ABSTRACT Electrochromism (EC) enables energy‐efficient, and non‐volatile optical modulation through reversible ion intercalation, with promising applications in smart windows, color displays, and advanced optical devices. Although numerous cations, such as H + , Li + , Na + , Mg 2+ , Zn 2+ , and Al 3+ , have been intensively examined for enhanced EC performance, a precise, quantitative understanding of how different cation species govern the evolution of optical constants ( n , k ) and the resultant coloration remains a challenge. Herein, we develop a Fabry‐Perot resonant nanocavity film (FPRNF) consisting of a WO 3 layer atop a metallic reflector. The FPRNF transduces subtle changes in optical constants ( n , k ) of WO 3 upon different cation intercalation into distinct spectral and color responses. We establish a precise quantitative relationship between these EC responses and the density of intercalated cation/charge, highlighting that the injected electrons accompanying cation intercalation are the primary determinant of coloration behavior. The intercalation of higher‐valence cations such as Al 3+ promotes substantial charge compensation, with electron injection over 400 mC/cm 2 ·µm. This significantly alters the optical constants ( n , k ), with achieved values of Δ n = 0.43 and Δ k = 0.50, and consequently drives a pronounced resonant wavelength shift of 160 nm and intensity modulation of approximately 40%, leading to greatly enhanced EC performance.
Chen et al. (Sat,) studied this question.