Structural design enabled a hypotoxic Na_3.36FeV(PO₄)₃ cathode with ultra-fast and ultra-long sodium storage

Among various cathode materials for sodium-ion batteries, Na₃V₂(PO₄)₃ has attracted much attention due to its outstanding electrochemical performance. However, the toxicity and expensive price of vanadium limit its practical application. Therefore, the substitution of vanadium with nontoxic and inexpensive transition metal elements is significant. We select the earth-abundant iron element to partially replace the vanadium element, and successfully synthesize Na_3.36FeV(PO₄)₃ with a Na superionic conductor structure. Furthermore, a Na_3.36FeV(PO₄)₃ cathode with an optimal carbon content can deliver an initial capacity of 97.6 mA h g^-1 at 0.5C with a high capacity retention of 96.4% after 200 cycles. In addition, it also delivers an initial capacity of 90.4 mA h g^-1 at 10C, and a capacity retention of 80% can be obtained after 5000 cycles. We also found that the lack of sodium in the material can be compensated by an electrochemical reaction. Furthermore, the in situ X-ray diffraction analysis reveals that the sodium storage process follows a pseudo-solid-solution reaction mechanism and the volume change ratio is less than 3% during charging/discharging. In order to study the practical application capability of Na_3.36FeV(PO₄)₃, we assemble the pre-activated cathode and a hard carbon anode into a full cell, which exhibits high initial discharge capacities of 103 and 91.3 mA h g^-1 at 0.5C and 10C, respectively. This work will provide new insights into the structural engineering of low-toxicity and ultralong-life NASICON-type cathode materials for SIBs.