Investigating Lithium Ordering and Electronic Evolutions in Wadsley–Roth Phases

The development of new materials for fast-charging lithium-ion batteries is critical to meet the growing demand for energy storage technologies. As a result of their high capacity and high-rate capabilities, the Wadsley–Roth block phases are promising anode materials for this application. Despite the diversity of structures and chemistries proposed within this family of materials, challenges remain in uncovering subtle differences in the lithiation behavior of these phases by traditional electrochemical methods. Here, we report an in-depth study of the electrochemical and structural properties of a few representative materials and identify potential structure–property relationships. First, operando synchrotron X-ray diffraction was used to reveal that independently of their structure, these selected Wadsley–Roth phases all exhibit a solid–solution intercalation process throughout their lithiation. However, connectivity between blocks was found to impact the lattice contraction–expansion behavior. By using the entropic potential technique, we uncovered ionic ordering behavior occurring at the early stages of lithiation; the ordering is sensitive to the metal coordination environment and the elemental content. However, we demonstrate using in situ electronic conductivity measurements that electronic contributions to the entropic potential are non-negligible, which complicates the analysis. Using this technique, the large increase in entropic potential at low lithium content was ascribed to an increase in configurational electronic entropy associated with a charge ordering process and a semiconductor-to-metal transition. Conversely, at large lithium content at which these phases are metallic, shallow increases were attributed to increased concentration of bands around the Fermi level. Together, these results yield a broader understanding of the lithiation mechanism in this group of materials and provide an example of the key role played by coordination and block size on the intercalation chemistry of Wadsley–Roth phases.