Targeted Re-Speciation of Ca2+ in Borohydride-Based Electrolytes
Authors: Melemed Aaron Max , Gallant Betar M. 
Published: July 2022

Given concerns about the low earth abundance of Li in Li-ion batteries, there is growing interest in developing a beyond-Li materials basis for rechargeable batteries. Divalent batteries based on calcium (Ca) have received attention due to their ~2000-fold higher concentration in the earth’s crust along with attractive theoretical metrics if Ca is used as the metallic anode. At present, only a small number of nonaqueous electrolytes have been identified that allow for Ca electrodeposition,1-5 limiting the design space for Ca battery development; thus, learning how to tailor Ca2+ speciation and electroactivity is of central importance to engineer next-generation battery electrolytes. Unlike many Li+ analogues in common battery electrolytes, Ca2+ cations are prone to coordinate with one or both anions in solution even at low concentrations (< 1 M), which can substantially impair their ability to participate in electrochemical processes. Recent studies have shown that in an electrolyte system of growing interest, Ca(BH4)2 in tetrahydrofuran (THF), unexpected promotion of active CaBH4
+ clusters from neutral Ca(BH4)2 ion pairs occurs at salt concentrations greater than 1 M, such that higher salt concentrations are necessary to provide charge mobility and high current densities.6 Learning how to break ion-pairing constraints and more flexibly modulate Ca2+ speciation over broader ranges of salt concentrations is therefore important to identify handles for next-generation electrolyte design.
This study investigates an alternative means to tailor the Ca2+-to-BH4
- ratio, and thus the generation of electroactive Ca ionic species, by using an exemplar dual-salt electrolyte, Ca(BH4)2 + Ca(TFSI)2 in THF, at varying anion ratios for a constant total salt concentration of 1 M Ca2+. Introduction of a more highly-dissociating source of Ca2+ in Ca(TFSI)2 effectively drives re-speciation of the more strongly ion-pairing Ca(BH4)2 as indicated by a 4x increase in ionic conductivity (Fig. 1a) and Raman spectroscopy measurements (Fig. 1b),7 generating larger populations of charged species and supporting a ~2x increase in plating current density. We further find that TFSI- enables Ca plating at high current densities when its concentration is less than that of Ca(BH4)2, but leads to a dramatic shut-down of plating activity when concentrations exceed that of Ca(BH4)2 (Fig. 1c),7 providing direct evidence of the role of coordination-shell chemistry on modulating Ca2+ plating activity. On the other hand, Ca stripping activity is suppressed by the presence of TFSI- at all salt concentrations (Fig. 1d),7 which decomposes onto the Ca surface, passivating the deposits with compounds comprising ~30% C, 35% O, and 10% F. Results are compared to that of a second dual-salt electrolyte system, Ca(BH4)2 + TBABH4 in THF, which enables enrichment of BH4
- concentrations to be higher than 2x that of Ca2+ and similarly experiences a 4x increase in ionic conductivity and ~2x increase in plating current density. This work reveals factors that modulate Ca2+ coordination and activity and highlights future directions to attain both high plating currents and reversibility for Ca-based electrolyte design.
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Figure 1