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 (MPII₃) is proposed, where the color-changing mechanism is based on the reversible redox reaction of I^-/I₃ ^- in solution, with 1-methyl-3-propylimidazolium cation (MPI⁺) forming a complex with I₃ ^- to suppress its shuttle effect. More importantly, the resulting ionic liquid MPII₃ 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 MPII₃ 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.