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

Abstract

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 Li37.5(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.