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.