Chemical and Structural Factors Affecting the Stability of Wadsley–Roth Block Phases

Abstract

Wadsley–Roth phases have emerged as highly promising anode materials for Li-ion batteries and are an important class of phases that can form as part of the oxide scales of refractory multiprinciple element alloys. An algorithmic approach is described to systematically enumerate two classes of Wadsley–Roth crystallographic shear structures. An analysis of algorithmically generated Wadsley–Roth phases reveals that a diverse set of oxide crystal structures belongs to the Wadsley–Roth family of phases. First-principles calculations enable the identification of crystallographic and chemical factors that affect Wadsley–Roth phase stability, pointing in particular to the importance of the number and nature of the edges shared by neighboring metal–oxygen octahedra. A systematic study of Wadsley–Roth phases in the Ti–Nb–O ternary system shows that the cations with the highest oxidation states segregate to octahedral sites that minimize the number of shared edges, while cations with the lowest oxidation state accumulate to edge-sharing octahedra at shear boundaries.

Redox Mechanisms, Structural Changes, and Electrochemistry of the Wadsley–Roth LixTiNb2O7 Electrode Material

“…The three collective displacement modes of Figure a also form a basis to describe octahedral distortions that reside within a T 2 u irreducible subspace, with each collective displacement involving four equatorial oxygen ions that distort perpendicular to their equatorial plane. These displacement modes measure the collateral distortions of the oxygen octahedra in response to the large relaxations that lead to an off-centering of edge-sharing transition-metal cations Figure b,c plots the average Euclidian distance of the amplitudes of the three orthogonal displacement modes for the TiO 6 and NbO 6 octahedra as a function of Li concentration.…”

“…The d 0 Ti 4+ and Nb 5+ cations are susceptible to second-order Jahn–Teller distortions when octahedrally coordinated by oxygen. , This causes displacement of the cations away from the center of their coordinating octahedra. The edge-sharing MO 6 octahedra of TiNb 2 O 7 also undergo significant distortions due to the strong electrostatic repulsion between neighboring Ti 4+ and Nb 5+ cations . This repulsion increases the distance between edge-sharing cations, causing a further off-centering of each cation that simultaneously induces collateral distortions of their surrounding oxygen octahedron …”

“…The arrangement with the lowest energy as predicted with DFT-PBE is shown in Figure 1 . 15 , 43 The higher oxidation state Nb 5+ cations prefer to occupy the corner-sharing octahedra at the center of the block, while lower oxidation state cations, such as Ti 4+ , tend to segregate to the edge-sharing octahedra at the peripheries of each block. Experimentally synthesized forms of TiNb 2 O 7 exhibit some degree of disorder among Ti and Nb because the phase is cooled from high temperatures.…”

“…The edge-sharing MO 6 octahedra of TiNb 2 O 7 also undergo significant distortions due to the strong electrostatic repulsion between neighboring Ti 4+ and Nb 5+ cations. 43 This repulsion increases the distance between edge-sharing cations, causing a further off-centering of each cation that simultaneously induces collateral distortions of their surrounding oxygen octahedron. 43 …”

“…The good agreement between predicted and experimentally measured magnetic moments and lattice parameter variations gives confidence in the validity of the predicted dimer redox mechanism. The insights of this work provide guidance as to how electrochemical properties can be tuned in early transition-metal oxide intercalation compounds by exploiting the rich structural and chemical diversity of Wadsley–Roth phases …”