Long Straczekite δ-Ca_0.24V₂O₅⋅H₂O Nanorods and Derived β-Ca_0.24V₂O₅ Nanorods as Novel Host Materials for Lithium Storage with Excellent Cycling Stability

Nanorods of δ-Ca_0.24V₂O₅⋅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 β geometry (β-Ca_0.24V₂O₅) through a vacuum annealing treatment. The generated β-Ca_0.24V₂O₅ still preserves the nanorod construction of δ-Ca_0.24V₂O₅⋅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 β-Ca_0.24V₂O₅ nanorods show better cycling performance (81.68 and 97.93 % capacity retention after 200 cycles at 100 and 400 mA g⁻¹, respectively) and excellent long-term cyclic stability with an average decay of 0.035 % per cycle over 500 cycles at 500 mA g⁻¹. Note that the double-layered δ-Ca_0.24V₂O₅⋅H₂O electrode irreversibly converts into β-Ca_xV₂O₅ phase during the initial Li⁺ insertion/extraction process, while in contrast, the β-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.