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 Ca²⁺ 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 Ca²⁺ 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 CaH₂, CaO, and CaF₂, while concurrent H₂ 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.