Sodium Rich Vanadium Oxy‐Fluorophosphate – Na_3.2Ni_0.2V_1.8(PO₄)₂F₂O – as Advanced Cathode for Sodium Ion Batteries

Abstract Conventional sodium‐based layered oxide cathodes are extremely air sensitive and possess poor electrochemical performance along with safety concerns when operating at high voltage. The polyanion phosphate, Na₃V₂(PO₄)₃ stands out as an excellent candidate due to its high nominal voltage, ambient air stability, and long cycle life. The caveat is that Na₃V₂(PO₄)₃ can only exhibit reversible capacities in the range of 100 mAh g⁻¹, 20% below its theoretical capacity. Here, the synthesis and characterizations are reported for the first time of the sodium‐rich vanadium oxyfluorophosphate, Na_3.2Ni_0.2V_1.8(PO₄)₂F₂O, a tailored derivative compound of Na₃V₂(PO₄)₃, with extensive electrochemical and structural analyses. Na_3.2Ni_0.2V_1.8(PO₄)₂F₂O delivers an initial reversible capacity of 117 mAh g⁻¹ between 2.5 and 4.5 V under the 1C rate at room temperature, with 85% capacity retention after 900 cycles. The cycling stability is further improved when the material is cycled at 50 °C within 2.8–4.3 V for 100 cycles. When paired with a presodiated hard carbon, Na_3.2Ni_0.2V_1.8(PO₄)₂F₂O cycled with a capacity retention of 85% after 500 cycles. Cosubstitution of the transition metal and fluorine in Na_3.2Ni_0.2V_1.8(PO₄)₂F₂O as well as the sodium‐rich structure are the major factors behind the improvement of specific capacity and cycling stability, which paves the way for this cathode in sodium‐ion batteries.