Ti-Doped Na 3 V 2 (PO 4 ) 3 Activates Additional Ti 3+ /Ti 4+ and V 4+ /V 5+ Redox Pairs for Superior Sodium Ion Storage

Na 3 V 2 (PO 4 ) 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 Na 3 V 1.9 Ti 0.1 (PO 4 ) 3 /C (Ti 0.1 -NVP/C) composite as a cathode for SIBs. Using Ti 4+ to replace V 3+ 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 Ti 3+ /Ti 4+ and V 4+ /V 5+ redox pairs. The synergistic effect of the two redox pairs makes the capacity of the Ti 0.1 -NVP/C electrode (123.3 mAh g –1 at 0.1 C) higher than the theoretical specific capacity of NVP. Ti 0.1 -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 Ti 0.1 -NVP/C electrode exhibits superior competitiveness. The reaction mechanism of the Ti 0.1 -NVP/C electrode is elucidated by ex-situ XRD and GITT measurements.