Why Lithium Ions Stick to Some Anions and Not Others.

Designing battery electrolytes for lithium-ion batteries has been a topic of extensive research for decades. The ideal electrolyte must have a large conductivity as well as high Li transference number. The conductivity is very sensitive to the nature of the anions and dynamical correlations between ions. For example, lithium bis(trifluoromethane)sulfonimide (LiTFSI) has a large conductivity, but the chemically similar lithium trifluoromethanesulfonate (LiOTf) shows poor conductivity. In this work, we study the binding of Li to these anions in an ethylene carbonate (EC) solvent using enhanced sampling metadynamics. The evaluated free energies display a large dissociation barrier for LiOTf compared to LiTFSI, suggesting long-lived ion-pair formation in the former but not the latter. We probe these observations via unbiased molecular dynamics simulations and metadynamics simulations of TFSI with a hypothetical OTF-like partial charge model indicating an electrostatic origin for those differences. Our results highlight the deleterious impact of sulfonate groups in lithium-ion battery electrolytes and provide a new basis for the assessment of electrolyte designs.