Stable Interfaces in a Sodium Metal-Free, Solid-State Sodium-Ion Battery with Gradient Composite Electrolyte

Composite electrolytes (CE) combining a ceramic filler and a polymer matrix is an effective way to enhance battery safety. But the increased ceramic filler mass fraction decreases the flexibility, which increases the interfacial resistance. To alleviate interfacial resistance further, a gradient composite electrolyte (GCE) using a Sc, Ge-doped Na₃Zr₂Si₂PO₁₂ (NZSP) as the ceramic filler and poly(ethylene oxide) (PEO) as the polymer matrix is proposed. The outer layer contains a low concentration of ceramic filler to improve interfacial contact, and the central layer contains a high concentration of ceramic filler to inhibit dendrite penetration. This GCE possesses an enhanced conductivity (4.0 × 10^-5 S cm^-1 at 30 °C) and a reduced interfacial resistance. Furthermore, the safety was boosted using Sn₄P₃@CNT/C as the high-capacity anode active material and Na₃V₂(PO₄)₃ (NVP) as the cathode active material. This ultrasafe sodium metal-free, solid-state sodium-ion battery (SSSIB) displays an impressive cycling performance.