In Situ Electrochemical Techniques for Investigating Lithium Ordering and Electronic Transitions in Wadsley-Roth Phases

Lithium-ion batteries (LIBs) with fast-charging capabilities have the potential to drive wider adoption of electric vehicles and eliminate a substantial CO2 emission source. LIBs typically utilize graphite negative electrodes due to their low cost, widespread availability, and high energy density. However, operation of these batteries at high current densities leads to lithium plating reactions, reducing the lifespan of the cell. This has motivated the search for alternative negative electrode materials, of which niobium-based Wadsley-Roth oxides (WRs) are one promising candidate. WRs crystallize into units of repeating “blocks”, which exhibit edge sharing between metal polyhedra at the block boundaries and corner sharing between polyhedra within the block. The resulting structure has wide tunnels for fast lithium diffusion and exhibits high capacities due to the multielectron redox capabilities of Nb⁵⁺ and the number of available lithiation sites in the structure. While numerous materials with varied block sizes, polyhedral coordination, and metal cation substitutions have been proposed, the impact of these factors on intercalation behavior remains uncertain. This presentation explores how block size, coordination type, and cation substitution influences electrochemical behavior and structural evolution during lithiation. The electrochemical cycling behavior of these phases is shown to be very similar, outside of the initial redox potential related to the substituted cation, while operando synchrotron XRD shows single phase behavior in all of them. However, subtle differences are observed in the lattice contraction/expansion behavior, suggesting that Li⁺ distribution may be different. In order to probe the ordering behavior, entropic potential measurements were carried out to identify changes in the entropy due to ordering at specific lithium concentrations. This technique reveals fascinating differences in the lithiation behavior of different WR phases. Nevertheless, a common pitfall of entropic potential measurements is that they are impacted by both configurational and electronic entropy evolutions. To address this issue, and to identify insulator-metal transitions during lithiation, in-situ conductivity measurements were employed. These results serve as guiding principles for designing new WRs for fast-charging applications, as well as providing a framework to extend this approach to other intercalation compounds to answer outstanding questions about their mechanisms.