A negative electrode composite material and its preparation and application

Abstract

This invention belongs to the field of battery technology, specifically relating to a sodium-ion battery anode composite material and its preparation and application. The sodium-ion battery anode composite material comprises a carbon fiber inner layer and a hard carbon outer layer; the surface of the carbon fiber inner layer is loaded with transition metal sulfides, including SnS2 and FeS2, with the FeS2 comprising pyrite and marcasite structures; the hard carbon outer layer coats the carbon fiber inner layer and the transition metal sulfides. This invention, by designing the SnS2 / FeS2 material into a hard carbon-coated fibrous structure, not only allows for sufficient contact between the electrolyte and the material, providing ample reactive sites, but also shortens the ion transport path and improves electrochemical reaction kinetics.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, specifically relating to a negative electrode composite material and its preparation and application. Background Technology

[0002] Due to the limited availability, uneven geographical distribution, and high cost of lithium resources, they are not suitable for large-scale energy storage applications. Sodium, however, belongs to the same group as lithium, sharing similar physicochemical properties and energy storage mechanisms. Furthermore, sodium resources are abundant on Earth, easy to extract, and inexpensive. Therefore, sodium-ion batteries have a very broad application prospect in large-scale energy storage.

[0003] Currently, carbon-based materials are attracting attention as anode materials for sodium-ion batteries due to their abundant resources, stable structure, and low manufacturing cost. Graphite anode materials, which have achieved great success in lithium-ion batteries, are limited in their application because sodium ions are difficult to intercalate. Hard carbon, with its large carbon interlayer spacing and unique microstructure, is considered a promising anode material for sodium-ion batteries. Resin-based materials are excellent precursors for preparing hard carbon, allowing for the design of ideal molecular structures. However, the increasing energy density requirements of sodium-ion batteries are driving the development of anode materials with higher sodium storage capacity. In the development of sodium-ion battery anode materials, transition metal sulfide (TMS) compounds can achieve higher sodium storage capacity relative to Na+. + / Na undergoes a reversible reaction with sodium at a suitable low voltage and possesses a large theoretical capacity. FeS2 is advantageous due to its low cost, common elemental composition, and high theoretical specific capacity of 894 mAh g. -1 FeS2 has attracted much attention due to its characteristics such as low conductivity, slow reaction kinetics, and capacity decay and structural degradation caused by large volume changes during the reaction. Currently, the rational structural engineering of bifunctional carbon-based composites has become an effective way to improve the anodic electrochemical performance of FeS2. The construction of heterostructures composed of binary or multi-metal sulfides has also improved sodium storage performance. The addition of multiple components to heterojunction interfaces increases active reaction sites and improves ion storage performance. However, heterojunction interfaces gradually passivate during repeated and asynchronous conversion / alloy-dealloying processes. Simultaneously, soluble intermediate reaction products continue to weaken reaction kinetics. In summary, there is a need to develop a new material that can provide higher sodium storage capacity while overcoming capacity decay and structural degradation problems and suppressing the passivation process. Summary of the Invention

[0004] This invention aims to solve the above problems and provides a sodium-ion battery anode composite material. By designing the SnS2 / FeS2 material into a hard carbon-coated fibrous structure, it not only allows the electrolyte to fully contact the material and has sufficient reactive sites, but also shortens the ion transport path and improves the electrochemical reaction kinetics.

[0005] According to the technical solution of the present invention, the negative electrode composite material includes a carbon fiber inner layer and a hard carbon outer layer; the surface of the carbon fiber inner layer is loaded with transition metal sulfides, the transition metal sulfides include SnS2 and FeS2, the FeS2 includes pyrite structure and marcasite structure; the hard carbon outer layer covers the carbon fiber inner layer and the transition metal sulfides.

[0006] Specifically, the transition metal sulfide is a discontinuous coating layer, and the hard carbon outer layer covers the transition metal sulfide and contacts the carbon fiber inner layer.

[0007] In this invention, the sodium-ion battery anode composite material exhibits an overall fibrous structure, which helps to shorten the electron / ion transport path. Among transition metal sulfides, SnS2 / FeS2 material has a high sodium storage specific capacity. By using SnS2 to regulate the heterostructure interface of FeS2 material, a three-phase interface is obtained, which contributes to interface stability. Specifically, FeS2 has two structures: pyrite and marcasite. The cubic structure of pyrite is superior to that of orthorhombic marcasite in terms of sodium storage performance; however, the passivation phenomenon of the two-phase heterojunction interface is exacerbated during charge and discharge. To further improve the electrochemical performance of FeS2 material, SnS2 is used to regulate the heterostructure interface of FeS2 material to obtain a three-phase interface, thereby further improving ion and electron transport and contributing to interface stability.

[0008] Furthermore, the diameter of the inner carbon fiber layer is 1-10 μm.

[0009] Furthermore, nitrogen is doped into both the inner carbon fiber layer and the outer hard carbon layer. Specifically, the total nitrogen doping content accounts for 2-5 wt% of the negative electrode composite material.

[0010] Furthermore, the thickness of the hard carbon outer layer is 5-10 nm, and it can be formed by calcining resin.

[0011] Furthermore, the transition metal sulfide satisfies at least one of the following conditions:

[0012] The transition metal sulfide accounts for 30-50% of the total mass of the negative electrode composite material (in the sodium-ion battery negative electrode composite material, the loading mass ratio of SnS2 / FeS2 is 30-50%).

[0013] In the transition metal sulfide, the ratio of iron to tin atoms in SnS2 and FeS2 is (3-5):1;

[0014] The particle size of the transition metal sulfide is 10-100 nm.

[0015] A second aspect of the present invention provides a method for preparing a sodium-ion battery anode composite material, comprising the following steps:

[0016] S1: Immerse the biomass material in a mixed solution of iron and tin sources, and freeze-dry it to obtain the mixed material;

[0017] S2: The mixed material and sulfur-containing resin are calcined in an inert gas atmosphere to obtain the sodium-ion battery negative electrode composite material.

[0018] Furthermore, the biomass material contains a fibrous structure and is selected from one or more of cellulose, bamboo fiber, coconut shell, rice straw, cotton stalk bark, and sugarcane bagasse.

[0019] Furthermore, in step S1, before the biomass material is immersed in the mixed solution of iron source and tin source, a step of removing impurities is also included.

[0020] Further, the specific steps for removing impurities include: first, washing the biomass material with alkali for 1.5-3 hours, then rinsing it with water until the pH value is 7-8; then, washing it with acid for 1.5-3 hours, then rinsing it with water until the pH value is 6-7; and finally, drying it.

[0021] Furthermore, the alkaline solution used in the alkaline washing is selected from one or both of potassium hydroxide solution and sodium hydroxide solution, and its concentration can be 0.5-1 mol / L;

[0022] The acid solution used for pickling is selected from one or more of hydrochloric acid, nitric acid, sulfuric acid, and phosphoric acid, and its concentration can be 0.5-1 mol / L, preferably hydrochloric acid with a concentration of 0.5-1 mol / L;

[0023] The solid-liquid ratio (mass-volume ratio of biomass material to acid or alkaline solution) for both alkaline and acid washing is 1:50-200 (g:mL).

[0024] The drying temperature is 70-90℃, and the time is 10-20h.

[0025] Furthermore, in step S1, the biomass material is immersed in a mixed solution of iron and tin sources for 10-16 hours.

[0026] Furthermore, the iron source is selected from one or more of ferric chloride, ferrous chloride, ferric fluoride, ferric acetate, ferric sulfate, and ferric nitrate;

[0027] The tin source is selected from one or more of tin chloride, stannous chloride, tin sulfate, and tin acetate;

[0028] In the mixed solution of the iron source and the tin source, the concentrations of both the iron source and the tin source are 0.01-0.1M.

[0029] Furthermore, in the mixed solution of the iron source and the tin source, the molar ratio of iron to tin is 1-3:1.

[0030] Furthermore, in step S1, the freeze-drying is carried out in liquid nitrogen for 1.5-4 hours, and after freezing, the product is placed in a vacuum freeze dryer for drying.

[0031] Furthermore, the inert gas is selected from one or more of nitrogen, argon, and helium, and the gas flow rate of the inert gas can be 40-80 mL / min.

[0032] Furthermore, in step S2, the mass ratio of the mixed material to the sulfur-containing resin is 1:5-10; the calcination temperature is 1000-1500℃, the heating rate can be 15-20℃ / min, and the calcination time is 2-5h.

[0033] Furthermore, the sulfur-containing resin, as a sulfur source and carbon source, can be vaporized at a certain temperature (the violent thermal decomposition temperature of sulfur-containing resin is 500-700℃), and can be selected from one or more of thiourea resin, polysulfide epoxy resin, polyphenylene sulfide (PPS resin), etc.

[0034] Preferably, in order to avoid impurities and obtain a product with higher purity, in step S2, when calcining the mixed material and the sulfur-containing resin, the mixed material and the sulfur-containing resin are placed in two adjacent containers (such as crucibles).

[0035] Furthermore, in step S2, after calcining the mixed material and the sulfur-containing resin, the step further includes calcining under an NH3 atmosphere to dope the carbon material with nitrogen.

[0036] Furthermore, the conditions for calcination under an NH3 atmosphere are as follows: NH3 gas flow rate of 40-80 mL / min, calcination temperature of 500-700℃, heating rate of 2-5℃ / min, and calcination time of 2-5 h.

[0037] A third aspect of the present invention provides a sodium-ion battery negative electrode sheet comprising the above-described sodium-ion battery negative electrode composite material or the sodium-ion battery negative electrode composite material prepared by the above-described preparation method.

[0038] A fourth aspect of the present invention provides a sodium-ion battery, including the above-described sodium-ion battery negative electrode sheet.

[0039] Compared with the prior art, the technical solution of the present invention has the following advantages: the overall design of the sodium-ion battery negative electrode composite material of the present invention is a fibrous structure (formed by coating with carbon fiber as the inner layer), which is beneficial to shortening the ion / electron transport path; it adopts a theoretical specific capacity of up to 894 mAh g. -1 FeS2 is used to coat carbon fibers, and then SnS2 is used to regulate the heterostructure of the FeS2 material to obtain a three-phase interface of SnS2 / pyrite FeS2 / marginite FeS2, which improves the transport of ions and electrons and helps stabilize the interface. The outermost hard carbon coating can effectively suppress and buffer volume expansion and enhance structural stability. The conductivity of the material can be further improved by doping with nitrogen. The preparation method is simple and easy to control, and the synthesized product has a stable structure and morphology, showing good application prospects. Attached Figure Description

[0040] Figure 1 The image shows the SEM image of the sodium-ion battery anode composite material (SnS2 / pyrite FeS2 / marginate FeS2@NCF) in Example 1.

[0041] Figure 2 This is a transmission electron microscope (TEM) image of the hard carbon outer layer in the sodium-ion battery anode composite material (SnS2 / pyrite FeS2 / marginite FeS2@NCF) in Example 1.

[0042] Figure 3 The image shows a transmission electron microscope (TEM) image of the three-phase interface in the sodium-ion battery anode composite material (SnS2 / pyrite FeS2 / marbled FeS2@NCF) in Example 1 (P: pyrite; M: marbled). Detailed Implementation

[0043] Biomass-based and resin-based materials are excellent precursors for preparing hard carbon, yielding hard carbon materials suitable for sodium storage. However, hard carbon materials are insufficient to meet the ever-increasing energy density requirements of sodium-ion batteries, prompting the development of anode materials with higher capacity. In the development of anode materials for sodium-ion batteries, transition metal sulfide (TMS) compounds can achieve higher capacity relative to Na+. + / Na undergoes a reversible reaction with sodium at a suitable low voltage and possesses a large theoretical capacity. FeS2 is advantageous due to its low cost, common elemental composition, and high theoretical specific capacity of 894 mAh g. -1 FeS2 has attracted much attention due to its characteristics such as low electrical conductivity, slow reaction kinetics, and capacity decay and structural degradation caused by large volume changes during the reaction. Furthermore, FeS2 has two structures: pyrite and marcasite. Cubic pyrite exhibits better sodium storage performance than orthorhombic marcasite; however, passivation at the two-phase heterojunction interface is exacerbated during charge and discharge.

[0044] Based on this, the present invention provides a sodium-ion battery anode composite material, comprising a carbon fiber inner layer and a hard carbon outer layer; a transition metal sulfide is loaded on the surface of the carbon fiber inner layer. The transition metal sulfide is a discontinuous coating layer, referred to as the intermediate layer, and its composition includes SnS2 and FeS2, with FeS2 comprising pyrite and marcasite structures; the hard carbon outer layer coats the carbon fiber inner layer and the intermediate layer.

[0045] Specifically, the transition metal sulfide layer has a three-phase interface, consisting of SnS2, pyrite FeS2, and marcasite FeS2. A hard carbon outer layer covers the intermediate layer and contacts the carbon fiber inner layer.

[0046] In some preferred embodiments, the transition metal sulfide accounts for 30-50% of the total mass of the sodium-ion battery anode composite material, for example, 30%, 35%, 40%, 45%, 50%, etc.; wherein, the iron-tin atomic ratio is 3-5:1, for example, 3:1, 3.5:1, 4:1, 5:1, etc.; the particle size of the transition metal sulfide is 10-100nm, for example, 10nm, 30nm, 50nm, 70nm, 100nm, etc.

[0047] In some preferred embodiments, nitrogen is doped into both the inner carbon fiber layer and the outer hard carbon layer. The doped nitrogen can further synergistically improve the conductivity of the composite material. The total amount of nitrogen doping accounts for 2-5 wt% of the negative electrode composite material, for example, 2 wt%, 3 wt%, 4 wt%, 5 wt%, etc.

[0048] The preparation method of the above-mentioned sodium-ion battery negative electrode composite material may include the following steps:

[0049] S1: The biomass material (selected from one or more of cellulose, bamboo fiber, coconut shell, rice straw, cotton stalk bark, and sugarcane bagasse) is soaked in a mixed solution of an iron source (selected from one or more of ferric chloride, ferrous chloride, ferric fluoride, ferric acetate, ferric sulfate, and ferric nitrate) and a tin source (selected from one or more of tin chloride, stannous chloride, tin sulfate, and tin acetate) for 10-16 hours, then immersed in liquid nitrogen for freezing for 1.5-4 hours. After freezing, it is placed in a vacuum freeze dryer for drying to obtain the mixed material.

[0050] S2: Under an inert gas atmosphere (selected from one or more of nitrogen, argon, and helium, with a gas flow rate of 40-80 mL / min), a mixture of materials with a mass ratio of 1:5-10 (e.g., 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, etc.) is calcined with a sulfur-containing resin (selected from one or more of thiourea resin, polysulfide epoxy resin, PPS resin, etc.). The calcination temperature is 1000-1500℃, the heating rate is 15-20℃ / min, and the calcination time is 2-5 h to obtain the sodium-ion battery negative electrode composite material.

[0051] In some preferred embodiments, the biomass material is further subjected to a step of removing impurities before being immersed in a mixed solution of iron and tin sources. Specifically, this includes: first, rinsing the biomass material with an alkaline solution (selected from one or both of potassium hydroxide and sodium hydroxide solutions) at a concentration of 0.5-1 mol / L (e.g., 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, etc.) for 1.5-3 hours (e.g., 1.5 hours, 2 hours, 2.5 hours, 3 hours, etc.), then rinsing with water until the pH value reaches 7-8 (e.g., 7, 7.2, 7.5, 7.7, 8, etc.); then rinsing with a 0.5 mol / L solution... Acid wash with a 1 mol / L (e.g., 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, etc.) acid solution (selected from one or more of hydrochloric acid, nitric acid, sulfuric acid, and phosphoric acid) for 1.5-3 hours (e.g., 1.5 hours, 2 hours, 2.5 hours, 3 hours, etc.), rinse with water until the pH value is 6-7 (e.g., 6, 6.2, 6.5, 6.7, 7, etc.); finally, dry at 70-90℃ (e.g., 70℃, 75℃, 80℃, 85℃, 90℃, etc.) for 10-20 hours (e.g., 10 hours, 12 hours, 15 hours, 18 hours, 20 hours, etc.). The solid-liquid ratio for both alkaline washing and acid washing is 1:50-200 (g:mL), for example, it can be 1:50 (g:mL), 1:100 (g:mL), 1:150 (g:mL), 1:200 (g:mL), etc.

[0052] In some preferred embodiments, the concentrations of both the iron source and the tin source in the mixed solution are 0.01-0.1M, for example, 0.01M, 0.03M, 0.05M, 0.07M, 0.09M, 0.1M, etc.; the molar ratio of iron to tin is 1-3:1, for example, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, etc.

[0053] In some preferred embodiments, to avoid impurities and obtain a product with higher purity, in step S2, when calcining the mixed material with the sulfur-containing resin, the mixed material and the sulfur-containing resin are placed in two adjacent containers (such as crucibles). Specifically, the container containing the sulfur-containing resin can be placed at the front end of the calcination container (such as a tube furnace). During the calcination process, the sulfur-containing resin vaporizes and reacts with the mixed material in the rear container to obtain a sandwich structure material with an inner layer of carbon fiber, a middle layer of SnS2 / FeS2 material, and an outermost layer of resin-derived hard carbon (carbon has low solubility in FeS2, and carbon will be deposited on the surface when S and Fe react), denoted as SnS2 / FeS2@CF sodium-ion battery anode composite material.

[0054] In some preferred embodiments, step S2, after calcining the mixed material and the sulfur-containing resin, further includes a step of calcining under an NH3 atmosphere to dope the carbon material with nitrogen.

[0055] Specifically, the calcination conditions under an NH3 atmosphere are as follows: the NH3 gas flow rate is 40-80 mL / min, for example, 40 mL / min, 50 mL / min, 60 mL / min, 70 mL / min, 80 mL / min, etc.; the calcination temperature is 500-700℃, for example, 500℃, 550℃, 600℃, 650℃, 700℃, etc.; the heating rate is 2-5℃ / min, for example, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, etc.; and the calcination time is 2-5h, for example, 2h, 3h, 4h, 5h, etc.; resulting in a sodium-ion battery anode composite material denoted as SnS2 / FeS2@NCF.

[0056] The resulting sodium-ion battery anode composite material (SnS2 / FeS2@CF or SnS2 / FeS2@NCF) can be used to prepare sodium-ion battery anode sheets and further used to prepare sodium-ion batteries.

[0057] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0058] Example 1

[0059] (1) Take 10g of cellulose and put it into 500mL of 1M potassium hydroxide solution and stir for 2h. Then rinse the material repeatedly with deionized water to make the pH value of the solution reach 7. Then put the material into 500mL of 1M hydrochloric acid solution and stir for 2h. Then rinse repeatedly with water to make the pH value of the solution reach 7. Place it in an 80℃ oven to dry for 12h.

[0060] (2) Take 1.2g of ferric nitrate and 1.2g of tin chloride and add them to 500mL of deionized water. Stir the mixture for 1h. Soak 5g of cellulose with impurities removed in the stirred mixture for 12h. Then immerse the mixture in liquid nitrogen and freeze for 2h. After freezing, put it into a vacuum freeze dryer for drying. The material obtained after drying is recorded as material No. 1.

[0061] (3) Take 3g of material No. 1, and then put material No. 1 and thiourea resin into two adjacent crucibles at a mass ratio of 1:5. The crucible containing thiourea resin is placed at the front. Then transfer the two crucibles into a tube furnace. The atmosphere inside the tube furnace is nitrogen, and the gas flow rate can be 80mL / min. The heating rate can be 15℃ / min, the calcination time is 2h, and the calcination temperature is 1000℃. After calcination, a sandwich structure material with an inner layer of carbon fiber, a middle layer of SnS2 / FeS2 material, and an outermost layer of resin-derived hard carbon is obtained, denoted as SnS2 / FeS2@CF.

[0062] (4) The SnS2 / FeS2@CF material is placed in a crucible and calcined in a tube furnace under an NH3 atmosphere. The gas flow rate can be 80 mL / min. The heating rate can be 2℃ / min, the calcination time is 2 h, and the calcination temperature is 500℃, to obtain a sodium-ion battery anode composite material denoted as SnS2 / FeS2@NCF.

[0063] Example 2

[0064] (1) Take 10g of cellulose and put it into 500mL of 1M sodium hydroxide solution and stir for 2h. Then rinse the material repeatedly with deionized water to make the pH value of the solution reach 7. Then put the material into 500mL of 1M hydrochloric acid solution and stir for 2h. Then rinse repeatedly with water to make the pH value of the solution reach 7. Place it in an 80°C oven to dry for 12h.

[0065] (2) Take 400 mg of ferric chloride and 600 mg of tin chloride and add them to 250 mL of deionized water. Stir the mixture for 1 hour. Soak 5 g of cellulose with impurities removed in the stirred mixture for 12 hours. Then immerse the mixture in liquid nitrogen and freeze for 2 hours. After freezing, put it into a vacuum freeze dryer for drying. The material obtained after drying is recorded as material No. 1.

[0066] (3) Take 3g of material No. 1, and then put material No. 1 and thiourea resin into two adjacent crucibles at a mass ratio of 1:5. The crucible containing thiourea resin is placed at the front. Then transfer the two crucibles into a tube furnace. The atmosphere inside the tube furnace is nitrogen, and the gas flow rate can be 80mL / min. The heating rate can be 15℃ / min, the calcination time is 2h, and the calcination temperature is 1000℃. After calcination, a sandwich structure material with an inner layer of carbon fiber, a middle layer of SnS2 / FeS2 material, and an outermost layer of resin-derived hard carbon is obtained, denoted as SnS2 / FeS2@CF.

[0067] (4) The SnS2 / FeS2@CF material is placed in a crucible and calcined in a tube furnace under an NH3 atmosphere. The gas flow rate can be 80 mL / min. The heating rate can be 5℃ / min, the calcination time is 2 h, and the calcination temperature is 500℃, to obtain a sodium-ion battery anode composite material denoted as SnS2 / FeS2@NCF.

[0068] Example 3

[0069] (1) Take 10g of coconut shell and put it into 500mL of 1M potassium hydroxide solution and stir for 2h. Then rinse the material repeatedly with deionized water to make the pH value of the solution reach 7. Then put the material into 500mL of 1M hydrochloric acid solution and stir for 2h. Then rinse repeatedly with water to make the pH value of the solution reach 7. Place it in an 80°C oven to dry for 12h.

[0070] (2) Take 2g of ferric sulfate and 1.2g of tin chloride and add them to 500mL of deionized water. Stir the mixture for 1h. Soak 5g of cellulose with impurities removed in the stirred mixture for 12h. Then immerse the mixture in liquid nitrogen and freeze for 2h. After freezing, put it into a vacuum freeze dryer for drying. The material obtained after drying is recorded as material No. 1.

[0071] (3) Take 3g of material No. 1, and then put material No. 1 and thiourea resin into two adjacent crucibles at a mass ratio of 1:5. The crucible containing thiourea resin is placed at the front. Then transfer the two crucibles into a tube furnace. The atmosphere inside the tube furnace is nitrogen, and the gas flow rate can be 80mL / min. The heating rate can be 15℃ / min, the calcination time is 2h, and the calcination temperature is 1000℃. After calcination, a sandwich structure material with an inner layer of carbon fiber, a middle layer of SnS2 / FeS2 material, and an outermost layer of resin-derived hard carbon is obtained, denoted as SnS2 / FeS2@CF.

[0072] (4) The SnS2 / FeS2@CF material is placed in a crucible and calcined in a tube furnace under an NH3 atmosphere. The gas flow rate can be 80 mL / min. The heating rate can be 2℃ / min, the calcination time is 2 h, and the calcination temperature is 500℃, to obtain a sodium-ion battery anode composite material denoted as SnS2 / FeS2@NCF.

[0073] Example 4

[0074] (1) Take 10g of bamboo fiber and put it into 500mL of 1M potassium hydroxide solution and stir for 2h. Then rinse the material repeatedly with deionized water to make the pH value of the solution reach 7. Then put the material into 500mL of 1M hydrochloric acid solution and stir for 2h. Then rinse it repeatedly with water to make the pH value of the solution reach 7. Then put it into an 80°C oven and dry for 12h.

[0075] (2) Take 2g of ferric sulfate and 1.2g of tin chloride and add them to 500mL of deionized water. Stir the mixture for 1h. Soak 5g of cellulose with impurities removed in the stirred mixture for 12h. Then immerse the mixture in liquid nitrogen and freeze for 2h. After freezing, put it into a vacuum freeze dryer for drying. The material obtained after drying is recorded as material No. 1.

[0076] (3) Take 3g of material No. 1, and then put material No. 1 and thiourea resin into two adjacent crucibles at a mass ratio of 1:5. The crucible containing thiourea resin is placed at the front. Then transfer the two crucibles into a tube furnace. The atmosphere inside the tube furnace is nitrogen, and the gas flow rate can be 80mL / min. The heating rate can be 15℃ / min, the calcination time is 2h, and the calcination temperature is 1000℃. After calcination, a sandwich structure material with an inner layer of carbon fiber, a middle layer of SnS2 / FeS2 material, and an outermost layer of resin-derived hard carbon is obtained, denoted as SnS2 / FeS2@CF.

[0077] (4) The SnS2 / FeS2@CF material is placed in a crucible and calcined in a tube furnace under an NH3 atmosphere. The gas flow rate can be 80 mL / min. The heating rate can be 2℃ / min, the calcination time is 2 h, and the calcination temperature is 500℃, to obtain a sodium-ion battery anode composite material denoted as SnS2 / FeS2@NCF.

[0078] Comparative Example 1

[0079] (1) Take 10g of cellulose and put it into 500mL of 1M potassium hydroxide solution and stir for 2h. Then rinse the material repeatedly with deionized water to make the pH value of the solution reach 7. Then put the material into 500mL of 1M hydrochloric acid solution and stir for 2h. Then rinse repeatedly with water to make the pH value of the solution reach 7. Place it in an 80°C oven and dry for 12h.

[0080] (2) Take 1.2g of tin chloride and add it to 500mL of deionized water and stir for 1h. Soak 5g of cellulose with impurities removed in the solution for 12h. Then immerse the mixture in liquid nitrogen and freeze for 2h. After freezing, put it into a vacuum freeze dryer for drying. The material obtained after drying is recorded as material No. 1.

[0081] (3) Take 3g of material No. 1, and then place material No. 1 and sulfur powder in two adjacent crucibles at a mass ratio of 1:5. Place the crucible containing sulfur powder at the front end, and then transfer the two crucibles to a tube furnace. The atmosphere inside the tube furnace is nitrogen, and the gas flow rate can be 80mL / min. The heating rate can be 5℃ / min, the calcination time is 2h, and the calcination temperature is 700℃. After calcination, a sodium ion anode composite material with carbon fiber as the inner layer and SnS2 as the outer layer is obtained, denoted as SnS2@CF.

[0082] Comparative Example 2

[0083] (1) Take 10g of cellulose and put it into 500mL of 1M potassium hydroxide solution and stir for 2h. Then rinse the material repeatedly with deionized water to make the pH value of the solution reach 7. Then put the material into 500mL of 1M hydrochloric acid solution and stir for 2h. Then rinse repeatedly with water to make the pH value of the solution reach 7. Place it in an 80°C oven and dry for 12h.

[0084] (2) Take 1.2g of ferric nitrate and 3.6g of tin chloride and add them to 500mL of deionized water. Stir the mixture for 1h. Soak 5g of cellulose with impurities removed in the stirred mixture for 12h. Then immerse the mixture in liquid nitrogen and freeze for 2h. After freezing, put it into a vacuum freeze dryer for drying. The material obtained after drying is recorded as material No. 1.

[0085] (3) Take 3g of material No. 1, and then put material No. 1 and thiourea resin into two adjacent crucibles at a mass ratio of 1:5. The crucible containing thiourea resin is placed at the front end. Then transfer the two crucibles into a tube furnace. The atmosphere inside the tube furnace is nitrogen, and the gas flow rate can be 80mL / min. The heating rate can be 15℃ / min, the calcination time is 2h, and the calcination temperature is 1000℃. After calcination, a sandwich structure material with an inner layer of carbon fiber, a middle layer of SnS2 / pyrite FeS2 material, and an outermost layer of resin-derived hard carbon is obtained, which is denoted as the SnS2 / pyrite FeS2@CF sodium ion anode composite material.

[0086] (4) Place the SnS2 / pyrite FeS2@CF material into a crucible and calcine it in a tube furnace under an NH3 atmosphere. The gas flow rate can be 80 mL / min. The heating rate can be 2℃ / min, the calcination time is 2 h, and the calcination temperature is 500℃. The material after calcination is recorded as the sodium ion anode composite material of SnS2 / pyrite FeS2@NCF.

[0087] Comparative Example 3

[0088] (1) Take 10g of cellulose and put it into 500mL of 1M potassium hydroxide solution and stir for 2h. Then rinse the material repeatedly with deionized water to make the pH value of the solution reach 7. Then put the material into 500mL of 1M hydrochloric acid solution and stir for 2h. Then rinse repeatedly with water to make the pH value of the solution reach 7. Place it in an 80°C oven and dry for 12h.

[0089] (2) Take 1.2g of ferric nitrate and add it to 500mL of deionized water and stir. Then soak 5g of cellulose with impurities removed in the stirred mixture for 12h. Then immerse the mixture in liquid nitrogen and freeze for 2h. After freezing, put it into a vacuum freeze dryer for drying. The material obtained after drying is recorded as material No. 1.

[0090] (3) Take 3g of material No. 1, and then put material No. 1 and sulfur powder into two adjacent crucibles at a mass ratio of 1:5. The crucible containing sulfur powder is placed at the front. Then transfer the two crucibles into a tube furnace. The atmosphere inside the tube furnace is nitrogen, and the gas flow rate can be 80mL / min. The heating rate can be 5℃ / min, the calcination time is 2h, and the calcination temperature is 700℃. After calcination, a sodium ion anode composite material with carbon fiber as the inner layer and pyrite FeS2 / marbled FeS2 as the outer layer is obtained, which is denoted as pyrite FeS2 / marbled FeS2@CF.

[0091] Results Analysis

[0092] Using the materials synthesized in Examples 1-4 and Comparative Examples 1-3 as the negative electrode active material, carbon black as the conductive agent, and CMC (carboxymethyl cellulose) as the binder, the negative electrode was prepared according to a ratio of negative electrode active material: conductive agent: binder = 8:1:1. A sodium metal sheet was used as the positive electrode, and 1.0 mol / L NaPF6 dissolved in diethylene glycol dimethyl ether was used as the electrolyte. NCF2032 button cells were assembled in an argon-filled glove box. The GCD (constant current charge-discharge test) was tested using the Land battery testing system within a voltage window of 0.01-2V.

[0093] (1) The specific test procedures for the first discharge capacity, first charge capacity and first coulombic efficiency are as follows:

[0094] Under the condition of 25℃, the initial discharge capacity was obtained by constant current discharge to 0.01V at a current density of 100mA / g; then, the initial charge capacity was obtained by constant current charging to 2V at a current density of 100mA / g. The initial coulombic efficiency = initial charge capacity / initial discharge capacity * 100%. The test results of initial discharge capacity, initial charge capacity and initial coulombic efficiency are shown in Table 1.

[0095] Table 1

[0096]

[0097]

[0098] Table 1 shows that the button cells prepared by the materials in Examples 1-4 all have a high initial discharge capacity of over 890 mAh / g, while the initial discharge capacity of Comparative Example 1 is only 107.8 mAh / g. Due to its suitable multiphase heterojunction structure, abundant active sites, fast ion transport channels, and structural stability, the fibrous SnS2 / FeS2@NCF material exhibits outstanding sodium storage capacity.

[0099] (2) The specific test procedure for capacity retention is as follows: Under the temperature condition of 25℃, discharge to 0.01V with a constant current density of 500mA / g; then charge to 2V with a constant current density of 500mA / g, and record the charging capacity at this time as the charging capacity of the first cycle; repeat the above charge and discharge steps for 200 cycles, and record the charging capacity of the 200th cycle; Capacity retention = Charging capacity of the 200th cycle / Charging capacity of the 1st cycle * 100%. The test results of capacity retention are shown in Table 2.

[0100] Table 2

[0101] Current density 500 mA / g Capacity retention rate (%) Example 1 91.8 Example 2 91.2 Example 3 90.3 Example 4 92.2 Comparative Example 1 82.5 Comparative Example 2 85.2 Comparative Example 3 83.6

[0102] Table 2 shows that the button cells prepared by the materials in Examples 1-4 all exhibited high cycle retention rates (up to 92.2%) and excellent cycle stability. In contrast, the capacity retention rates of Comparative Examples 1-3 were all lower than those of the Examples.

[0103] In summary, a well-designed SnS2 / FeS2@NCF fiber structure exhibits excellent electrical conductivity, while its superior mechanical properties mitigate volume changes during cycling. Through appropriate process adjustments, not only is the material's structural stability utilized, but its high specific capacity is also achieved, thereby enhancing the overall electrochemical performance of the electrode.

[0104] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A negative electrode composite material, characterized in that, The material comprises a carbon fiber inner layer and a hard carbon outer layer; the surface of the carbon fiber inner layer is loaded with transition metal sulfides, including SnS2 and FeS2, wherein the FeS2 includes pyrite and marcasite structures; the transition metal sulfides have a three-phase interface of SnS2 / pyrite / marcasite; the hard carbon outer layer covers the carbon fiber inner layer and the transition metal sulfides. The preparation method of the negative electrode composite material includes the following steps: S1: Immerse the biomass material in a mixed solution of iron and tin sources, and freeze-dry it to obtain the mixed material; S2: The mixed material and sulfur-containing resin are calcined in an inert gas atmosphere to obtain the negative electrode composite material.

2. The negative electrode composite material as described in claim 1, characterized in that, At least one of the following conditions must be met: The diameter of the carbon fiber inner layer is 1-10 μm; Nitrogen is doped into the inner layer of the carbon fiber and the outer layer of hard carbon fiber. The thickness of the hard carbon outer layer is 5-10 nm.

3. The negative electrode composite material as described in claim 1, characterized in that, The transition metal sulfide satisfies at least one of the following conditions: The transition metal sulfide accounts for 30-50% of the total mass of the negative electrode composite material; In the transition metal sulfide, the ratio of iron to tin atoms in SnS2 and FeS2 is (3-5):1; The particle size of the transition metal sulfide is 10-100 nm.

4. The method for preparing a negative electrode composite material as described in claim 1, characterized in that, Includes the following steps, S1: Immerse the biomass material in a mixed solution of iron and tin sources, and freeze-dry it to obtain the mixed material; S2: The mixed material and sulfur-containing resin are calcined in an inert gas atmosphere to obtain the negative electrode composite material.

5. The preparation method according to claim 4, characterized in that, In step S1 The biomass material is selected from one or more of cellulose, bamboo fiber, coconut shell, rice straw, cotton stalk bark, and sugarcane bagasse; The biomass material is further subjected to a step of removing impurities before being immersed in a mixed solution of iron and tin sources; The iron source is selected from one or more of ferric chloride, ferrous chloride, ferric fluoride, ferric acetate, ferric sulfate, and ferric nitrate. The tin source is selected from one or more of tin chloride, stannous chloride, tin sulfate, and tin acetate; In the mixed solution of the iron source and the tin source, the molar ratio of iron to tin is (1-3):1; The molar ratio of the biomass material to the iron source in the mixed solution is (2000-500) g: 1 mol; The freeze-drying process involves freezing in liquid nitrogen for 1.5-4 hours, followed by vacuum drying.

6. The preparation method according to claim 4, characterized in that, In step S2 The mass ratio of the mixed material to the sulfur-containing resin is 1:(5-10); During the calcination process, the mixed materials do not come into direct contact with the sulfur-containing resin; The calcination temperature is 1000-1500℃, and the time is 2-5 hours.

7. The preparation method according to claim 4, characterized in that, In step S2, after calcining the mixed material and the sulfur-containing resin, the step further includes calcining under an NH3 atmosphere.

8. The preparation method according to claim 7, characterized in that, The conditions for calcination under an NH3 atmosphere are: NH3 gas flow rate of 40-80 mL / min, calcination temperature of 500-700℃, and time of 2-5 h.

9. A negative electrode sheet, characterized in that, The negative electrode composite material comprising any one of claims 1-3 or the negative electrode composite material prepared by any one of claims 4-8.

10. A sodium-ion battery, characterized in that, Includes the negative electrode sheet as described in claim 9.

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