Enhancement of Phosphotungstic Acid Cathode for Lithium Sulfur Battery

At present, lithium-sulfur battery is a kind of battery system with high energy density. Its theoretical energy density is as high as 2600Wh/Kg, which is much higher than other lithium-ion battery systems. Usually, lithium sulfur battery uses sulfur or sulfur based composite as positive electrode and lithium metal as negative electrode. Lithium-sulfur batteries have become a hot spot in battery research because of their high theoretical specific capacity (1675 mAh / g), low price, abundant reserves and relatively friendly environment.

Phosphotungstate materials (H₃PW₁₂O₄₀) have high thermal stability, diversity in structure and composition, reversible and continuous multi-electron redox, transport and storage of electrons and protons, and provide a stable structure for the charge-discharge cycle of sulfur cathode materials. In view of the tremendous role of phosphotungstic acid, some scholars added it to the cathode of lithium-sulfur batteries to prepare nano-sphere phosphotungstate/sulfur cathode materials. The steps are as follows:

(1)Synthesis of nano-sphere phosphotungstate: inorganic salt solution and phosphotungstic acid aqueous solution were mixed, reacted at a certain temperature and stirring speed, reacted for a certain time, and then cooled, filtered, washed and dried to obtain solid-phase nano-sphere phosphotungstate material. The inorganic salts are potassium chloride, potassium nitrate, potassium acetate, potassium sulfate and ammonium chloride.

(2)Synthesis of nano-sphere phosphotungstate/sulfur composites: The nano-sphere phosphotungstate/sulfur composites were prepared by mixing the nano-sphere phosphotungstate and sulfur monomer in ball milling, and the sulfur monomer was dispersed into the pores and surfaces of the nano-sphere phosphotungstate/sulfur composites by melting method. The reaction temperature is 40-80 C, the stirring speed is 300-800r/ min, and the reaction time is 4h-12h.

(3)Preparation of electrode materials: nano-sphere phosphotungstate/sulfur composite material, conductive agent, binder dispersed in the solvent, stirred evenly, formed slurry, coated on the collector, dried, sliced.

It was found that the H₃PW₁₂O₄₀ nanospheres possessed a variety of structures and compositions, high thermal stability, reversible and continuous multi-electron redox, transport and storage capacity for electrons and protons, and provided a stable structure for the charge-discharge cycle of sulfur cathode materials. It was also found that nano-sphere phosphotungstate material (H₃PW₁₂O₄₀ ) possessed abundant pore structure and high porosity compared with the existing porous carbon materials, which could effectively increase the sulfur content in the positive electrode. Additionally, Bronstein acidity had strong chemisorption effect on polysulfide ions, and abundant pore structure possessed sulfur possession. With physical restriction, the cycle life and rate performance of lithium sulfur battery are effectively improved.

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