Tailored TiNb2O7 Particle Size, Defects, and Crystallinity Accelerate Lithiation

Abstract

TiNb₂O₇ (TNO) is a promising anode material for durable, fast-charging lithium-ion batteries that combine fast lithium diffusion with minimal lithiation strain based on a Wadsley–Roth crystal structure. Battery performance depends on a convolution of defect-sensitive material properties (lithium diffusivity and electrical resistivity) in conjunction with the architecture (feature size, porosity, and charge-transfer surface). All of these attributes connect to synthetic conditions where there is an opportunity to improve performance by understanding the interplay of changes during crystallization. Nanostructured TNO was prepared via spray drying, where variable crystallization temperature simultaneously influenced crystal structure/defects while coarsening the architecture. Comprehensive X-ray analysis (WAXS, SAXS, and XRD) and microscopy (SEM) characterized the crystallization and coarsening progress. The galvanostatic lithiation capacities (700–1100 °C) were similar at 0.1C (∼300 mAh/g); however, sample TNO-800 exhibited the highest capacity at 5C (260 ± 3 mAh/g). Intermittent current interruption (ICI) analysis revealed increasing diffusivity with calcination temperature and a nonmonotonic trend in cell resistance, minimized for TNO-800. Detailed X-ray near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and Rietveld analyses identified that crystallization led to progressive point/extended defect elimination and increasing octahedral distortion. With cell resistance including both electrical resistance and charge-transfer resistance, this overall trend reflects competition of generally improving electronic properties with calcination temperature against the coarsening architecture that progressively increases the charge-transfer resistance. The optimal TNO-800 exhibited a remarkable 10C capacity of 233 ± 1 mAh/g, which compares favorably with leading TNO precedents. This study highlights the complex convolution of atomic structure and architecture changes that occur during crystallization, which may advance other known battery materials.