Composite Polymer Electrolyte With Functional 2D GCN for Improved Na‐Ion Transport and Na Metal Compatibility of All‐Solid‐State Sodium‐Ion Batteries

Polymer electrolytes based on polyethylene oxide are promising for all‐solid‐state sodium‐ion batteries due to commendable processability and scalability; however, their practical realization is impeded by poor ionic conductivity and highly unstable electrode–electrolyte interfaces. Herein, a facile 2D g‐C 3 N 4 (GCN) functional filler–reinforced composite polymer electrolyte is demonstrated to synergistically regulate ion transport and interfacial chemistry. The GCN, enriched with Lewis center, weakens Na + –anion coordination, enhancing salt dissociation, while 2D morphology provides large polymer–filler interfacial area, inducing long‐range interfacial amorphous regions as preferential Na + transport pathways. Consequently, the electrolyte exhibits improved Na‐ion transference number (0.51), ionic conductivity (0.39 mS cm −1 at 55°C), electrochemical stability window (>4.5 V) and reduced ion concentration polarization. Meanwhile, GCN promotes inorganic‐enriched solid electrolyte interphase formation, effectively suppressing dendrite nucleation and enabling long‐term Na plating/stripping exceeding 2000 h at 0.2 mA cm −2 . When integrated with Zn‐doped Na 3 V 2 (PO 4 ) 3 /C (NVP/C) based cathode, all‐solid‐state battery delivers impressive rate capability till 2C and cyclability over 500 cycles at 0.5C with 95% retention, while proof‐of‐concept pouch cell validates its safety and commercial viability. This study demonstrates incorporating 2D functional‐filler represents a powerful CPE design strategy to simultaneously improve interfacial stability and ion transport, offering a versatile platform for developing robust all‐solid‐state sodium batteries.