To separate the nanopore scattering contribution from the electrolyte (+ carbon) structure factor and the lowq particle scattering (Supplementary Fig. ), we first determine the electrolyte structure factor using a modified Porod fit of the form -.*/*0 = 2 3 4 + 5 • 78(3) ⁄ at 6.5 nm -1 < q < 9 nm -1 . 78(3) is the measured scattering intensity of the bulk electrolyte, 5 a multiplicative constant, and P the porod constant. The latter two parameters are determined by the fit. Since the nanopore scattering intensity is fitted with a simple two-phase model (see below), density fluctuations within the carbon matrix are neglected. After subtracting the electrolyte structure factor contribution 5 • 78(3), the particle scattering contribution is determined from a power law fit of the form -. /9:);< = = 3 .? + @ ⁄ at 0.4 nm -1 < q < 1 nm -1 , with B and C being the unknown fit parameters. The exponent of 3.73 is determined from a power law fit on the empty AC electrode. The obtained particle scattering = 3 .? ⁄ is then subtracted from the experimental SAXS intensities.