A Long‐Life Battery‐Type Electrochromic Window with Remarkable Energy Storage Ability

Electrochromic (EC) windows with controllable transmittances according to ambient temperature and solar irradiation strength are highly desired for energy‐efficient buildings. However, traditional EC windows operate via consuming electrical energy to trigger the chromogenic reactions, with negligible energy storage ability. Herein, a facile battery‐type Prussian blue (PB, Fe 4 III [Fe II (CN) 6 ] 3 )/Zn EC window with excellent EC properties (transmittance modulation = 84.9% at 633 nm), remarkable energy storage ability (average output voltage = 1.24 V and areal capacity = 78.9 mAh m −2 ), fast switching time ( t bleaching = 4.1 s and t coloring = 4.6 s), as well as an ultralong cycling lifetime (7000 cycles with 60.7% capacity retention and 92.7% transmittance modulation retention) is reported, thanks to the rational electrode and electrolyte design. As an EC window, this device can effectively promote energy efficiency and occupational comfort of buildings by regulating visible light transmittance. As a battery, this device can work as backup power or a component of smart grids, making the buildings more sustainable and functional. The design concept of this bifunctional device is helpful to further the development of next‐generation EC windows.