Superior Coloration Efficiencies in Reflective Multicolor Electrochromics with Birefringent WO₃ Nanogratings

Abstract The degree of optical modulation per unit charge per area, known as the coloration efficiency, is a crucial figure of merit for evaluating the performance of electrochromic devices. Typically, the coloration efficiencies of most organic and inorganic electrochromic materials have lagged behind those of liquid crystal devices, reaching values in the range of one to two orders of cm²/C. As reflective electrochromics gain importance in applications like colored e‐paper and smart labels, enhancing their coloration efficiency becomes essential. Here, a wide range of vibrant colors is demonstrated resulting from birefringent mechanisms in a WO₃ nanograting and achieve reflective coloration efficiencies exceeding 2100 cm² C⁻¹ upon lithiation. The lithiation reduces the grating birefringence, dramatically enhancing the blackness, which results in superior coloration efficiencies beyond improvements from engineering the intrinsic properties of WO₃. Additionally, unlike other strategies based on resonant cavities that display changing colors during intensity variations due to the modified refractive index upon lithiation, the approach ensures consistent hues over all intensity levels as the colors are primarily determined by the fixed period rather than the shifting refractive index. These results suggest new pathways for achieving high‐performance reflective electrochromic devices.