Thermal Properties and Ionic Conductivity in Salt-Containing POEM/PEO Polymer Blend Electrolytes

Polymer blend electrolytes are one possible pathway to developing solid electrolytes with improved properties when compared to the widely studied poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) system. However, many previously studied polymer blend electrolytes are not fully miscible at varying salt concentrations across the full blend composition window. In this study, we design a polymer blend electrolyte system that is fully miscible and characterize its thermal and ion transport properties. Poly(oligo ethylene oxide methacrylate) (POEM) is an ion conducting polymer with ethylene oxide side chains that can be varied in length. Our polymer blend electrolyte system contains PEO, POEM, and LiTFSI, where we vary POEM monomer chemistry to determine the effect of side chain length and molecular weight on blend properties. All PEO/POEM/LiTFSI blends were miscible, regardless of the POEM monomer structure, blend composition, or salt concentration. We characterize the blend properties using differential scanning calorimetry (DSC) and variable-temperature electrochemical impedance spectroscopy (EIS). Using the blend glass-transition temperature and a modified Fox equation, we are able to determine the salt partitioning within the system and quantify changes in the segmental dynamics of the polymer blend. We find that while Li ions are primarily solvated by PEO, the POEM monomer structure determines the extent. Through EIS, we find that longer side chains more easily solvate the Li ions and have higher ionic conductivities. This study shows that engineering the solvation structure and segmental dynamics of salt-doped polymer blends can lead to developing electrolytes with improved ion transport properties for battery applications.