To understand how the intercalation of different sized cations influences the electrochromic spectral response, we must first consider the size of the interstitial sites in the hexagonal lattice. The hexagonal crystal structure has three interstitial voids that vary in size. The first are the large hexagonal channels that run along the c-axis of the crystal structure. The second are the smaller trigonal channels that run parallel to the c-axis as well. The last set of pathways ions can potentially diffuse through are the square channels that run orthogonal to the other two. To compare the approximate size of these different voids in the material, a circle was inscribed in the channel and the radius was used to compare the relative sizes of the different sites (Figure and). The approximate sizes are 236 pm, 78 pm, and 129 pm for the hexagonal, trigonal, and square sites, respectively. The ionic radii of the four cations used in this study are 60 pm, 95 pm, 133 pm, and 494 pm for Li + , Na + , K + , and TBA + , respectively. Comparing the relative sizes of the ionic radii to the sizes calculated for the different sites in the material, we expect certain cations to be inhibited from diffusing through certain sites. The ionic radius of the lithium cation is small enough that it is expected to be able to freely diffuse through all the sites. The opposite can be said for TBA + . Its much larger size limits its ability to intercalate into the material at all, which restricts it to purely surface charging. Na + is small enough that it is expected to be able to diffuse through the square and hexagonal channels, while K + is only expected to be able to diffuse through the hexagonal channels. Based on this analysis and the observed electrochromism, it appears that the ability for Li + and Na + to diffuse through and occupy sites within the smaller square or trigonal channels leads to a secondary mode of absorption during electrochemical reduction.