Vanadium oxides are widely seen as strong candidates for next-generation energy-saving electrochemical devices, ranging from their use as cathode materials in inherently safe high energy all-solid-state batteries to smart windows that employ their wide color range of electrochromic response. However, critical questions about these materials remain largely unanswered: interfacial reactions and the evolution of the electrode material as delithiation takes place. Distinguishing between topotactic (i.e., reversible) intercalation, conversion, and alloying reactions in ion tunable vanadium oxide devices, in operando, at a resolution that matches the size of structural building units, is a particularly challenging task. In this work, we investigated the effects of lithiation on the structural and optical characteristics of a model thin film system -Li x V 2 O 5 -as a function of depth, using several highly sensitive and nondestructive spectroscopic methods with different depth sensitivities. We exploit (1) Li x V 2 O 5 electrochromic properties to utilize in operando optical response, (2) depthresolved cathodoluminescence spectroscopy (DRCLS), and (3) Raman spectroscopy to monitor the changes in Li x V 2 O 5 electronic structure from the surface to the bulk of the thin film with nanoscale resolution. We find that the degradation of electrochemical performance with deep discharge of Li x V 2 O 5 is associated with drastic band structure changes that accompany octahedral distortion, rather than with a chemical conversion reaction. Elongation along the c axis and charge redistribution induced by varying levels of V(3d)−O(2p) hybridization in the presence of the Li considerably affect the electronic band structure. The coexistence of multiple metastable phases, strong electron correlation, and deviation from an ideal cubic symmetry results in lower structural reversibility with a higher bandgap. Beyond these specific inferences, these results suggest that these optical techniquesRaman, optical absorption/reflection, and cathodoluminescencecan be a powerful combination to reveal electrochemical behavior of ion-tunable transition metal oxides materials and associated reaction mechanisms.

V2O5 electronic band structure from cathodoluminescence reveals how lithiation creates octahedral distortion and not oxygen vacancies or conversion reactions.

A combination of depth-resolved cathodoluminescence spectroscopy (DRCLS) and X-ray photoemission depth profiling (XPS) measured the pronounced changes in both the electronic density of states and lithium composition near the nanoscale Li x V 2 O 5 /LiPON interface. DRCLS studies of electrochemically lithiated bare V 2 O 5 and the sputterdeposited V 2 O 5 plus LiPON overlayer electrochemically lithiated in stages both showed that in the bulk the luminescence intensity of the "split-off" hybridized bonding density of states was anticorrelated with XPSmeasured Li content, decreasing as the Li content increased. However, the LiPON overlayer was found to modify the band structure of the underlying Li x V 2 O 5 (LVO) to a depth of at least 30 nm beneath the V 2 O 5 interface. DRCLS spectra near the electrochemically lithiated LiPON/ LVO interface showed a significant intensity of the split-off band, implying a low Li content. However, XPS depth profiling revealed a pronounced negative gradient of Li extending from a maximum Li content at the intimate LiPON boundary to its lowest content of ∼30 nm into the V 2 O 5 in the same region, indicating a strong interaction between band structure and Li electrochemical potential near this heterojunction. These results provide evidence for substantial effects on the local band structure near an electrolyte/cathode interface and insights into the electrochemical interface behavior of solid-state batteries in general.