Why Bis(fluorosulfonyl)imide Is a "Magic Anion" for Electrochemistry

Ionic liquids consisting of bis(fluorosulfonyl)imide (FSI-) anion show promise as electrolytes for Li-ion-based electric storage devices, as they exhibit relatively low viscosity, high chemical stability, and form robust solid electrolyte interphase (SET) protecting liquid electrolyte from further breakdown on the electrode. These ionic liquids have been reported to inhibit dendrite formation on lithium metal and lithiated graphite electrodes, which also relates to the unusual SEI properties. In this study, we examine the chemistry aspects that may account for this behavior. Radiolysis was used to induce redox reactions of FSI- anions in model systems, and matrix isolation electron paramagnetic resonance was used to identify radical (ion) intermediates generated in these reactions. Our results suggest that qualitative differences between such ionic liquid electrolytes versus common carbonate electrolytes reflect ease of mineralization of the reduced anion without the concurrent generation of organic radicals and/or elimination of gaseous products in side reactions of the corresponding radical intermediates.