Lithium Intercalation Mechanisms in Wadsley-Roth Phases
Authors: Saber Muna , Van der Ven Anton 
Published: September 2023

Wadsley-Roth crystallographic shear phases are complex inorganic compounds that intercalate alkali ions at a rapid rate, allowing for fast charging lithium-ion battery electrodes with high power densities. Wadsley-Roth crystallographic shear phases consist of mxn size blocks of corner-sharing octahedra. These mxn blocks of octahedra share edges with other blocks of corner sharing octahedra, thereby forming what are referred to as crystallographic shear planes. A variety of Wadsley-Roth phases have been synthesized with differing structural features including the size of the blocks of corner-sharing transition metal octahedra, transition metal chemistry, and disorder among the transition metal sites. Using first-principles statistical mechanics methods, we have predicted the electrochemical properties of PNb9O25, TiNb2O7, and Nb2O5 (Figure 1), three important Wadsley-Roth shear phases, to explore the interplay between crystal structure, transition metal chemistry, Li site preferences, and the voltage profile. The computational approach relied on a triptych of ab initio density functional theory calculations, cluster expansion methods and finite temperature Monte Carlo methods. The calculations provide insights about lithium site preferences, strain evolution[1], and electronic structure [2, 3] as a function of state of charge in representative Wadsley-Roth phases.
For the PNb9O25 compound, we find that lithium prefers to initially fill pyramidal sites at the crystallographic shear planes. It is not until higher lithium concentrations that Li begins to fill window sites at the center of the 3 by 3 octahedral blocks. This inversion in Li-Va ordering is paired with an increase of crystallographic strain and a decrease in octahedral distortions. In contrast to PNb9O25, the TiNb2O7 compound, a commercialized Wadsley-Roth phase, is characterized by Ti-Nb disorder on the transition metal sublattice. We find that the Nb, due to its higher oxidation state, prefers octahedral sites that share fewer edges with neighboring octahedra, relegating the lower oxidation state Ti cations to octahedra at the block edges. First-principles calculations and Monte Carlo simulations predict that the lithium site stability is paired to the local transition metal chemistry and ordering, with Li preferring to fill sites coordinated by TiO6 octahedra. With the use of symmetry adapted collective displacement order parameters we uncover changes in octahedral distortions that differ between the TiO6 and NbO6 octahedra upon intercalation of lithium. Furthermore, we find a similar evolution in strain upon intercalation as is predicted for PNb9O25. To study the effect of octahedral block size on electrochemical properties of Wadsley-Roth phases, we performed an additional in-depth study on Nb2O5, a Wadsley-Roth structure with 4 by 4 corner-sharing octahedral blocks. Predictions for this phase indicate that the dilute limit Li site preference changes when increasing block sizes. Unlike TiNb2O7 and PNb9O25, which have 3 by 3 octahedral block structures, lithium window sites at the center of the block are stable in Nb2O5 at the dilute limit. We attribute this to differences in octahedral distortions in the 4 by 4 and 3 by 3 blocks. Our results indicate that there are a wide variety of structural and chemical levers with which to modify and further optimize the electrochemical properties of Wadsley-Roth phases.

References

Saber, M., Preefer, M.B., Kolli, S.K., Zhang, W., Laurita, G., Dunn, B., Seshadri, R. and Van der Ven, A., 2021. Role of Electronic Structure in Li Ordering and Chemical Strain in the Fast Charging Wadsley–Roth Phase PNb9O25. Chemistry of Materials, 33(19), pp.7755-7766.
Preefer, M.B., Saber, M., Wei, Q., Bashian, N.H., Bocarsly, J.D., Zhang, W., Lee, G., Milam-Guerrero, J., Howard, E.S., Vincent, R.C. and Melot, B.C., 2020. Multielectron redox and insulator-to-metal transition upon lithium insertion in the fast-charging, Wadsley-Roth phase PNb9O25. Chemistry of Materials, 32(11), pp.4553-4563.
Baek, S.W., Preefer, M.B., Saber, M., Zhai, K., Frajnkovič, M., Zhou, Y., Dunn, B.S., Van der Ven, A., Seshadri, R. and Pilon, L., 2022. Potentiometric entropy and operando calorimetric measurements reveal fast charging mechanisms in PNb9O25. Journal of Power Sources, 520, p.230776.

Figure 1