 40℃ for 3h to prepare 0.4mol / L doping solution.

[0050] In-situ vacuum impregnation doping: vacuum degree -0.06MPa, solid-liquid ratio 1g:10mL, impregnation at 70℃ for 4h.

[0051] Low-temperature pre-oxidation stabilization: pre-oxidation at 210℃ for 1 hour.

[0052] Gradient carbonization activation: 600℃ for 1 hour → 800℃ for 1 hour → 1000℃ for 2 hours, potassium carbonate as activator, mass ratio 3:1.

[0053] Carbon coating by vapor deposition: methane was used as the carbon source, and deposition was carried out at 800℃ for 30 min.

[0054] Gradient acid washing purification: stepwise acid washing with 0.3 mol / L dilute hydrochloric acid and 0.1 mol / L dilute sulfuric acid, 60 min per step.

[0055] Multi-stage vacuum drying and sieving: drying at 70℃ for 2 hours, drying at 90℃ for 1 hour, and sieving to 300 mesh.

[0056] Plasma surface modification: 200W treatment for 30s yields negative electrode material.

[0057] Example 4: Multi-stage pretreatment of biomass carbon source: Corn straw was selected as the carbon source, crushed and sieved to 200 mesh, soaked in deionized water for 15 h, boiled in 0.8 mol / L hydrochloric acid for 3 h, and vacuum dried at 60℃ for 12 h to obtain pretreated straw carbon source.

[0058] Preparation of nitrogen-oxygen dual-doped hard carbon precursor: straw carbon source, dicyandiamide and oxalic acid were mixed in a mass ratio of 10:2:1, ball-milled at 300 r / min for 4 h, and pre-carbonized at 300℃ in an argon atmosphere for 4 h at a flow rate of 50 mL / min.

[0059] Preparation of tin-based complex doping solution: Dissolve tin acetate in N-methylpyrrolidone, add potassium sodium tartrate, stir at 60℃ for 1 h to prepare 0.1 mol / L doping solution.

[0060] In-situ vacuum impregnation doping: vacuum degree -0.08MPa, solid-liquid ratio 1g:5mL, impregnation at 50℃ for 8h.

[0061] Low-temperature pre-oxidation stabilization: Pre-oxidation at 180℃ for 2 hours.

[0062] Gradient carbonization activation: 500℃ for 2 hours → 700℃ for 2 hours → 900℃ for 3 hours, zinc oxide as activator, mass ratio 1:1.

[0063] Carbon coating by vapor deposition: Ethanol vapor was used as the carbon source, and deposition was carried out at 600℃ for 60 min.

[0064] Gradient acid washing purification: stepwise acid washing with 0.1 mol / L dilute hydrochloric acid and 0.05 mol / L dilute sulfuric acid, 30 min per step.

[0065] Multi-stage vacuum drying and sieving: drying at 60℃ for 3 hours, drying at 80℃ for 2 hours, and sieving to 200 mesh.

[0066] Plasma surface modification: 100W treatment for 60s yields negative electrode material.

[0067] Example 5: Multi-stage pretreatment of biomass carbon source: Walnut shells were selected as the carbon source, crushed and sieved to 180 mesh, soaked in deionized water for 14 hours, boiled in 1.5 mol / L sulfuric acid for 1 hour, and vacuum dried at 75℃ for 10 hours to obtain pretreated walnut shell carbon source.

[0068] Preparation of nitrogen-oxygen dual-doped hard carbon precursor: Walnut shell carbon source, urea and glucose were mixed at a mass ratio of 10:3:2, ball-milled at 450 r / min for 2.5 h, pre-carbonized at 380 ℃ in a nitrogen atmosphere for 2.5 h, and flow rate was 70 mL / min.

[0069] Preparation of tin-based complex doping solution: Dissolve stannous chloride and stannous chloride composite tin source in ethylene glycol, add trisodium citrate, stir at 55℃ for 2.5h to prepare 0.35mol / L doping solution.

[0070] In-situ vacuum impregnation doping: vacuum degree -0.07MPa, solid-liquid ratio 1g:7mL, impregnation at 65℃ for 5h.

[0071] Low-temperature pre-oxidation stabilization: pre-oxidation at 220℃ for 1.2h.

[0072] Gradient carbonization activation: 580℃ for 1.2h → 780℃ for 1.2h → 980℃ for 2.2h, potassium hydroxide and potassium carbonate composite activator, mass ratio 2.5:1.

[0073] Carbon coating by vapor deposition: acetylene and methane composite carbon source, deposition at 750℃ for 40 min.

[0074] Gradient acid washing purification: stepwise acid washing with 0.25 mol / L dilute hydrochloric acid and 0.07 mol / L dilute sulfuric acid, 50 min per step.

[0075] Multi-stage vacuum drying and sieving: drying at 68℃ for 2.2 hours, drying at 88℃ for 1.8 hours, and sieving to 220 mesh.

[0076] Plasma surface modification: 180W treatment for 40s yields negative electrode material.

[0077] Comparative Example 1: Coconut shells were selected as the carbon source and only underwent crushing, washing, and drying. Pure hard carbon material was obtained by one-step carbonization at 600°C. There was no tin doping, no nitrogen and oxygen doping, no carbon coating, and no plasma modification. The remaining processes were the same as in Example 2.

[0078] Comparative Example 2: Tin / hard carbon composite material was prepared by conventional mechanical ball milling: pure hard carbon prepared in Example 1 was mixed with tin powder at a mass ratio of 9:1 and ball milled at 300 r / min for 6 h. There was no in-situ impregnation, no pre-oxidation, and no carbon coating process. The other conditions were the same as in Example 2.

[0079] Performance testing and results analysis: The physical structure and electrochemical properties of the materials prepared in Examples 2-5 and Comparative Examples 1-2 were tested. Physical property testing employed X-ray diffraction, a specific surface area and pore size analyzer, scanning electron microscopy, and transmission electron microscopy. Electrochemical property testing used CR2032 coin cells with a 1 mol / L NaPF6 / EC+DEC electrolyte (volume ratio 1:1) and a sodium sheet as the counter electrode. The test voltage range was 0.01–3.0 V vs Na / Na. + .

[0080] Table 1 .

[0081] Table 2 Electrochemical performance parameters of materials in the examples and comparative examples .

[0082] From Tables 1 and 2, we can see that: The materials prepared in the embodiments of this invention have the characteristics of hierarchical porous structure, uniform dispersion of tin particles, no agglomeration, and ultra-thin carbon coating layer, and their physical structure is far superior to that of the comparative examples. Example 3 has the best overall performance, with a 0.1C discharge capacity of 589mAh / g, a capacity retention rate of 89.5% after 1000 cycles at 1C, and an interface impedance of only 98Ω, which is far superior to pure hard carbon and mechanically mixed tin / hard carbon materials. Comparative example 1 has low capacity, poor rate capability, and poor cycle stability of pure hard carbon. Comparative example 2 has severe agglomeration of tin particles, uncontrollable volume expansion, rapid capacity decay, and extremely high interface impedance.

[0083] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a tin-based doped hard carbon sodium-ion battery anode material, characterized in that, Includes the following steps: (1) Multi-stage pretreatment of biomass carbon source: Take agricultural and forestry waste biomass carbon source, and successively crush and screen it, soak it in deionized water to remove ash, boil it in dilute acid solution to remove minerals, dehydrate it in anhydrous ethanol, and vacuum dry it to obtain pretreated biomass carbon source. (2) Preparation of nitrogen-oxygen dual-doped hard carbon precursor: The pretreated biomass carbon source is mixed with nitrogen and oxygen sources, and dry ball milling is used to obtain a uniformly mixed powder. Low-temperature pre-carbonization is carried out under an inert atmosphere to obtain nitrogen-oxygen dual-doped hard carbon precursor. (3) Preparation of tin-based complex doping solution: Dissolve soluble tin salt in a polar complexing solvent, add complexing agent, and stir at constant temperature until complete complexation to obtain tin-based complex doping solution; (4) In-situ vacuum impregnation doping: The nitrogen-oxygen dual-doped hard carbon precursor is placed in a vacuum impregnation vessel, and after vacuuming, a tin-based complex doping solution is injected. The precursor is impregnated at a constant temperature until tin ions are fully penetrated to obtain the doped precursor. (5) Low-temperature pre-oxidation stabilization: The doped precursor is placed in an air atmosphere and subjected to low-temperature pre-oxidation treatment to obtain a pre-oxidized intermediate; (6) Gradient carbonization activation: The pre-oxidized intermediate is placed in an inert atmosphere and carbonized by a three-stage gradient heating method. At the same time, an activator is added for in-situ activation to obtain carbonized activated material. (7) Carbon coating by vapor deposition: The carbonized activated material is placed in a chemical vapor deposition furnace, an organic carbon source gas is introduced, and a carbon coating layer is formed by high-temperature vapor deposition to obtain the coated material; (8) Gradient acid washing purification: The coated material is acid washed stepwise with dilute acid solution to remove free impurities, and then washed with deionized water until neutral; (9) Multi-stage vacuum drying and sieving: The washed material is subjected to multi-stage vacuum drying and sieving to obtain tin-based doped hard carbon semi-finished product; (10) Plasma surface modification: The semi-finished product is placed in an argon plasma device for surface modification treatment to obtain tin-based doped hard carbon sodium-ion battery anode material.

2. The method for preparing a tin-based doped hard carbon sodium-ion battery anode material according to claim 1, characterized in that, The biomass carbon source of agricultural and forestry waste mentioned in step (1) is one or more of coconut shell, rice husk, corn stalk, and walnut shell; the particle size after crushing and screening is 100-200 mesh; the dilute acid solution is 0.5-1.5 mol / L hydrochloric acid or sulfuric acid solution, and the boiling time is 1-3 h; the vacuum drying temperature is 60-80℃, and the time is 6-12 h.

3. The method for preparing a tin-based doped hard carbon sodium-ion battery anode material according to claim 1, characterized in that, The nitrogen source mentioned in step (2) is one of urea, melamine, and dicyandiamide; the oxygen source is one of citric acid, glucose, and oxalic acid; the mass ratio of carbon source, nitrogen source, and oxygen source in the pretreated biomass is 10:2-4:1-3; the dry ball milling speed is 300-500 r / min and the time is 2-4 h; the low-temperature pre-carbonization temperature is 300-400℃ and the time is 2-4 h; the inert atmosphere is argon or nitrogen and the flow rate is 50-100 mL / min.

4. The method for preparing a tin-based doped hard carbon sodium-ion battery anode material according to claim 1, characterized in that, The soluble tin salt mentioned in step (3) is one of stannous chloride, stannous chloride, and stannous acetate; the polar complexing solvent is one of ethanol, ethylene glycol, and N-methylpyrrolidone; the complexing agent is one of ethylenediaminetetraacetic acid, trisodium citrate, and potassium sodium tartrate; the concentration of the soluble tin salt in the complexing doping solution is 0.1-0.5 mol / L; the constant temperature stirring temperature is 40-60℃, and the time is 1-3h.

5. The method for preparing a tin-based doped hard carbon sodium-ion battery anode material according to claim 1, characterized in that, In step (4), the vacuum degree of the vacuum impregnation vessel is -0.08 to -0.06 MPa; the constant temperature impregnation temperature is 50-70℃ and the impregnation time is 4-8h; the solid-liquid ratio of the nitrogen-oxygen dual-doped hard carbon precursor to the tin-based complex dopant solution is 1g:5-10mL.

6. The method for preparing a tin-based doped hard carbon sodium-ion battery anode material according to claim 1, characterized in that, In step (5), the low-temperature pre-oxidation temperature is 180-220℃ and the time is 1-2h; in step (6), the first stage of the three-stage gradient heating carbonization is heated to 500-600℃ and held for 1-2h; the second stage is heated to 700-800℃ and held for 1-2h; the third stage is heated to 900-1000℃ and held for 2-3h; the activator is one of potassium hydroxide, potassium carbonate, and zinc oxide, and the mass ratio of the activator to the pre-oxidation intermediate is 1-3:

1.

7. The method for preparing a tin-based doped hard carbon sodium-ion battery anode material according to claim 1, characterized in that, In step (7), the organic carbon source gas is one of methane, acetylene, or ethanol vapor; the vapor deposition temperature is 600-800℃, the deposition time is 30-60 min, and the inert carrier gas flow rate is 30-50 mL / min.

8. The method for preparing a tin-based doped hard carbon sodium-ion battery anode material according to claim 1, characterized in that, In step (8), the stepwise dilute acid solutions are 0.1-0.3 mol / L dilute hydrochloric acid and 0.05-0.1 mol / L dilute sulfuric acid, with each step of acid washing taking 30-60 min and the acid washing temperature being 25-40℃; in step (9), the first stage temperature of the multi-stage vacuum drying is 60-70℃ and the time is 2-3 h; the second stage temperature is 80-90℃ and the time is 1-2 h; the sieve particle size is 200-300 mesh; in step (10), the plasma power is 100-200 W and the processing time is 30-60 s.

9. Tin-based doped hard carbon sodium-ion battery anode material prepared by any one of the preparation methods described in claims 1-8.

10. The application of the tin-based doped hard carbon sodium-ion battery anode material according to claim 9 in the preparation of batteries.

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