Tetrabutylammonium-Based Organosulfurs: A New Family of Catalysts for Lithium-Sulfur Batteries

Lithium-ion batteries hold a prominent position in the market for personal electronic devices, electric vehicles and are promising options for grid storage systems. The need for increased energy density underscores the significance of optimizing cathode materials. Among these materials, sulfur stands out for its exceptional promise. It boasts a remarkably high theoretical specific capacity and energy density, at 1,675 mAh/g and approximately 2,600 Wh/kg, respectively, over five times the energy density of current Lithium-ion batteries. Moreover, a sulfur cathode offers potentially increased safety due to lower battery voltage (around 2.4 V vs. Li⁰/Li⁺), reduced cost, and lower toxicity than some existing cathode materials.

However, there are challenges to be overcome. Typically, Li-S batteries employ a lithium metal anode, liquid organic electrolyte, and composite sulfur cathode. During charge/discharge, sulfur and the polysulfide intermediates exhibit high resistance, and some species are insoluble in the electrolyte. They also exhibit slow reaction rates due to their reaction mechanism, leading to limited long-term cycling. Various approaches are being pursued to address these, one of which involves catalysts. Previous research has focused on utilizing catalysts to enhance the reaction rate of polysulfide intermediates. These catalysts encompass metal oxides, sulfides, phosphides, and nitrides, which either absorb or convert the polysulfide intermediates.

For the first time, we identified a family of compounds with and without organosulfur anions as catalysts for Li-S batteries. The concept of 'families' establishes a structural baseline, enabling the observation of the effects of minor differences in molecules within these families, such as variations in elements or their quantities. Tetrabutylammonium-based compounds are soluble in traditional Li-S battery electrolytes, and their addition to the electrolyte enhances the cell's performance. These catalysts have been shown to significantly improve cycling performance, with enhancements of up to 52% observed after 50 cycles at C/5 (5 hours of charge and 5 hours of discharge) and a high coulombic efficiency of 98%. Insights into the mechanism have been proposed using operando UV-VIS and Raman spectroscopy, in addition to electrochemical methods. This novel family of catalysts paves the way for further optimization of Li-S batteries, bringing them closer to practical applications.