mirrors, [2] electronic paper, [3] spacecraft thermal control, [4] and military camouflage, [5] etc.Generally, there are five functional layers in one electrochromic device (ECD): two conductive layers, one electrochromic layer, one electrolyte layer, and one counter electrode layer (i.e., the ion storage layer). Among those, the electrochromic layer is responsible for the color transformation. The counter electrode layer coordinates the redox reaction of the electrochromic layer, storing the counter ions to achieve the charge balance concurrently. [6,7] Therefore, the counter electrode layer plays an essential role in the optical performance and long-term stability of ECDs. Compared to the mature research on electrochromic materials, [8] counter electrode materials (CEMs) have been largely ignored. Due to the very limited options for CEMs, many reported works just simply use bare tin-doped indium oxide (ITO), without a counter electrode layer, when evaluating electrochromic materials in a device set-up. This is a simple way to construct an ECD, whereas, the redox process of ITO is irreversible, leading to serious degradation in its conductivity and transparency, [9] which makes it unsuitable for the industrial application of ECDs.

Electrochromism is a reversible color change triggered by the external electrical field, which can be utilized in smart windows, displays, etc. The ability to display multiple colors in a single electrochromic device is highly beneficial for the versatile applications of the electrochromic technique. Yet, achieving the ability to adopt multiple colors in one electrochromic device has proven to be a challenging task by merely using the conventional chemical structure engineering of the electrochromic materials. In this study, a thermochromic electrolyte is introduced. A multicolor electrochromic prototype device is produced by integrating the thermochromic engineered electrolyte together with electrochromic material in which both can independently contribute to the coloring of the device. This “color palette” strategy gives an additional degree of freedom and extensive flexibility to the design of multicolor electrochromic devices. It introduces a new pathway for producing multicolor electrochromic devices and removes a bottleneck imposed by conventional strategies merely depending on the electrochromic material structure engineering. Moreover, it inspires further development of electrolytes, that share the responsibility of tuning the optical properties of electrochromic devices together with the electrochromic materials, thereby accelerating and diversifying the applications of electrochromics in industry.