Ca-dimers, solvent layering, and dominant electrochemically active species in Ca(BH4)2 in THF

Ana Sanz Matias, Fabrice Roncoroni, Siddharth Sundararaman and David Prendergast ()
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Ana Sanz Matias: Lawrence Berkeley National Laboratory
Fabrice Roncoroni: Lawrence Berkeley National Laboratory
Siddharth Sundararaman: Lawrence Berkeley National Laboratory
David Prendergast: Lawrence Berkeley National Laboratory

Nature Communications, 2024, vol. 15, issue 1, 1-12

Abstract: Abstract Divalent ions (Mg, Ca, and Zn) are being considered as competitive, safe, and earth-abundant alternatives to Li-ion electrochemistry, but present challenges for stable cycling due to undesirable interfacial phenomena. We explore the formation of electroactive species in the electrolyte Ca(BH4)2∣THF using molecular dynamics coupled with a continuum model of bulk and interfacial speciation. Free-energy analysis and unsupervised learning indicate a majority population of neutral Ca dimers and monomers with diverse molecular conformations and an order of magnitude lower concentration of the primary electroactive charged species – the monocation, $${\rm{CaBH}}_{4}^{+}$$ CaBH 4 + – produced via disproportionation of neutral complexes. Dense layering of THF molecules within ~1 nm of the electrode surface strongly modulates local electrolyte species populations. A dramatic increase in monocation population in this interfacial zone is induced at negative bias. We see no evidence for electrochemical activity of fully-solvated Ca2+. The consequences for performance are discussed in light of this molecular-scale insight.

Date: 2024
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DOI: 10.1038/s41467-024-45672-7

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