Thermodynamics of Litfsi in Poly(ethylene oxide)-Based Electrolytes

We present the thermodynamic characterization of lithium bis(trifluoromethane) imidate (LiTFSI) in Poly(ethylene oxide) (PEO) electrolytes using molecular dynamics (MD) simulations in experimentally relevant salt concentration regimes. The mean ionic activity coefficient generally increases with salt concentration but experiences a plateau around a critical concentration of =1/6 [Li⁺]/[EO]. In addition, the end-to-end distance of the PEO chain shows a nonmonotonic dependence on the salt concentration where the turning point is also located around . These unusual thermodynamic and conformational behaviors observed in our simulations are in good agreement with experimental results. Microscopic information such as the pair correlation function, coordination number and the ion cluster distribution has also been analyzed. The plateau in activity coefficient can be explained by the strong ion-ion correlation when lithium ions are abruptly released from complete solvation by PEO around . Such abrupt change of the ionic solvation environment around further gives rise to the reversal of polymer chain size, as well as the pronounced change of ion dissociation and clustering.