Ionic Liquid MPII3 Elevates Electrochromic Battery Capacity to Practical Applications

Electrochromic batteries are multifunctional devices that integrate optical modulation with energy storage capabilities through electrochemical reactions. Traditional electrochromic batteries rely on solid-state thin films, but their limited material loading constrains capacity to ≈100–300 mAh m −2 , failing to meet practical demands. Herein, an electrochromic battery based on ionic liquid 1-methyl-3-propylimidazolium triiodide (MPII 3 ) is proposed, where the color-changing mechanism is based on the reversible redox reaction of I − /I 3 − in solution, with 1-methyl-3-propylimidazolium cation (MPI + ) forming a complex with I 3 − to suppress its shuttle effect. More importantly, the resulting ionic liquid MPII 3 demonstrates exceptional reaction kinetics, enabling rapid and extensive charge transfer, thereby significantly enhancing the energy storage potential of electrochromic batteries. The fabricated devices achieve a high capacity of 56396 mAh m −2 at a current density of 0.5 mA cm −2 . Additionally, these MPII 3 electrochromic batteries exhibit an optical modulation as high as 68.1% and excellent cycling stability, with 92.3% capacity retention after 20 000 cycles. These findings represent a significant advancement and are expected to promote the practical application of electrochromic batteries in smart windows, energy-efficient buildings, and other fields.