On the Structural Origin of Fast Li-Ion Cycling in Tetragonal Bronze-Type Nb8W9O47.

Abstract

Bronze and bronze-derived, mixed metal oxides are materials of growing interest for lithium-ion battery anodes due to their high-rate capabilities and the potential for high energy densities via multielectron redox. We report on the synthesis and electrochemical properties of the tetragonal tungsten bronze-type phase Nb₈W₉O₄₇ and our investigation into the structural evolution of this phase upon lithium (de)-intercalation, with particular attention to how it relates to the fast-cycling capabilities. Electrochemical cycling shows Nb₈W₉O₄₇ can achieve greater than one Li⁺ per transition metal at rates of C/2 or slower and maintains a capacity equivalent to 0.65 Li⁺ per transition metal at a rate of 20C. Sequential Rietveld analysis of operando X-ray diffraction data reveals anisotropic changes to the unit cell parameters during discharge, accompanied by an off-centering of the transition metals within their octahedral environments. While higher capacities can be accessed below 1.2 V, cells discharged to 1.0 V exhibit a significant expansion of the unit cell volume and reduced Coulombic efficiencies. This report suggests that rigid crystal structures incapable of undergoing polyhedral rotational deformations can instead exploit displacive distortions of the transition metal redox center to enable fast Li-ion cycling with minimal atomic rearrangements.