The Unusual Conductivity of Na⁺ in PEO-Based Statistical Copolymer Solid Electrolytes: When Less Means More

The low conductivity of Na⁺ electrolytes in solid polymer electrolytes (SPEs) curtails the development of Na polymer batteries. In this study, NaClO₄ (3-24 wt %, 90-9:1 O:Na) is dissolved in statistical copolymers of ethylene oxide (EO) and propylene oxide (PO) (0-20 mol %). Remarkably, the conductivity of these SPEs increases as the concentration of Na⁺ decreases, thus departing from the usual Nernstian behavior. Using a combination of calorimetric measurements and molecular dynamic simulations, this unusual phenomenon is attributed to the presence of physical cross-links generated by Na⁺ . As a result, polymers containing a low salt concentration (3 wt %) display a drastically enhanced ionic conductivity (up to 0.2 10^-4  S cm^-1 at 25 °C), thus paving the way for the design of all-solid-state PEO-based sodium batteries operational at room temperature.