Enhancement in High-Rate Performance of Graphite Anodes via Interface Modification Utilizing Ca(BF4)2 as an Electrolyte Additive in Lithium Ion Batteries

Herein, we report the use of inorganic calcium tetrafluoroborate (Ca(BF4)2) as an electrolyte additive to achieve a significant improvement in the reversible capacities of graphite anodes at high rates for lithium ion batteries (LIBs). Even in a low concentration (0.01 M), the addition of Ca(BF4)2 distinctively modifies the composition of a solid-electrolyte-interface (SEI) film on a graphite surface. While maintaining solid-state diffusivity, the SEI film modified by Ca(BF4)2 enables facile Li+ transport, which leads to a noticeable decrease in over-potentials for Li+ intercalation/de-intercalation and finally to a remarkable increase in reversible capacities at high rates. For example, the capacity enhancement by ca. 80% can be achieved by adding Ca(BF4)2 at 0.3 A⋅g−1 (346 vs. 192 mAhg−1). The Ca(BF4)2 additive effect is not graphite-morphology dependent. This beneficial effect of Ca(BF4)2 is ascribed to a synergistic combination of Ca2+ cations and BF4− anions. Ca2+ salts with either different anionic species (Ca(TFSI)2 and Ca(ClO4)2) or different cationic species (LiBF4) also show a certain improvement in capacities, but not as significant as that of Ca(BF4)2. We hope that the formation of a robust and more ionically conductive SEI layer via this simple addition of Ca(BF4)2 will be useful in designing fast-charging LIBs.