Hot‐Pressing Enhances Mechanical Strength of PEO Solid Polymer Electrolyte for All‐Solid‐State Sodium Metal Batteries

Abstract Poly(ethylene oxide) (PEO)‐based solid polymer electrolytes (SPEs) are widely utilized in all‐solid‐state sodium metal batteries (ASSSMBs) due to their excellent flexibility and safety. However, poor ionic conductivity and mechanical strength limit its development. In this work, an emerging solvent‐free hot‐pressing method is used to prepare mechanically robust PEO‐based SPE, while sodium superionic conductors Na 3 Zr 2 Si 2 PO 12 (NZSP) and NaClO 4 are introduced to improve ionic conductivity. The as‐prepared electrolyte exhibits a high ionic conductivity of 4.42 × 10 −4 S cm −1 and a suitable electrochemical stability window (4.5 V vs Na/Na + ). Furthermore, the SPE enables intimate contact with the electrode. The Na||Na 3 V 2 (PO 4 ) 3 @C ASSSMB delivers a high‐capacity retention of 97.1% after 100 cycles at 0.5 C and 60 °C, and exhibits excellent Coulombic efficiency (CE) (close to 100%). The ASSSMB with the 20 µm thick electrolyte also demonstrates excellent cyclic stability. This study provides a promising strategy for designing stable polymer‐ceramic composite electrolyte membranes through hot‐pressing to realize high‐energy‐density sodium metal batteries.