High-Performance Aqueous Electrochromic Battery for Smart Window Application: Mechanistic Insights of Al-Ion (De)Intercalation Kinetics in Thickness-Optimized WO₃

Aqueous-based electrochromic batteries have significant potential for use in color indicator energy storage, including energy-efficient buildings and wearable devices with visible energy level performance. While many materials have the ability to change color upon ion intercalation, they often have low optical contrasts and capacities. To address these issues, multivalent ions can provide a solution by providing multiple electrons, resulting in a higher optical contrast and capacity. Herein, we propose an Al-ion electrochromic battery with a unique design, utilizing WO 3 as the cathode material, Al metal as the anode, and AlCl 3 as the aqueous electrolyte. Ex situ X-ray diffraction results reveal that the peak position and interlayer spacing of the optimized WO 3 porous thin film (∼170 nm) change during Al-ion intercalation and deintercalation, indicating the high structural stability of the optimized WO 3 electrode. A double cathode-electrochromic battery (DC-EES) device consisting of two cathodes on either side of an Al metal film anode was fabricated and demonstrated superior electrochemical performance due to its increased active surface area and open circuit potential. The device exhibited an open circuit potential of ∼1.2 V and a capacity of ∼155 mAh/m 2 at 0.5 A/m 2, which are sufficient values for powering an electronic device. Additionally, the developed device exhibited excellent electrochromic behavior upon discharge with an optical contrast of 68% (transparent and blue). These results show the great potential of Al-ion electrochromic batteries as a cost-effective and high-safety material for applications in smart windows and energy storage displays in modern infrastructures.