Reductive Decomposition Kinetics and Thermodynamics That Govern the Design of Fluorinated Alkoxyaluminate/Borate Salts for Mg-Ion and Ca-Ion Batteries

The rational design of electrolytes has been a long-standing
challenge
in chemistry and materials science. In this work, we demonstrate a
computational rationale for improving the performance of weakly coordinating
electrolytes in currently challenging multivalent-ion battery applications,
based on enhanced thermodynamic and kinetic stability against reductive
decomposition. A series of fluorinated alkoxyborate and alkoxyaluminate
salts are systematically examined based on their reduction and oxidation
potentials and, motivated by NMR spectroscopy, detailed reductive
decomposition pathways involving the breaking of Al/B–O, C–O,
or C–F bonds are obtained. Based on the decomposition kinetics,
the hexafluoro-tert-isopropoxy (hfip) ligand for
borates and the trifluoro-tert-butoxy (tftb) ligand
for aluminates are identified as promising ligands for constructing
the salt anions. This borate prediction corroborates previous experimental
work on Mg­[B­(hfip)4]2 and Ca­[B­(hfip)4]2, in which excellent electrochemical properties were
reported. We find that steric factors govern the B–O bond-breaking
decomposition kinetics while electronic factors are more important
for aluminate salts. There is more charge transfer character in the
aluminate transition states compared with borates for Al/B–O
bond-breaking decomposition and thus electron-withdrawing ligands
tend to stabilize the aluminate transition states. Such molecular-level
understandings allow for better design principles for developing new
electrolytes with improved stability and performance.