Tailoring Diffusion Dynamics in Energy Storage Ionic Conductors for High‐Performance, Multi‐Function, Single‐Layer Electrochromic Supercapacitors

Abstract Electrochromic (EC) energy storage devices represent a cutting‐edge technology visually indicating stored energy status in real time through color change. Herein, extremely simple single‐layer electrochromic supercapacitors (SL‐ECSs) based on energy storage EC ion gels are proposed. The operation of SL‐ECSs follows Fick's law because of the diffusive nature of mass transport of all redox species. Therefore, the diffusion dynamics are tuned by tailoring fundamental parameters, the diffusion coefficient, and concentration gradient, and the availability of energy storage is maximized by reducing residual charges that cannot be extracted during the normal discharge process. In terms of the two critical metrics of ECSs, areal capacitance (C_areal) and transmittance contrast between charged and discharged states (ΔT), the performance of SL‐ECSs (C_areal ≈ 43.0 mF cm^–2 and ΔT ≈ 96.8%) is favorably compared with previously reported ECSs. The capacitance retention remains at >80% even after continuous charge/discharge cyclic operations for 3000 min without specific encapsulation. Moreover, the practical multi‐functionality of the SL‐ECS is successfully demonstrated as a power source and applied force monitoring platform. It is expected that the SL‐ECS will be a simple but versatile component of high‐performance, ultra‐small functional electronics.