Rechargeable Aqueous Electrochromic Batteries Utilizing Ti‐Substituted Tungsten Molybdenum Oxide Based Zn2+ Ion Intercalation Cathodes

Batteries are used in every facet of human lives. Desirable battery architectures demand high capacity, rechargeability, rapid charging speed, and cycling stability, all within an environmentally friendly platform. Many applications are limited by opaque batteries; thus, new functionalities can be unlocked by introducing transparent battery architectures. This can be achieved by incorporating electrochromic and energy storage functions. Transparent electrochromic batteries enable new applications, including variable optical attenuators, optical switches, addressable displays, touch screen devices, and most importantly smart windows for energy-efficient buildings. However, this technology is in the incipient state due to limited electrochromic materials having satisfactory optical contrast and capacity. As such, triggering electrochromism via Zn²⁺ intercalation is advantageous: Zn is abundant, safe, easily processed in aqueous electrolytes and provides two electrons during redox reactions. Here, enhanced Zn²⁺ intercalation is demonstrated in Ti-substituted tungsten molybdenum oxide, yielding improved capacity and electrochromic performance. This technique is employed to engineer cathodes exhibiting an areal capacity of 260 mAh m^-2 and high optical contrast (76%), utilized in the fabrication of aqueous Zn-ion electrochromic batteries. Remarkably, these batteries can be charged by external voltages and self-recharged by spontaneously extracting Zn²⁺ , providing a new technology for practical electrochromic devices.