Surface modification and interfacial engineering of Na3SbS4 solid electrolytes by polymethylsiloxane-based polymer for high-performance all-solid-state sodium-ion batteries

All-solid-state batteries are regarded as promising next-generation energy-storage systems due to their potential to achieve energy densities exceeding 400 Wh kg-1. However, their practical implementation remains limited by interfacial instability, non-uniform metal deposition, and continuous electrolyte decomposition during cycling. In this work, a dual-sided polymer coating was introduced onto Na3SbS4 solid electrolyte pellets using a siloxane-based sodiated polyelectrolyte (AAM950), combined with sodium bis(trifluoromethylsulfonyl)imide, as an artificial interfacial layer. The conformal coating partially filled surface pores and reduced interfacial gaps, leading to improved solid-solid contact between the electrolyte and electrode. The optimized 7 µL polymer-coated Na3SbS4 (NSS) pellet exhibited an ionic conductivity of 0.348 mS cm-1 at 55 °C. In addition, the Na2/3Fe1/2Mn1/2O2|Na-AAM950-NSS-Na-AAM950|Na cell delivered an initial discharge capacity of 137.4 mAh g-1, compared with 83.59 mAh g-1 for the bare NSS system at 0.02 A g-1, together with a capacity retention of 89.6% and an average Coulombic efficiency of 99.56% over 50 cycles at 55 °C. Although the polymer layer did not substantially increase the intrinsic bulk ionic conductivity of NSS, it effectively stabilized the electrode/electrolyte interface and promoted more homogeneous Na+ transport during cycling. These findings demonstrate that dual-sided polymer modification provides an effective strategy for improving the practical cycling stability of Na3SbS4-based sodium all-solid-state batteries.