Sodium Bis(fluorosulfonyl)imide/Poly(ethylene oxide) Polymer Electrolytes for Sodium‐Ion Batteries

Abstract Sodium‐ion batteries (SIBs), a promising substitute for lithium‐ion batteries (LIBs), are considered to have the potential to be employed in large‐scale energy storage systems with lower cost and enhanced safety as primary concerns. Solid polymer electrolyte (SPE)‐based SIBs will more likely meet these demands, because of their good flame resistance and excellent flexibility compared with conventional organic liquid‐electrolyte‐based SIBs. Here, we describe an SPE composed of sodium bis(fluorosulfonyl)imide (NaFSI) and poly(ethylene oxide) (PEO). The NaFSI/PEO (molar ratio of EO/Na + =20) blended polymer electrolyte exhibits a low glass transition temperature (i.e. −37.9 °C), relatively high ionic conductivity (i.e. ≈4.1×10 −4 S cm −1 at 80 °C), and enough electrochemical and thermal stability for application in SIBs. Most importantly, the NaFSI/PEO blended polymer electrolyte displays excellent interfacial stability with Na metal in Na/Na cells and good cycling performance in prototype cells with Na 0.67 Ni 0.33 Mn 0.67 O 2 as the cathode material. All of these properties make NaFSI‐based SPEs promising candidates for use in SIBs.