Electronic/Ionic Dual Functional Layer-Coated Na3V2(PO4)2F3 Cathode with High Sodium Storage Performance

NASICON-structured Na₃V₂(PO₄)₂F₃ (NVPF) is considered to be a promising cathode for sodium-ion batteries due to its high work potential and large theoretical capacity. However, the poor electronic conductivity severely limits its electrochemical performance. Herein, a dual functional layer composed of N-doped C and β″-Al₂O₃ is constructed on the NVPF surface to improve electronic and ionic conductivity simultaneously. The dual functional layer-coated NVPF cathode exhibits superior capacities of 126.7 and 106.4 mAh g^–1 at 0.2 and 10 C, respectively. Furthermore, an outstanding capacity retention of 95.5% can be achieved after 500 cycles at 10 C. In full cell systems, it also has a superior electrochemical performance, with a capacity retention of 96.1% over 120 cycles at 1 C and a capacity of up to 78.5 mAh g^–1 at 5 C. Unexpectedly, due to the compensation of irreversible Na⁺ loss during the formation of cathode/electrolyte interface film by Na⁺ in β″-Al₂O₃, the full cell has an initial Coulombic efficiency as high as 91.9%. Therefore, this modification strategy of combining β″-Al₂O₃ with carbon is feasible and effective and can be extended to other electrode materials with poor electronic/ionic conductivity.