Competing Substrate and Free-Surface Effects on Ion Transport in Ultrathin Polymer Electrolyte Films
Ceramic nanoparticles incorporated into solid polymer electrolytes have been reported to both enhance and suppress ionic conductivity, but the origins of these effects remain unclear due to difficulties in isolating interfacial phenomena in disordered bulk systems. Here, we characterize ion transport in ultrathin films of poly(ethylene oxide)–lithium bis(trifluoromethanesulfonyl)imide on silica and employ analytical modeling to elucidate how interfaces might control thickness-dependent behavior. Ionic conductivity is suppressed for the thinnest films at all salt concentrations and temperatures, though modest enhancement appears in some films at low salt concentration. The extent of modulation, however, varies with concentration alone. A modified Vogel–Fulcher–Tammann model incorporating both substrate and free-interface effects on polymer dynamics best captures the observed behavior, outperforming models based on nonconducting layers, interfacial salt partitioning, or substrate-only interface effects. This indicates that both inorganic surfaces and free interfaces can substantially influence polymer mobility and conductivity, suggesting that strategies to optimize composite polymer electrolytes should address the coupled behavior of interfacial polymer dynamics.