In situ Raman and in situ SAXS/WAXS have shown that oxidation of I -in the confinement of carbon nanopores generates not only I3 -and I5 -polyiodides, but to a large extend solid iodine nanoparticles. Induced by the confinement of the carbon nanopores solid I2 dissolves only slowly during self-discharge. The derived reaction mechanism explains all recorded in situ spectroscopic, in situ scattering, and electrochemical data and suggests pathways to increase capacity densities and further reduce self-discharge. We demonstrate that an effective strategy to improve the iodine pore filling capacities and reduce self-discharge lies in reducing the concentration of iodide in the carbon nanopores in the charged state using an inert supporting electrolyte.