Mechanistic Insight into the Protective Action of Bis(oxalato)borate and Difluoro(oxalate)borate Anions in Li-Ion Batteries.

Lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFOB) are uniquely efficient as electrolyte additives in carbonate-based Li-ion batteries. Electrochemical redox reactions of these anions facilitate the formation of robust solid-electrolyte interphases (SEIs) at the graphite and oxide electrodes. We used electron paramagnetic resonance (EPR) spectroscopy to demonstrate that oxidation of these anions causes the elimination of a carbon dioxide molecule and the formation of a stable acyl radical, whereas the reduction of these anions results in the loss of oxalate (for BOB) or fluoride anions (for DFOB) and the concurrent formation of oxalatoboryl adducts. The latter species enters a previously identified radical cycle implicated in SEI formation. Recombination of the acyl radicals at the oxide–electrolyte interface yields difluoroborane dimers that (being strong Lewis acids) form strong B–O bonds with oxygens at the surface, thereby passivating the electrode and preventing oxidation of the electrolyte.