Electrochromic materials that can dynamically modulate the transmittance of infrared and visible light independently are essential for the development of energy-saving window technologies. Electrochemical charging and discharging modulates the optical properties in response to a potential bias, resulting in reversible coloring and bleaching. By controlling the transmittance of solar radiation, we can limit unwanted heating and regulate natural lighting, improving building energy efficiency. Both amorphous and crystalline forms of tungsten oxide absorb visible and near infrared light when electrochemically reduced, making it one of the most heavily studied materials for electrochromic window technologies. More recent research has shown that nanocrystalline forms of tungsten oxide demonstrate localized surface plasmon resonance (LSPR), allowing selective optical modulation in the near infrared. LSPR is a property of conductive nanoparticles where the free charge carriers oscillate collectively in resonance with the electric field of incident light, leading to strong absorption. This absorption occurs throughout the infrared for various conductive metal oxides, making it ideal for controlling the transmittance of infrared light in solar radiation. LSPR can be modulated electrochemically through oxidation or reduction, which varies the population of free charge carriers that contribute to light absorption. Electrochemical modulation of LSPR has been described as due to purely capacitive charging, which is a surface property where charge is stored in the electrical double layer at the interface between the electrolyte and electrode surface. The modulation of the LSPR of Sb:SnO2 and Sn:In2O3 NCs has been shown to be mostly independent of the electrolyte cation used, with only small differences between electrolytes containing lithium and tetrabutylammonium (TBA + ) cations. Differences in the spectral response arising at more strongly reducing potentials have been ascribed to the irreversible intercalation of lithium and the reduction of either Sb or Sn. The crystal structures of these materials do not contain easily accessible interstitial voids or channels for cations to diffuse through reversibly during electrochemical cycling. However, plasmonic tungsten oxide NCs do contain cation-accessible interstitial sites. Tungsten oxide exists in various phases, all containing different arrangements of WO6 octahedra. The various crystal phases of tungsten oxide allow for cation intercalation into the interstitial voids between the tungsten octahedra. The impact of ion intercalation on the LSPR of tungsten oxide NCs is not well understood.