Salt Activity Coefficients, Chain Statistics, and Scattering Behaviors in Polymer Electrolytes*

A fundamental understanding of the of mixtures of polymer electrolytes and lithium salts is important for the rational design of solid-state lithium-ion batteries. We report the molecular dynamics studies of the mixtures of Poly(ethylene oxide) (PEO) and lithium bis(trifluoromethane) imidate (LiTFSI) in experimentally relevant salt concentration regime. Near the critical salt concentration, rc=1/6 [Li+]/[EO], the simulation results show the emergence of a small plateau of salt activity coefficient. At the same concentration, the dependence of polymer chain size on salt concentration exhibits a reversal from contraction to expansion. These unusual thermodynamic and chain conformational behaviors are attributed to qualitative changes of the solvation structure near rc.The scattering profile shows two peaks as the salt concentration increases. The high-q peak is due to neighboring correlation between PEO segments, whereas the low-q peak is entirely due to the local correlation between TFSI anions. These simulation results are in good agreement with experimental observations.