Regulation of Carrier Transport at Grain Boundaries of Na ₃ Zr ₂ Si ₂ PO ₁₂ Solid Electrolyte with Ionic Conductor Filler for Superior Solid‐State Sodium Batteries

Abstract Solid‐state sodium‐metal batteries (SSSMBs) have attracted great attention as a promising next‐generation power source due to their low cost, excellent safety, and high energy density. However, the inefficient transmission of Na + and poor cycling stability remain crucial challenges that need to be settled. Herein, the ionic conductor of Na 3 AlF 6 (NAF) is employed to fill grain boundaries (GBs) to regulate the carrier transport in the solid electrolyte (SE) of Na 3 Zr 2 Si 2 PO 12 . The injection of NAF can construct a 3D ion transport network at GBs to reduce the energy barrier for ion transport and simultaneously block electron transport, thereby inhibiting the nucleation and growth of Na filaments within the electrolyte and at the interface with the sodium anode. For the optimized SE, it suggests a high ionic conductivity of 1.08 × 10 −3 S cm −1 and a large critical current density of 1.05 mA cm −2 . Furthermore, the assembled sodium symmetric cell can deliver an ultra‐long cycle life of more than 8000 h at 0.1 mA cm −2 , and the full cell is able to operate stably with a high capacity retention of 91.3% after 400 cycles at 0.5 C. This work may offer an appealing route to design the GBs of SE for high‐performance SSSMBs.