In the WAXS regime (q > 6 nm -1 ), distinct intensity peaks form around 7.5 nm -1 , 16 nm -1 , and 21.5 nm -1 (Fig. ). To better visualize the peak formation, we subtracted the structure factor contribution of the 1 M NaI electrolyte (that is, the WAXS intensity at 0 V cell voltage). We attribute the peak around 7.5 nm -1 to I3 - -I3 -correlations of by a highly concentrated triiodide solution in the cavities formed by nanoporous carbon and solid I2, justified by the mean distance between I3 -species being in the order of 2π/7.5 nm -1 = 0.84 nm.