Carbon coated Na_3+xV_2-xCu_x(PO₄)₃@C cathode for high-performance sodium ion batteries

Na₃V₂(PO₄)₃ is considered as one of the most promising cathodes for sodium ion batteries owing to its fast Na⁺ diffusion, good structural stability and high working potential. However, its practical application is limited by its low intrinsic electronic conductivity. Herein, a carbon coated Cu²⁺-doped Na₃V₂(PO₄)₃ cathode was prepared. The carbon coating not only improve its apparent conductivity, but also inhibit crystal growth and prevent agglomeration of particles. Moreover, Cu²⁺ doping contributes to an enhanced intrinsic conductivity and decreased Na⁺ diffusion energy barrier, remarkably boosting its charge transfer kinetics. Based on the structure characterizations, electrochemical performances tests, charge transfer kinetics analyses and theoretical calculations, it's proved that such an elaborate design ensures the excellent rate performances (116.9 mA h g^-1 at 0.1C; 92.6 mA h g^-1 at 10C) and distinguished cycling lifespan (95.8 % retention after 300 cycles at 1C; 84.8 % retention after 3300 cycles at 10C). Besides, a two-phase reaction mechanism is also confirmed via in-situ XRD. This research is expected to promote the development of Na₃V₂(PO₄)₃-based sodium ion batteries with high energy/power density and excellent cycling lifespan.