Abstract
Studying batteries in real time is essential for understanding their operation and degradation mechanisms. Operando methods such as X-ray diffraction, NMR, and electron microscopy provide detailed insights but are costly, complex, and require specialized cell designs, making them impractical for long-term cycling or screening. Here, we propose using operando diffuse reflectance spectroscopy (DRS) to probe battery electrodes as they charge and discharge. This technique measures subtle changes in the reflectance spectrum of battery electrodes caused by electronic structure changes during cycling. By correlating these optical property changes with the state-of-charge of the battery, we can reconstruct a 'second view' of the electrochemistry of the battery. We show that DRS can be used to determine heterogeneity in state-of-charge, study solid state lithium-ion diffusion, uncover the origin of first cycle capacity losses, and study surface limited behaviour. We apply operando DRS to a wide variety of battery materials to show that the proposed method enables the extraction of information that previously was only accessible using methods that are orders of magnitude more expensive.