Revisiting the interfacial chemistry of calcium metal anodes: the importance of inorganic-rich solid/electrolyte interfaces derived from an aggregation-dominated electrolyte†

Metallic calcium (Ca) is a promising anode for rechargeable batteries; however, it is plagued by poor reversibility of Ca²⁺ plating/stripping due to the lack of an idealized solid/electrolyte interface (SEI). This is intrinsically related to the fact that little knowledge is available on species that may be more favourable. Herein, this study reveals that the degradation of native SEIs is attributed to organic-rich species with insufficient electrical insulation, resulting in the continuous decomposition of conventionally used carbonic ester or ether solvents. On this basis, we propose a new insight that regulating the Ca²⁺ solvent sheath to obtain inorganic-rich SEI is a decisive step toward developing reversible Ca metal anodes. With the screening of theoretical calculations, an aggregation (AGG) electrolyte is proposed by involving a small-sized and high-binding-energy anion (BF₄⁻) into the Ca²⁺ solvation sheath to realize the preferential reductive decomposition of anions. By this method, the derived inorganic fluorides and borates improve reversible Ca plating/stripping. Consequently, the Ca‖Ca symmetric cell exhibits a long-cycling stability over 350 h with low polarization. Finally, the density functional theory confirmed that the fundamental mechanism of working the hybrid inorganic-rich SEI is a low diffusion energy barrier and high electronic insulation that ensure fast Ca²⁺ diffusion through the SEI film and reversible plating/stripping on the Ca metal surface.