Quantifying Species Populations in Multivalent Borohydride Electrolytes

Multivalent batteries represent an important beyond Li-ion energy storage concept. The prospect of calcium batteries, in particular, has emerged recently due to novel electrolyte demonstrations, especially that a ground-breaking combination the borohydride salt Ca(BH4)2 dissolved tetrahydrofuran. Recent analysis magnesium and versions this led identification divergent speciation pathways for Mg2+ Ca2+ despite identical anions solvents, owing differences cation size attendant flexibility coordination. To test these proposed equilibria develop more quantitative understanding thereof, we have applied pulsed-field-gradient nuclear magnetic resonance dielectric relaxation spectroscopy study electrolytes. Concentration-dependent variation anion diffusivities solution dipole relaxations, interpreted with aid molecular dynamics simulations, confirms pathways. These results provide description electroactive species populations. We find are present relatively small quantities, even highly active Ca(BH4)2/tetrahydrofuran electrolyte. This finding helps interpret previous characterizations metal deposition efficiency morphology control thus provides fundamental insight into dynamic properties multivalent electrolytes next-generation batteries.