(310d) Electrochromism as an in Operando Tool to Deconvolute Dynamic Li-Ion Charging Processes in Nanocrystal Electrodes

Li-ion batteries are commercially important because of their high specific capacities and stability at room temperature. Typical Li-ion insertion electrodes store and release charge through an assortment of reversible interactions, including surface capacitance, single-phase Li insertion and two-phase transformations. An assortment of in situ and in operando characterization tools have been developed to deconvolute and quantify these processes to engineer better batteries. However, it is very difficult to resolve how batteries charge in operando at the time and length scales relevant to these processes simultaneously. Color is a simple, yet often overlooked, indicator of the convoluted processes that occur in electrodes. Indeed, electrochromism is commonly observed in many ion-insertion electrodes (e.g. graphite, Li₄Ti₅O₁₂, Li_xFePO₄ and Li_xCoO₂). Transitions due to the band-gap (UV-visible), local defects (visible), electronic oscillations (near-IR) cause distinct features, which can be used to deconvolute charging processes. Coupled with electrochemistry, these measurements reveal rate-limiting processes and track the state-of-charge (SoC) of independent charging transformations.

Here, we demonstrate how optical characterization of nanostructured TiO₂ thin-film electrodes can serve as an effective probe to deconvolute dynamic nanoscale charging processes. Anatase TiO₂ nanocrystal films, synthesized with nm-scale control of size and shape, are studied as a model Li-ion anode material. Using optical measurements during charge titration experiments, coupled with phase-field models of individual particles, we find that particle size systematically tunes the (de)lithiation potentials of nanocrystalline TiO₂. We demonstrate how the distribution of critical transformation potentials in an ensemble of particles changes non-equilibrium charging rates. These measurements yield an unexpected relationship between the initial state-of-charge and charging rates across a range of different nanocrystal particle morphologies. We conclude that mesoscopic Li diffusion and the hysteretic 'memory' of the particle ensemble dictate practical charging rates in nanostructured TiO₂. More broadly, these results indicate a path forward to deconvolute non-equilibrium charging behavior in nanostructured systems through a combination of precise colloidal synthesis, first-principles modeling, and in operando resolution of dynamic processes with optical spectroscopy.