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

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 alpha-phase at 30 s and to the epsilon-phase at 30 min. The epsilon-phase continues to grow in spatial extent for the remaining 2 h duration. In contrast, the hydrothermally grown nanowires convert to the alpha-phase after 30 s and have the epsilon-phase as the predominant surface species after just 1 min. The CVD grown nanoplatelets show a much accelerated response with the epsilon-phase nucleated within just 30 s and the Li-rich epsilon'-phase stabilized after 5 min. After 30 min of lithiation, these nanowires convert to the delta/gamma phase and are subsequently irreversibly amorphized after 2 h. Chemical delithiation is seen to result in reversion to the alpha-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 delta/gamma-phase nanoplatelet arrays.