Li⁺ Ion Insertion in TiO₂ (Anatase). 1. Chronoamperometry on CVD Films and Nanoporous Films

Abstract

Electrochemical insertion of lithium in nanoporous and CVD samples of TiO2 (anatase) was studied by chronoamperometry. The currents following cathodic and anodic potential steps were monitored as a function of film thickness, temperature, and electrolyte concentration. The time dependence of the currents generally exhibit the behavior of a diffusion-limited process. It is demonstrated that the magnitude of the currents scales directly with the inner area of the electrodes. The potential dependence on the rate of insertion and extraction indicates that the reduction of Ti4+ and oxidation of Ti3+ is kinetically hindered. The double-layer capacitance and the adsorbate concentration at 0 V of the nanoporous structure were determined to be 30−40 μF/cm2 and 1.7−2.4 mol/cm2, respectively. The chemical diffusion coefficient at 25 °C for insertion and extraction in the nanoporous structure was 1 × 10-17 and 4 × 10-17 cm2/s, respectively. The corresponding values for the CVD samples, using the projected area, were 2 × 10-15 cm2/s for insertion and 6 × 10-15 cm2/s for extraction. The activation energy for lithium insertion and extraction was 0.35 and 0.38 eV for the nanoporous films and 0.54 and 0.78 eV for the CVD samples.