LiFSA and Urea/Dimethylurea Eutectic Electrolytes for Lithium Metal Batteries

Eutectic electrolytes are attracting increasing attention as liquid electrolytes for secondary batteries as they are potentially lower cost, less/nonflammable, and more environmentally benign than the current state-of-the-art. We have successfully developed eutectic electrolytes combining the LiN(SO2F)2 (LiFSA) with urea and dimethyl urea and investigated the physicochemical properties, solution structures, and applicability of these binary and ternary melts to lithium batteries. Solution structure analysis and electrochemical measurements suggest that reducing free solvent enhances anodic stability, while mitigating solvent clustering through hydrogen bonding improves cathodic stability. Moreover, LiFSA concentrated electrolytes of LiFSA/urea/dimethylurea (1:1:1) and LiFSA/dimethylurea (1:2) exhibited excellent compatibility with lithium metal due to the formation of an organic–inorganic hybrid solid electrolyte interphase. These electrolytes enable the reversible plating and stripping of lithium metal, as well as the stable operation of 4 V-class lithium metal batteries.