Structural Stability from Crystallographic Shear in TiO₂–Nb₂O₅ Phases: Cation Ordering and Lithiation Behavior of TiNb₂₄O₆₂

The host structure and reversible lithium insertion and extraction of an intercalation compound, TiNb 24 O 62, are described. Neutron diffraction, applied for the first time to TiNb 24 O 62, 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) TiNb 24 O 62 . Nevertheless, prior to lithium stuffing, TiNb 24 O 62 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 TiO 2 ·Nb 2 O 5 phase diagram is examined and electrochemical results are compared to related superstructures of crystallographically sheared blocks of octahedra in the TiO 2 ·Nb 2 O 5 homologous series including the H–Nb 2 O 5 end member.