Biphasic interactions between Na4Fe3(PO4)2P2O7 and Na3V2(PO4)3 to achieve ultra-long lifespan cathode material for sodium-ion batteries

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) is considered a promising candidate for the cathode in sodium-ion batteries (SIBs) due to its low cost and structural stability. However, it faces poor conductivity and low operating voltage. In this work, we design a step-by-step solid-state approach to heterogeneously compositing NFPP with Na₃V₂(PO₄)₃ (NVP) with outstanding electrochemical performance. The biphasic interactions between NVP and NFPP are revealed in the synthesis process and electrochemical reactions. In the core-shell structure of NFPP@NVP (NVFPP), the robust NFPP can suppress the expansion of NVP and stabilize the electrode structure, while NVP elevates the operating voltage up to 3.05 V and significantly decreases the particle size via steric effect. The carbon-coated NVFPP@C delivers a reversible capacity of 125 mAh g⁻¹ at 0.1C, corresponding to an energy density of 382 Wh kg⁻¹. At a high current rate of 10C, it maintains a specific capacity of 96 mAh g⁻¹ and retains 75.3 % capacity over 10,000 cycles. This study presents a novel heterogeneous compositing strategy for designing high-performance materials for SIBs.