High‐ionicity fluorophosphate lattice via aliovalent substitution as advanced cathode materials in sodium‐ion batteries

Abstract As a cathode for sodium‐ion batteries (SIBs), Na 3 V 2 (PO 4 ) 2 F 3 (NVPF) with 3D open framework is a promising candidate due to its high working voltage and large theoretical capacity. However, the severe capacity degradation and poor rate capability hinder its practical applications. The present study demonstrated the optimization of Na‐storage performance of NVPF via delicate lattice modulation. Aliovalent substitution of V 3+ at Na + in NVPF induces the generation of electronic defects and expansion of Na + ‐migration channels, resulting in the enhancement in electronic conductivity and acceleration of Na + ‐migration kinetics. It is disclosed that the formed stronger NaO bonds with high ionicity than VO bonds lead to the significant increase in structural stability and ionicity in the Na + ‐substituted NVPF (NVPF‐Na x ). The aforementioned effects of Na + substitution achieve the unprecedented electrochemical performance in the optimized Na 3.14 V 1.93 Na 0.07 (PO 4 ) 2 F 3 (NVPF‐Na 0.07 ). As a result, NVPF‐Na 0.07 delivers a high‐rate capability (77.5 mAh g −1 at 20 C) and ultralong cycle life (only 0.027% capacity decay per cycle over 1000 cycles at 10 C). Sodium‐ion full cells are designed using NVPF‐Na 0.07 as cathode and Se@reduced graphene oxide as anode. The full cells exhibit excellent wide‐temperature electrochemical performance from −25 to 25°C with an outstanding rate capability (96.3 mAh g −1 at 20 C). Furthermore, it delivered an excellent cycling performance over 300 cycles with a capacity retention exceeding 90% at 0.5 C under different temperatures. This study demonstrates a feasible strategy for the development of advanced cathode materials with excellent electrochemical properties to achieve high‐efficiency energy storage.