Effect of Temperature on Intermediate Phases of Na₃V₂(PO₄)₃ during Cycling by Operando X-ray Diffraction

Na₃V₂(PO₄)₃ (NVP) has gained a lot of attention due to its remarkable properties, such as its robust crystal structure, cycle life, rate capabilities, and so on. Nevertheless, NVP undergoes a substantial decrease in its rate capability at low temperatures, which limits its practical applications. In this study, the performance of NVP at low, room, and high temperatures during cycling is thoroughly investigated using synchrotron operando X-ray diffraction. The (de)insertion of two sodium ions from Na₃V₂(PO₄)₃ to Na₁V₂(PO₄)₃ appeared to occur via two intermediate phases (Na₂V₂(PO₄)₃ and Na_1.64V₂(PO₄)₃). The Na_1.64V₂(PO₄)₃ phase which is observed for the first-time during operando XRD measurements of NVP, exhibited limited stability at high temperatures. The increase in the quantity of these intermediate phases from high to low temperatures, especially at high C-rates, could be anticipated to be one of the contributing factors of poor rate capabilities of NVP at low temperatures. This study encourages the exploration of suitable strategies to enhance the performance of NVP at low temperatures and high C-rates.