High Performance of All-Solid-State Batteries with PEO:NaTFSI at 40 °C

Developing polymer electrolytes as an alternative to aprotic liquid electrolytes for lithium and sodium-ion batteries aims to enhance their safety, durability, and cost. Among these, polyethylene oxide (PEO) is a favorite choice due to its wide availability, excellent versatility, and mechanical properties. PEO:NaTFSI and PEO:NaFSI are stable and efficient ion-conducting solid polymer electrolytes compared to other PEO:NaX matrices (for instance, X = [PF₆]⁻, [ClO₄]⁻). We tested Na/PEO:NaTFSI/NVP cells at low temperatures (40 °C) and carried out a series of electrochemical measurements to extract vital performance metrics such as diffusion coefficient, transference number, conductivity, and activation energy. Our findings emphasize the important role of the anions’ nature in the properties of polymeric electrolytes like those based on PEO, in which there are strong interactions between the ions and the oxygen atoms. Highly important are the interfacial interactions of the anions with the electrodes, which form surface species that determine stability and charge-transfer kinetics. Significant impact of TFSI anions on the cathode-electrolyte interfacial properties may explain the exceptional performance of sodium batteries at 40 °C, which is below the melting temperature of PEO. Our findings may open the door for an effective operation of solid-state batteries at room temperatures.