Solvation‐Mediated Shift From Solvent‐ to Anion‐Derived Solid Electrolyte Interphases for Stable Calcium Metal Anodes

Calcium metal batteries represent a promising next‐generation energy storage device due to the high abundance and high capacity of Ca, but remain almost irreversible in ether‐based electrolytes. By elucidating the Ca 2 + solvation structures, we uncover that these detrimental passivation components originate from thermodynamically unstable Ca 2 + –DME coordination, which promotes the formation of highly reactive solvent‐separated ion pairs (SSIPs). This understanding motivates a co‐solvent design strategy that reshapes the coordination environment through selective solvent capture using ionic liquids. Guided by theoretical screening, 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI) is identified as the most effective additive. In the optimized electrolyte, strong interactions between EMIM + and DME reduce the population of unstable SSIPs, while TFSI − anions are preferentially incorporated into Ca 2 + solvation shells, yielding a contact‐ion‐pair (CIP)‐rich configuration. This fundamentally switches the electrolyte decomposition pathway from solvent‐derived organics to an inorganic‐rich SEI from TFSI − fragments, thus resulting in a denser and ion‐permeable solid‐electrolyte interphase on Ca metal surface. Consequently, Ca||Ca symmetric cells exhibit stable cycling for 160 h with low overpotentials, whereas the base electrolyte fails after the initial cycle. Further improvement is achieved by replacing TFSI − with more compatible OTf − anions, extending stable cycling to 180 h at an overpotential of 0.3 V.