Capacity Limiting Effects for Freestanding, Monolithic TiO<sub>2</sub> Nanotube Electrodes with High Mass Loadings
Wei Wei, Charlotte Ihrfors, Fredrik Björefors, Leif Nyholm
Abstract
Galvanostatic and cyclic voltammetric experiments have been used to identify the main capacity limiting phenomenon for TiO2 nanotube electrodes with nanotube lengths between 4.5 and 40.5 μm and mass loadings up to 10.5 mg cm–2. The results for the nanotube electrodes, which were synthesized by using a two-step anodization and evaluated in pouch cell batteries containing lithium metal counter electrodes, show that higher capacities could be obtained by using voltammetric rather than galvanostatic cycling and that the capacity is limited by the TiO2 lithiation step. The maximum average TiO2 lithiation degree (which correspond to an average composition of about Li0.55TiO2) is a result of a decrease in the lithium ion diffusion rate with an increasing concentration of LixTiO2 in the nanotubes. This saturation effect is also responsible for the diffusion-controlled decrease in the capacity seen when increasing the constant current cycling rate. The different electrochemical lithiation and delithiation behaviors are explained based on the differences between the LixTiO2 and TiO2 concentration profiles obtained in the nanotubes. During the lithiation, the increasing LixTiO2 concentration in the nanotubes gives rise to a decreasing lithiation voltage when the LixTiO2 concentration becomes sufficiently high. The areal capacity of the nanotube electrodes can be increased from 0.18 to 1 mAh cm–2 (at a rate of C/5) by increasing the length of the nanotubes from 4.5 to 40.5 μm. Although the cell resistance is shown to be practically independent of the nanotube length, the increasing mass loading and hence current required at a given cycling rate result in larger iR drops for the longer nanotubes. The data also indicate the presence of a lithium-ion trapping effect due to two-way diffusion of lithium ions in the lithiated nanotubes in analogy with the behavior previously found for lithium-alloy-forming electrode materials.