Quantifying Species Populations in Multivalent Borohydride Electrolytes

Hahn, Nathan T.; Self, Julian; Han, Kee Sung; Murugesan, Vijayakumar; Mueller, Karl T.; Persson, Kristin A.; Zavadil, Kevin R.

doi:10.1021/acs.jpcb.1c00263

DOE OSTI · 1778071

Quantifying Species Populations in Multivalent Borohydride Electrolytes

Hahn, Nathan T.·Self, Julian·Han, Kee Sung·Murugesan, Vijayakumar·Mueller, Karl T.·Persson, Kristin A.·Zavadil, Kevin R.

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Abstract

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 of a ground-breaking combination of the borohydride salt Ca(BH 4 ) 2 dissolved in tetrahydrofuran. Recent analysis of magnesium and calcium versions of this electrolyte led to the identification of divergent speciation pathways for Mg 2+ and Ca 2+ despite identical anions and solvents, owing to differences in cation size and attendant flexibility of coordination. To test these proposed speciation equilibria and develop a more quantitative understanding thereof, we have applied pulsed-field-gradient nuclear magnetic resonance and dielectric relaxation spectroscopy to study these electrolytes. Additionally, concentration-dependent variation in anion diffusivities and solution dipole relaxations, interpreted with the aid of molecular dynamics simulations, confirms these divergent Mg 2+ and Ca 2+ speciation pathways. These results provide a more quantitative description of the electroactive species populations. We find that these species are present in relatively small quantities, even in the highly active Ca(BH 4 ) 2 /tetrahydrofuran electrolyte. This finding helps interpret previous characterizations of metal deposition efficiency and morphology control and thus provides important fundamental insight into the dynamic properties of multivalent electrolytes for next-generation batteries.

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Hahn, Nathan T., Self, Julian, Han, Kee Sung, Murugesan, Vijayakumar, Mueller, Karl T., Persson, Kristin A., Zavadil, Kevin R.. 2021-04-02. Quantifying Species Populations in Multivalent Borohydride Electrolytes. https://doi.org/10.1021/acs.jpcb.1c00263

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