Calcium batteries: synergistic effects of Ca(BF₄)₂ and acetonitrile for calcium deposition/stripping

Abstract Details of the interfacial reactions at calcium metal electrodes using three different electrolytes—Ca(BF 4 ) 2 or Ca(TFSI) 2 in acetonitrile (ACN), and Ca(BF 4 ) 2 in an ethylene carbonate: propylene carbonate (EC:PC) mixture—are here revealed using a set of electrochemical techniques: cyclic voltammetry, open-circuit potential, electrochemical impedance spectroscopy, and electrochemical quartz crystal microbalance. Furthermore, the deposition/stripping of calcium on electrodes/substrates of calcium metal and stainless steel (SS), and the products formed, are assessed by x-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. Combined, these show that calcium undergoes corrosion, resulting in the formation of pits, if polarized at sufficient positive potentials, but also that spontaneous formation of passivation layers proceeds through two or more steps at different rates in different electrolytes. The cathodic degradation of ACN is suppressed by the presence of Ca salts in the electrolyte, while the calcium deposition is strongly inhibited in both the Ca(TFSI) 2 in ACN and the Ca(BF 4 ) 2 in EC:PC electrolytes due to the simultaneous formation of the passivation layer. Deposition from the Ca(BF 4 ) 2 in ACN electrolyte, however, is enabled by the formation of a passivation layer that proceeds at lower rates, and stripping can proceed at an appreciable rate. On inert SS substrates/electrodes, the calcium deposition goes through two stages, the first being underpotential deposition at a potential of approximately 1 V more positive than the thermodynamic potential. Altogether, we showcase ways to truly deposit/strip calcium, paving the way for improved calcium batteries.