Capacity Limiting Effects for Freestanding, Monolithic TiO₂ Nanotube Electrodes with High Mass Loadings

Abstract

Galvanostatic and cyclic voltammetric experiments have been used to identify the main capacity limiting phenomenon for TiO 2 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 twostep 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 TiO 2 lithiation step. The maximum average TiO 2 lithiation degree (which correspond to an average composition of about Li 0.55 TiO 2 ) is a result of a decrease in the lithium ion diffusion rate with an increasing concentration of Li x TiO 2 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 Li x TiO 2 and TiO 2 concentration profiles obtained in the nanotubes. During the lithiation, the increasing Li x TiO 2 concentration in the nanotubes gives rise to a decreasing lithiation voltage when the Li x TiO 2 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.

Lithium‐Diffusion Induced Capacity Losses in Lithium‐Based Batteries

“…[ 39 ] recently discovered a mass transfer dependent memory effect analogous to the diffusion‐controlled Li‐trapping effect seen for Li + insertion in TiO 2 nanotube electrodes. [ 36 ] In both cases, the capacity was found to gradually decrease during cycling at rates of 5C and 10C although the full capacity was regained once the cycling was continued at a lower rate.…”

“…As the diffusion‐controlled Li‐trapping effect is caused by the presence of Li concentration gradients in the electrode, this trapping effect should, however, not be seen if the material can be homogeneously lithiated. While this can be realized for, for example, TiO 2 nanotube electrodes, [ 36 ] a homogeneous lithiation is unlikely to be reached for conventional electrode materials due to the longer Li diffusion path lengths (when increasing this length from 1 nm to 1 µm, the diffusion time would be increased by a factor of one million).…”

“…When discussing Li‐trapping in metals, it should also be mentioned that the effect should involve elemental Li rather than Li + , as lithium ions (for obvious electroneutrality reasons) cannot diffuse into a metal in the absence of a counter ion. [ 36 ]…”

“…Wei et al. [ 36 ] investigated the capacity limiting effects for binder‐ and additive‐free monolithic TiO 2 nanotube electrodes. By comparing the results of galvanostatic and voltammetric experiments it was found that the capacity was limited by the lithiation step and that the lithiation capacity was limited by the Li + diffusion rate in the electrode.…”

“…The latter is in good agreement with previous results obtained for negative electrode materials such as Si [ 20 ] and TiO 2 . [ 36 ]…”