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

Abstract

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_0.8Co_0.1Mn_0.1O₂ (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.