Li⁺ Ion Insertion in TiO₂ (Anatase). 2. Voltammetry on Nanoporous Films

Electrochemical properties of Li+ ion insertion in nanoporous TiO2 (anatase) electrodes were studied by voltammetry. Linear and cyclic potential scans were recorded as a function of electrolyte concentration, film thickness, and temperature. The currents were directly proportional to the inner electrode area of the electrodes. The reduction of Ti4+ and oxidation of Ti3+ are sluggish and follows irreversible kinetics. The standard rate constant was (3.5 ± 0.5) × 10-10 cm/s. The transfer coefficient was close to 0.5, indicating that the potential drop appears mainly across the Helmholtz layer. The capacitive currents govern largely the shape of the i−v curves, except within a region near the peak potential where diffusion-limited insertion and extraction of Li+ ions in the anatase lattice are dominating. The diffusion coefficient at 25 °C in the nanoporous structure was approximately 2 × 10-17 cm2/s for insertion and 4 × 10-17 cm2/s for extraction. The activation energy was 0.4 eV for insertion and 0.5 eV for extraction. The maximum obtained mole fraction of Li+ in LixTiO2 was x = 0.47.