Enhancement of the Electrochemical Performance of Na ₃ MnTi(PO ₄ ) ₃ Cathode Material by Nb Doping for Sodium-Ion Batteries

Despite the promising sodium-storage characteristics of NASICON-type Na3MnTi(PO4)3 arising from its stable three-dimensional framework and high ionic conductivity, its practical performance remains limited by the low Ti3+/Ti4+ redox potential, poor electronic conductivity, and Mn3+-induced Jahn–Teller distortion. In this study, Na3MnTi1–xNbx(PO4)3 (x = 0, 0.025, 0.05, 0.075, 0.1) was synthesized via a sol–gel method. X-ray diffraction confirms that Nb5+ substitution at Ti sites preserves the NASICON structure while slightly expanding the lattice and promoting Na+ transport. Electrochemical tests show that appropriate Nb doping not only increases the high-voltage capacity corresponding to the Mn2+→ Mn4+ redox process, but also improves the rate capability, cycling stability, and reversibility of Na3MnTi1–xNbx(PO4)3 cathodes. The Na3Ti0.95Nb0.05Mn(PO4)3 sample delivered an initial capacity of 139.0 mAh g–1 and maintained 74.1 mAh g–1 after 500 cycles at 180 mA g–1. Moreover, it exhibits enhanced rate capability, maintaining 88.6 mAh g–1 as the current density increases to 900 mA g–1. These findings indicate that Nb-doping is an effective strategy to enhance the electrochemical performance and structural stability of Na3MnTi(PO4)3.