A soft and hard carbon composite material and its preparation and application

Abstract

This invention belongs to the field of sodium-ion battery technology, and specifically relates to a soft-hard carbon composite material and its preparation and application. The soft-hard carbon composite material has a core-shell structure with hard carbon coating soft carbon and dual sulfur enrichment. The soft carbon, serving as the core, is a sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon, while the hard carbon, serving as the shell, is sulfur-doped PEDOT-based hard carbon. This invention constructs a core-shell structure through surface chemical modification of mesophase carbon microspheres, pore structure regulation, and soft-hard carbon composites, solving the technical problem of improving the sodium storage capacity of pitch-based soft carbon anodes. It achieves considerable intercalation capacity by widening the carbon interlayer spacing and obtains abundant sodium storage sites to realize a large proportion of surface capacitance storage, thereby improving the initial charge-discharge capacity, initial efficiency, and cycle performance of sodium-ion batteries (sodium batteries).

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, and in particular relates to a soft and hard carbon composite material and its preparation and application. Background Technology

[0002] In recent years, with the development of large-scale energy storage and electric vehicles, sodium-ion batteries have regained attention due to their abundant raw materials and low cost, becoming one of the alternatives to lithium-ion batteries. The cathode materials for sodium-ion batteries have drawn on research findings from lithium-ion batteries, achieving significant progress. However, graphite-based materials, the commercially available anode materials for lithium-ion batteries, are unsuitable for use as anode materials in sodium-ion batteries. Carbon materials with larger interlayer spacing have become a new choice for anode materials in sodium-ion batteries.

[0003] Sodium-ion batteries (SIBs) are low-cost alternatives to lithium-ion batteries in large-scale energy storage systems and electric vehicles. Anode materials (negative electrode materials) are crucial for the commercialization of SIBs. SIB anode materials can be categorized into five types: carbon materials, titanium-based materials, alloy materials, metal compound materials, and organic materials. Among carbon materials, mesophase carbon microspheres and mesophase pitch, as stage products of pitch thermal condensation, possess abundant graphite microcrystals and exhibit good conductivity. However, they suffer from small interlayer spacing and few surface defects, resulting in very limited sodium storage capacity for pitch-based soft carbon anodes. Hard carbon precursors typically possess naturally porous structures and abundant surface functional groups. While contributing to sodium storage capacity, they also introduce problems such as poor rate performance, low initial coulombic efficiency, and sodium metal deposition. Among titanium-based materials, those with low redox potential (Ti) are preferred. 3+ / Ti 4+ Titanium-based compounds, with their low lattice stress, low cost, and non-toxicity, are well-suited as anode materials for SIBs. However, their relatively low specific capacity (<120 mAh / g) and initial coulombic efficiency are major obstacles to their commercialization. Alloy and metal compound materials typically exhibit high sodium storage capacity, but significant volume expansion can lead to poor rate and cycle performance. Organic compounds are highly attractive anode materials due to their multi-electron reactions, tunable voltage range, and low cost; however, their low initial coulombic efficiency and poor cycle stability remain unresolved.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a soft and hard carbon composite material and its preparation and application. Through surface chemical modification of mesophase carbon microspheres, pore structure regulation, and soft and hard carbon compositing, a soft and hard carbon composite material is constructed. The core is soft carbon with sulfur-doped mesophase carbon microspheres and layered porous carbon, and the outer shell is sulfur-doped PEDOT-based hard carbon. This invention aims to solve the technical problem of improving the sodium storage capacity of pitch-based soft carbon anodes. It can achieve considerable intercalation capacity by widening the carbon interlayer spacing and obtain abundant sodium storage sites to achieve a large proportion of surface capacitance storage, thereby improving the first charge and discharge capacity, first efficiency, and cycle performance of sodium-ion batteries (sodium batteries).

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] In a first aspect, there is a soft-hard carbon composite material having a core-shell structure in which hard carbon coats soft carbon and is rich in sulfur, wherein the soft carbon as the core is a sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon, and the hard carbon as the shell is a sulfur-doped PEDOT-based hard carbon.

[0008] Furthermore, the carbon interlayer spacing of the soft and hard carbon composite material is 0.340-0.410 nm;

[0009] And / or, the sulfur doping content of the soft and hard carbon composite material is 6.5 to 8.8 at%.

[0010] And / or, in the soft and hard carbon composite material, the content of active carbon sulfur bonds (CSC) accounts for 96-100% of the sulfur-containing organic bonds.

[0011] Furthermore, the soft carbon is obtained by using sulfur-doped mesophase carbon microspheres, obtained by etching the surface of mesophase carbon microspheres with H2S, as a precursor, followed by sulfur doping and pore-conditioning modification using magnesium sulfate as a sulfur source template, and then carbonization.

[0012] Furthermore, the mass ratio of magnesium sulfate to sulfur-doped mesophase carbon microspheres is 1 to 10:1; and / or the carbonization temperature is 600 to 890°C.

[0013] Secondly, a method for preparing a soft-hard carbon composite material as described in the first aspect includes the following steps:

[0014] The soft carbon is obtained by carbonization after using sulfur-doped mesophase carbon microspheres as a precursor, which are modified by sulfur doping and pore adjustment using magnesium sulfate as a sulfur source template.

[0015] The soft and hard carbon composite material is obtained by using the soft carbon as the core, guiding the oxidative polymerization of EDOT monomers on its surface, and then carbonizing it at high temperature and inertly, which is sulfur-doped PEDOT-based hard carbon.

[0016] The soft and hard carbon composite material has a core-shell structure with hard carbon coating soft carbon and double sulfur enrichment.

[0017] Furthermore, the sulfur-doped mesophase carbon microspheres are prepared by pre-oxidation treatment and H2S gas etching using mesophase carbon microspheres as a precursor.

[0018] Furthermore, during the preparation of the soft carbon, the mass ratio of magnesium sulfate to sulfur-doped mesophase carbon microspheres is 1 to 10:1.

[0019] Furthermore, in the preparation of the soft and hard carbon composite material, the mass ratio of EDOT monomer to soft carbon is 1-20:80-99.

[0020] Thirdly, the application of a soft-hard carbon composite material as described in the first or second aspect is as follows:

[0021] A sodium-ion battery anode material, comprising the soft and hard carbon composite material or the soft and hard carbon composite material prepared by the preparation method.

[0022] A sodium-ion battery comprising the soft and hard carbon composite material or the soft and hard carbon composite material prepared by the preparation method.

[0023] Compared with the prior art, the present invention has at least the following beneficial effects:

[0024] 1. The present invention provides a soft and hard carbon composite material with a dual sulfur-rich core-shell structure. The soft carbon serves as the core, which is a sulfur-doped mesophase carbon microsphere-based layered porous carbon, and the hard carbon serves as the outer shell, which is a sulfur-doped PEDOT-based hard carbon. This structure improves the sodium storage capacity of the mesophase carbon microspheres. It can achieve considerable intercalation capacity by widening the carbon interlayer spacing, and can also obtain abundant sodium storage sites to achieve a large proportion of surface capacitance storage. When applied to sodium batteries, it is beneficial to improve the first charge-discharge capacity, first efficiency, and cycle performance.

[0025] Notably, the construction of the core-shell structure unexpectedly enabled the re-sulfur doping of the PEDOT-based hard carbon shell by a sulfur-containing soft carbon core at high temperatures, providing additional active sulfur-containing functional groups (active carbon-sulfur bonds, CSCs) for the PEDOT-based hard carbon while widening the carbon interlayer spacing. Specifically, using sulfur-doped mesophase carbon microspheres as the polymerization center, the organic sulfur-containing compound monomer EDOT was guided to polymerize layer by layer on its surface. During the high-temperature inert carbonization process, as the outer carbon shell formed, sulfur elements on the surface of the polymerization center escaped outward in gaseous form, re-doping the outer carbon shell with sulfur, resulting in a dual sulfur-rich core-shell structure. The sulfur-doped mesophase carbon microspheres-based layered porous carbon core, rich in graphite microcrystals, contributes to good conductivity, while simultaneously achieving re-sulfur doping of the PEDOT-based hard carbon shell. This increases the pseudocapacitive contribution, widens the carbon interlayer spacing, and improves the intercalation capacity. Furthermore, the PEDOT-based hard carbon shell exhibits a layer-by-layer cross-linked structure, with close contact between the soft and hard carbons, which facilitates the full activation of active sites and the rapid transport and diffusion of electrolyte ions.

[0026] 2. The method for preparing soft and hard carbon composite materials of the present invention can improve the sodium storage capacity of mesophase carbon microspheres through surface chemical modification, pore structure regulation and soft and hard carbon composites. It can not only obtain considerable intercalation capacity by widening the carbon layer spacing, but also obtain abundant sodium storage sites to achieve a large proportion of surface capacitance storage.

[0027] In the preparation process, firstly, mesophase carbon microspheres were selected as the precursor and subjected to sulfur doping through surface modification to obtain sulfur-doped mesophase carbon microspheres. Secondly, using sulfur-doped mesophase carbon microspheres as the precursor, magnesium sulfate was selected as the hard template and sulfur source to perform secondary sulfur doping while controlling the mesoporous structure, thus preparing sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon with high sulfur content and high mesoporosity. Thirdly, using sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon as the polymerization center, the organic sulfur-containing compound monomer EDOT was selected to polymerize layer by layer to form a sulfur-containing polymer shell for tertiary sulfur doping. Finally, the sulfur-containing polymer shell was carbonized, and during the high-temperature inert carbonization process, the sulfur element on the surface of the polymerization center was released outward in gaseous form, and the sulfur-containing polymer shell was subjected to quaternary sulfur doping, ultimately preparing a soft and hard carbon composite material with a dual sulfur-rich core-shell structure, with sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon as the core and sulfur-doped PEDOT-based hard carbon as the shell.

[0028] Generally, the main contributions to charge storage include diffusion-controlled intercalation and surface-driven (pseudo)capacitance. The different sulfur doping methods used in this invention lead to variations in the sodium storage mechanism. Single-stage sulfur doping using H2S etching significantly improves the carbon interlayer spacing, sp2 carbon content, and sulfur doping amount. Although the initial coulombic efficiency (first efficiency) decreases slightly, sodium storage performance is significantly improved in three aspects: interlayer intercalation, pseudocapacitance, and surface adsorption, achieving a balance between intercalation capacity and surface adsorption capacity. The resulting sulfur-doped mesophase carbon microspheres exhibit sodium storage behavior primarily driven by surface adsorption and pseudocapacitance, with capacitance playing a dominant role, which is key to their excellent rate performance.

[0029] Secondary sulfur doping using magnesium sulfate as a sulfur source to prepare hierarchical porous carbon increases the surface capacitance contribution at the expense of carbon interlayer spacing, utilizing SO4 in magnesium sulfate. 2- -CH in mesophase carbon microspheres x A mild reaction occurs with the C-C bond to form a C-S covalent bond (active carbon-sulfur bond). Due to the abundance of active carbon-sulfur bonds, the initial coulombic efficiency is actually achieved through -SO x The selective removal of oxygen-containing functional groups and active carbon atoms through oxidation or S atom substitution reactions significantly enhances the process. Simultaneously, the pore-forming effect of MgO crystal templates made from magnesium sulfate enables the preparation of mesoporous hierarchical porous carbon with ultra-high sulfur doping. Mesoporous hierarchical porous carbon materials possess advantages such as high specific surface area, excellent electrical conductivity, physical and chemical stability, gas-liquid permeability, and tunable pore structure. The obtained sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon exhibits redox reactions between its surface sulfur-containing functional groups (active carbon-sulfur bonds, CSC) and sodium ions. This contributes abundant pseudocapacitance and maintains the structural stability of the soft carbon material during cycling, resulting in good rate capability and cycling performance.

[0030] Tertiary sulfur doping involves hard carbon coating soft carbon, utilizing the room-temperature polymerization properties of organic monomers to achieve uniform growth of hard carbon on the soft carbon surface. EDOT is an organic sulfur-containing monomer that can be used to synthesize the conductive polymer PEDOT through room-temperature oxidative polymerization. The sulfur in PEDOT provides abundant active sites for sodium ion storage. Furthermore, PEDOT-based hard carbon materials have a large carbon interlayer spacing, which facilitates the reversible insertion and extraction of sodium ions between carbon layers. However, PEDOT-based hard carbon materials have a limited sp2 carbon content, resulting in poor conductivity.

[0031] The fourth sulfur doping process involves doping the outer shell with sulfur-containing cores, especially surface sulfur, under a high-temperature inert atmosphere. This process not only widens the carbon interlayer spacing but also fully utilizes active sulfur-containing functional groups (active carbon-sulfur bonds, CSC) to improve the performance of PEDOT-based hard carbon materials.

[0032] The method of this invention can further adjust the amount of sulfur-doped mesophase carbon microspheres and the carbonization temperature of soft carbon to achieve precise control of carbon interlayer spacing (0.340-0.410 nm), sulfur doping amount (6.5-8.8 at%), and active carbon sulfur bond (CSC) content (96-100%), thereby endowing the soft and hard carbon composite materials with a hybrid energy storage mechanism dominated by surface capacitance, which improves the first charge-discharge capacity, first efficiency, and cycle performance of sodium batteries. Attached Figure Description

[0033] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0034] Figure 1 This is a SEM image of the soft and hard carbon composite material of the present invention. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0036] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0037] This invention discloses a soft and hard carbon composite material, its preparation and application, and has the following embodiments:

[0038] According to a first aspect of the present invention, a soft-hard carbon composite material has a core-shell structure in which hard carbon coats soft carbon and is rich in sulfur, wherein the soft carbon as the core is sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon, and the hard carbon as the shell is sulfur-doped PEDOT-based hard carbon.

[0039] As an optional embodiment of the soft and hard carbon composite material of the present invention, the carbon interlayer spacing of the soft and hard carbon composite material is 0.340-0.410nm (e.g., 0.345nm, 0.350nm, 0.355nm, 0.360nm, 0.365nm, 0.370nm, 0.375nm, 0.380nm, 0.385nm, 0.390nm, 0.395nm, 0.400nm, 0.405nm).

[0040] As an optional embodiment of the soft and hard carbon composite material of the present invention, the sulfur doping content of the soft and hard carbon composite material is 6.5 to 8.8 at% (e.g., 6.6 at%, 6.8 at%, 7.0 at%, 7.2 at%, 7.4 at%, 7.6 at%, 7.8 at%, 8.0 at%, 8.2 at%, 8.4 at%, 8.6 at%).

[0041] As an optional embodiment of the soft and hard carbon composite material of the present invention, the content of active carbon sulfur bonds (CSC) in the soft and hard carbon composite material is 96-100% (e.g., 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, 99.5%).

[0042] As an optional embodiment of the soft and hard carbon composite material of the present invention, the soft carbon is obtained by using sulfur-doped mesophase carbon microspheres, obtained by etching the surface of mesophase carbon microspheres with H2S, as a precursor, followed by sulfur doping and pore-conditioning modification using magnesium sulfate as a sulfur source template, and then carbonization. Further optionally, the mass ratio of magnesium sulfate to sulfur-doped mesophase carbon microspheres is 1–10:1 (e.g., 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1); and / or, the carbonization temperature is 600–890℃ (e.g., 610℃, 630℃, 650℃, 670℃, 690℃, 710℃, 730℃, 750℃, 770℃, 790℃, 810℃, 830℃, 850℃, 870℃).

[0043] According to a second aspect of the present invention, a method for preparing a soft and hard carbon composite material as described in the first aspect includes the following steps:

[0044] The soft carbon is obtained by carbonization after using sulfur-doped mesophase carbon microspheres as a precursor, which are modified by sulfur doping and pore adjustment using magnesium sulfate as a sulfur source template.

[0045] The soft and hard carbon composite material is obtained by using the soft carbon as the core, guiding the oxidative polymerization of EDOT monomers on its surface, and then carbonizing it at high temperature and inertly, which is sulfur-doped PEDOT-based hard carbon.

[0046] The soft and hard carbon composite material has a core-shell structure with hard carbon coating soft carbon and double sulfur enrichment.

[0047] Furthermore, the general preparation steps of the soft and hard carbon composite material may include the following:

[0048] S1. Preparation of sulfur-doped mesophase carbon microspheres

[0049] The preparation of sulfur-doped mesophase carbon microspheres involves using mesophase carbon microspheres as a precursor, followed by pre-oxidation treatment and H2S gas etching. Specific steps may include:

[0050] S11. Place the mesophase carbon microspheres in a mixed acid of nitric acid and sulfuric acid with a volume ratio of 1.5 to 2.5:1 (e.g., 1.6:1, 1.8:1, 2:1, 2.2:1, 2.4:1) for oxidation. Wash with water until neutral and then dry to obtain pre-oxidized mesophase carbon microspheres for later use.

[0051] S12. Pre-oxidized mesophase carbon microspheres are loaded into an alumina boat and placed in a tube furnace. High-temperature carbonization is then carried out under a mixed atmosphere of argon and hydrogen sulfide, followed by natural cooling to room temperature to obtain sulfur-doped mesophase carbon microspheres. The specific high-temperature carbonization process is as follows:

[0052] First, a pre-carbonization stage is carried out in an argon atmosphere, where the temperature is raised from room temperature to 350-450℃ (e.g., 360℃, 380℃, 400℃, 420℃, 440℃) at a certain heating rate, such as 2℃ / min, and then held at that temperature for 0.5-1.5h (e.g., 0.6h, 0.8h, 1h, 1.2h, 1.4h).

[0053] Secondly, the carbonization stage is carried out in a mixed atmosphere (argon and hydrogen sulfide volume ratio of 8-10:1, such as 8.2:1, 8.4:1, 8.6:1, 8.8:1, 9:1, 9.2:1, 9.4:1, 9.6:1, 9.8:1). The temperature is increased from the pre-carbonization stage holding temperature to 650-750℃ (such as 660℃, 680℃, 700℃, 720℃, 740℃) at a certain heating rate, such as 5℃ / min, and held for 2-4 hours (such as 2.5 hours). h, 3h, 3.5h), the gas flow rates of argon and hydrogen sulfide in the mixed atmosphere are 120-360 ml / min (e.g. 130 ml / min, 150 ml / min, 170 ml / min, 190 ml / min, 210 ml / min, 230 ml / min, 250 ml / min, 270 ml / min, 290 ml / min, 310 ml / min, 330 ml / min, 350 ml / min).

[0054] In this invention, mesophase carbon microspheres (MCMB) are micron-sized spherical carbon materials with a nematic liquid crystal layered stacking structure generated by the thermal condensation polymerization of heavy aromatic compounds such as pitch.

[0055] Preparation of S2 and soft / hard carbon composite materials

[0056] S21. Preparation and Pretreatment of Soft Carbon

[0057] Sulfur-doped mesophase carbon microspheres were ultrasonically dispersed in DMF solvent. Magnesium sulfate was then added, with a mass ratio of magnesium sulfate to sulfur-doped mesophase carbon microspheres of 1–10:1 (e.g., 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1). The mixture was magnetically stirred, and the solvent was evaporated in a rotary evaporator to obtain a solid. The solid was loaded into an alumina boat and placed in a tube furnace for high-temperature carbonization under an inert gas atmosphere such as nitrogen. The carbonization process involved raising the temperature from room temperature to a specified carbonization temperature at a certain rate, such as 5℃ / min, and holding at that temperature. The carbonization temperatures ranged from 600℃ to 890℃ (e.g., 610℃, 630℃, 650℃, 670℃, 690℃, 710℃, 730℃, 750℃, 770℃, 790℃, 810℃, 830℃, 850℃, 870℃). After natural cooling to room temperature, soft carbon is obtained, namely sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon. The soft carbon is then acid-washed and dried to obtain pretreated soft carbon. Acid washing is necessary to remove inorganic metal salts and other acid-soluble impurities.

[0058] Preparation of S22 and soft / hard carbon composites

[0059] Sodium dodecylbenzenesulfonate was dissolved in deionized water, and EDOT monomer and pretreated soft carbon were added sequentially in a mass ratio of 1–20:80–99 (specifically, typical but not limited to 2:98, 3:97, 4:96, 5:95, 6:94, 7:93, 8:92, 9:91, 10:90, 11:89, 12:88, 13:87, 14:86, 15:85, 16:84, 17:83, 18:82, 19:81). The mixture was stirred for a certain time, such as 30 minutes, to obtain a mixed solution. A chain initiator, such as ammonium persulfate, was then dissolved... Dissolved in deionized water, the mixture is added dropwise to the above mixed solution; after polymerization at room temperature for a certain time, such as 24 hours, the mixture is centrifuged and washed with deionized water until colorless, then dried to obtain a black powder; the dried black powder is loaded into an alumina boat and placed in a tube furnace, heated to a certain high temperature, such as 700℃, under an argon or nitrogen atmosphere at a certain heating rate, and carbonized for a certain time, then naturally cooled to room temperature to obtain a hard carbon-soft carbon composite material with hard carbon coating soft carbon, that is, soft carbon with a sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon core and a sulfur-doped PEDOT-based hard carbon shell, with both the core and shell being sulfur-rich.

[0060] In this invention, PEDOT is a polymer of EDOT (3,4-ethylenedioxythiophene monomer), with the following molecular structure:

[0061] Where R is hydrogen.

[0062] PEDOT is characterized by its simple molecular structure, small band gap, and high conductivity.

[0063] The present invention relates to a soft and hard carbon composite material, which improves sodium storage capacity through surface chemical modification, pore structure regulation and soft and hard carbon compositing. It can achieve considerable intercalation capacity by widening the carbon layer spacing, and obtain abundant sodium storage sites to achieve a large proportion of surface capacitance storage.

[0064] First, the sodium storage capacity of pitch-based soft carbon anodes is improved through surface chemical modification. Pitch-based mesophase carbon microspheres are selected as the precursor for several reasons: first, their spherical structure facilitates dispersion in the slurry, allowing for uniform distribution when current passes through the anode surface; second, they reduce sodium ion consumption at sharp material edges and irregular morphologies, preventing excessive SEI growth; and third, their high packing density results in higher volumetric energy. Surface chemical modification (primary sulfur doping) is achieved by introducing active sulfur elements onto the surface of the mesophase carbon microspheres using H2S gas etching, aiming to improve the sodium storage capacity while maintaining the initial coulombic efficiency. This invention does not impose specific requirements on the particle size of the mesophase carbon microspheres; the particle size can be selected between 2-20 μm, typically but not limited to 3 μm, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, 15 μm, 17 μm, and 19 μm.

[0065] Secondly, from the perspective of improving sodium storage capacity through pore structure regulation, a suitable hard template was selected to prepare mesoporous hierarchical porous carbon. Specifically, using H2S-etched mesophase carbon microspheres (sulfur-doped mesophase carbon microspheres) as a precursor, magnesium sulfate was selected as both a template and a sulfur source to prepare secondary sulfur-doped sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon. The aim was to simultaneously improve interlayer intercalation capacity, pseudocapacitive contribution, and surface adsorption energy storage through increased sulfur content and mesoporous structure. Specifically, SO4 in magnesium sulfate was utilized... 2- -CH in mesophase carbon microspheres x A mild reaction occurs between the C and C bonds to form CSC covalent bonds (active carbon-sulfur bonds). Simultaneously, the pore-forming effect of the MgO crystal template made of magnesium sulfate is utilized to achieve the preparation of mesoporous hierarchical porous carbon with ultra-high sulfur doping. This mesoporous hierarchical porous carbon possesses advantages such as high specific surface area, excellent electrical conductivity, physical and chemical stability, gas-liquid permeability, and tunable pore structure. Furthermore, its surface sulfur-containing functional groups (active carbon-sulfur bonds, CSC) undergo redox reactions with sodium ions, contributing abundant pseudocapacitance and maintaining the structural stability of the soft carbon material during cycling to achieve good rate capability and cycling performance.

[0066] Finally, a soft-hard carbon composite was prepared by oxidative polymerization of sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon and EDOT organic sulfur-containing compound monomers, followed by high-temperature inert carbonization. During oxidative polymerization, sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon (sulfur-containing soft carbon) served as the polymerization center, guiding EDOT to polymerize layer by layer on its surface to form an outer shell, thus constructing a core-shell structure while simultaneously performing three sulfur doping processes. During high-temperature inert carbonization, the construction of the core-shell structure enabled the sulfur elements on the surface of the sulfur-containing soft carbon to escape in gaseous form at high temperatures. However, due to the inert atmosphere, the sulfur elements were not completely converted into volatile sulfur-containing substances, but rather the outer carbon shell underwent four sulfur doping processes. This provided additional active sulfur-containing functional groups (active carbon-sulfur bonds, CSCs) for the PEDOT-based hard carbon, while widening the carbon interlayer spacing. Ultimately, a dual-sulfur-rich soft-hard carbon composite material was formed, with sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon as the core and layer-by-layer cross-linked, sulfur-doped PEDOT-based hard carbon as the outer shell. The sulfur-doped mesophase carbon microspheres with layered porous carbon cores and abundant graphite microcrystals contribute to good conductivity. At the same time, the outer carbon shell is further doped with sulfur, which can provide additional active carbon-sulfur bonds, increase pseudocapacitance contribution, and widen the carbon layer spacing to improve intercalation capacity. The outer carbon shell has a layer-by-layer cross-linked structure with close contact between soft and hard carbon, enabling rapid transport of electrolyte ions.

[0067] In the preparation of soft and hard carbon composite materials, the inventors, through extensive research, discovered that different sulfur doping methods lead to differences in sodium storage mechanisms. H₂S etching for sulfur doping significantly improves the carbon interlayer spacing, sp₂ carbon content, and sulfur doping amount. Although the initial coulombic efficiency decreases slightly, the sodium storage performance is significantly improved in three aspects: interlayer intercalation, pseudocapacitance, and surface adsorption, achieving a balance between intercalation capacity and surface adsorption capacity. Layered porous carbon prepared using magnesium sulfate as the sulfur source increases the surface capacitance contribution at the expense of carbon interlayer spacing. Due to the abundance of sulfur bonds in the active carbon, the initial coulombic efficiency is actually improved through -SO₄²⁻. xThe selective removal of oxygen-containing functional groups and active carbon atoms by oxidation or S atom substitution reactions significantly improves performance. When PEDOT-based hard carbon materials are coated with soft carbon, the sulfur in PEDOT provides abundant active sites for sodium ion storage and has a large carbon interlayer spacing, which facilitates the reversible insertion and extraction of sodium ions between carbon layers. Furthermore, during high-temperature inert carbonization, the escape of core sulfur, especially core surface sulfur, leads to sulfur doping of the outer shell, widening the carbon interlayer spacing while fully utilizing the active carbon sulfur bond (CSC), thus improving the performance of PEDOT-based hard carbon materials. Ultimately, by adjusting the amount of sulfur-doped mesophase carbon microspheres and the carbonization temperature, the carbon interlayer spacing (0.340-0.410 nm), sulfur doping amount (6.5-8.8 at%), and active carbon sulfur bond (CSC) content (96-100%) of the soft and hard carbon composite materials can be precisely controlled. This endows the soft and hard carbon composite materials with a hybrid energy storage mechanism dominated by surface capacitance, thereby improving the first charge-discharge capacity, first efficiency, and cycle performance of sodium batteries. Therefore, the soft and hard carbon composite material of the present invention, when used as a sodium battery anode material, can significantly improve the initial charge-discharge capacity, initial efficiency, and cycle performance of sodium batteries.

[0068] The present invention will be further described below with reference to specific embodiments and comparative examples.

[0069] The sulfur-doped mesophase carbon microspheres involved in the examples and comparative examples were prepared as follows:

[0070] S11. Place the mesophase carbon microspheres with a particle size of 8-15μm in a mixed acid of nitric acid and sulfuric acid with a volume ratio of 2:1 for oxidation, wash with water until neutral, and then dry to obtain pre-oxidized mesophase carbon microspheres for later use.

[0071] S12. The pre-oxidized mesophase carbon microspheres are loaded into an alumina boat and placed in a tube furnace. First, a pre-carbonization stage is carried out in an argon atmosphere, with the temperature increased from room temperature to 400℃ and held for 1 hour at a heating rate of 2℃ / min. Second, a carbonization stage is carried out in a mixed atmosphere (argon and hydrogen sulfide volume ratio of 9:1), with the temperature increased from 400℃ to 700℃ at a heating rate of 5℃ / min and held for 3 hours. The gas flow rates of argon and hydrogen sulfide in the mixed atmosphere are 240 ml / min. Finally, the mixture is naturally cooled to room temperature to obtain sulfur-doped mesophase carbon microspheres.

[0072] Example 1

[0073] 4g of sulfur-doped mesophase carbon microspheres were ultrasonically dispersed in 400ml of DMF solvent for 30min. Then, 20g of magnesium sulfate was added, and the mixture was magnetically stirred for 12h. The solvent was then evaporated in a rotary evaporator. The solid was loaded into an alumina boat and placed in a tube furnace for high-temperature carbonization under a nitrogen atmosphere. The carbonization process involved raising the temperature from room temperature to the specified carbonization temperature at a rate of 5℃ / min and holding at that temperature for 1h. The carbonization temperature was 600℃. After natural cooling to room temperature, soft carbon, i.e., sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon, was obtained. The soft carbon was acid-washed and dried to obtain pretreated soft carbon.

[0074] 5g of sodium dodecylbenzenesulfonate was dissolved in 50ml of deionized water, and 1ml of EDOT monomer and pretreated soft carbon material (mass ratio of EDOT monomer to pretreated soft carbon was 10:90) were added sequentially. The mixture was stirred for 30min. 4g of ammonium persulfate was dissolved in 10ml of deionized water and added dropwise to the above mixed solution. After polymerization at room temperature for 24h, the mixture was centrifuged and washed with deionized water until colorless. It was then dried to obtain a black powder. The dried black powder was loaded into an alumina boat and placed in a tube furnace. It was carbonized at 700℃ for 2h under an argon or nitrogen atmosphere with a heating rate of 3℃ / min. It was then naturally cooled to room temperature to obtain a hard carbon-soft carbon composite material with hard carbon coating. The core is a sulfur-doped mesophase carbon microsphere-based layered porous carbon soft carbon, and the outer shell is a sulfur-doped PEDOT-based hard carbon. Both the core and the outer shell are rich in sulfur.

[0075] Example 2

[0076] 4g of sulfur-doped mesophase carbon microspheres were ultrasonically dispersed in 400ml of DMF solvent for 30min. Then, 20g of magnesium sulfate was added, and the mixture was magnetically stirred for 12h. The solvent was then evaporated in a rotary evaporator. The solid was loaded into an alumina boat and placed in a tube furnace for high-temperature carbonization under a nitrogen atmosphere. The carbonization process involved raising the temperature from room temperature to the specified carbonization temperature at a rate of 5℃ / min and holding at that temperature for 1h. The carbonization temperature was 650℃. After natural cooling to room temperature, soft carbon, i.e., sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon, was obtained. The soft carbon was acid-washed and dried to obtain pretreated soft carbon.

[0077] 5g of sodium dodecylbenzenesulfonate was dissolved in 50ml of deionized water, and 1ml of EDOT monomer and pretreated soft carbon material (mass ratio of EDOT monomer to pretreated soft carbon was 10:90) were added sequentially. The mixture was stirred for 30min. 4g of ammonium persulfate was dissolved in 10ml of deionized water and added dropwise to the above mixed solution. After polymerization at room temperature for 24h, the mixture was centrifuged and washed with deionized water until colorless. It was then dried to obtain a black powder. The dried black powder was loaded into an alumina boat and placed in a tube furnace. It was carbonized at 700℃ for 2h under an argon or nitrogen atmosphere with a heating rate of 3℃ / min. It was then naturally cooled to room temperature to obtain a hard carbon-soft carbon composite material with hard carbon coating. The core is a sulfur-doped mesophase carbon microsphere-based layered porous carbon soft carbon, and the outer shell is a sulfur-doped PEDOT-based hard carbon. Both the core and the outer shell are rich in sulfur.

[0078] Example 3

[0079] 4g of sulfur-doped mesophase carbon microspheres were ultrasonically dispersed in 400ml of DMF solvent for 30min. Then, 20g of magnesium sulfate was added, and the mixture was magnetically stirred for 12h. The solvent was then evaporated in a rotary evaporator. The solid was loaded into an alumina boat and placed in a tube furnace for high-temperature carbonization under a nitrogen atmosphere. The carbonization process involved raising the temperature from room temperature to the specified carbonization temperature at a rate of 5℃ / min and holding at that temperature for 1h. The carbonization temperature was 700℃. After natural cooling to room temperature, soft carbon, i.e., sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon, was obtained. The soft carbon was acid-washed and dried to obtain pretreated soft carbon.

[0080] 5g of sodium dodecylbenzenesulfonate was dissolved in 50ml of deionized water, and 1ml of EDOT monomer and pretreated soft carbon material (mass ratio of EDOT monomer to pretreated soft carbon was 10:90) were added sequentially. The mixture was stirred for 30min. 4g of ammonium persulfate was dissolved in 10ml of deionized water and added dropwise to the above mixed solution. After polymerization at room temperature for 24h, the mixture was centrifuged and washed with deionized water until colorless. It was then dried to obtain a black powder. The dried black powder was loaded into an alumina boat and placed in a tube furnace. It was carbonized at 700℃ for 2h under an argon or nitrogen atmosphere with a heating rate of 3℃ / min. It was then naturally cooled to room temperature to obtain a hard carbon-soft carbon composite material with hard carbon coating. The core is a sulfur-doped mesophase carbon microsphere-based layered porous carbon soft carbon, and the outer shell is a sulfur-doped PEDOT-based hard carbon. Both the core and the outer shell are rich in sulfur.

[0081] Examples 4-6 and Comparative Examples 1-2

[0082] The difference from Example 3 is only in the carbonization temperature during soft carbon treatment, which is 750℃, 800℃, 850℃, 900℃, and 950℃ respectively. All other settings are the same as in Example 3.

[0083] Comparative Example 3

[0084] 4 g of sulfur-doped mesophase carbon microspheres were ultrasonically dispersed in 400 ml of DMF solvent for 30 min. Then, 20 g of magnesium sulfate was added, and the mixture was magnetically stirred for 12 h. The solvent was then evaporated in a rotary evaporator. The solid was loaded into an alumina boat and placed in a tube furnace for high-temperature carbonization under a nitrogen atmosphere. The carbonization process involved increasing the temperature from room temperature to the specified carbonization temperature at a rate of 5 °C / min and holding at that temperature for 1 h. The carbonization temperature was 700 °C. After natural cooling to room temperature, sulfur-doped mesophase carbon microsphere-based layered porous carbon was obtained. After acid washing and drying, soft carbon was obtained.

[0085] Comparative Example 4

[0086] 4g of mesophase carbon microspheres were ultrasonically dispersed in 400ml of DMF solvent for 30min. Then, 20g of magnesium sulfate was added, and the mixture was magnetically stirred for 12h. The solvent was then evaporated in a rotary evaporator. The solid was loaded into an alumina boat and placed in a tube furnace for high-temperature carbonization under a nitrogen atmosphere. The carbonization process involved raising the temperature from room temperature to the specified carbonization temperature at a rate of 5℃ / min and holding at that temperature for 1h. The carbonization temperature was 700℃. After naturally cooling to room temperature, the carbon was acid-washed and dried to obtain soft carbon.

[0087] Comparative Example 5

[0088] 4g of mesophase carbon microspheres were ultrasonically dispersed in 400ml of DMF solvent for 30min. Then, 20g of magnesium sulfate was added, and the mixture was magnetically stirred for 12h. The solvent was then evaporated in a rotary evaporator. The solid was loaded into an alumina boat and placed in a tube furnace for high-temperature carbonization under a nitrogen atmosphere. The carbonization process involved raising the temperature from room temperature to the specified carbonization temperature at a rate of 5℃ / min and holding at that temperature for 1h. The carbonization temperature was 700℃. After naturally cooling to room temperature, the carbon was acid-washed and dried to obtain pretreated soft carbon.

[0089] Dissolve 5g of sodium dodecylbenzenesulfonate in 50ml of deionized water, then add 1ml of EDOT monomer and pretreated soft carbon material (mass ratio of EDOT monomer to pretreated soft carbon is 10:90) and continue stirring for 30min. Dissolve 4g of ammonium persulfate in 10ml of deionized water and add it dropwise to the above mixed solution. After polymerization at room temperature for 24h, centrifuge the mixture and wash it with deionized water until it is colorless. Dry the mixture to obtain a black powder. Place the dried black powder into an alumina boat and put it in a tube furnace. Carbonize it at 700℃ for 2h under an argon or nitrogen atmosphere with a heating rate of 3℃ / min. Allow it to cool naturally to room temperature to obtain a soft and hard carbon composite material.

[0090] Tests and Results

[0091] The carbon materials obtained in the examples and comparative examples were tested as follows:

[0092] (1) Carbon interlayer spacing was measured using an X-ray diffractometer. The XRD data was obtained with a Cu target wavelength of λ = 0.154056 nm and a diffraction angle of 2θ of 10–90°. The carbon interlayer spacing was calculated by substituting the (002) diffraction angle of the XRD into the Scherrer formula.

[0093] (2) Sulfur content was tested by elemental analysis using energy dispersive X-ray spectroscopy (EDS) on a field emission scanning electron microscope.

[0094] (3) CSC content test: analysis was performed using an infrared combined carbon and sulfur analyzer.

[0095] The carbon material obtained in the examples and comparative examples was used as the negative electrode material for sodium batteries. It was coated with conductive agent, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in a mass percentage of 94.5:2:1.5:2 to form the negative electrode. The sodium metal sheet was used as the positive electrode, and the separator was a glass fiber separator. The sodium salt in the electrolyte was 1.5M NaPF6, and the solvent was EC and DMC in a volume ratio of 1:1. There was also 5% volume of EMC additive. The battery was placed in an argon-filled glove box, and then charge and discharge tests were performed in a Blue Electric test cabinet. The charge and discharge rate was 0.1C (voltage range 0-2V), and the nominal capacity was set to 320mAh / g. The first charge and discharge capacity, the first coulombic efficiency (first efficiency), and the capacity retention rate after 100 cycles were recorded. The first efficiency is the ratio of the first discharge capacity to the first charge capacity.

[0096] The test results are shown in Table 1.

[0097] Table 1

[0098]

[0099]

[0100] As can be seen from the comparison of the embodiments and comparative examples, the soft and hard carbon composite material of the present invention, with a core of sulfur-doped mesophase carbon microsphere-based layered porous carbon and an outer shell of sulfur-doped PEDOT-based hard carbon, has achieved improvements in the first charge-discharge capacity, first efficiency, and cycle retention of sodium batteries. This should be the result of improving the sodium storage capacity of mesophase carbon microspheres, possibly by achieving considerable intercalation capacity through widened carbon interlayer spacing and obtaining abundant sodium storage sites to achieve a large proportion of surface capacitance storage, thereby improving the first charge-discharge capacity, first efficiency, and cycle performance of sodium batteries.

[0101] As can be seen from the comparison of Examples 1-6 and Comparative Examples 1-2, the soft and hard carbon composite materials of the present invention, when carbonized with sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon, show an initial increase followed by a decrease in the first charge-discharge capacity, first efficiency, and 100-cycle retention rate of sodium batteries as the temperature increases. In particular, the highest values ​​are reached at 700℃ (Example 3), which are 335mAh / g, 290.1mAh / g, 86.6%, and 99.1%, respectively. However, at 900℃ and above, the values ​​are significantly reduced to below 305mAh / g, 240mAh / g, 80%, and 91%, respectively.

[0102] By comparing Example 3 with Comparative Example 3, and Comparative Examples 4 and 5, it is evident that the soft-hard carbon composite material improves the initial charge-discharge capacity, initial efficiency, and cycle retention of sodium batteries. The improvement is more significant with the soft-hard carbon composite material of this invention, particularly in initial efficiency and cycle retention. Further comparison of the total sulfur doping amount and the core sulfur doping amount reveals that the construction of the core-shell structure not only introduces more sulfur through the outer shell but also unexpectedly achieves re-sulfur doping of the PEDOT-based hard carbon shell by the sulfur-containing soft carbon core at high temperatures. This provides additional active carbon-sulfur bonds (CSCs) for the PEDOT-based hard carbon while widening the carbon interlayer spacing.

[0103] By comparing Example 3 with Comparative Examples 4 and 5, and comparing Comparative Examples 3 and 4, it can be seen that for the modification of mesophase carbon microspheres, the four-stage sulfur doping improvement of this invention—first sulfur doping through hydrogen sulfide etching surface modification, second sulfur doping through magnesium sulfate pore-forming adjustment, third sulfur doping through hard carbon coating of soft carbon, and fourth sulfur doping through a high-temperature inert environment core to the outer shell—is more effective, resulting in better initial charge-discharge capacity, initial efficiency, and cycle retention in sodium batteries. This is because: H2S etching for sulfur doping can significantly improve the carbon interlayer spacing, sp2 carbon content, and sulfur doping amount. Although the initial coulombic efficiency is slightly reduced, the sodium storage performance is significantly improved in three aspects: interlayer intercalation, pseudocapacitance, and surface adsorption, achieving a balance between intercalation capacity and surface adsorption capacity. The hierarchical porous carbon prepared using magnesium sulfate as the sulfur source increases the surface capacitance contribution at the expense of the carbon interlayer spacing. Due to the abundance of sulfur bonds in the active carbon, the initial coulombic efficiency is actually improved through -SO x The selective removal of oxygen-containing functional groups and active carbon atoms by oxidation or S-atom substitution reactions is significantly enhanced. When hard carbon coats soft carbon, the sulfur in PEDOT provides abundant active sites for sodium ion storage and has a large carbon interlayer spacing, which facilitates the reversible insertion and extraction of sodium ions between carbon layers. Furthermore, during high-temperature inert carbonization, the escape of core sulfur, especially core surface sulfur, leads to sulfur doping of the outer shell. This not only widens the carbon interlayer spacing but also fully utilizes the active carbon-sulfur bonds (CSCs), improving material properties.

[0104] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A soft and hard carbon composite material, characterized in that: The soft and hard carbon composite material has a core-shell structure with hard carbon coating soft carbon and double sulfur richness. The soft carbon as the core is sulfur-doped mesophase carbon microsphere-based hierarchical porous carbon, and the hard carbon as the shell is sulfur-doped PEDOT-based hard carbon. The sulfur doping content of the soft and hard carbon composite material is 6.5~8.8 at%; In the aforementioned soft and hard carbon composite material, active carbon sulfur bonds (CSCs) account for 96-100% of the sulfur-containing organic bonds. The soft carbon is obtained by using sulfur-doped mesophase carbon microspheres, obtained by etching the surface of mesophase carbon microspheres with H2S, as a precursor, followed by sulfur doping and pore-conditioning modification with magnesium sulfate as a sulfur source template, and then carbonization. Sulfur-doped mesophase carbon microspheres were prepared as follows: S11. Place the mesophase carbon microspheres with a particle size of 8-15μm in a mixed acid of nitric acid and sulfuric acid with a volume ratio of 2:1 for oxidation, wash with water until neutral, and then dry to obtain pre-oxidized mesophase carbon microspheres for later use. S12. The pre-oxidized mesophase carbon microspheres are loaded into an alumina boat and placed in a tube furnace. First, a pre-carbonization stage is carried out in an argon atmosphere, where the temperature is raised from room temperature to 400°C and held for 1 hour at a heating rate of 2°C / min. Second, a carbonization stage is carried out in a mixed atmosphere, where the temperature is raised from 400°C to 700°C at a heating rate of 5°C / min and held for 3 hours. The gas flow rates of argon and hydrogen sulfide in the mixed atmosphere are 240 ml / min, and the volume ratio of argon to hydrogen sulfide is 9:

1. Finally, the mixture is naturally cooled to room temperature to obtain sulfur-doped mesophase carbon microspheres.

2. The soft and hard carbon composite material as described in claim 1, characterized in that: The carbon interlayer spacing of the soft and hard carbon composite material is 0.340-0.410 nm.

3. The soft and hard carbon composite material as described in claim 1, characterized in that: The mass ratio of magnesium sulfate to sulfur-doped mesophase carbon microspheres is 1~10:1; and / or the carbonization temperature is 600~890℃.

4. A method for preparing a soft and hard carbon composite material as described in any one of claims 1-3, characterized in that: Includes the following steps: The soft carbon is obtained by carbonization after using sulfur-doped mesophase carbon microspheres as a precursor, which are modified by sulfur doping and pore adjustment using magnesium sulfate as a sulfur source template. The soft and hard carbon composite material is obtained by using the soft carbon as the core, guiding the oxidative polymerization of EDOT monomers on its surface, and then carbonizing it at high temperature and inertly, which is sulfur-doped PEDOT-based hard carbon. The soft and hard carbon composite material has a core-shell structure with hard carbon coating soft carbon and double sulfur enrichment.

5. The method for preparing the soft and hard carbon composite material as described in claim 4, characterized in that: In the preparation of the soft carbon, the mass ratio of magnesium sulfate to sulfur-doped mesophase carbon microspheres is 1~10:

1.

6. The method for preparing the soft and hard carbon composite material as described in claim 4, characterized in that: In the preparation of the soft and hard carbon composite material, the mass ratio of EDOT monomer to soft carbon is 1~20:80~99.

7. A sodium-ion battery anode material, characterized in that: This includes the soft and hard carbon composite materials as described in any one of claims 1-3 or the soft and hard carbon composite materials prepared by the preparation method as described in any one of claims 4-6.

8. A sodium-ion battery, characterized in that: This includes the soft and hard carbon composite materials as described in any one of claims 1-3 or the soft and hard carbon composite materials prepared by the preparation method as described in any one of claims 4-6.

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