Isotropic Steric Hindrance of Acyclic Cations Enables Thermally Robust Anion-Derived Interphase for High-Temperature Lithium Metal Batteries at 100 °C

Abstract High-temperature lithium metal batteries are critical for desert energy storage and deep-field exploration, yet the molecular-level understanding of how solvation structure and interphase evolution govern thermal safety remains elusive. Conventional carbonate electrolytes undergo reductive decomposition at the lithium metal anode, while cyclic ionic liquids, constrained by charge-localized, anisotropic cations, fail to regulate high-temperature interfacial chemistry. Herein, we propose an acyclic ionic liquid electrolyte based on the tributylmethylammonium (TBMA+) cation, whose flexible acyclic skeleton and uniformly delocalized charge impart isotropic steric hindrance. Theory and experiments prove that TBMA+ can reshape the solvation structure and build an anion-dominated solvation sheath. This leads to the formation of a compact, anion-derived inorganic-rich SEI at high temperatures, which effectively suppresses solvent decomposition and dendrite growth. This strategy enables Li|NCM90 cells to cycle stably from room temperature to 100 °C. This work establishes cation configuration–interfacial stability relationships, guiding electrolyte design for extreme-condition lithium metal batteries