Lithium metal batteries (LMBs) offer exceptional theoretical energy density and an ultra-low reduction potential, making them a leading candidate for next-generation energy storage. However, challenges such as dendritic lithium growth and electrolyte instability hinder their commercial viability by causing capacity decline and safety risks. This study presents an electrolyte formulation based on a single-salt, single-solvent system of lithium bis(fluorosulfonyl)imide (LiFSI) in diethylene glycol diethyl ether (DEGDEE). The key advantage of this system stems from a unique, anion-participating solvation structure, engineered through the molecular design of the DEGDEE solvent. This structure, particularly at an optimized concentration of 1.75 M LiFSI in DEGDEE, facilitates the formation of protective layers on both the anode and cathode that effectively stabilize interfacial side-reactions, leading to a significant enhancement in cycle life. The resulting Li||Cu cells exhibit an average Coulombic efficiency of ~98% at both 25 °C and 60 °C, and Li||Li symmetric cells demonstrate ultra-stable cycling for over 1500 h with a minimal polarization of ~0.02 V. When paired with practical LiFePO4 cathodes, the full cell achieves a specific capacity of 147 mAh g−1 attaining 85.4% capacity retention over 1000 cycles at 25 °C and 163 mAh g−1 with 95.7% over 200 cycles at 60 °C, all while maintaining a high efficiency (99.8%) at 1.0 C. This work demonstrates that engineering the Li+ solvation structure through rational solvent design provides a powerful strategy for creating highly stable interfaces, advancing LMBs toward practical, high-performance energy storage.
