Outstanding long cycle stability provide by bismuth doped Na₃V₂(PO₄)₃ enwrapped with carbon nanotubes cathode for sodium-ion batteries

Na₃V₂(PO₄)₃ (NVP), possessing good ionic conduction properties and high voltage plateau, has been deemed as the most prospective material for sodium ion batteries. However, the weak intrinsic electronic conductivity has hindered its further commercialization. Herein, an ingenious strategy of Bi³⁺ substitution at V³⁺ site in NVP system is proposed. The ionic radius of Bi³⁺ is slightly larger than that of V³⁺, which can further expand the crystal structure inside the NVP, thus accelerating the migration of Na⁺. Meanwhile, the appropriate amount of carbon coating and carbon nanotubes (CNTs) enwrapping construct an effective three-dimensional network, which provides a conductive framework for electronic transfer. Furthermore, the introduction of CNTs also inhibit the agglomeration of active grains during the sintering process, reducing the particle size and shortening the diffusion path of Na⁺. Comprehensively, the conductivity, ionic diffusion ability and structural stability of the modified Na₃V_2-xBi_x(PO₄)₃/C@CNTs (0 ≤ x ≤ 0.05) sample are significantly improved. The Na₃V_1.97Bi_0.03(PO₄)₃/C@CNTs sample obtains a reversible capacity of 97.8 mAh g^-1 at 12C and maintains a value of 80.6 mAh g^-1 after 9000 ultra-long cycles. As for the super high rate at 80C, it exhibits a high capacity of 84.34 mAh g^-1 and retains a capacity of 73.34 mAh g^-1 after 6000 cycles. The superior electrochemical performance is derived from the enhancement of the crystal structure by Bi³⁺ doping and the highly conductive network consisting of carbon coating layers and enwrapped CNTs.