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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2 Wang, D. et al. Plating and stripping calcium in an organic electrolyte. Nature Materials 17, 16 (2017).

3 Shyamsunder, A., Blanc, L. E., Assoud, A. & Nazar, L. F. Reversible Calcium Plating and Stripping at Room Temperature Using a Borate Salt. ACS Energy Letters 4, 2271-2276 (2019).

4 Li, Z., Fuhr, O., Fichtner, M. & Zhao-Karger, Z. Towards stable and efficient electrolytes for room-temperature rechargeable calcium batteries. Energy & Environmental Science (2019).

5 Kisu, K. et al. Monocarborane cluster as a stable fluorine-free calcium battery electrolyte. Scientific Reports 11, 7563 (2021).

6 Hahn, N. T. et al. The critical role of configurational flexibility in facilitating reversible reactive metal deposition from borohydride solutions. Journal of Materials Chemistry A 8, 7235-7244 (2020).

7 Melemed, A. M., Skiba, D. A., Gallant, B. M. Toggling Calcium Plating Activity and Reversibility through Modulation of Ca²⁺ Speciation in Borohydride-based Electrolytes. Manuscript in revision.

8 Rey, I. et al. Spectroscopic and Theoretical Study of (CF₃SO₂)2N^- (TFSI^-) and (CF₃SO₂)2NH (HTFSI). The Journal of Physical Chemistry A (1998).

9 Tchitchekova, D. S. et al. On the Reliability of Half-Cell Tests for Monovalent (Li⁺, Na⁺) and Divalent (Mg²⁺, Ca²⁺) Cation Based Batteries. Journal of The Electrochemical Society 164, A1384-A1392 (2017).