Cation Disorder and Lithium Insertion Mechanism of Wadsley–Roth Crystallographic Shear Phases from First Principles

Wadsley–Roth
crystallographic shear phases form a family
of compounds that have attracted attention due to their excellent
performance as lithium-ion battery electrodes. The complex crystallographic
structure of these materials poses a challenge for first-principles
computational modeling and hinders the understanding of their structural,
electronic and dynamic properties. In this article, we study three
different niobium–tungsten oxide crystallographic shear phases
(Nb12WO33, Nb14W3O44, Nb16W5O55) using an enumeration-based
approach and first-principles density-functional theory calculations.
We report common principles governing the cation disorder, lithium
insertion mechanism, and electronic structure of these materials.
Tungsten preferentially occupies tetrahedral and block-central sites
within the block-type crystal structures, and the local structure
of the materials depends on the cation configuration. The lithium
insertion proceeds via a three-step mechanism, associated with an
anisotropic evolution of the host lattice. Our calculations reveal
an important connection between long-range and local structural changes:
in the second step of the mechanism, the removal of local structural
distortions leads to the contraction of the lattice along specific
crystallographic directions, buffering the volume expansion of the
material. Niobium–tungsten oxide shear structures host small
amounts of localized electrons during initial lithium insertion due
to the confining effect of the blocks, but quickly become metallic
upon further lithiation. We argue that the combination of local, long-range,
and electronic structural evolution over the course of lithiation
is beneficial to the performance of these materials as battery electrodes.
The mechanistic principles we establish arise from the compound-independent
crystallographic shear structure and are therefore likely to apply
to niobium–titanium oxide or pure niobium oxide crystallographic
shear phases.