Impact of Differential Ca²⁺ Coordination in Borohydride-Based Electrolyte Blends on Calcium Electrochemistry and SEI Formation

Abstract

Despite growing interest in calcium (Ca)-based batteries for sustainable energy storage, there are a limited number of electrolytes that enable the reversible plating/stripping of Ca metal. Consequently, an understanding of the interplay between electrolyte formulation, solid electrolyte interphase (SEI) composition, and electrochemical performance lags behind that of other alkali and alkaline-earth batteries. In this context, this study examines how plating/stripping behavior and SEI formation are modulated by differential changes in solvent composition and the resulting Ca 2+ −anion speciation in borohydride electrolytes comprising ether solvent blends. Starting with the baseline electrolyte 1 M Ca(BH 4 ) 2 in tetrahydrofuran (THF), THF was systematically replaced by 1,2-dimethoxyethane (G1) or bis(2-methoxyethyl) ether (G2) over a range of Gx:Ca 2+ molar ratios (0−4:1 G1:Ca 2+ or 0−3:1 G2:Ca 2+ ). Replacement of THF by glymes increased the plating overpotential and decreased the Coulombic efficiency of Ca plating/stripping, and a marked decrease in electrochemical activity occurred at a composition threshold for each glyme. Comparison between the X-ray photoelectron spectra of electrolyte-soaked Ca and electrodeposited Ca revealed a relative increase in organic C−O species and a decrease in borate species in the electrochemically-formed SEI with increased glyme proportion in the electrolyte. NMR and Raman spectroscopy connected SEI composition to differential Ca 2+ coordination, as glymes displace THF from the Ca 2+ coordination environment, weaken Ca 2+ −BH 4 − interactions, and prompt BH 4 − reorganization. Overall, BH 4 − -facilitated solvent decomposition governs Ca electrochemistry in these systems, as coordinated THF promotes beneficial borate formation but coordinated glymes limit such phases, leading to Ca 2+ -blocking phases in the SEI. These findings delineate future directions to modulate Ca(BH 4 ) 2 coordination toward improving electrolyte reversibility by considering the O-donating ability of the coordinating solvent.