Structural Stability from Crystallographic Shear in TiO₂-Nb₂O₅ Phases: Cation Ordering and Lithiation Behavior of...

The host structure and reversible lithium insertion and extraction of an intercalation compound, TiNb₂₄O₆₂, are described. Neutron diffraction, applied for the first time to TiNb₂₄O₆₂, allowed an accurate refinement of the complex block superstructure, particularly with respect to the oxygen sublattice. Analysis of the transition-metal sites revealed significant cation ordering in the mixed-metal oxide. Electrochemical analysis demonstrated highly reversible lithium intercalation with ca. 190 mA·h·g^-1 after 100 cycles (C/10 rate, 3 months). The effect of the potential window on the capacity, polarization, and reversibility was carefully examined; a minimum voltage limit of 1.1-1.2 V is critical for efficient and reversible cycling. The galvanostatic intermittent titration technique revealed three solid-solution regions, with different lithium diffusivities, in addition to the two-phase plateau that was clearly observed in the V versus Q discharge/charge profile. Lithium-ion diffusion decreases by over 3 orders of magnitude from the dilute lithium limit early in the discharge to the lithium-stuffed phase Li_37.5(1.0)TiNb₂₄O₆₂. Nevertheless, prior to lithium stuffing, TiNb₂₄O₆₂ possesses intrinsically rapid lithium-ion kinetics, as demonstrated by the high-rate performance in thick films of ca. 10 μm particles when interfaced with a carbon-coated aluminum foil substrate. The TiO₂·Nb₂O₅ phase diagram is examined and electrochemical results are compared to related superstructures of crystallographically sheared blocks of octahedra in the TiO₂·Nb₂O₅ homologous series including the H-Nb₂O₅ end member.