Robust MnO<sub>2</sub>–WO<sub>3</sub> Complementary\nElectrochromic Device Enabled by Reversible Electrodeposition of MnO<sub>2</sub>

Reversible electrodeposition and\ndissolution of manganese\noxide\n(MnO<sub>2</sub>) represent an emerging electrochromic system. However,\nchallenges such as “dead MnO<sub>2</sub>” formation,\nlimited optical modulation across a narrow wavelength range, and difficulties\nin scaling up have significantly hindered the development of MnO<sub>2</sub> reversible electrodeposition-based electrochromic windows.\nIn this work, we introduced Fe<sup>2+</sup>/Fe<sup>3+</sup> mediator\nions into the electrolyte to suppress the Jahn–Teller effect,\nthereby preventing the formation of “dead MnO<sub>2</sub>”\nand achieving stable and reversible MnO<sub>2</sub> deposition/dissolution.\nFurthermore, by employing WO<sub>3</sub> as the counter electrode,\nwe developed a complementary electrochromic device based on ion insertion\nand metal oxide deposition. This complementary system exhibits color\nneutrality in the colored state and high optical contrast across the\nentire visible spectrum, with an average optical modulation of 67.3%,\nexcellent cycling stability (85.0% retention after 3000 cycles), and\ncapability to switch uniformly over areas as large as 100 cm<sup>2</sup>.