Finite size effects on the structural progression induced by lithiation of V2O5: a combined diffraction and Raman spectroscopy study

Abstract

Developing an understanding of the structural changes induced during the insertion of Li-ions into the layered framework of nanostructured V₂O₅ is necessary to unravel the origin of the dramatically increased power densities characteristic of nanostructured electrodes. In this work, we have contrasted the sequence of structural progressions induced within V₂O₅ micron-sized powders, hydrothermally grown nanowires, and CVD-grown nanoplatelet arrays as a function of chemical lithiation using powder diffraction and Raman spectroscopy. Raman spectroscopy serves as a powerful and highly sensitive probe for investigating the local structure of the lithiated V₂O₅ phases. We note a profound size dependence of the structural progression with the kinetics of Li-ion uptake following: CVD-grown nanoplatelet arrays ≫ hydrothermally grown nanowires > micron-sized powders. For bulk powders, Raman spectroscopy indicates conversion to the α-phase at 30 s and to the ε-phase at 30 min. The ε-phase continues to grow in spatial extent for the remaining 2 h duration. In contrast, the hydrothermally grown nanowires convert to the α-phase after 30 s and have the ε-phase as the predominant surface species after just 1 min. The CVD grown nanoplatelets show a much accelerated response with the ε-phase nucleated within just 30 s and the Li-rich ε′-phase stabilized after 5 min. After 30 min of lithiation, these nanowires convert to the δ/γ phase and are subsequently irreversibly amorphized after 2 h. Chemical delithiation is seen to result in reversion to the α-phase for bulk and hydrothermally grown nanowire powders for chemical lithiations up to 2 h. In contrast, the unlithiated orthorhombic phase is recovered upon delithiation of the δ/γ-phase nanoplatelet arrays.