Defect engineering of TiNb₂O₇ compound for enhanced Li-ion battery anode performances

Low-crystalline TiNb₂O₇ shows a high practical capacity of ∼280 mAh g⁻¹ by size and morphology controls, composites, doping foreign atoms. However, the defect chemistry of TiNb₂O₇ has rarely been investigated. We report an effect of point/line defects on a crystal structure and lithium storage properties of TiNb₂O_7−x engineered by different calcination temperatures and vacuum conditions. X-ray diffraction studies reveal an evolution of oxygen vacancies aligned to (h¯0l) and (0k0) planes can cause an increase in lattice constants and a cell volume expansion to ∼3%. Transmission electron microscopy observation shows calcination at 800℃ in vacuum results in not only a size reduction of particles but also a development of edge/screw dislocations generated on (3¯03) plane as a slip system, which can drive microstrain correlated to the lattice parameter change. TiNb₂O_7−x shows remarkable lithium storage properties including charge (lithiation)/discharge (delithiation) capacities of 324/284 mAh g^{− 1} and capacity retentions of 100% at a 2C rate over 500 cycles and at a 5C rate over 1500 cycles, respectively. The achievement is attributed to a synergetic effect of short Li⁺ ion path by a size reduction and reduce Li⁺ ion migration by an expansion of the lattice constants and the cell volume.