Water‐Mediated SEI in Fluorinated Alkoxylborate Electrolytes for Long‐Cycling Calcium Metal Anodes

The practical implementation of calcium metal anodes in rechargeable batteries is persistently hindered by surface passivation, which impedes Ca 2+ transport and induces inhomogeneous Ca deposition. Herein, we demonstrate that introducing a tailored trace amount of water (∼139 ppm) fundamentally modifies the interfacial chemistry in a state‐of‐the‐art Ca[B(hfip) 4 ] 2 /DME electrolyte. Mechanistic investigations reveal that instead of triggering deleterious bulk side reactions, the trace water stably enters Ca 2+ solvation sheath, directing a controlled electrochemical hydrolysis pathway during cycling. This process generates a thin, uniform bilayer solid electrolyte interphase (SEI) comprising an outer hybrid organic‐inorganic layer and an inner inorganic layer rich in CaH 2 , CaO, and CaF 2 , while concurrent H 2 evolution effectively eliminates the passivating native oxide layer. Consequently, this tailored SEI enables highly stable Ca plating/stripping for over 400 h at 0.2 mA cm −2 and over 300 h at 1 mA cm −2 . Furthermore, the high anodic stability of the electrolyte enables reliable operation of various high‐voltage cathodes within a wide voltage window (up to 4.5 V) for up to 70 cycles. These results highlight the critical role of a stable SEI for Ca metal anodes, but also illustrate how subtle electrolyte modification can profoundly regulate the interfacial chemistry toward high‐performance multivalent batteries.