Li<sup>+</sup> Ion Insertion in TiO<sub>2</sub> (Anatase). 2. Voltammetry on Nanoporous Films
Henrik Lindström, Sven Södergren, Anita Solbrand, Håkan Rensmo, Johan Hjelm, Anders Hagfeldt, Sten‐Eric Lindquist
Abstract
Electrochemical properties of Li + ion insertion in nanoporous TiO 2 (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 Ti 4+ and oxidation of Ti 3+ 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 cm 2 /s for insertion and 4 × 10 -17 cm 2 /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 Li x TiO 2 was x = 0.47.