Abstract: 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 Ca 2+ speciation and electroactivity is of central importance to engineer next-generation battery electrolytes. Unlike many Li + analogues in common battery electrolytes, Ca 2+ 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(BH 4 ) 2 in tetrahydrofuran (THF), unexpected promotion of active CaBH 4 + clusters from neutral Ca(BH 4 ) 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 Ca 2+ 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 Ca 2+ -to-BH 4 - ratio, and thus the generation of electroactive Ca ionic species, by using an exemplar dual-salt electrolyte, Ca(BH 4 ) 2 + Ca(TFSI) 2 in THF, at varying anion ratios for a constant total salt concentration of 1 M Ca 2+ . Introduction of a more highly-dissociating source of Ca 2+ in Ca(TFSI) 2 effectively drives re-speciation of the more strongly ion-pairing Ca(BH 4 ) 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(BH 4 ) 2 , but leads to a dramatic shut-down of plating activity when concentrations exceed that of Ca(BH 4 ) 2 (Fig. 1c), 7 providing direct evidence of the role of coordination-shell chemistry on modulating Ca 2+ 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(BH 4 ) 2 + TBABH 4 in THF, which enables enrichment of BH 4 - concentrations to be higher than 2x that of Ca 2+ and similarly experiences a 4x increase in ionic conductivity and ~2x increase in plating current density. This work reveals factors that modulate Ca 2+ coordination and activity and highlights future directions to attain both high plating currents and reversibility for Ca-based electrolyte design. 1 Ponrouch, A., Frontera, C., Bardé, F. & Palacín, M. R. Towards a calcium-based rechargeable battery. Nature Materials 15 , 169 (2015). 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 Ca 2+ Speciation in Borohydride-based Electrolytes. Manuscript in revision. 8 Rey, I. et al. Spectroscopic and Theoretical Study of (CF 3 SO 2 )2N - (TFSI - ) and (CF 3 SO 2 )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 (Mg 2+ , Ca 2+ ) Cation Based Batteries. Journal of The Electrochemical Society 164 , A1384-A1392 (2017). Figure 1