The ability for these smaller cations to access more sites in the suggests an explanation for the differing electrochromic properties for NC films reduced in these electrolytes. The occupation of these sites leads to an increase in the lowest binding energy W 4f peak seen by XPS, corresponding to a more reduced state of tungsten. The more reduced state then contributes to the higher energy optical transitions observed in the UV-Vis-NIR spectra. This understanding illustrates that selecting cations for intercalation into specific interstitial sites is a viable design strategy to tune the spectral response of electrochromic materials. This approach can be used in combination with tuning the morphology of NCs to control the spectral line shape of the electrochromism to tailor it to solar radiation, customize the optical tint, and better optimize spectrally selective NIR blocking smart windows to enable for increased building energy efficiency.