High-performance Dawson-type polyoxometalate-based electrochromic energy storage devices with Co-doped MnO2 counter electrodes

Electrochromic energy storage devices (EESDs) represent an innovative solution for future intelligent energy systems by integrating energy storage with visual status indication. Yet the inherent low electrical conductivity and color interference of conventional MnO₂ electrodes limit both energy storage capacity and optical contrast in EESDs. To overcome these limitations, this study introduces an innovative cobalt doping strategy that simultaneously optimizes the Mn³⁺/Mn⁴⁺ ratio and oxygen vacancy concentration in MnO₂ electrodes. This optimization led to remarkable improvements in both electrochemical and optical performance: the resulting EESD exhibits an outstanding optical contrast of 76.5% at 700 nm, fast response (3.2/2.7 s for coloration / bleaching respectively), excellent coloring efficiency (150.3 cm²/C), and substantially improved areal capacitance reaching 12.45 mF/cm² (88% improvement compared to undoped devices). The synergistic enhancement stems from improved charge transfer kinetics and Li⁺ diffusion efficiency (D = 1.494 × 10^-8 cm²/s), along with excellent stability demonstrated by 80% optical modulation retention after 150 cycles and memory time up to 10 hours (14% decay). These findings offer a fundamental materials development approach for developing EESDs that integrate superior energy storage performance with real-time optical display functions.