Redox Mechanisms, Structural Changes, and Electrochemistry of the Wadsley–Roth Li_xTiNb₂O₇ Electrode Material

Abstract

The TiNb2O7 Wadsley–Roth phase is a promising anode material for Li-ion batteries, enabling fast cycling and high capacities. While already used in commercial batteries, many fundamental electronic and thermodynamic properties of Li x TiNb2O7 remain poorly understood. We report on an in-depth first-principles study of the redox mechanisms, structural changes, and electrochemical properties of Li x TiNb2O7 as a function of Li concentration. First-principles electronic structure calculations reveal an unconventional redox mechanism upon Li insertion that results in the formation of metal–metal bonds. This metal dimer redox mechanism has important structural consequences as it results in a shortening of cation-pair distances, which in turn affects lattice parameters of the host and thereby alters Li site preferences as the Li concentration is varied. The new insights about redox mechanisms in TiNb2O7 and their effect on the structure and Li site preferences provide guidance on how the electrochemical properties of a promising class of anode materials can be tailored by exploiting the tremendous structural and chemical diversity of Wadsley–Roth phases.

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

“…We note that edge-sharing pairs of cations in a lower oxidation state (e.g., Nb 4+ ) can form metal–metal bonds, which would then lead to a contraction of the metal–metal distance. This has been predicted to occur during the lithiation of Wadsley–Roth phases. , …”

“…These particular collective displacement modes characterize a second-order Jahn–Teller distortion that many d 0 transition metal cations, such as Ti 4+ and Nb 5+ , are susceptible to when octahedrally coordinated by oxygen. , An MO 6 octahedron has a total of 15 symmetry-adapted collective displacement modes that form a basis with which to describe an arbitrary distortion of the octahedron . We analyzed the deformations in the 44 fully relaxed TiNb 2 O 7 structures by decomposing the coordinates of each MO 6 octahedron as a linear superposition of the 15 symmetry-adapted collective displacements of an octahedron as described in Saber and Van der Ven Figure b,c shows histograms of the amplitudes of the off-centering displacement modes of Figure a for the TiO 6 and NbO 6 octahedra, respectively.…”

“…Figure b,c shows histograms of the amplitudes of the off-centering displacement modes of Figure a for the TiO 6 and NbO 6 octahedra, respectively. The displacement amplitudes were calculated as the Euclidean distance of the amplitudes of the three symmetry-adapted collective displacement modes of Figure a, as described in Saber and Van der Ven …”

Redox Mechanisms upon the Lithiation of Wadsley–Roth Phases

“…The first is an atom-centric redox mechanism involving t 2g orbitals of early transition metals residing in corner-sharing octahedra that dominates at dilute Li concentrations. At less dilute Li concentrations, a different redox mechanism that relies on the formation of metal–metal dimers between edge-sharing MO 6 octahedra was recently predicted to dominate in Li x TiNb 2 O 7 . The t 2g orbitals (i.e., d xy , d yz , and d xz ) of early transition metal cations, having lobes that point between pairs of oxygen, can hybridize with neighboring t 2g orbitals to form bonding and antibonding states, as schematically illustrated in Figure .…”

“…The t 2g orbitals, such as d xy , of early transition metal cations that reside in edge-sharing octahedra made of oxygen anions can hybridize to form metal–metal bonds that can host two electrons. Reproduced from Saber et al …”