Tracking Lithium Intercalation in Battery Electrodes via Their Electrochromic Properties Using <i>Operando</i> Ellipsometry

A deep understanding of charge storage mechanisms in lithium-ion battery electrodes is necessary to advance next-generation materials for fast charging and high-power-density applications. Under high-rate conditions, disentangling charge storage contributions (e.g., intercalation, alloying, pseudocapacitance, capacitance, interphase formation, electrolyte degradation, and other processes) is complex, especially for thin film and nanosized materials that typically exhibit surface-dominated behavior. Often overlooked, the electrochromic properties of lithium-ion battery electrodes are innately related to their charge storage properties. In this work, we demonstrate how <i>operando</i> ellipsometry provides detailed insights into charge storage kinetics in battery electrodes. Using a model TiO₂-anatase thin film, we validate the energy loss function (ELF) derived from the complex dielectric constant as a key indicator for tracking the lithiation state and thus state of charge. Further, we demonstrate that d(ELF)/dV acts as an "opto-voltammogram" that can be used to selectively probe and deconvolute redox reactions, analogous to differential capacity analysis (dQ/dV) or cyclic voltammetry. These findings establish spectroscopic ellipsometry as a powerful technique for optimizing materials in energy storage applications.