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

Journal Article · 14 February 2024 · Nature Communications

DOI: https://doi.org/10.1038/s41467-024-45672-7 · OSTI ID:2305556

Sanz Matias, Ana ; Roncoroni, Fabrice ; Sundararaman, Siddharth ; Prendergast, David

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(BH ₄ ) 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, $$$${\rm{CaBH}}_{4}^{+}$$$$ ${\mathrm{CaBH}}_4^{+}$ – 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 Ca ²⁺ . The consequences for performance are discussed in light of this molecular-scale insight.

• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 2305556
• Alternate ID(s): OSTI ID: 2433800
• Journal Information: Nature Communications, Journal Name: Nature Communications Journal Issue: 1 Vol. 15; ISSN 2041-1723
• Publisher: Nature Publishing GroupCopyright Statement
• Country of Publication: United Kingdom
• Language: English