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

Abstract

Na3V2(PO4)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 Na3V2(PO4)3/C–CeO2@CNTs system is constructed by a facile sol-gel method for the first time. The much larger Ce3+ 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 Mo6+ 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 CeO2 phase is clearly identified combining with carbon layers to construct favorable bilayer conductive networks, which effectively facilitate electronic transport. Thus, the modified Na3V1.79Ce0.07Mo0.07(PO4)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.