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. Solid polymer electrolytes from synthesized statistical copolymers of ethylene oxide and propylene oxide were mixed with various amounts of NaClO4. Physical crosslinking with cations is more prone to happen at high contents in both salt and comonomers, reducing the ionic conductivity. Low salt content (3 wt %) is thus preferred, contrasting with classical Nernstian behavior, leading to ionic conductivity of 0.2×10−4 S cm−1 at 25 °C.