Investigation of the Li-Ion Insertion Mechanism for Amorphous and Anatase TiO2Thin-Films

Titania is considered an interesting anode candidate for Li+-ion batteries, as it offers a high theoretical capacity (1280 mAh cm−3 or 336 mAh g−1) and long term cycling stability. Unfortunately, the most commonly investigated anatase structure never reaches the theoretical capacity at practical charging rates (i.e. above 1 C). In this work, we compare amorphous (am-TiO2) to anatase TiO2 thin-films, and investigate the exceptional performance of am-TiO2 as Li+-ion insertion electrode. An in-depth electrochemical characterization using cyclic voltammetry (CV), constant current lithiation and delithiation, and potentiostatic intermittent titration technique (PITT) is performed. From CV, the insertion and extraction kinetics of am-TiO2 is found to be unrestricted by diffusion, contrary to anatase. Based on our combined electrochemical results, two different mechanisms are formulated for anatase and am-TiO2. Whereas anatase is filled from the "top-down", with a buildup of Li near the electrode/electrolyte interface, am-TiO2 shows a "bottom-up" filling mechanism. This discrepancy is ascribed to the difference in diffusion coefficient measured for am-TiO2 and anatase. This work highlights the differences of Li-ion insertion into amorphous TiO2 compared to anatase, and gives guidance on material development for high capacity and fast charging electrodes.