A sulfur cathode material and its preparation method

Abstract

This invention discloses a sulfur-containing cathode material and its preparation method, belonging to the field of lithium-sulfur battery technology. The method uses a quaternary ammonium polymer containing nitrate as a cathode binder, dispersed with a conductive agent and a sulfur-containing cathode material in deionized water to obtain a cathode slurry. The slurry is then coated onto aluminum foil and vacuum dried to obtain a cathode sheet. The quaternary ammonium cation backbone in the binder can adsorb polysulfides, improving lithium-ion migration capacity, while simultaneously preventing the rapid and uncontrollable consumption of nitrate, thereby extending the lifespan of the lithium-sulfur battery.

Description

Technical Field

[0001] This invention discloses a sulfur cathode material and its preparation method, belonging to the field of lithium-sulfur battery technology. Background Technology

[0002] Lithium-sulfur batteries have an extremely high theoretical energy density (2600Wh / kg). -1 The energy density of lithium-ion batteries is far higher than that of currently commercialized lithium-ion batteries, making them one of the most promising next-generation battery systems that combine cost-effectiveness and high energy density. Furthermore, due to their low cost and environmental friendliness, they have great potential for application in large-scale energy storage devices and power grids. However, the commercial application of lithium-sulfur batteries is limited by the dissolution of discharge products (polysulfides) during charging and discharging, and the subsequent shuttle effect. The dissolution of polysulfides leads to many problems: (1) high self-discharge rate and low coulombic efficiency; (2) polysulfides can shuttle to the surface of the negative electrode and easily react with the lithium negative electrode, damaging it. To overcome the above problems, improvements have been made to the binders, separators, electrolytes, and electrode interfaces of lithium-sulfur batteries. For example, patent CN 113224309 B discloses a lithium-sulfur battery binder of a cross-linked network polymer containing side chains of thiol functional groups; patent CN 114464954 A discloses an MXene@WS2 heterostructure material for lithium-sulfur batteries and its application; patent CN 110137572 A discloses the application of terthiophene as an electrolyte additive for lithium-sulfur batteries; and patent CN 112409976 A discloses a method for forming a solid electrolyte using sulfonic acid-modified polyurethane to protect the lithium anode of lithium-sulfur batteries.

[0003] Commonly used binders can mechanically bond active materials and conductive carbon additives together to maintain the integrity of the macroscopic structure of the battery electrode, but they only play a single bonding role. During the cycling process of lithium-sulfur batteries, polysulfides continuously accumulate and dissolve in the electrolyte. Since traditional sulfur cathode binders lack the ability to confine polysulfides, they cannot effectively improve the performance of sulfur cathodes. Therefore, developing a dual-functional, low-cost polymer binder that can capture polysulfides and prevent their loss from the sulfur cathode is one of the most effective means to improve the capacity of lithium-sulfur batteries. Summary of the Invention

[0004] The purpose of this invention is to solve the problem by providing a sulfur cathode material and its preparation method. In this method, the cationic backbone of the binder can adsorb polysulfides and simultaneously improve lithium-ion migration ability; by immobilizing nitrate ions in the polymer, the positive effects of nitrate ions on the cathode of lithium-sulfur batteries can be better utilized, while avoiding the rapid and uncontrollable consumption of nitrate ions.

[0005] The technical solution adopted by the present invention to solve the above problems is as follows: a sulfur positive electrode material includes a sulfur-containing active material, a conductive agent, and a binder, wherein the binder is a quaternary ammonium polymer containing nitrate, polydiallyldimethylammonium nitrate (PDDANO3); the sulfur-containing active material, the conductive agent, and the binder are connected by a mixed crosslinking.

[0006] The steps for preparing the sulfur cathode material are as follows:

[0007] The PDDANO3, conductive agent, and sulfur-containing cathode material are dispersed in deionized water at a mass ratio of 0.5-2:1-4.2:5-8.5 at room temperature using ball milling or stirring to obtain a sulfur cathode material slurry. The slurry is then coated onto aluminum foil using roller coating or spray coating and dried at 50-130°C for 8-24 hours to obtain the sulfur cathode material.

[0008] The molecular weight of the PDDANO3 is 4×10⁴ to 3×10⁶.

[0009] The positive electrode slurry has a solid content of 10-30%;

[0010] The conductive agent is one or more of carbon nanotubes, graphene, conductive carbon black, and acetylene black.

[0011] The sulfur-containing active material is one or more of elemental sulfur, carbon-sulfur composite materials, and organic sulfur materials;

[0012] The thickness of the sulfur cathode material coating is 40–300 μm.

[0013] The beneficial effects of this invention are:

[0014] (1) The cationic backbone in the binder can adsorb polysulfides and improve the lithium ion migration ability. By fixing nitrate in the polymer, the positive effects of nitrate on the positive and negative electrodes of lithium-sulfur batteries can be better utilized, while avoiding the rapid and uncontrollable consumption of nitrate.

[0015] (2) The binder has strong confinement and conversion capabilities for polysulfides, effectively suppressing problems such as polysulfide ion shuttle effect. The polymer backbone is composed of strongly cationic quaternary ammonium ions, which can suppress the effects of polysulfide shuttle; NO3- as anion provides additional redox sites to improve the kinetics of the positive electrode reaction, promote the conversion of soluble polysulfides to elemental sulfur during oxidation, and greatly improve the utilization rate of active materials. Attached Figure Description

[0016] Figure 1 Nuclear magnetic resonance spectra of the prepared nitrate-containing quaternary ammonium polymer

[0017] In the figure: (a) is the NMR spectrum of PDDA, and (b) is the NMR spectrum of the prepared nitrate-containing quaternary ammonium polymer.

[0018] The X-axis represents chemical shift in ppm; the Y-axis represents peak intensity in au.

[0019] Figure 2 Infrared spectrum of the prepared nitrate-containing quaternary ammonium polymer

[0020] In the figure: — represents the infrared characterization of the prepared nitrate-containing quaternary ammonium polymer, … represents the infrared characterization of PDDA, and — represents the infrared characterization of silver nitrate.

[0021] The X-axis represents the wave number, in cm. -1 The Y-axis represents the transmittance of the signal, expressed as a percentage.

[0022] Figure 3 Adsorption properties of different substances for polysulfides

[0023] In the figure: ■ represents a polysulfide solution, ● represents a polysulfide solution containing the prepared quaternary ammonium polymer with nitrate ions, and ▲ represents a polysulfide solution with LA133 added;

[0024] The X-axis represents wavelength, measured in nm; the Y-axis represents signal strength, measured in au.

[0025] Figure 4 Cross-sectional SEM image of sulfur positive electrode sheet

[0026] In the figure: (a) LA133 is used as a binder, and (b) the prepared nitrate-containing quaternary ammonium polymer is used as a binder.

[0027] Figure 5 Cycle performance diagram of the prepared nitrate-containing quaternary ammonium polymer lithium-sulfur battery cathode material

[0028] In the figure: ▲● are the battery discharge specific capacity curves of the nitrate-containing quaternary ammonium polymer and LA133 respectively, and △○ are the battery coulombic efficiency curves of the nitrate-containing quaternary ammonium polymer and LA133 respectively.

[0029] The X-axis represents the number of cycles, in units of 1; the left Y-axis represents the discharge specific capacity, in units of mAh / g; the right Y-axis represents the coulombic efficiency, in units of %. Detailed Implementation

[0030] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0031] This invention designs and prepares a novel quaternary ammonium cationic polymer containing nitrate ions—poly(N,N-diallyl-N,N-dimethyl)ammonium nitrate (PDDANO3)—through a simple ion exchange method. When applied as an aqueous binder for sulfur-containing cathode materials, the strongly cationic quaternary ammonium ions in the polymer backbone can limit the migration of polysulfides through electrostatic interactions, suppressing the effects of polysulfide shuttle; furthermore, it can promote lithium-ion diffusion and improve lithium-ion migration ability. NO3, as an anion... - This provides additional redox sites to improve the kinetics of the positive electrode reaction, promoting the conversion of soluble polysulfides to elemental sulfur during oxidation and greatly improving the utilization rate of active materials. Due to the common ion effect, nitrate ions in the positive electrode binder reduce the consumption rate of nitrate-containing additives (lithium nitrate) in the electrolyte. The role of lithium nitrate in the battery is to react with the lithium anode surface to form an SEI layer, and this reaction is dynamic. Therefore, the reduced consumption rate of lithium nitrate allows the SEI formation to be maintained for a longer period. The common ion effect also slows down the consumption of lithium nitrate in the electrolyte, thus forming a long-term stable SEI layer on the lithium anode surface, effectively protecting the lithium anode and improving coulombic efficiency. Based on these multiple effects, this polymer can effectively improve the overall performance of lithium-sulfur batteries.

[0032] Example 1

[0033] Poly(N,N-diallyl-N,N-dimethyl)ammonium nitrate was prepared by anion substitution reaction. Its NMR and IR spectra were compared with those of polydiallyldimethylammonium chloride. Figure 1 and Figure 2 As shown in the figure, the NMR spectrum indicates that the polymer backbone structure was maintained before and after anion exchange. Simultaneously, due to the influence of nitrate, all peak shapes shifted, indicating the occurrence of the anion exchange reaction. Furthermore, surface chemical identification of the reaction products using infrared spectroscopy revealed the presence of characteristic peaks for both the polymer backbone and nitrate. Therefore, it can be determined that NO3-... - It was immobilized on the cationic polymer backbone of PDDA. The adsorption effect of PDDANO3 on polysulfides was investigated through static adsorption experiments. Figure 3 The UV-Vis spectrophotometric results show that after standing for 12 hours, the S of PDDANO3... x 2- The characteristic peak intensity of (4≤x≤8) disappeared, while the characteristic peak of polysulfides was still present in the solution containing LA133, which further reflects that PDDANO3 has a stronger chemical adsorption effect on polysulfides.

[0034] PDDANO3 was used as the positive electrode binder, mixed with conductive carbon black Super P and elemental sulfur at a mass ratio of 1:3:6 at room temperature, and dispersed evenly in deionized water. The mixture was then ball-milled to obtain a positive electrode slurry. The slurry was then coated onto an aluminum foil current collector by roller coating and dried to obtain a lithium-sulfur battery positive electrode sheet. The electrode coating thickness was 200 μm. The prepared positive electrode sheet was applied to a lithium-sulfur battery using a conventional lithium-sulfur electrolyte, with LA133 used as an aqueous positive electrode binder as a comparison.

[0035] Depend on Figure 4 As can be seen, comparing the sulfur cathode sheets prepared with the two binders, the sulfur cathode material prepared with PDDANO3 is smoother and has better adhesion to aluminum foil. Therefore, compared with LA133, PDDANO3 has stronger adhesion to solid particles and substrate surfaces. Figure 5 The graph shows the cycle performance of the sulfur cathode materials prepared in Example 2 and the comparative example in lithium-sulfur batteries. It can be seen that the lithium-sulfur battery using the sulfur cathode material prepared in Example 2 has an initial discharge specific capacity of 1506 mAh / g, which is much higher than the comparative sample; and it maintains long-term cycle stability, demonstrating good application performance.

Claims

1. A sulfur positive electrode material, characterized by, The sulfur cathode material comprises a sulfur-containing active material, a conductive agent, and a binder; the binder is polydiallyldimethylammonium nitrate (PDDANO3) with nitrate anions chemically bonded to the quaternary ammonium cation backbone; the sulfur cathode material is prepared by the following method: Step 1: Disperse the PDDANO3, conductive agent, and sulfur-containing cathode material in deionized water at a mass ratio of 0.5-2:1-4.2:5-8.5 at room temperature using ball milling or stirring to obtain a sulfur cathode material slurry; Step 2: The slurry is coated onto aluminum foil using a roller coating or spray coating method, and dried at 50-130℃ for 8-24 hours to obtain sulfur cathode material; the structure of sulfur-containing active material, conductive agent and binder is a mixed crosslinking; the PDDANO3 is used to simultaneously realize electrostatic adsorption and redox catalysis of polysulfides.

2. The sulfur cathode material according to claim 1, characterized in that: The PDD ANO3 has a molecular weight of 4 x 10 4 ~ 3 x 10 6 ; The positive electrode slurry has a solid content of 10-30% by mass. The conductive agent is one or more of carbon nanotubes, graphene, and conductive carbon black. The sulfur-containing active material is one or more of elemental sulfur, carbon-sulfur composite materials, and organic sulfur materials; The thickness of the sulfur cathode material coating is 40–300 μm; The sulfur-containing active material, conductive agent, and binder are connected by a mixed crosslinking structure.

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