Dynamics of Lithium Insertion in Electrochromic Titanium Dioxide Nanocrystal Ensembles

Nanocrystalline anatase TiO₂ is a robust model anode for Li-insertion in batteries. The influence of nanocrystal size on the equilibrium potential and kinetics of Li-insertion is investigated with in operando spectroelectrochemistry of thin film electrodes. Distinct visible and infrared responses correlate with Li-insertion and electron accumulation, respectively, and these optical signals are used to deconvolute Li-insertion from other electrochemical responses, such as double-layer capacitance and electrolyte leakage. Electrochemical titration and phase-field simulations reveal that a difference in surface energies between anatase and lithiated phases of TiO₂ systematically tunes Li-insertion potentials with particle size. However, particle size does not affect the kinetics of Li-insertion in ensemble electrodes. Rather, Li-insertion rates depend on applied overpotential, electrolyte concentration, and initial state-of-charge. We conclude that Li diffusivity and phase propagation are not rate-limiting during Li-insertion in TiO₂ nanocrystals. Both of these processes occur rapidly once the transformation between the low-Li anatase and high-Li orthorhombic phases begins in a particle. Instead, discontinuous kinetics of Li accumulation in TiO₂ particles prior to the phase transformations limits (dis)charging rates. We demonstrate a practical means to deconvolute non-equilibrium charging behavior in nanocrystalline electrodes through a combination of colloidal synthesis, phase field simulations and spectroelectrochemistry.