Electron passivation in CaF₂ on calcium metal anodes

Current electrolytes in calcium-ion batteries suffer from a lack of stability and degradation caused by reduction from the anode. The solid-electrolyte interphase (SEI) that forms on the anodes during operation stems the flow of electrons from the anode to the electrolyte. CaF₂ is a common inorganic compound found in the SEI, and is derived from electrolyte salts such as Ca(PF₆)₂. CaF₂ can exist in crystalline, polycrystalline, and amorphous phases in the SEI, and as recent work has shown, different phases of the same compound can have vastly different electronic conductivities. Using the non-equilibrium Green's function technique with density functional theory (NEGF-DFT), we find that amorphous phase systems enhance electron tunneling in thin CaF₂ films by 1-2 orders of magnitude when compared to crystalline and polycrystalline CaF₂ systems. Transport through several amorphous structures was considered showing that, despite their random structures, their conductance properties are similar. Through analysis of the decay constant β and the low-bias conductance of each system, we show that crystalline and polycrystalline CaF₂ offer greater protection of the electrolyte than amorphous CaF₂.