Enhanced performance of Sn-doped Na₃V₂(PO₄)₃ with CNT integration for high-efficiency sodium-ion batteries

The development of Na₃V₂(PO₄)₃ (NVP) has been severely hindered by low conductivity and unstable crystal structure. A simultaneously optimized strategy of Na-rich and Sn substitution is proposed for the first time. SnX-NVP@CNTs with different doping gradients are successfully prepared by the facile sol-gel method. Notably, more hole carriers can be generated by introducing Sn²⁺, thus improving its electron transport efficiency. In addition, since Sn²⁺ ions have a larger ion radius; when replacing V³⁺ ions at pillar positions, the lattice spacing can be enlarged to improve the structural stability of electrode materials. Meanwhile, it is beneficial to the movement of deep-level Na⁺ ions and improves the utilization rate of electrode materials. Moreover, to achieve charge compensation, it is necessary to introduce excess Na⁺ to the Sn-doped NVP system, which will increase the number of Na⁺ involved in the deintercalation process and improve its reversible capacity. Furthermore, the dense coating of CNTs can form an efficient conductive network structure, which improves the electron transport rate and inhibits the accumulation of active grains to accelerate Na⁺ diffusion. Under the synergistic adjustment of Sn²⁺ doping and CNTs enwrapping, the prepared Sn0.07-NVP@CNTs exhibit a high reversible capacity of 115.1 mAh/g at 0.1C, and the capacity retention rate reaches 89.35 % after 2000 cycles at 10C. Even after 10,000 cycles at 60C, its reversible capacity dropped from the initial 75.9 to 51.3 mAh/g, with a capacity loss of only 0.003 % per cycle. Besides, the Sn0.07-NVP@CNTs//CHC full battery releases a capacity of 139.9 mAh/g, highlighting its great potential for actual applications.