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.