Gas Evolution Markers of Dynamic Solid Electrolyte Interphase Formation on Calcium in a Borohydride-Based Electrolyte

Abstract

Although calcium (Ca)-based batteries are of growing interest for sustainable energy storage, a limited number of electrolytes enable reversible plating/stripping of Ca metal, making understanding the behavior of successful electrolytes paramount. Ca(BH₄)₂ in tetrahydrofuran (THF) is one of the more widely-tested electrolytes and a current benchmark given its highest-to-date Coulombic efficiency (CE) of Ca plating/stripping. Yet, there is limited understanding of the reactions forming the solid electrolyte interphase (SEI) and the resulting degree of SEI stability, which impedes understanding into this electrolyte's function. To begin to address these knowledge gaps, this study employs gas chromatography (GC) analysis to track gas evolution markers of SEI formation in Ca(BH₄)₂/THF across a range of cell conditions and timescales. In general, both H₂ and linear hydrocarbon (RH) gases are detected as the major evolved gas species, with the former predominantly evolving from reaction between Ca(BH₄)₂ and residual water, and RH products evolving from reaction between Ca and THF. RH evolution is found to be dynamic for cells at rest, with distinct timescales attributed to initial and extended SEI formation and spanning several days. When cells are polarized during Ca plating, RH evolution arrests after a certain plated capacity is reached, signaling the formation of a protective yet functional SEI that continues to be permissive to Ca plating to higher capacities. Yet, this SEI is metastable, as further chemical changes are observed when polarization is relaxed. Given the significant role of water in contributing to H₂ evolution, we further systematically investigate the impact of water on SEI formation dynamics and composition. Increased water content is observed to mainly shorten SEI formation timescales, but its impact diminishes over extended cycling, and the SEI composition ultimately remains largely independent of initial water content. Despite the ability of the Ca plating electrochemistry to tolerate a degree of water in cells, the associated H₂ evolution presents challenges to any practical implementation of this electrolyte.