Plating and Stripping Ca-Metal Batteries Using Potassium Hexafluorophosphate in Hybrid Solid Electrolyte Interphases

The use of calcium (Ca) metal anodes in batteries is currently challenged by the development of a passivation layer that forms in the Solid Electrolyte Interphase (SEI). The passivation layer is formed from the degradation of the electrolyte onto the surface of the calcium and produces either calcium fluoride (CaF₂) or calcium chloride (CaCl₂). This insulating phase significantly reduces cation movement while continually forming new layers with freshly plated calcium. Reversible plating and stripping of calcium in such systems has been possible but has required either large overpotentials or operating at elevated temperatures. The use of a hybrid SEI is a strategy to mitigate the uncontrolled production of the passivation layer and reduce the overpotentials needed for the plating and stripping of calcium. Here, we report the development of a hybrid Potassium (K) /Ca SEI using symmetrical Ca // Ca cells. The calcium symmetrical cells are cycled for 200 hours at a capacity of 0.15 mAh/cm² with a current density of 0.025 mA/cm². Throughout the lifetime of cycling, the symmetrical cells were able to maintain overpotentials below 2V. Ex-situ X-Ray Diffraction (XRD) of cycling reveals plating and stripping of both calcium and potassium. Scanning Electron Microscope (SEM) cross-sectional views of the calcium electrodes outline clear distinctions between the bulk electrode and the interphase formed from cycling while Energy Dispersive X-Ray (EDX) maps confirm the elements present in the XRD analysis.