Sodium Rich Vanadium Oxy‐Fluorophosphate – Na_3.2Ni_0.2V_1.8(P₄)₂F₂O – as Advanced Cathode for Sodium Ion Batteries

<p dir="ltr">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, Na₃_.2Ni_0.2V_1.8(PO₄)₂F₂O, a tailored derivative compound of Na₃V₂(PO₄)₃, with extensive electrochemical and structural analyses. Na₃_.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, Na₃_.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 Na₃_.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.</p><h2>Other Information</h2><p dir="ltr">Published in: Advanced Science<br>License: <a href="http://creativecommons.org/licenses/by/4.0/" target="_blank">http://creativecommons.org/licenses/by/4.0/</a><br>See article on publisher's website: <a href="https://dx.doi.org/10.1002/advs.202301091" target="_blank">https://dx.doi.org/10.1002/advs.202301091</a></p>