TUNING ELECTROLYTE COMPOSITION FOR ENHANCED PERFORMANCE OF LITHIUM-SULFUR BATTERIES

Abstract

As the world faces a growing need to electrify and reduce carbon emissions, batteries offer much needed energy storage for electric cars, mobile devices, and the grid. The lithium-sulfur (Li-S) battery combines low weight, non-toxicity, low cost, and high capacity. With one of the highest theoretical gravimetric capacities (1675 mAh/g-S) of any conversion-type cathode, the pursuit of low cost, long-lasting Li-S batteries is a global research focus. However, it is practically difficult to attain full capacity because of the polysulfide dissolution and subsequent reaction at the Li anode surface, depleting the active material in the cathode. Current electrolytes are not effective at managing the polysulfide dissolution and have the negative side effects of high viscosity and high cost. In this work, low concentration electrolytes were investigated as a possible solution to these challenges. Low concentration electrolytes offer low viscosities, which ease access to sulfur in tortuous cathodes. First, the low concentration regime (<0.2M) was applied to traditional electrolyte salts and solvents: LiTFSI, dimethoxyethe, and dioxolane. The low viscosity and enhanced wettability of such an electrolyte system enabled strong cycling performance as well as better access to active sulfur materials in high loading cathodes. In order to further limit polysulfide dissolution in Li-S cells, an electrolyte solvent with very low polysulfide solubility (1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (HFE)) was explored. Sulfolane was used as a co-solvent to the dissociate the Li salt and provide Li+ ion transport. By gradually altering the solvent ratio, we discovered a change in discharge behavior as the proportion of HFE increases. Polysulfide solubility tests plus computational modeling suggest dramatic suppression of the long-chain polysulfide formation, shifting the discharge to a quasi-solid-state mechanism. This work demonstrates the promising performance characteristics of low concentration electrolytes for S cathodes and provides new scientific insights into the lithium-sulfur discharge mechanism. When paired with high loading cathodes and polysulfide-suppressing solvents, low concentration electrolytes may enable lightweight batteries with high mass loadings, thus showing multiple avenues for future practical applications.