1, The University of California, Santa Barbara, Santa Barbara, California, United States
3, Rice University, Houston, Texas, United States
Anatase TiO2 is a promising anode material that shows robust cyclability and high specific energy and power. TiO2 is often studied as a model insertion electrode due to the well-characterized first-order phase transformation that occurs between the oxidized anatase TiO2 phase and reduced Li0.6TiO2 phase. The relatively slow transport of Li+ cations through TiO2 can impede the switching speed of these electrodes, so nanostructured electrodes have been engineered with high specific surface area and shortened diffusion path lengths. However, both the energetics and kinetics of charging can change dramatically once the electrode is structured with mesoporous nanocrystalline grains. Despite a wealth of synthetic strategies, and detailed theoretical models of lithium insertion in nanostructured electrodes, significant uncertainty remains about the microscopic behavior of lithium insertion and macroscopic ramifications of nanostructuring in real phase-transforming insertion electrodes. This talk will investigate the transformation pathways in nanocrystalline anatase films using a robust in situ optical characterization technique relying on the electrochromic visible color change of TiO2 during lithiation.
The distribution of sizes and shapes of nanocrystal grains in a mesoporous electrode can convolute the effects of surface faceting and grain morphology on electrochemical transformations. A surfactant-mediated colloidal synthesis is used to create monodisperse ensemble electrodes of particles with controlled morphologies ranging from isotropic 10nm particles to 100nm x 15nm platelets. Potentiostatic charge titration experiments reveal that nanocrystal size and shape impact the onset potential of lithium insertion. The kinetics of this transformation are studied by observing the electrochromic color change that occurs upon lithium insertion. This electrochromic response, attributed to localized charged Ti3+ defects in the lattice, occurs only upon lithium insertion, and is not confounded by capacitive charge compensation. The kinetics of lithium insertion and de-insertion are observed and modeled for different initial charge states and applied overpotentials, revealing nucleation and growth kinetics that correlate with nanocrystal size and shape. A qualitative nucleation and growth model suggests that lithium insertion is geometrically constrained, consistent with current models of 1-dimensional diffusion in anatase TiO2. These results are placed in context with existing studies of nanocrystalline insertion electrodes, and the role of grain morphology, solid-solid interfaces and ensemble behaviors are explored. A general strategy for probing electrochemical transformations through in situ spectroelectrochemical techniques is developed, and applied to nanocrystalline TiO2 as a functional model system for solid-state ion conductors and battery electrodes.