RESUMO
Na3V2(PO4)3(NVP) is an ideal cathode material for sodium ion battery due to its stable three-dimensional frame structure and high operating voltage. However, the low intrinsic conductivity and serious structural collapse limit its further application. In this work, a simultaneous optimized Na3V1.96Ru0.04(PO4)3/C@CNTs cathode material is synthesized by a simple sol-gel method. Specifically, the ionic radius of Ru3+ is slightly larger than that of V3+ (0.68 Å vs 0.64 Å), which not only ensures the feasibility of Ru3+ replacing V3+ site, but also appropriately expands the migration channel of sodium ions in NVP and stabilizes the structure, effectively improving the diffusion efficiency of sodium ions. Moreover, CNTs construct a three-dimensional conductive network between the grains, reducing the impedance at the interface and effectively improving the electronic conductivity. Ex-situ XRD analysis at different SOC were performed to determine the change in the crystal structure of Ru3+doped Na3V2(PO4)3, and the refinement results simultaneously demonstrate the relatively low volume shrinkage value of less than 3 % during the de-intercalation process, further verifying the stabilized crystal construction after Ru3+ substitution. Furthermore, the ex-situ XRD/SEM/CV/EIS after cycling indicate the significantly improved kinetic characteristics and enhanced structural stability. Notably, the modified Na3V1.96Ru0.04(PO4)3/C@CNTs reveals superior rate capability and ultralong cyclic performance. It submits high capacities of 82.3/80.9 mAh g-1 at 80/120C and maintains 71.3/59.6 mAh g-1 after 14800/6250 cycles, indicating excellent retention ratios of 86.6 % and 73.6 %, respectively. This work provides a multi-modification strategy for the realization of high-performance cathode materials, which can be widely applied in the optimization of various materials.
RESUMO
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 Na3V2-xBix(PO4)3/C@CNTs (0 ≤ x ≤ 0.05) sample are significantly improved. The Na3V1.97Bi0.03(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.