Molecular-level interfacial engineering unlocking Na3Zr2Si2PO12/poly(vinylidene fluoride-co-hexafluoropropylene) composite electrolytes toward quasi-solid-state sodium metal batteries.

Composite solid-state electrolytes (CSSEs) are a promising candidate for quasi-solid-state sodium metal batteries (QSSSMBs) by combining the advantages of ceramic electrolytes and solid polymer electrolytes. However, poor interfacial compatibility between ceramic and polymer components greatly limits their full potential. Herein, we design and fabricate a CSSE by solution-casting a suspension containing polydopamine (PDA)-modified Na3Zr2Si2PO12 (NZSP), denoted as PDA@NZSP, poly(vinylidene fluoride-co-hexafluoropropylene) and NaClO4. The PDA coating acts as an amphiphilic interfacial bridge improving the interfacial compatibility between inorganic filler and polymer matrix, which promotes a more homogeneous distribution of PDA@NZSP and facilitates interfacial Na+ transport. The polar groups of PDA reconstruct the local Na+ClO4- coordination environment, suppress salt crystallization, and favor the formation of more weakly associated Na+ species, contributing to a room-temperature ionic conductivity of 0.233 mS cm-1 and a Na+ transference number of 0.73. Depth-resolved X-ray photoelectron spectroscopy of cycled Na anodes further reveals that PDA@NZSP modification is associated with a distinct distribution of NO-containing and fluorinated interphase species compared with pristine NZSP. Together with the favorable interfacial-resistance evolution and long-term Na plating/stripping behavior, these results demonstrate improved Na/electrolyte interfacial stability in the optimized CSSE-based quasi-solid-state configuration.