Salt Activity Coefficient and Chain Statistics in Poly(ethylene oxide)-Based Electrolytes

A fundamental understanding of mixtures of poly­(ethylene oxide) (PEO) electrolytes and lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) is important for developing solid electrolytes for rechargeable lithium batteries. Here, we quantify the salt activity coefficient and polymer chain statistics of the PEO/LiTFSI mixture at experimentally relevant salt concentrations using molecular dynamics simulations. Near the critical salt concentration, r c = 1/6 [Li+]/[EO], our simulation shows the emergence of a small plateau in the dependence of salt activity coefficient on salt concentration. At the same concentration, the dependence of polymer chain size on salt concentration exhibits a reversal from contraction to expansion. The simulation results are in good agreement with experimental observations. These unusual thermodynamic and chain conformational behaviors are attributed to the qualitative changes of the solvation structure near r c: the solvation cage for Li+ below this concentration is largely provided by adjacent monomers on a single PEO chain, but the polymer segments are increasingly replaced by TFSI– above this concentration. The existence of a critical salt concentration is entirely because of the connectivity of polymers; in the absence of connectivity, the solvation structure changes continuously with the added salt.