Multilevel gradient framework of Ce/Mo co-substituted Na3V2(PO4)3/C–CeO2@CNTs system with ultralong cyclic stability for half and full sodium ion cells

Na 3 V 2 (PO 4 ) 3 (NVP) has attracted much attention due to its excellent potential for sodium storage property. However, low electronic and ionic conductivities severely limit further development. Herein, a dimensional gradient structure of Ce/Mo co-doped and carbon nanotubes (CNTs) enwrapped Na 3 V 2 (PO 4 ) 3 /C–CeO 2 @CNTs system is constructed by a facile sol-gel method for the first time. The much larger Ce 3+ can be used as the pillar ions to buffer the crystal deformation and enhance the structural stability . Moreover, the high charge and strong self-polarization ability of Ce can suppress the increase of charge transfer resistance during the charge/discharge process, which is beneficial to improve the electrochemical performance . Meanwhile, the doping of high-valent Mo 6+ generates beneficial vacancies to elevate the Na + diffusion kinetics . The presence of encapsulated CNTs inhibits particle growth, optimizing particle size and shortening ion delivery paths. Significantly, a new conductive CeO 2 phase is clearly identified combining with carbon layers to construct favorable bilayer conductive networks, which effectively facilitate electronic transport. Thus, the modified Na 3 V 1.79 Ce 0.07 Mo 0.07 (PO 4 ) 3 /C@CNTs ( CeMo0.07@CNTs ) exhibits a reversible capacity of 110.3 mAh g −1 at 1C, with 91% retention after 1000 cycles. It releases a capacity of 74.9 mAh g −1 at 30C and maintains at 73.9 mAh g −1 after 12000 cycles with a high retention rate of 98.7%. Furthermore, a high retention rate of 83.2% can be obtained after cycling 15000 cycles at 80C, corresponding to a low decay rate of 0.0011% each cycle. In addition, the CHC//CeMo0.07@CNTs asymmetric full cell reveals high specific capacity of 91.59 mAh g −1 , demonstrating its excellent practical prospects.