Long Straczekite δ-Ca0.24V2O5•H2O Nanorods and Derived β-Ca0.24V2O5 Nanorods as Novel Host Materials for Lithium Storage with Excellent Cycling Stability

Nanorods of delta-Ca0.24V2O5 center dot H2O, a straczekite group mineral with an open double-layered structure, have been successfully fabricated by a facile hydrothermal method and can be transformed into the tunnel beta geometry (beta-Ca0.24V2O5) through a vacuum annealing treatment. The generated beta-Ca0.24V2O5 still preserves the nanorod construction of delta-Ca0.24V2O5 center dot H2O without substantial sintering and degradation of the nanostructure. As cathode materials, both calcium vanadium bronzes exhibit high reversible capacity, good rate capability, as well as superior cyclability. Compared with the hydrated vanadium bronze, the beta-Ca0.24V2O5 nanorods show better cycling performance (81.68 and 97.93% capacity retention after 200 cycles at 100 and 400 mAg(-1), respectively) and excellent long-term cyclic stability with an average decay of 0.035% per cycle over 500 cycles at 500 mAg(-1). Note that the double-layered delta-Ca0.24V2O5 center dot H2O electrode irreversibly converts into beta-CaxV2O5 phase during the initial Li+ insertion/extraction process, while in contrast, the beta-phase calcium vanadium bronze electrode shows excellent structural stability during cycling. The excellent electrochemical performance demonstrates that the two calcium vanadium bronzes are potential cathode candidates for rechargeable lithium-ion batteries.