Aligned TiO ₂ nanotube arrays as durable lithium-ion battery negative electrodes

Vertically aligned amorphous titania (TiO ₂) nanotubes are produced by anodizing Ti foils at various applied potentials in a neutral electrolyte solution containing fluoride ions. Pore size and wall thickness are tuned in the range from 30 to 70 nm and 17 to 35 nm, respectively, by adjusting the applied potential, in addition to tuning the tube length from 355 to 550 nm. Utilizing all of these films as negative electrode materials in lithium-ion batteries delivers stable capacities of 130-230 mAh g ^-1 and 520-880 mAh cm ^-3 up to 200 cycles. Microstructural analysis shows that there is no structural change or mechanical degradation in the active material, and the amorphous active material maintains good contact with the substrate/current collector. A continuum elasticity model for the tubular geometry is presented to understand the diffusion-induced stresses, fracture tendency, and stability in TiO ₂ nanotubes. Modeling results indicate that the fracture tendencies of nanotubes with the dimensions in this work are very small; stable reversible capacity retention results from the high ratio of inner to outer diameter of the tubes. In other words, tubes with thinner walls more easily accommodate expansion or contraction during the lithiation/delithiation process. A guideline for designing lithium-ion battery nanotube electrodes is given such that under specific conditions the fracture tendency is small and volumetric charge density is high.