Glass Transition Temperature And Ion Binding Determine Conductivity And Lithium-Ion Transport In Polymer Electrolytes

Polymer electrolytes with high Lit-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 Lit ion contribution to it.