Glass Transition Temperature and Ion Binding Determine Conductivity and Lithium-Ion Transport in Polymer Electrolytes.

Polymer electrolytes with high Li + -ion conductivity provide a route toward improved safety and performance of Li + -ion batteries. However, most polymer electrolytes suffer from low ionic conduction and an even lower Li + -ion contribution to the conductivity (the transport number, t + ), with the anion typically transporting over 80% of the charge. Here, we show that subtle and potentially undetected associations within a polymer electrolyte can entrain both the anion and the cation. When removed, the conductivity performance of the electrolyte can be improved by almost 2 orders of magnitude. Importantly, while some of this improvement can be attributed to a decreased glass transition temperature, T g , the removal of the amide functional group reduces interactions between the polymer and the Li + cations, doubling the Li + t + to 0.43, as measured using pulsed-field-gradient NMR. This work highlights the importance of strategic synthetic design and emphasizes the dual role of T g and ion binding for the development of polymer electrolytes with increased total ionic conductivity and the Li + ion contribution to it.