Supercooled, Low-Melting Lithium Salt Stabilizes Cathode-Electrolyte Interfaces in Sulfide-Based Solid-State Batteries

Abstract All-solid-state batteries (ASSBs) are next-generation energy storage systems with potential advantages in energy density and safety. However, loss of physical contact at the cathode−electrolyte interface during cycling leads to capacity fading. Here, we demonstrate that a supercooled, low-melting lithium salt enables a strategy for stabilizing cathode−electrolyte interfaces in sulfide-based ASSBs. The lithium salt, referred to as a single-cation ionic liquid (SCIL), exhibits a melting point of 76 °C and maintains a supercooled liquid state over periods. Through solvent-free processing at 90 °C, molten SCIL infiltrates interparticle voids without undesirable decomposition reactions, maintaining an ionic conductivity of 1.0 mS cm−1. When applied to composite cathodes, SCIL improves the initial Coulombic efficiency (from 76.8 to 85.2%), capacity retention (from 89.8 to 94.9% after 100 cycles), and rate capability (from 70 to 104 mAh g−1 at 1C). These findings demonstrate that supercooled lithium salts provide an interfacial stabilization strategy for sulfide-based ASSBs.