Influence of Ether Solvent and Anion Coordination on Electrochemical Behavior in Calcium Battery Electrolytes

The emergence of magnesium and calcium batteries as potential beyond Li ion energy storage technologies has generated significant interest into the fundamental aspects of alkaline earth metal cation coordination in multivalent electrolytes and the impact of coordination on application-critical electrolyte properties such as solubility, transport, and electrochemical stability. Understanding these details in calcium electrolytes is of immediate importance due to recent, unprecedented demonstrations of reversible calcium metal electrodeposition in a limited number of ethereal solvent-based systems. In this work, we provide insight connecting Ca2+ coordination tendencies to important calcium battery electrolyte properties. Our results demonstrate a clear solvent:Ca2+ coordination strength trend across a series of cyclic ether and linear glyme solvents that controls the extent of ion association in solutions of "weakly" coordinating salts. We apply understanding gained from these results to rationalize relative anion:Ca2+ coordination tendencies and attendant Ca2+ coordination structures using two oxidatively stable anions of particular interest for current battery electrolytes. Armed with this understanding of solvent and anion interactions with Ca2+, we demonstrate and interpret differences in electrochemical calcium deposition behavior across several electrolyte exemplars with varying solvent and anion coordination strengths. Our findings demonstrate that solvents exhibiting especially strong coordination to Ca2+, such as triglyme, can inhibit reversible calcium deposition despite effective elimination of anion:Ca2+ coordination while solvents exhibiting more modest coordination strength, such as 1,2-dimethoxyethane, may enable deposition provided anion:Ca2+ coordination is substantially limited. These results reveal that the strength of coordination of both anion and solvent should be considered in the design of electrolytes for calcium batteries.