Constructing hierarchical porous Fe/F-codoped Na3V2(PO4)3/C composite enwrapped with carbon nanotubes as high-performance cathode for symmetric sodium ion batteries

Na3V2(PO4)3(NVP) is regarded as a promising cathode material. Herein, a dual strategy to optimize the crystal structure and conductive characteristics of NVP is proposed. F-doping can decrease the particle size and diminish the pathway of Na+, increasing the ionic conductivity. The substitution of Fe3+ is facilitated to stabilize the structural framework. Meanwhile, hierarchical porous construction is generated with Fe3+ doping, dramatically increasing the contact areas between active particles and electrolyte, as well as providing sufficient space for cell shrinkage. The coated carbon layers and enwrapped carbon nanotubes (CNTs) construct an effective conductive framework for accelerated electronic migration. The twining CNTs promote to inhibit the growth of the sintered grains, reducing the particle size to provide shortened pathways for Na+. The optimized Na3V1.95Fe0.05(PO4)2.95F0.15/[email protected] reveals an outstanding rate capability (92.2 mAh g−1 at 120 C; 80.1 mAh g−1 at 450 C) and long cyclic stability (capacity retention of 65.1% at 120 C after 3000 cycles). The symmetric full cell presents potential applicability. Moreover, the ex-situ electrochemical impedance spectroscopy illuminates the variation rules of kinetic parameters with different state-of-charge definitely. The stimulation of large current during prolonged cyclic process promotes the graphitization of CNTs, which is demonstrated by the morphological investigations after cycling.