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

ABSTRACT 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.