Ether‐Oxygen‐Functionalized Ionic Liquid Electrolytes Enabling Intrinsically Safe 535 Wh Kg −1 Lithium Metal Batteries

High‐energy‐density lithium metal batteries (LMBs) suffer from the flammability of carbonate electrolytes and the high reactivity of lithium metal anodes. Ionic liquids (ILs) offer a nonflammable alternative, but their high viscosity and sluggish Li + transport hinder practical use. Herein, we introduce an ether oxygen (C‐O‐C) group into the alkyl side chain of a pyrrolidinium‐based IL cation. This molecular‐level modification weakens cation–anion interactions, lowers viscosity, and enhances ionic conductivity. Concurrently, the solvation structure shifts from large anion aggregates (AGGs) to uniform contact ion pairs (CIPs), accelerating Li + transport and promoting a robust, inorganic‐rich solid electrolyte interphase (SEI). The resulting electrolyte enables a Li||LiNi 0.9 Co 0.05 Mn 0.05 O 2 (NCM90) coin cell to retain 82.1% of its capacity after 240 cycles at 1 C, and a 3.8 Ah pouch cell to achieve an energy density of 535.3 Wh kg −1 while passing nail penetration and overcharge tests without thermal runaway. Accelerating rate calorimetry (ARC) measurements show that both the self‐heating onset temperature (T 1 ) and the thermal runaway threshold temperature (T 2 ) exceed those of carbonate electrolytes by more than 1.5 times. This work establishes a clear structure–property–performance relationship and provides molecular‐level guidance for designing low‐viscosity, nonflammable IL electrolytes toward high‐energy‐density and intrinsically safe LMBs.