An Ultrathin Asymmetric Composite Polymer Electrolyte for High-Performance Solid-State Sodium–Metal Batteries

Solid-state sodium–metal batteries have attracted much attention due to their high theoretical capacity and relatively low cost associated with abundant sodium resources. However, the poor interface problems of solid-state electrolytes matching with both the rough composite cathode and the highly reactive sodium anode urgently need to be addressed. In this study, an asymmetric composite polymer electrolyte (PEO|PTFE|PEO@MOF) with an ultrathin thickness of only 16 μm is successfully developed through an asymmetric coating of poly(ethylene oxide) (PEO) and PEO@metal–organic framework (PEO@MOF) electrolytes on the two sides of a porous, high-strength poly(tetrafluoroethylene) (PTFE) membrane combined with a hot-pressing process. The ultrathin thickness of the electrolyte suggests shorter ionic transport paths with faster ionic conductance between a cathode and an anode. The softer PEO side enables a close interface contact with the rough cathode. The harder PEO@MOF side with abundant MOF pores can not only guide the transportation of sodium ions for uniform sodium deposition but also avoid the electrochemical reaction between PTFE and the sodium anode. Therefore, the batteries with such asymmetric electrolytes exhibit much superior rate capability and cycling stability to their reference symmetric electrolytes. This study also provides a highly operational route for the low-cost and scalable manufacturing of ultrathin asymmetric composite electrolytes.