Thin films based on electrochromic materials for energy storage performance and smart windows applications: a review

This review covers electrochromic (EC) cells that use different ion electrolytes. In addition to EC phenomena in inorganic materials, these devices can be used as energy storage systems. Lithium-ion (Li⁺) electrolytes are widely recognized as the predominant type utilized in EC and energy storage devices. These electrolytes can exist in a variety of forms, including solid layers, such as Li:Ta₂O₅ or LiAlO_x/Ta₂O₅/LiAlO_x. These cells have a significantly longer cycling life, ranging from 5000 to 10,000 cycles. Additionally, the coloration efficiency of the electrochromic device (ECD) containing the ITO glass/WO₃/Li⁺-polyvinyl butyral-based gel electrolyte was 175.34 cm²/C. ECDs utilizing Na⁺ or K⁺ ions commonly integrate a Prussian blue compound owing to its open-framework structure. The utilization of Zn²⁺ ion electrolytes as energy storage systems has been observed, demonstrating compatibility with aqueous electrolytes and showcasing a notable capacity of 820 mAh/g. EC cells and batteries can use aluminum-ion (Al³⁺) electrolytes. The cathode is indium hexacyanoferrate, and the anode is amorphous WO₃, giving a power density of 2433.8 mW/m². The morphology and structure of the EC layer are critical to improving the EC performance of protons (H⁺) as an electrolytic ion. In an effort to enhance the performance and stability of energy storage systems and ECDs, multi-ion electrolytes have emerged as a recent trend. These electrolytes consist of Zn²⁺ and Al³⁺ ions, Li⁺ and Al³⁺ ions, and electrolytes based on Zn²⁺ and K⁺ ions. By regulating the switching time, these devices are capable of attaining a remarkable optical modulation of 75%.