Tailoring NVP-NFP heterostructures: A high capacity and stable dual polyanionic cathode for sodium-ion batteries

NASICON-type polyanionic cathodes with exceptional structural robustness, variable redox activity, relatively high Na + intercalation voltage, theoretical capacity and rapid Na + transport, are becoming attractive for high-performance sodium-ion battery. However, poor electronic conductivity and sluggish kinetics restrict their achievable capacity. Herein, a heterostructure composite of Na 3 V 2 (PO 4 ) 3 and Na 3 Fe 2 (PO 4 ) 3 cathode material embedded within a conducting carbon network is designed which broadens the operating voltage window, enhances Na + storage capacity, and significantly improves electronic transport. The sample NVFP 900 shows three redox active plateaus of Fe 2+ /Fe 3+ , V 3+ /V 4+ , V 4+ /V 5+ , and a specific capacity of ∼136 mAh g −1 at 10 mA g −1 current density and stable cycling. The ex-situ XPS at different state of charge reveals valence state evolution of Fe and V, confirming the three-stage redox mechanism of NVFP 900 during Na + intercalation–deintercalation. The structural stability is further established by ex-situ XRD. The electrochemical impedance spectroscopy at different state of charge shows variation in charge transfer resistance, and the calculated diffusion coefficient falls within 10 −10 - 10 −13 cm 2 s −1 range. The sodium ion full cell with hard carbon anode, provides ∼100 mAh g −1 discharge capacity, ∼89% rate capability, and energy density of ∼177 Wh kg −1 .