Vanadium K-Edge X-ray Absorption Spectroscopy as a Probe of the Heterogeneous Lithiation of V₂O₅: First-Principles Modeling and Principal Component Analysis

Understanding the diffusion mechanisms of Li ions through host materials and the resulting phase evolution of intercalated phases is of paramount importance for designing electrode materials of rechargeable batteries. The formation of lithiation gradients and discrete domains during intercalation leads to the development of strain within the host material and is responsible for the observed capacities of most cathode materials being well below theoretically predicted values. Such mesoscale heterogeneity has also been implicated in the loss of capacity upon cycling. Due to their inherent complexity, the analysis of such heterogeneity is rather complex and precise understanding of the evolution of metal sites remains underexplored. In this work, we use phase-pure, single-crystalline V₂O₅ nanowires with dimensions of 183 ± 50 nm and lengths spanning tens of microns as a model cathode material and demonstrate that V K-edge X-ray absorption near-edge structure can be used as an effective probe of the local valence and geometry of vanadium sites upon lithiation. We demonstrate that a highly lithiated phase is nucleated and grows at the expense of a homogeneous low-lithium-content α-phase without mediation of a solid-solution with intermediate lithium content. Density functional theory calculations allow for assignment of the pre-edge feature to dipolar transitions that are particularly sensitive to the V 3d-O 2p hybridization of the vanadyl bond and the local geometry of the distorted [VO₅] square pyramid. The quantitative analysis of multiple vanadium sites and their evolution as a function of Li-ion content provides insight into the mechanism of phase evolution and the nature of lithiation gradients. The phase coexistence and segregation is further observed in scanning transmission X-ray microscopy images of individual lithiated V₂O₅ nanowires. The mechanisms and the dynamics of nucleation and growth unraveled here are of great importance for the design and discovery of Li-ion cathode materials.