Rationalizing Calcium Electrodeposition Behavior by Quantifying Ethereal Solvation Effects on Ca2+ Coordination in Well-Dissociated Electrolytes

Ca-ion electrochemical systems have been pushed to the forefront of recent multivalent energy storage advances due to their use of earth-abundant redox materials and their high theoretical specific densities in relation to monovalent or even other more widely explored multivalent-charge carriers. However, significant pitfalls in metal plating and stripping arise from electrolyte decomposition and can be related to the coordination environment around Ca2+ with both the negatively charged anion and the organic-aprotic solvent. In this study, we apply multiple spectroscopic techniques in conjunction with density functional theory to evaluate the coordination environment of Ca2+ across a class of ethereal solvents. Through the combination of X-ray absorption fine structure and time-dependent density functional theory, descriptive measures of the local geometry, coordination, and electronic structure of Ca-ethereal complexes provide distinct structural trends depending on the extent of the Ca2+-solvent interaction. Finally, we correlate these findings with electrochemical measurements of calcium tetrakis(hexafluoroisopropoxy)borate (CaBHFIP2) salts dissolved within this class of solvents to provide insight into the preferred structural configuration of Ca2+ electrolytic solutions for optimized electrochemical plating and stripping.