Sodium Rich Vanadium Oxy-Fluorophosphate - Na_3.2 Ni_0.2 V_1.8 (PO₄ )₂ F₂ O - as Advanced Cathode for Sodium Ion...

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, Na₃ V₂ (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 Na₃ V₂ (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_3.2 Ni_0.2 V_1.8 (PO₄ )₂ F₂ O, a tailored derivative compound of Na₃ V₂ (PO₄ )₃ , with extensive electrochemical and structural analyses. Na_3.2 Ni_0.2 V_1.8 (PO₄ )₂ F₂ O 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_3.2 Ni_0.2 V_1.8 (PO₄ )₂ F₂ O cycled with a capacity retention of 85% after 500 cycles. Cosubstitution of the transition metal and fluorine in Na_3.2 Ni_0.2 V_1.8 (PO₄ )₂ F₂ O 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.