Electron transport in CaF₂: The importance of structural phases at calcium-anode interfaces

Current electrolytes in calcium-ion batteries (CIBs) suffer from reduction-driven degradation caused by electron flow from the anode. The solid-electrolyte interphase (SEI) that forms on CIB anodes during operation stems the flow of electrons from the anode to the electrolyte, making SEI formation a self-limiting process. CaF₂ is a common inorganic compound found in the SEI of CIBs, and is derived from salts in the electrolyte 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 electron conductances. 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 compared to crystalline CaF₂, while polycrystalline systems show similar transport properties to crystalline CaF₂. Through analysis of the decay constant and the low-bias conductance, we show that crystalline and polycrystalline CaF₂ offer greater protection of the electrolyte than amorphous CaF₂. This work will provide key insights into the rational design of SEI layers with low electronic conductivity for use in calcium-ion batteries.