Researchers have successfully engineered an affordable and high-performance aqueous transparent battery with colour modulation capabilities that can be used in smart windows. Such windows remain transparent in a fully charged state during the daytime, thus allowing sunlight into the room while simultaneously functioning as an energy storage system.
During the night, the stored energy can power electronic devices within the room, and the electrochromic battery transitions to a dark blue state, ensuring privacy.
Transparent batteries require careful consideration of the thickness of the electrochromic active layer to maintain optimal transparency and energy storage capabilities. The thickness plays an important role in balancing competing requirements. Increasing the thickness of the electrochromic layer increases the energy storage capacity, but it introduces a trade-off by reducing the transparency value in both the bleached and coloured states of the smart window. Achieving an optimal balance between these conflicting factors is essential for the overall performance and effectiveness of transparent battery technology.
A team of researchers from Centre for Nano and Soft Matter Sciences (CeNS), Bengaluru, an autonomous institute under Department of Science and Technology, have developed a transparent battery that features a distinctive design employing aluminium-ion battery technology. The design integrates a cathode material composed of thickness-optimized (170 nm) electrochromic tungsten oxide (WO3) and aluminium as the anode, showcasing the charging and discharging process through visible transitions.
To enhance the switching speed and capacity of the battery, the research team recognized the critical role of electron mobility. They employed a two-order higher ion-conductive aqueous electrolyte, surpassing conventional non-aqueous electrolytes found in typical battery devices. This strategic choice resulted in rapid charging, improved switching speed, a discharge capacity of 155 mAh/m2, and an impressive optical contrast of 68% between colored and transparent states. The work has been recently published in the journal ACS Applied Energy Materials.
Dr. Ashutosh Kumar Singh, the lead scientist highlighted that these transparent energy storage device technologies hold significant potential for integration into smart window applications, offering both energy storage capabilities with adaptive transparency. “The use of aqueous electrolytes contributes to their cost-effectiveness, high performance, and elevated safety levels, making them well-suited for use in smart windows and energy storage applications within modern infrastructures. Its commercialization would require minimal investment and we can engage with potential smart glass manufacturers for collaborative efforts in this direction,” he added.
"We opted for a unique approach by employing two cathodes, deviating from the conventional single cathode configuration found in typical electrochromic devices. This not only enhances the specific capacity but also elevates the optical contrast. We have highlighted our exploration of capacity contributions in both single and double cathode-based transparent battery designs,” Mr. Rahuldeb Roy, a Ph.D. scholar and co-author of the research pointed out.
Configuration of transparent battery with a single and double cathode design (left), a visual representation of the device in charged and discharged states (right).
This study is part of an ongoing technology project to develop affordable and functional electrochromic smart windows which is supported by the Technology Development Board of the Department of Science and Technology, Govt. of India.
Publication Link: DOI: doi.org/10.1021/acsaem.3c02237
