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.