Long Straczekite δ-Ca0.24 V2 O5 ⋅H2 O Nanorods and Derived β-Ca0.24 V2 O5 Nanorods as Novel Host Materials for Lithium Storage with Excellent Cycling Stability

Abstract

Nanorods of δ-Ca_0.24 V₂ O₅ ⋅H₂ O, 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_0.24 V₂ O₅ ) through a vacuum annealing treatment. The generated β-Ca_0.24 V₂ O₅ still preserves the nanorod construction of δ-Ca_0.24 V₂ O₅ ⋅H₂ O 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_0.24 V₂ O₅ 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_0.24 V₂ O₅ ⋅H₂ O electrode irreversibly converts into β-Ca_x V₂ O₅ 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.

Keywords: cathode materials; lithium storage; straczekite; vanadium bronze.