The 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.