Influence of ligands on lithium salt properties: A comprehensive approach to the structure-function relationship

Abstract

Complexes of lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiODFB) and lithium difluorobis(oxalato) phosphate (LiDFBOP) are the promising additives for advanced lithium ion batteries because of their high reactivity and easy synthesis, in which the changeable central atom and ligand bring diverse properties for salts. In this study, their structure-function relationship has been surveyed by exploring the reduction pathways. According to the analysis of experiments and quantum chemistry calculations, we have established that the steric-hindrance effect of ligands is the key effect on the bond-broken of these lithium salts. Specifically, more oxalate ligands are beneficial to the breaking of C–O and the bond of the central atom (M) with O atom, resulting in CO and CO2 release as well as the generation of MO2-type soluble products. The as-generated MO2-type soluble products undergo a further self-aggregate process to form macromolecular complexes. In contrast, fewer oxalate ligands just encourage the bond breaking of the M − O bond, leading to CO2 release and the generation of MF2-type soluble products. In addition, the universal promoting effect of soluble products on ethylene carbonate decomposition is verified. These results offer theoretic support for the molecular design of advanced functional chelae lithium salts.