Understanding the Na-Ion Storage Mechanism in Na_{3+ x}V_{2- x}M_x(PO₄)₃(M = Ni²⁺, Co²⁺, Mg²⁺x = 0.1-0.5) Cathodes

Bivalent cations (M = Ni2+, Co2+, Mg2+) with different doping contents (x = 0.1, 0.2, 0.3, 0.4, 0.5) were incorporated for vanadium in the Na3+xV2-xMx(PO4)3 (NVP), yielding enhanced rate performance and capacity retention. Successful doping of these cations in the NVP structure was confirmed by powder X-ray diffraction (PXRD), vibrational FT-IR spectroscopy, and scanning electron microscopy (SEM) techniques. The improved electrochemical performance of substituted NVP cathode has been correlated to effective Na ion migration, which improved kinetics of charging and discharging properties. Mg2+ was possible to dope up to x = 0.5 in the NVP structure, which exhibited a superior electrochemical performance compared to that of Ni2+- and Co2+-doped NVP samples. The Mg2+-doped NVP electrode exhibited fast Na ion kinetics with a specific capacity of 70 mAh g-1 at a 20 C rate. The oxidation state of the vanadium in the Mg2+-substituted NVP was investigated by using X-ray photoelectron spectroscopy (XPS).