From a broader perspective, the work shows the necessity of applying advanced in situ techniques for detailed insights into electrochemical mechanisms and to derive practical conclusions for real devices. We show that properties and function of complex emerging systems for electrochemical energy storage are equally controlled by chemistry (in situ spectroscopy) and the nanoscale structure (in situ scattering). In combination with stochastic modeling, in situ SAXS/WAXS allowed for quantifying the iodine pore filling and visualizing the (sub-)nm iodine phase evolution during electrochemical cycling. Hence, in situ SAXS proves as a powerful experimental method to study the phase evolution of active materials in nanoporous carbons and beyond.