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Divalent ions, such as Mg, Ca, and Zn, are being considered as competitive,safe, and earth-abundant alternatives to Li-ion electrochemistry. However, thechallenge remains to match electrode and electrolyte materials that stablycycle with these new formulations, based primarily on controlling interfacialphenomena. We explore the formation of electroactive species in the electrolyteCa(BH$_4$)$_2$ in THF through molecular dynamics simulation. Free-energyanalysis indicates that this electrolyte has a majority population of neutralCa dimers and monomers, albeit with diverse molecular conformations as revealedby unsupervised learning techniques, but with an order of magnitude lowerconcentration of possibly electroactive charged species, such as themonocation, CaBH$_4^+$ , which we show is produced via disproportionation ofneutral Ca(BH$_4$)$_2$ complexes. Dense layering of THF molecules within 1 nmof the electrode surface (modeled here using graphite) hinders the approach ofreducible species to within 0.6 nm and instead enhances the local concetrationof species in a narrow intermediate-density layer from 0.7-0.9 nm. A dramaticincrease in the monocation population in this intermediate layer is induced atnegative bias, supplied by local dimer disproportionation. We see no evidenceto support any functional role of fully-solvated Ca$^{2+}$ in theelectrochemical activity of this electrolyte. The consequences for performanceand alternative formulations are discussed in light of this molecular-scaleinsight.

Ca-dimers and solvent layering determine electrochemically active species in Ca(BH$_4$)$_2$ in THF