Investigation of Structural Evolution of Li_1.1V₃O₈ by In Situ X-ray Diffraction and Density Functional Theory Calculations

Li_1.1V₃O₈ is a promising cathode material for Li-ion batteries, because of its high theoretical capacity (362 mAh g^-1) and good rate performance. In this study, the structural evolution of Li_1.1V₃O₈ material during electrochemical dis(charge) processes was investigated using a combination of theoretical calculations and experimental data. Density functional theory (DFT) was used to predict the intermediate structures at various lithiation states, as well as the stability of major phases. In order to validate these predictions, in situ X-ray diffraction (XRD) data was collected operando, allowing for the phase transformations to be monitored under current load and eliminating the possibilities of structural relaxation processes and environmental oxidation. Rietveld refinement was performed to fit the diffraction data with the DFT-derived structures and to analyze the fractions of major phases as a function of dis(charge). The DFT calculations identified three stable states that were validated by the in situ XRD result: a Li-poor α-phase (Li₁), a Li-rich α-phase (Li_2.5), and a β-phase (Li₄). The DFT-predicted particle shape based on the surface energy of the (100), (001), and (010) planes rationalized the preferential orientation of Li_1.1V₃O₈ particles along the [010] direction in the electrode. Furthermore, the onset and offset of the α → β transition, as well as the phase fractions of α and β determined via in situ XRD, related well with the DFT-derived relative stability of each phase. Thus, by integrating DFT calculations with experimental work, this work provides a thorough understanding of the structural transformations in Li_1.1V₃O₈ during electrochemical dis(charge).