Anode-Driven <i>In Situ</i> -Formed Electrolytes Enable Rechargeable Calcium Metal Batteries

Calcium’s high reactivity typically causes surface passivation, limiting its applicability in electrochemical devices, such as batteries. Here we harness this very reactivity to generate an electrolyte both ex situ and, most importantly, in situ, by reacting Ca metal with a Ca-free precursor solution comprising tris(2,2,2-trifluoroethyl) borate (B(Otfe)3; Otfe = OCH2CF3) and 2-methoxyethylamine (MOEA) in 1,2-dimethoxyethane (DME). The resulting electrolyte enables reversible Ca electrochemistry in symmetric, half-, and full cells, including at −10 °C. By combining electrochemistry with FTIR spectroscopy, XPS, and DFT calculations, we are able to propose a synergistic mechanism in which B(Otfe)3 reacts at the Ca metal surface to form anionic borate species and a Ca2+-conducting interphase, while the Ca2+ released from the anode is stabilized in the solution/electrolyte by MOEA coordination. This metal-driven, interfacial strategy provides an alternative to the salt-in-solvent paradigm for electrolyte preparation and suggests a broadly applicable route for electrolyte generation directly from reactive-metal anodes.