The modeled in situ SAXS intensities fit well to the reduced experimental in situ SAXS intensities measured during positive polarization (Fig. -oxidation proceeds from red to blue). The increasing scattering intensity is caused by the increasing amount of solid iodine as visualized on cross-sections and 3D cut-outs in Fig. (see Supplementary Video of the 3D model at maximum pore filling). The overall shape of the scattering curve remains similar to the shape of the empty nanopore structure (Fig. and bottom red curve in Fig. at the beginning of I -oxidation). This finding is attributed to the similar characteristic feature size of the solid iodine structure and the empty carbon nanopores on the one hand (compare fit parameters lZ and lY, Supplementary Fig. ), and the correlation between the iodine and carbon structure on the other hand (fit parameter ). Given the size of the confining carbon pore structure and the crystallite sizes obtained from the Scherrer analysis (Supplementary Fig. ), the iodine particles should not be much larger than the mean pore size of 0.81 nm. The I2 pore occupancy reaches a maximum of 30% at the maximum positive cell voltage of 0.8 V (Fig. ). This I2 pore occupancy fits well with the value estimated from Faradaic capacity, assuming that all capacity forms I2 (blue line; in reality, a certain fraction also reacts to polyiodides). Given the poor electronic conductivity of I2, such high degree of pore filling is remarkable. We conclude that solid I2 rather than dissolved polyiodides represent the primary, capacity-relevant fraction of oxidized species. While in situ Raman data confirms the existence of significant amounts of I3 -and I5 -, relative to solid I2, their absolute amount is small at least up to charging capacities of 200 mAh gC -1 . The quantification via plurigaussian random fields, presented here, confirms the qualitative data interpretation from above. Contrary to the current state-of-the-art, oxidation of iodide in nanoporous carbon electrodes results in solid iodine nanoparticles, as well as dissolved I3 -and I5 -polyiodides. Processes in the positive electrode of a hybrid NaI supercapacitor are comparable to those in common conversion-type battery electrodes, such as Li-S or Li-O2 battery cathodes, where solid Li2S or Li2O2 precipitate from solution species . During charging and discharging, the solid active material is deposited within the nanopore network of a carbon cathode. The pore occupancy of active material hence determines achievable capacities and reversibility.