Modified sulfur positive electrode material for metal-sulfur battery, preparation method thereof and metal-sulfur battery

Abstract

The application discloses a modified sulfur positive electrode material for a metal-sulfur battery, a preparation method of the modified sulfur positive electrode material and the metal-sulfur battery. The modified sulfur positive electrode material comprises an S / C composite material and an HNBR layer uniformly coated on the surface of the S / C composite material. The preparation method of the modified sulfur positive electrode material comprises the following steps: (1) obtaining an S / C composite material; (2) uniformly dispersing S / C composite material powder in 3-20 wt% HNBR glue solution, treating the obtained mixture at 30-80 DEG C for 5-60 minutes, then heating and treating the mixture at 170-200 DEG C for 20-60 minutes, and finally separating and drying to obtain the modified sulfur positive electrode material. The application provides a metal-sulfur battery, wherein the positive electrode comprises the modified sulfur positive electrode material. Compared with unmodified S / C material, the modified sulfur positive electrode material provided by the application improves the specific capacity and cycle stability of the metal-sulfur battery.

Description

Technical Field

[0001] This invention belongs to the field of new energy materials technology, specifically relating to a modified sulfur cathode material for metal-sulfur batteries, its preparation method, and a metal-sulfur battery using the modified sulfur cathode material. Background Technology

[0002] Metal-sulfur batteries are considered a promising next-generation high-energy-density battery system due to their extremely high theoretical specific capacity and energy density, as well as the advantages of abundant sulfur resources, low cost, and environmental friendliness.

[0003] However, the numerous problems with sulfur cathode materials severely hinder their practical application, mainly including the following aspects: (1) Low intrinsic conductivity (close to insulation): Elemental sulfur has extremely low intrinsic conductivity, making it difficult to use as an electrode material. This ultimately requires the addition of two other components to the positive electrode, namely conductive additives and polymer binders, to form a conductive electrode.

[0004] (2) Polysulfide shuttle effect: The basic electrochemistry of Li-S batteries can be represented by S⁸ + 16Li + + 16e - → A simplified representation of the overall discharge mechanism of 8Li2S is used to describe it ( Figure 12 In S8 and Li2S x The solubility of Li-S redox products varies significantly at each stage of the conversion reaction. Electroactive materials exhibit different solubilities in the electrolyte medium, with the initial S8 active material and the deeply discharged Li2S exhibiting different solubilities. x (x≤2) are insoluble solids, while higher-order polysulfides (such as Li₂S₈, Li₂S₆) are easily soluble. These soluble intermediates easily diffuse through the membrane to the lithium anode, where they undergo side reactions to form insoluble Li₂S that deposits on the anode surface, while the cathode loses its active material. This leads to problems such as rapid capacity decay, low coulombic efficiency, loss of active material, lithium anode corrosion, and severe self-discharge.

[0005] (3) Volume expansion (~80%): The densities of the insoluble electroactive material S8 and the final product Li2S are 2.03 and 1.66 g / mL, respectively. This nearly 80% volume change will induce significant mechanical stress in the carbon / binder framework of the cathode. With a larger amount of Li2S... x Repeated deposition of discharge products leads to the appearance of cracks and fissures in the positive electrode composite material, causing the electrical connection area to break and ultimately the device to fail.

[0006] To address these shortcomings, researchers have developed various modification strategies, such as compositing with conductive substrates (to solve poor conductivity), physical confinement and encapsulation (to alleviate shuttle effect and volume expansion), chemical adsorption and bonding (to suppress shuttle effect), catalytic conversion (to accelerate reaction kinetics), and binder optimization (to improve structural stability).

[0007] Polymer materials are widely used in the modification of the cathode of lithium-sulfur batteries. Their applications include using polymers as binders, cathode active materials (sulfur active materials or sulfur / carbon composite active materials), or coating materials for the entire cathode, and cathode active materials.

[0008] Polymers are used as coatings for positive electrode active materials to confine Li-S redox reactions within the cathode and reduce the effective diffusion of soluble lithium polysulfides. Ideally, polymers used in these coatings should possess electrochemical properties that promote charge and / or ion conduction to the electroactive material. Additionally, the polymer coating should have mechanical properties capable of accommodating significant volume changes in Li-S redox products. Furthermore, the coating should exhibit electrochemical stability within a potential window of 1.7–2.7 V and chemical stability against alkaline environments and nucleophilic polysulfides during Li-S battery operation. The ease of preparation and the feasibility of scale-up production are relevant factors considering the ultimate goal of commercializing Li-S technology. Moreover, the mass and volume loading of the polymer coating should be minimized to ensure maximum total electroactive sulfur loading without reducing the energy density of the cathode. Currently, polymer encapsulation of sulfur / carbon composite active materials mainly focuses on the use of PEG, PEO and PEO / PVP blends, conjugated polymers with good conductivity (including PEDOT, PANI, PPy and polyphenylene), some polymers with polar / ionic side chains (such as PVP and Nafion), and bio-inspired polymers such as branched starch and polydopamine.

[0009] The main role of polymers used as positive electrode active materials in Li-S batteries is as a source of S-S bonds for electrochemical reduction (discharge) and oxidation (charge). Post-polymerization vulcanization of polyacrylonitrile (PAN) has attracted widespread attention as a route for preparing high-performance positive electrode active materials. Wang et al. heated sulfur and PAN at 300°C under an argon atmosphere to obtain S-PAN material. Electrochemical performance evaluation of S-PAN showed that it is effective as a positive electrode active material; the positive electrode prepared with S-PAN exhibited a reversible capacity greater than 600 mAh / g after 50 cycles.

[0010] Hydrogenated nitrile butadiene rubber (HNBR) is a special rubber material modified by catalytic hydrogenation of a random copolymer of acrylonitrile and butadiene (nitrile butadiene rubber, NBR). In recent years, the application of HNBR in lithium-ion batteries has received attention. HNBR is mainly used as a binder for cathode materials or a dispersant for conductive slurries, but its application as a modified cathode material (sulfur / carbon composite active material) in metal-sulfur batteries has not been reported. This invention aims to provide a modified sulfur cathode material using HNBR in metal-sulfur batteries, its preparation method, and a metal-sulfur battery using the modified sulfur cathode material. Summary of the Invention

[0011] This invention provides a modified sulfur cathode material for metal-sulfur batteries. This modified sulfur cathode material can effectively suppress the loss of electroactive materials, improve the structural stability of the electrode, and enhance the specific capacity and cycle stability of metal-sulfur batteries.

[0012] The second problem to be solved by the present invention is to provide a simple and easily scaled-up method for preparing modified sulfur cathode materials for metal-sulfur batteries, which can form an HNBR coating layer of a certain thickness on the surface of S / C composite materials.

[0013] The third problem to be solved by the present invention is to provide a metal-sulfur battery using the modified sulfur cathode material, which has improved specific capacity and cycle stability.

[0014] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: In a first aspect, the present invention provides a modified sulfur cathode material for metal-sulfur batteries, the modified sulfur cathode material comprising an S / C composite material and an HNBR layer uniformly coated on the surface of the S / C composite material.

[0015] The HNBR layer described in this invention is an S-HNBR coating layer obtained by chemical reaction between HNBR and sulfur in the S / C composite material at the interface.

[0016] Preferably, the modified sulfur cathode material contains HNBR at a mass percentage of 1.8%-10.2%.

[0017] In a second aspect, the present invention provides a method for preparing a modified sulfur cathode material for a metal-sulfur battery according to the first aspect, comprising the following steps: (1) Obtain S / C composite powder; (2) The S / C composite material powder is uniformly dispersed in 3-20wt% HNBR adhesive solution. The resulting mixture is first treated at 30-80℃ for 5-60 minutes, then heated at 170-200℃ for 20-60 minutes, and finally separated and dried to obtain the modified sulfur cathode material.

[0018] In step (1) of this invention, the S / C composite material can be obtained by calcining a sulfur source and a carbon source. The sulfur source can be sublimated sulfur, sulfides (e.g., sodium sulfide), sulfurized polymers (e.g., polyphenylene sulfide), etc.; the carbon source can be conductive carbon black (Super P, Ketjen black, etc.), activated carbon (e.g., activated carbon with model YP50F), ordered porous carbon (e.g., mesoporous carbon CMK-3), carbon nanotubes, metal-organic frameworks or other organic carbon sources derived from porous carbon, biomass carbon, etc., preferably at least one of Ketjen black, activated carbon and mesoporous carbon.

[0019] In the modified sulfur cathode material of the present invention, the mass content of HNBR is preferably 1.8%-10.2%.

[0020] The solvent of the HNBR adhesive solution in this invention is required to disperse HNBR into an adhesive solution in which the S / C composite material is insoluble. Preferably, it is at least one of N-methylpyrrolidone (NMP), methyl isopropyl ketone, and xylene, more preferably NMP or methyl isopropyl ketone, and even more preferably NMP. The mass concentration of HNBR in the HNBR adhesive solution is preferably 5-10 wt%, more preferably 5 wt%.

[0021] In step (2) of this invention, the obtained mixture is first treated at 30-80℃ for 5-60 minutes, then treated at 170-200℃ for 20-60 minutes, and finally separated and dried to obtain the modified sulfur cathode material. The first step treatment temperature is 30-80℃, for example 30℃, 40℃, 50℃, 60℃, 70℃, 80℃, preferably 50-70℃, more preferably 60℃, and the first step treatment time is 5-60 minutes, preferably 15-60 minutes, more preferably 15 minutes; the second step treatment temperature is 170-200℃, and the second step treatment time is preferably 20-40 minutes.

[0022] In step (2) of this invention, the separation can be performed by centrifugation, precipitation, or other operations. The drying can be ordinary pressure drying or vacuum drying.

[0023] Thirdly, the present invention provides a metal-sulfur battery, including a positive electrode, wherein the positive electrode includes the modified sulfur positive electrode material described in the first aspect.

[0024] This invention does not particularly limit the composition, assembly, and testing of the metal-sulfur battery, except for the positive electrode. The positive electrode is obtained by coating a positive electrode slurry containing the modified sulfur positive electrode material, a conductive agent, and a binder onto a current collector and then drying it. The conductive agent can be at least one of Super P, Ketjen Black, CNTs, graphene, etc., and the binder can be at least one of PVDF, CMC, PEO, polyacrylic acid (PAA), sodium alginate (SA), etc. The sulfur loading in the positive electrode is preferably 0.5~5 mg cm⁻¹. -2 .

[0025] Compared with the prior art, the beneficial effects and differences of the present invention are as follows: (1) This invention provides a novel modified sulfur cathode material for metal-sulfur batteries, which for the first time coats HNBR onto the surface of S / C composite material particles, and HNBR undergoes a sulfidation reaction with S in the S / C composite material at the interface to form an S-HNBR coating layer, wherein the S in the S-HNBR coating layer X It exhibits similar electrochemical activity to S8 with Li. The S-HNBR coating can suppress the loss of active materials, improve the structural stability of the electrode, and enhance the specific capacity and cycle stability of the battery system.

[0026] (2) The preparation method of the present invention is simple to operate and easy to scale up industrially. It involves two-step treatment of the mixture. On the one hand, a certain thickness of HNBR coating layer is formed on the surface of S / C composite material particles. On the other hand, HNBR undergoes a sulfurization reaction with S in S / C composite material at the interface, forming an S-HNBR coating layer with stronger interfacial bonding.

[0027] (3) The present invention provides a high-performance metal-sulfur battery using modified sulfur cathode material as cathode material. The battery performance is significantly better than that of batteries using unmodified S / C composite material as cathode material. The specific capacity and cycle stability of the battery are significantly improved. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of a typical structure of S / C@S-HNBR; Figure 2 This is a TEM image of the modified sulfur cathode material described in Example 1; Figure 3 This is a TEM image of the modified sulfur cathode material described in Example 14; Figure 4 This is a TEM image of the modified sulfur cathode material described in Example 15; Figure 5 SEM images of the S / C (a) and modified sulfur cathode (b) materials described in Comparative Example 1 and Example 1; Figure 6 SEM images (top view) of the electrodes made from the S / C (a) and modified sulfur cathode (b) materials described in Comparative Example 1 and Example 1. Figure 7 SEM images (interface diagrams) of the electrodes made from the S / C (a) and modified sulfur cathode (b) materials described in Comparative Example 1 and Example 1. Figure 8 The second-cycle charge-discharge curves of a lithium-sulfur battery using the S / C and modified sulfur cathode materials described in Comparative Example 1 and Example 1 as cathode materials are shown, where the modified sulfur cathode material is represented by S / C@S-HNBR. Figure 9 EIS diagram of the battery using the S / C and modified sulfur cathode materials described in Comparative Example 1 and Example 1 as cathode materials, wherein the modified sulfur cathode material is represented by S / C@S-HNBR; Figure 10 The cyclic voltammograms of the battery using the modified sulfur cathode material described in Examples 1 and 2 as the cathode material are shown. Figure 11 The cyclic voltammogram is for a battery using the modified sulfur cathode material described in Example 7 as the cathode material.

[0029] Figure 12 A diagram illustrating the overall discharge mechanism of the basic electrochemical reactions in a lithium-sulfur battery. Figure 13 These are optical photographs of the insoluble phase S-HNBR and the solution phase S-HNBR prepared in Example 14; Figure 14 The images show (A) scanning electron microscope image and elemental distribution map of the insoluble phase S-HNBR product prepared in Example 14: (B) layered image, (C) S elemental distribution and (D) C elemental distribution.

[0030] Figure 15 The images shown are FT-IR images (a) and magnified images (b) of HNBR sample 1 and two types of S-HNBR used in Example 14. Figure 16 These are the Raman spectra of the two S-HNBRs prepared in Example 14; Figure 17 This is the NMR spectrum of the insoluble phase S-HNBR prepared in Example 14. Detailed Implementation

[0031] To facilitate understanding of the present invention, the technical solution of the present invention will be fully and completely described below with reference to the embodiments. Obviously, the scope of protection of the present invention is not limited to the embodiments.

[0032] This invention provides a novel modified sulfur cathode material, comprising an S / C composite material and an HNBR layer uniformly coated on the surface of the S / C composite material, wherein the S in the S / C composite material and the HNBR undergo a chemical reaction at the interface to form an S-HNBR coating layer.

[0033] In some embodiments, the HNBR content in the modified sulfur cathode material is 1.8%-10.2% by mass. In some preferred embodiments, the battery is a lithium-sulfur battery, and the HNBR content in the modified sulfur cathode material is 4.5%-10.0% by mass, more preferably 4.8%-9.8%. In some preferred embodiments, the battery is a sodium-sulfur battery, and the HNBR content in the modified sulfur cathode material is 4%-6% by mass, more preferably 4.9%-5.6%. In some preferred embodiments, the battery is a magnesium-sulfur battery, and the HNBR content in the modified sulfur cathode material is 2.0%-10.2% by mass.

[0034] The aforementioned HNBR, or hydrogenated nitrile butadiene rubber, is obtained by catalytically hydrogenating nitrile butadiene rubber (NBR) to convert the double bonds in the NBR into single bonds. Its structural formula is shown below:

[0035] In some embodiments, the mass content of acrylonitrile repeating units (ACN) in the HNBR is 18%-43%, including but not limited to 18%, 28%, 35%, 43%, or any range between the two, preferably 28%-35%, and more preferably 30%-35%. The ACN content in the HNBR can be determined by referring to the following standard method: ISO24698-2:2008 / SH / T1157.2-2015 Determination of bound acrylonitrile content in acrylonitrile-butadiene rubber (NBR) Part 2: Kjeldahl method.

[0036] In some embodiments, the iodine value of the HNBR is ≤27, preferably ≤8. The iodine value is used to characterize the degree of hydrogenation of the NBR. The iodine value of the HNBR can be determined according to the following standard method: SH / T 1763-2020 Determination of Residual Unsaturation in Hydrogenated Nitrile Butadiene Rubber (HNBR) - Iodine Value Method.

[0037] In some embodiments, the Mooney viscosity value ML(1+4)@100℃ of the HNBR is 40-49, such as 40, 42, 43, 49, or any range between the two, preferably 40-45, and more preferably 40-43. The Mooney viscosity is an important indicator characterizing the flowability and processing performance of polymer materials under specific conditions, reflecting the molecular weight and distribution range of the polymer. The Mooney viscosity value is usually expressed as ML(1+4)@100℃, where ML refers to the use of a large rotor (L-shaped rotor, diameter 38.1 mm), 1+4 refers to a preheating time of 1 minute and a formal testing time of 4 minutes, and 100℃ refers to the testing temperature. The Mooney viscosity value ML(1+4)@100℃ of the HNBR can be determined according to the following standard method: GB / T 1232.1-2016 / ISO289-1:2014 Determination of uncured rubber using a disc shear viscometer – Part 1: Determination of Mooney viscosity.

[0038] Understandably, the aforementioned hydrogenated nitrile butadiene rubber can be obtained commercially, or it can be synthesized by purchasing nitrile butadiene rubber (both commercially available) and following conventional methods, or it can be synthesized by purchasing the corresponding monomers (all commercially available) and following conventional methods.

[0039] This invention provides a method for preparing a modified sulfur cathode material, comprising the following steps: (1) Obtain S / C composite powder; (2) The S / C composite material powder is uniformly dispersed in 3-20wt% HNBR adhesive solution. The resulting mixture is first treated at 30-80℃ for 5-60 minutes, then treated at 170-200℃ for 20-60 minutes, and finally separated and dried to obtain the modified sulfur cathode material.

[0040] The S / C composite material described in this invention is the commonly used S / C composite material in metal-sulfur battery cathode materials, wherein the S mass content in the S / C composite material is generally 35%-75%. The S / C composite material can be prepared using existing technologies. In some embodiments, the S / C composite material is obtained by calcining a mixture of a sulfur source and a carbon source. The sulfur source can be sublimated sulfur, sulfides (e.g., sodium sulfide), sulfurized polymers (e.g., polyphenylene sulfide), etc.; the carbon source can be conductive carbon black (Katjenblack, Super P, Ketjen black, etc.), activated carbon (e.g., activated carbon of type YP50F), ordered porous carbon (e.g., mesoporous carbon CMK-3), carbon nanotubes, metal-organic frameworks, or porous carbon derived from other organic carbon sources, biomass carbon, etc. In some preferred embodiments, the sulfur source is sulfur powder, and the carbon source is at least one selected from Ketjen black, activated carbon, and mesoporous carbon. After mixing the sulfur and carbon sources, they are first homogenized by ball milling, and then calcined. The calcination is carried out under inert gas protection at a temperature of 140-165°C for 2-15 hours. The carbon and sulfur sources are added according to the desired sulfur mass content in the S / C composite material, for example, according to m... S / (m S +m C )×100%=40%-80% of the material is added.

[0041] In some embodiments, in step (2), the mass concentration of HNBR in the HNBR adhesive is 3-20 wt%, preferably 5-10 wt%, and more preferably 5 wt%.

[0042] In some embodiments, the solvent of the HNBR adhesive in step (2) is at least one of NMP, methyl isopropyl ketone, and xylene, preferably NMP or methyl isopropyl ketone, and more preferably NMP.

[0043] In some embodiments, in step (2), the feeding ratio of the S / C composite material and the HNBR adhesive is 1 g: 5-15 mL, preferably 1 g: 8-12 mL, and more preferably 1 g: 10 mL.

[0044] In step (2) of the present invention, the obtained mixture is first treated at 30-80℃ for 5-60 minutes, and then treated at 170-200℃ for 5-60 minutes.

[0045] In some embodiments, in step (2), the first step processing temperature is 30-80°C, such as 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, or any two of the above, preferably 50-70°C, and more preferably 60°C; the first step processing time is 5-60 minutes, such as 5, 10, 15, 20, 30, 60 minutes, or any two of the above, preferably 15-60 minutes, and more preferably 15 minutes.

[0046] In some embodiments, in step (2), the second step processing temperature is 170-200°C, such as 170, 180, 190, 200°C or any two of the above, and the second step processing time is 5-60 minutes, such as 5, 20, 30, 40, 50, 60 minutes or any two of the above, and more preferably 20-40 minutes.

[0047] In step (2) of this invention, the separation can be performed by centrifugation, precipitation, or other operations. The drying can be ordinary pressure drying or vacuum drying.

[0048] The present invention provides a metal-sulfur battery, comprising a positive electrode containing the modified sulfur positive electrode material, a metal negative electrode, an electrolyte, and a separator.

[0049] In some embodiments, the positive electrode is obtained by uniformly coating a positive electrode slurry containing the modified sulfur positive electrode material, a conductive agent, and a binder onto a current collector and then drying it. The conductive agent can be at least one of Super P, Ketjen Black, CNTs, graphene, etc. The binder can be at least one of PVDF, CMC, PEO, polyacrylic acid (PAA), sodium alginate (SA), etc. The current collector can be aluminum foil, carbon-coated aluminum foil, carbon paper, or carbon fiber. In some preferred embodiments, the conductive agent is Super P; the binder is PVDF; the mass ratio of the modified sulfur positive electrode material, conductive agent, and binder is 50-90%:5-25%:5-25%, for example, 7:2:1; and the current collector is carbon-coated aluminum foil. In some preferred embodiments, the sulfur loading in the positive electrode is 0.5~5 mg cm⁻¹. -2 .

[0050] This invention does not impose any particular limitations on the composition, assembly, and testing of the metal-sulfur battery other than the positive electrode.

[0051] In some embodiments, the metal anode is selected as lithium, sodium, or magnesium.

[0052] The electrolyte typically comprises an organic solvent and a metal salt. The organic solvent may be selected from one or more of the following: ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butenyl carbonate, fluoroethylene carbonate (FEC), methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, diethyl sulfone, 1,2-dimethoxyethane (DME), and 1,3-dioxolane (DOL). The metal salt may be at least one of lithium, sodium, and magnesium salts. The lithium salt may be one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorophosphate (LiPO2F2), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP). The sodium salt may be one or more of sodium perchlorate (NaClO4), sodium hexafluorophosphate (NaPF6), sodium trifluoromethanesulfonate (NaCF3SO3, NaOTf), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), and sodium difluorosulfonylimide (NaFSI). The magnesium salt can be one or both of bis(trifluoromethyl)sulfonylimide magnesium (Mg(TFSi)2) and bis(fluorosulfonyl)imide magnesium (Mg(FSI)2). The concentration of the metal salt in the electrolyte can be 0.5-3.0 mol / L, such as 0.5, 1, 1.5, 2, 2.5, 3.0 mol / L, or any range between the two mentioned above.

[0053] In addition to metal salts and organic solvents, additives can be added to the electrolyte as needed. For example, in the electrolyte used in lithium-sulfur batteries with LiTFSi as the metal salt and DOL / DME as the solvent, an appropriate amount of LiNO3 can be added as an additive.

[0054] In some embodiments, the organic solvent in the electrolyte is a mixture of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) in a volume ratio, the metal salt is LiTFSi, and an appropriate amount of LiNO3 is added as an additive, wherein the concentration of LiTFSi is 1-1.5 mol / L and the concentration of LiNO3 is 0.005-0.015 g / mL (e.g., 0.01 g / mL).

[0055] In other embodiments, the organic solvent of the electrolyte is a mixture of ethylene carbonate (EC) and propylene carbonate (PC) in a volume ratio of 1:1, and the metal salt is NaClO4, wherein the concentration of NaClO4 is 1-1.5 mol / L.

[0056] In other embodiments, the organic solvent in the electrolyte is a mixture of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) in a volume ratio, and the metal salt is Mg(TFSi)2, wherein the concentration of Mg(TFSi)2 is 1-1.5 mol / L.

[0057] In some embodiments, the diaphragm is made of polypropylene.

[0058] The following examples describe the contents of this application in more detail. These examples are for illustrative purposes only. All reagents used in the examples are commercially available or synthesized by conventional methods, and the instruments used in the examples are commercially available.

[0059] The sources of some of the raw materials used in the embodiments of this invention are as follows: Super P: Swiss Temico; PVDF: ARKEMA, model HSV900; Activated carbon YP50F: Kuraray Ltd., Japan; Ketjen Black ECP: Taiyuan Lizhiyuan Technology Co., Ltd.; Mesoporous carbon CMK-03: Jiangsu Xianfeng Nanomaterials Technology Co., Ltd.

[0060] The HNBR used in this embodiment of the invention was provided by Zhejiang Zansheng New Materials Co., Ltd., and its parameters are shown in Table 1 below: Table 1 Parameters of the HNBR products described in Examples 1-17

[0061] In Table 1, the parameters of the HNBR samples are within the following range: ACN content of 18% to 43%, iodine value of ≤27, and Mooney viscosity value of 40 to 49.

[0062] To better illustrate the different implementation methods of the embodiments, the embodiments are divided into three types of batteries for explanation: lithium-sulfur battery, sodium-sulfur battery, and magnesium-sulfur battery.

[0063] Comparative Example 1: Lithium-sulfur battery

[0064] The preparation method of S / C cathode material is as follows: sublimed sulfur powder and activated carbon YP50F are mixed in a mass ratio of 7:3, ball milled at 500 rpm for 2 h to make the two uniformly mixed, and then calcined at 155 ℃ for 10 h in an argon atmosphere to obtain S / C composite powder.

[0065] The preparation method of the positive electrode sheet is as follows: using S / C composite powder as the positive electrode material, the S / C composite powder, conductive agent Super P, and binder PVDF are mixed evenly in NMP solvent at a mass ratio of 7:2:1 to obtain a positive electrode slurry. This slurry is coated onto a current collector (carbon-coated aluminum foil) and dried at 60℃ for 12 h under vacuum to obtain the positive electrode sheet. The sulfur loading on the positive electrode sheet is 2 mg cm⁻¹. -2 .

[0066] The electrolyte was prepared as follows: 2.87 g of LiTFSi and 0.1 g of LiNO3 were weighed into a 10 mL volumetric flask in an argon-protected glove box (H2O and O2 concentrations were both less than 0.01 ppm). 10 mL of DOL / DME (volume ratio 1:1) mixture was added to the volumetric flask and stirred until the lithium salt was completely dissolved. 2 g of 3A molecular sieve was added and the mixture was left to stand in the glove box for 12 h to obtain the electrolyte.

[0067] Battery assembly: Button cells are prepared in a glove box. The positive electrode is the above-mentioned positive electrode sheet with a diameter of 12 mm. The separator is a polypropylene separator. The negative electrode is metallic lithium with a diameter of 16 mm.

[0068] Examples 1-7: Lithium-sulfur batteries

[0069] This embodiment provides a modified sulfur cathode material and its application in lithium-sulfur batteries to improve the performance of lithium-sulfur batteries.

[0070] The preparation method of the S / C composite material is the same as that of Comparative Example 1.

[0071] The modified sulfur cathode material was prepared as follows: 1 g of the S / C composite powder prepared in Examples 1-7 was uniformly dispersed in 10 mL of HNBR solution, with the HNBR sample and solution concentrations shown in Table 2. NMP was used as the solvent. The resulting mixture was subjected to two heating treatments at the temperatures and times shown in Table 2, with air as the reaction atmosphere. The resulting suspension was centrifuged, and the resulting solid was dried at 60°C for 12 h to obtain the modified sulfur cathode material.

[0072] The modified sulfur cathode material obtained was used as the cathode material, and the cathode preparation, electrolyte preparation and battery assembly processes were kept the same as those in Comparative Example 1.

[0073] Comparative Example 2: Sodium-sulfur battery

[0074] The preparation method of the S / C composite material is the same as that of Comparative Example 1, except that the carbon material is replaced with mesoporous carbon CMK-3.

[0075] The S / C composite material obtained above was used as the positive electrode material to prepare a positive electrode sheet. The preparation method of the positive electrode sheet was the same as that of Comparative Example 1.

[0076] The electrolyte was prepared as follows: 0.71 g NaClO4 was weighed into a 10 mL volumetric flask in an argon-protected glove box (H2O and O2 concentrations were both less than 0.01 ppm), and 10 mL of EC / PC (volume ratio 1:1) mixture was added to the volumetric flask. The mixture was stirred until all sodium salts were dissolved, and 2 g of 3A molecular sieve was added. The mixture was then allowed to stand in the glove box for 12 h to obtain the electrolyte.

[0077] Battery assembly: Coin cells are prepared in a glove box. The positive electrode is the above-mentioned positive electrode sheet with a diameter of 12 mm. The separator is a polypropylene separator. The negative electrode is metallic sodium with a diameter of 16 mm.

[0078] Examples 8-12: Sodium-sulfur batteries

[0079] This embodiment provides a modified sulfur cathode material and its application in sodium-sulfur batteries to improve the performance of sodium-sulfur batteries.

[0080] The preparation method of the S / C composite material is the same as that of Comparative Example 2, and Examples 8 and 9 have different S:C ratios.

[0081] Preparation method of modified sulfur cathode material: The S / C composite materials (1 g) obtained in Examples 8-12 were uniformly dispersed in 10 mL of HNBR solution, where the HNBR sample and solution concentrations are shown in Table 2. NMP was used as the solvent. The resulting mixture was subjected to two heating treatments at the temperatures and times shown in Table 2, with air as the reaction atmosphere. The resulting suspension was centrifuged, and the solid obtained after centrifugation was dried at 60 °C for 12 h to obtain the modified sulfur cathode material.

[0082] The modified sulfur cathode material obtained was used as the cathode material, and the cathode preparation, electrolyte preparation and battery assembly processes were kept the same as those in Comparative Example 2.

[0083] Comparative Example 3: Magnesium-Sulfur Battery

[0084] The preparation method of the S / C composite material is the same as that of Comparative Example 1, except that the carbon material is replaced with Ketjen Black.

[0085] The S / C composite material obtained above was used as the positive electrode material to prepare a positive electrode sheet. The preparation method of the positive electrode sheet was the same as that of Comparative Example 1.

[0086] The electrolyte was prepared as follows: 5.85 g of Mg(TFSi)2 was weighed into a 10 mL volumetric flask in an argon-protected glove box (H2O and O2 concentrations were both less than 0.01 ppm), and 10 mL of DOL / DME (volume ratio 1:1) mixture was added to the volumetric flask. The mixture was stirred until all the magnesium salt was dissolved, and 2 g of 3A molecular sieve was added. The mixture was then allowed to stand in the glove box for 12 h to obtain the electrolyte.

[0087] Battery assembly: Coin cells are prepared in a glove box. The positive electrode is the above-mentioned positive electrode sheet with a diameter of 12 mm. The separator is a polypropylene separator. The negative electrode is metallic magnesium with a diameter of 16 mm.

[0088] Examples 13-17: Magnesium-sulfur batteries

[0089] This embodiment provides a modified sulfur cathode material and its application in magnesium-sulfur batteries to improve the performance of magnesium-sulfur batteries.

[0090] The preparation method of the S / C composite material is the same as that of Comparative Example 3.

[0091] Preparation method of modified sulfur cathode material: The S / C composite materials (1 g) obtained in Examples 13-17 were uniformly dispersed in 10 mL of HNBR solution, where the HNBR parameters and solution concentration are shown in Table 2, and NMP was used as the solvent. The resulting mixture was subjected to two heating treatments at the temperatures and times shown in Table 2, with air as the reaction atmosphere. The resulting suspension was centrifuged, and the solid obtained after centrifugation was dried at 60 °C for 12 h to obtain the modified sulfur cathode material.

[0092] The modified sulfur cathode material obtained was used as the cathode material, and the cathode preparation, electrolyte preparation and battery assembly processes were kept the same as those in Comparative Example 3.

[0093] The batteries obtained from the comparative examples and embodiments described above were subjected to electrochemical tests. The electrochemical test methods are as follows: Constant current charge-discharge testing was performed using a LAND test system (CT2001A) with a test current of 0.2C. Electrochemical impedance spectroscopy (EIS) analysis was performed using an Ivium electrochemical workstation with a frequency range of 0.01 Hz–100 kHz and an amplitude of 5 mV. Cyclic voltammetry (CV) testing was performed using an Ivium electrochemical workstation with a scan rate of 0.1 mV·s. -1 The voltage testing windows for lithium-sulfur, sodium-sulfur, and magnesium-sulfur batteries are 1.7–2.8 V, 0.5–3.0 V, and 0.5–2.5 V, respectively. All electrochemical tests were conducted at room temperature (25°C).

[0094] The test results are shown in Table 2. The sulfur content in the sulfur-carbon composite material in Table 2 was determined by thermogravimetric analysis under the following conditions: air atmosphere, heating rate 10℃ / min; specific capacity was calculated using sulfur as the positive electrode active material. The method for determining the HNBR content in the products obtained in Examples 1-17 was as follows: First, the nitrogen content in each final product was determined (organic elemental analyzer, UNICUBE, Germany, Research Dog testing platform); then, the nitrogen content of each sample (HNBR) was calculated from the ACN content, from which the HNBR content in the final product could be calculated.

[0095] Table 2 Comparison of electrochemical performance of the three types of battery systems in Examples 1-17 and Comparative Examples 1-3

[0096] Example 14: Mechanism Analysis

[0097] 1. Sample preparation

[0098] In the preparation of modified sulfur cathode materials, the reaction between sulfur and HNBR occurs at the interface between the S / C composite material and the HNBR coating layer, making its characterization extremely difficult. This manifests in two ways: ① the reaction interface has a small proportion, potentially exceeding the detection limit of the testing method, resulting in a weak signal; ② the outer HNBR layer interferes with the S-HNBR formed at the interface, further weakening the signal. To address this issue, sublimated sulfur powder was used to replace the S / C composite material in the preparation of S-HNBR. The preparation process is consistent with the method described in Example 7 for preparing modified sulfur cathode materials, except that 1 g of the S / C composite material in Example 7 was replaced with 0.5 g of sublimated sulfur powder. After a two-step heating treatment as described in Example 7, the resulting suspension was filtered to obtain filter residue. After drying to remove the solvent, the insoluble phase S-HNBR was obtained (see optical photograph for details). Figure 13 The resulting filtrate was thoroughly mixed with CS2 solvent at a volume ratio of 1:10 (S is soluble in CS2, while HNBR is insoluble in CS2) to remove residual elemental sulfur. The mixture was then filtered to obtain filter residue, which was dried to obtain the solution phase S-HNBR (see optical photograph for details). Figure 13 ).

[0099] 2. Test methods and test results

[0100] (1) Scanning electron microscopy and elemental distribution tests were performed on the two S-HNBR products, which confirmed that both contained S and C and were uniformly distributed. Figure 14 Scanning electron microscope (SEM) images and elemental distribution maps of the insoluble phase S-HNBR products are shown. It can be seen that S and C are uniformly distributed on the products, with an S content of 46% (mass fraction). The S content in the solution phase S-HNBR is 66% (mass fraction).

[0101] (2) Solubility and material analysis: HNBR is soluble in NMP but insoluble in CS2; elemental sulfur is readily soluble in CS2 but insoluble in NMP; while the insoluble phase S-HNBR and the solution phase S-HNBR are insoluble in NMP and CS2, respectively, and both contain S and C. This indicates that a chemical reaction occurred between HNBR and S, producing a new substance.

[0102] (3) Spectral test results

[0103] To determine the chemical bonding between S and HNBR, the S-HNBR samples were characterized using FT-IR, Raman spectroscopy, and NMR techniques. The results are as follows: Figure 15 , 16 As shown in Figure 17.

[0104] FT-IR: Compared to HNBR, the -CH2- bonds in the insoluble phase and the solution phase of S-HNBR (2292 / 2850 cm⁻¹) -1 ) and -CN key (2236 cm -1 The decrease was significant, which may indicate that the -CN bond underwent a cyclization reaction with the adjacent CC-CN bond, and a new CS bond (465 cm) also appeared. -1 This indicates that HNBR underwent sulfidation, resulting in the formation of S-HNBR.

[0105] Raman: Both the insoluble and soluble phases of S-HNBR exhibit distinct SS (471 cm⁻¹). -1 ) and CSSC (437 cm) -1 )Signal.

[0106] 13 Solid-state NMR: Fitting the NMR spectrum of the insoluble S-HNBR sample confirmed the presence of CS bonds (44 ppm). The solution-phase S-HNBR is in a rubbery state and cannot be subjected to solid-state NMR testing.

[0107] (4) Explanation of the sulfidation mechanism

[0108] Based on the results of solubility analysis, EDS, FT-IR, Raman spectroscopy, and NMR, it can be confirmed that S and HNBR are chemically bonded. There are two possible ways for the CS bond to form: ① S on or near the surface of the S / C matrix bonds with the residual C=C double bonds in the HNBR, forming a CS bond. Considering that the hydrogen saturation of sample 1 is above 99%, this method is less common. ② S on or near the surface of the S / C matrix induces a chain dehydrogenation reaction at high temperature, involving the ortho-hydrogen of the methylene (-CH2-) or cyano group in the HNBR, resulting in a cyclization reaction. This reaction is commonly seen in the vulcanization process of polyacrylonitrile (PAN). The mechanism is as follows: elemental sulfur (S8) decomposes into short-chain sulfur species (such as S2 or S3) at high temperatures. These reactive sulfur intermediates undergo dehydrogenation reactions with hydrogen atoms on the PAN chain, resulting in the removal of hydrogen atoms from the acrylonitrile unit and the formation of unsaturated bonds. Subsequently, the dehydrogenated segments undergo intramolecular cyclization reactions to generate heterocyclic structures, such as polypyridine rings. This process is accompanied by the formation of a conjugated system, transforming the backbone from a linear structure to a ladder polymer or a two-dimensional network structure, thereby enhancing thermal stability and mechanical strength. Simultaneously, short-chain sulfur bonds (such as -S2- and -S3-) are covalently bonded to the carbon backbone to form CS and SS bonds. Sulfur atoms are connected in the form of covalent bonds, replacing some cyano (-C≡N) groups to generate thiocyclic heterocyclic structures. FT-IR spectroscopy can confirm the weakening of the -CN bond and the formation of the CS bond. From the FT-IR spectrum, it can be seen that the -CN peak of the insoluble phase S-HNBR disappears or is significantly weaker than that of HNBR, indicating that the -CN bond of HNBR is transformed in the presence of S. Raman spectroscopy confirmed the presence of SS and CSSC bonds. NMR results confirmed the presence of CS bonds.

[0109] The innovative features and performance of this invention will be further explained below with reference to the accompanying drawings.

[0110] Figure 1 This is a schematic diagram of the modified sulfur cathode material. The overall structure is a core-shell structure, with S / C particles as the core and a uniformly coated HNBR layer as the outer layer. The HNBR at the core-shell interface has undergone sulfidation to form S-HNBR.

[0111] Figure 2-4 TEM images of the modified sulfur cathode materials prepared in Examples 1, 14, and 15 are shown. The images show a uniform coating layer with a thickness of approximately 10 nanometers formed on the outer surface of the S / C particles.

[0112] Figure 5-7 SEM images of the materials and electrodes prepared in Comparative Example 1 and Example 1 were compared. Figure 5It can be seen that the S / C particles prepared from sublimated sulfur and activated carbon YP50F are relatively uniform in size, with no obvious agglomeration; while the particle size of the modified sulfur cathode material is significantly larger, indicating that the S / C particles are more tightly linked after HNBR coating and sulfidation. This structure can better connect the conductive agent and binder during the cathode preparation process, which is beneficial to improving the mechanical strength and integrity of the cathode. Figure 6 It can be seen that the electrodes made of modified sulfur cathode material are connected by special filaments, which is significantly different from the structure of S / C electrodes without HNBR, and can be attributed to the effect of the HNBR coating layer. From Figure 7 It can be seen that the particles in the modified sulfur cathode material coating are more tightly connected, and the contact with the carbon-coated aluminum foil current collector is significantly better than that of the S / C coating.

[0113] Figure 8 The first-week charge-discharge curves of the batteries prepared in Comparative Example 1 and Example 1 are shown. It can be seen that the battery using the modified sulfur cathode material has a higher capacity than the battery using the unmodified S / C composite material as the cathode material, especially with a significantly increased capacity contribution in the high-potential discharge range. This can be attributed to the HNBR coating layer effectively suppressing the loss of active material. Furthermore, the overpotential of the battery using S / C@S-HNBR material as the cathode material is significantly lower than that of the battery using S / C material as the cathode material, mainly due to improvements in electrode structure and kinetics. Figure 9 The EIS plots are for two batteries prepared in Comparative Example 1 and Example 1. It can be seen from the plots that the charge transfer impedance of the S / C@S-HNBR electrode is significantly lower than that of the S / C electrode.

[0114] Figure 10 The cyclic voltammograms are for batteries using the materials described in Examples 1 and 2 as cathode materials. The difference between the two materials is that the concentration of HNBR solution used in Example 2 is higher, and the HNBR content in S / C@S-HNBR is much higher than that in Example 1. The cyclic voltammograms show that the material obtained in Example 1 has higher current intensity and higher reversible capacity, indicating that a higher HNBR content is not necessarily better.

[0115] Figure 11 The image shows the cyclic voltammogram of the battery obtained in Example 7.

[0116] Table 2 comprehensively presents the preparation conditions, chemical composition, and core performance of Comparative Examples 1-3 and Examples 1-17. The table shows that, in all three metal-sulfur battery systems, the modified sulfur cathode material significantly outperforms the unmodified S / C material, maintaining higher initial discharge capacity and capacity retention. This demonstrates that the modified sulfur cathode material improves the specific capacity and cycle stability of the metal-sulfur battery system, exhibiting significant development potential and potentially providing new solutions and ideas for future widespread application.

Claims

1. A modified sulfur cathode material for metal-sulfur batteries, characterized in that: The modified sulfur cathode material comprises an S / C composite material and an HNBR layer uniformly coated on the surface of the S / C composite material; the modified sulfur cathode material for metal-sulfur batteries is prepared by a method comprising the following steps: (1) Obtain S / C composite powder; (2) The S / C composite material powder is uniformly dispersed in 3-20wt% HNBR adhesive solution. The resulting mixture is first treated at 30-80℃ for 5-60 minutes, then heated at 170-200℃ for 20-60 minutes, and finally separated and dried to obtain the modified sulfur cathode material.

2. The modified sulfur cathode material for metal-sulfur batteries as described in claim 1, characterized in that: In the modified sulfur cathode material, the mass content of HNBR is 1.8%-10.2%.

3. The modified sulfur cathode material for metal-sulfur batteries as described in claim 2, characterized in that: The metal-sulfur battery is a lithium-sulfur battery, and the HNBR mass content in the modified sulfur cathode material is 4.5-10.0%.

4. The modified sulfur cathode material for metal-sulfur batteries as described in claim 2, characterized in that: The metal-sulfur battery is a sodium-sulfur battery, and the HNBR content in the modified sulfur cathode material is 4%-6%.

5. The modified sulfur cathode material for metal-sulfur batteries as described in claim 2, characterized in that: The metal-sulfur battery is a magnesium-sulfur battery, and the HNBR mass content in the modified sulfur cathode material is 2.0-10.2%.

6. The modified sulfur cathode material for metal-sulfur batteries as described in any one of claims 1-5, characterized in that: The HNBR contains 18%~43% acrylonitrile repeating units by mass, has an iodine value of ≤27, and a Mooney viscosity value ML(1+4)@100℃ of 40-49.

7. A method for preparing a modified sulfur cathode material for metal-sulfur batteries as described in any one of claims 1-6, comprising the following steps: (1) Obtain S / C composite powder; (2) The S / C composite material powder is uniformly dispersed in 3-20wt% HNBR adhesive solution. The resulting mixture is first treated at 30-80℃ for 5-60 minutes, then heated at 170-200℃ for 20-60 minutes, and finally separated and dried to obtain the modified sulfur cathode material.

8. The preparation method according to claim 7, characterized in that: The solvent of the HNBR adhesive is at least one of N-methylpyrrolidone, methyl isopropyl ketone, and xylene, and the mass concentration of HNBR in the HNBR adhesive is 5-10 wt%.

9. The preparation method according to claim 7, characterized in that: In step (2), the first step processing temperature is 50-70℃ and the first step processing time is 15-60 minutes; the second step processing temperature is 170-200℃ and the second step processing time is 20-40 minutes.

10. The preparation method according to claim 9, characterized in that: In step (2), the first step processing temperature is 60℃ and the first step processing time is 15 minutes.

11. A metal-sulfur battery, comprising a positive electrode, characterized in that: The cathode comprises the modified sulfur cathode material according to any one of claims 1-6.

Images