Hierarchical Ru-doped sodium vanadium fluorophosphates hollow microspheres as a cathode of enhanced superior rate capability and ultralong stability for sodium-ion batteries

Novel hierarchical Ru-doped Na3V2O2(PO4)(2)F hollow microspheres were synthesized via a low -temperature solvothermal method. The individual unique microspheres were formed from assembly of numerous nanoparticles with diameters of 20-30 nm. When used as a cathode material for sodium -ion batteries (SIBs), the microspheres exhibited superior rate performance with a capacity of 72.6 mAh.g(-1) at 10 C. Furthermore, their rate performance-could be-significantly improved by-coating them-with a thin conductive RuO2 layer. Fot instance, high specific capacities of 102.5 mAh g(-1) and 44.9 mAh g(-1) were achieved at current rates of 20 C and 100 C, respectively. These materials exhibited impressive long-term cycling stability. A reversible capacity of approximately 55.0 mAh g(-1) was maintained even after 7500 charge/discharge cycles. Density functional theory (DFT) calculations increased our understanding of how H+ facilitates the formation of the hierarchical microsphere superstructure which is beneficial to achieve a good rate capability.