Ti-Doped Na3V2(PO4)3 Activates Additional Ti3+/Ti4+ and V4+/V5+ Redox Pairs for Superior Sodium Ion Storage

Na3V2(PO4)3 (NVP) is considered as a potential cathode material for next-generation sodium ion batteries (SIBs) because of its open Na+ diffusion channels and high operating voltage. In this paper, we design a Na3V1.9Ti0.1(PO4)3/C (Ti0.1-NVP/C) composite as a cathode for SIBs. Using Ti4+ to replace V3+ can not only stabilize the crystal structure of NVP, but also generate Na vacancies to promote Na+ diffusion and improve the intrinsic electronic conductivity of NVP. Meanwhile, the coated carbon layer provides a surface channel for the electron transport of NVP. More importantly, Ti-doped NVP activates additional Ti3+/Ti4+ and V4+/V5+ redox pairs. The synergistic effect of the two redox pairs makes the capacity of the Ti0.1-NVP/C electrode (123.3 mAh g–1 at 0.1 C) higher than the theoretical specific capacity of NVP. Ti0.1-NVP/C cathode also exhibits excellent rate capability (89.5 mAh g–1 at 30 C) and long cycle performance (retention of 62.3% at 20 C after 8000 cycles). Furthermore, the symmetric full cell of the Ti0.1-NVP/C electrode exhibits superior competitiveness. The reaction mechanism of the Ti0.1-NVP/C electrode is elucidated by ex-situ XRD and GITT measurements.