The Role of Ion Correlation on Transport Properties of Concentrated Ionic Liquids for Battery Electrolytes

Recent pioneering work on Na batteries has drawn attention on ionic liquid electrolytes, yet most published works ignore the effects of ion-ion correlation. Using classical all-atom molecular dynamics and fully-correlated theory we highlight the drastic differences in fundamental understanding when using the systematically overlooked correlated approach. As a case study, we look at the promising Na/bis(fluorosulfonyl)imide (NaFSI) in room temperature ionic liquid N-propyl-N-methylpyrrolidinium/FSI. A previously undetected negative transference number emerges for sodium molar fractions lower than 0.2, effectively hinting to high degrees of ion-ion correlation that should not be dismissed. The spatial correlation is explored further by employing a variant of the single-linkage clustering algorithm. The approach we use enables us to explain the concentration-driven trends in ionic conductivity and transference numbers, and is generally able to correctly capture the transport physics of highly-correlated systems such as concentrated electrolytes.