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

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 Ca2+-anion speciation in borohydride electrolytes comprising ether solvent blends. Starting with the baseline electrolyte 1 M Ca(BH4)(2) in tetrahydrofuran (THF), THF was systematically replaced by 1,2-dimethoxyethane (G1) or bis(2-methoxyethyl) ether (G2) over a range of Gx:Ca2+ molar ratios (0-4:1 G1:Ca2+ or 0-3:1 G2:Ca2+). 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 Ca2+ coordination, as glymes displace THF from the Ca2+ coordination environment, weaken Ca2+-BH4- interactions, and prompt BH4- 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 Ca2+-blocking phases in the SEI. These findings delineate future directions to modulate Ca(BH4)(2) coordination toward improving electrolyte reversibility by considering the O-donating ability of the coordinating solvent.

Automated summary

This study investigates how the plating/stripping behavior and solid electrolyte interphase (SEI) formation of calcium metal are influenced by changes in solvent composition in borohydride electrolytes. The results show that adding glymes to the electrolyte increases the plating overpotential and decreases the Coulombic efficiency of calcium plating/stripping. X-ray photoelectron spectroscopy reveals that an increase in glyme proportion leads to a higher relative abundance of organic C-O species and a decrease in borate species in the SEI.