Synergistic strategy of interfacial ion channels and a cross-linked network in a composite electrolyte for dendrite-free sodium-metal batteries

Solid-state electrolytes (SSEs) can address the safety concerns of liquid electrolytes in sodium-metal batteries, but their low ionic conductivity and high interfacial resistance limit practical use. To tackle these problems, this study develops a flexible and tunable composite solid electrolyte (CSE) by integrating Na₃Zr₂Si₂PO₁₂ (NZSPO) active fillers into a polymer matrix that combines the high dielectric constant of polyacrylonitrile (PAN) with an interpenetrating and cross-linked polyethylene oxide (PEO) network. The resulting electrolyte achieved a high Na⁺ transference number of 0.64 and an exceptional ionic conductivity of >1.2 × 10^-3 S cm^-1 at 30 °C. It enabled stable Na plating/stripping for over 2500 hours in symmetric cells, and when paired with a Na₃V₂(PO₄)₃ (NVP) cathode, delivered outstanding cycling stability (95.2 mAh g^-1 after 500 cycles at 0.5C) with an ultra-low decay rate of 0.029% per cycle due to the good interface contact based on the good external flexibility and high internal mechanical stability of the composite solid electrolyte. Theoretical calculation has demonstrated that Na⁺ cations have low transferring energy barriers in both NZSPO particles and the polymer matrix. The work highlights a synergistic filler-polymer strategy for advancing solid-state batteries.