(649a) Ion Transport in Polymer Electrolytes - Role of Ion Solvation and Dynamics

Polymer electrolytes are important class of solid electrolytes for enabling high-energy density lithium batteries. Fundamentally, ion-polymer coordination, inter-connectivity of solvation sites, and corresponding ion-solvating polymer dynamics are critical in understanding the limits of ionic conductivity. Here, the importance of these effects is highlighted in a series of combined experimental and computational studies on model lithium-ion conducting polymer electrolytes. In the first study, thin film block copolymer electrolytes (BCEs) consisting of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt were self-assembled to form single-grain, parallel lamellar structures on interdigitated electrodes. Ion conductivity measurements on these defect-free, single-grain structures allowed for more precise determination of the influence of PS and PEO domain interface on the intrinsic ionic conductivity. Moreover, our findings emphasize the critical role of the solvation site network for determining the ionic conductivity trends as a function of salt concentration in BCEs. In the second study, our analysis is extended to graft polymer architectures of poly[(oligo ethylene oxide) methyl ether methacrylate] (POEM) with LiTFSI of varying oligo side-chain lengths. The observed differences in ionic conductivity between the POEM derivatives cannot be adequately explained by differences in T_g. Instead, graft architecture of POEM leads to nonuniform, position-dependent relaxation and ion solvation behaviors along the side chain. Importantly, the limits of ionic conductivity are dictated by the segmental mobility of the ethylene oxide units that form effective solvation sites, rather than system-wide dynamics.