Molten salt electrolytes with enhanced Li + -transport kinetics for fast-cycling of high-temperature lithium metal batteries

Ideal high-temperature lithium metal battery (LMB) electrolytes should have good thermal stability and compatibility with highly reactive cathodes/anodes. Yet, conventional liquid electrolytes usually show severe degradation and substantial safety risks at high temperatures due to the presence of unstable organic solvents. Herein, we report a solvent-free molten salt electrolyte (SFMSE) composed solely of alkali bis(fluorosulfonyl)imide salts, enabling LMB's high thermal tolerance and fast-cycling ability. In addition to non-volatility and non-flammability, the designed Li–Cs electrolyte shows low activation energy and high Li + conductivity owing to the strong cation–cation concerted effect, thus exhibiting fast Li + transport kinetics and excellent electrochemical performance at practical capacities. The electrolyte/electrode interphases derived exclusively from anions are inorganic-rich with great robustness, as evidenced by the high Coulombic efficiency of 98.8% for Li plating/stripping and the stable cycling performance of LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) cells. As a result, the Li–Cs electrolyte achieves a discharge capacity beyond 100 mA h g −1 for NCM811/Li half cells at 10C, and a record capacity retention of 75% for NCM811 (3 mA h cm −2 )/Li (40 μm) full cells after 280 cycles at 80 °C. The proposed molten salt electrolyte could inspire further advancements in high-energy-density, high-safety, and high-temperature lithium metal batteries.