Ultrastable Calcium Metal Anodes Enabled by a Strongly Coordinated Electrolyte Derived Bilayer Solid Electrolyte Interphase

Abstract Calcium (Ca) metal battery is a promising alternative to current lithium battery chemistry due to the high crustal abundance of Ca element and potentially dendrite‐free cycling of high‐capacity Ca metal anodes. However, reversible Ca metal stripping and plating have been hindered by the lack of effective electrolytes and the formation of obstructive solid electrolyte interphase (SEI) layers. Here a strongly coordinated electrolyte system by incorporating LiB(hfip) 4 into Ca[B(hfip) 4 ] 2 /glyme solutions is introduced. The highly coordinated glyme molecules and B(hfip) 4 − anions are ready to decompose into organic rich compounds and CaH 2 , CaB 2 O 4 nanocrystals in the SEI layers on Ca metal surface. Transmission electron microscopy observations reveal that these ionically conductive inorganic particles are embedded beneath the organic‐rich outer layer, thus forming a bilayer SEI configuration. This unique structure facilitates efficient Ca‐ion transfer while preventing further electrolyte decomposition. Effectiveness of this electrolyte is evidenced by the ultrastable Ca//Ca symmetrical cells (over 1450 h with low potentials of <0.5 V vs. Ca/Ca 2+ at a high current density of 2 mA cm −2 ) and the high‐energy Ca//polyaniline full cells (energy densities of above 200 Wh kg −1 over 200 cycles), which set new benchmarks in the field of room‐temperature Ca metal batteries.