Potassium Bis(fluorosulfonyl)imide Is an Effective Additive for Improving Anode Cyclability in Sulfolane-Based Electrolytes for Li–S Batteries

Lithium–sulfur (Li–S) batteries are considered ideal candidates for next-generation batteries with high energy densities. However, the practical use of sulfur cathodes is hindered by the dissolution of Li polysulfides in the electrolyte during the charging and discharging processes, resulting in reduced sulfur utilization and enhanced shuttle reactions. The use of a highly concentrated sulfolane-based electrolyte is a solution to this problem in Li–S batteries; however, this electrolyte results in low reversibility of the Li-metal anode. The use of a fluorinated additive in nonaqueous electrolytes is a simple and effective solution for improving the reversibility of Li anodes. Therefore, we attempted to add bis(fluorosulfonyl)imide (FSI) anions, but the additional LiFSI was hardly soluble in the electrolyte. Here, we found that potassium FSI (KFSI) is highly soluble as an electrolyte additive, forming a stable solid electrolyte interface (SEI) at the Li anode. Consequently, the Coulombic efficiency of Li was significantly improved from 87.5 to 98.8% by preparing a sulfolane-based electrolyte containing an optimal amount of 7 wt % KFSI. The improved Li deposition morphology, lower interfacial impedance, and large amounts of Li fluoride in the SEI were identified as factors contributing to the improved reversibility of the Li anode. The electrolyte containing KFSI also plays an important role in improving the cycle stability of Li–S cells. These results indicate that KFSI is an effective additive for the protection of Li-metal anodes and the cycling stability of Li–S batteries.