Development of Smart Battery That Changes Color During Charging and Discharging

KAIST Professor Kim Il-du's Research Team
Develops Electrochromic Polymer Cathode Material
Incorporating 'Pi (π) Bond Spacer'

A smart secondary battery that changes color to indicate the timing for recharging once the stored electricity is fully used has been developed.

The Korea Advanced Institute of Science and Technology (KAIST) announced on the 21st that a research team led by Professor Il-Doo Kim from the Department of Materials Science and Engineering, in collaboration with Professor Tae-Kwang Yoon's team from Myongji University, developed an electrochromic polymer cathode material embedded with a ‘pi (π) bond spacer’ that enhances the mobility of electrons and ions. They created a core material for a smart electrochromic-zinc ion battery that visualizes the charging and discharging process.

With the rapid growth of the smart electronic device and wearable markets, smart secondary battery systems that change color beyond simple energy storage functions are gaining attention. However, existing electrochromic materials have limitations such as low electrical conductivity, resulting in poor electron and ion mobility, low energy storage capacity, and difficulty in application to flexible/wearable energy technologies.

Batteries integrated with electrochromic functionality visualize the charging and discharging states through color changes and can be used as display devices that regulate solar absorption, thereby reducing indoor cooling energy consumption. This represents a groundbreaking smart battery technology.

The research team succeeded in implementing a flexible electrochromic-smart zinc ion battery that maintains excellent electrochromic performance and superior electrochemical properties even after prolonged air exposure and mechanical deformation. To maximize electron and ion mobility, they designed and synthesized for the first time a polymer cathode material embedded with a ‘pi (π) bond spacer’ based on theoretical modeling. The pi (π) bond enhances electron transport within the structure, significantly accelerating ion mobility and maximizing ion adsorption efficiency, which also increases energy storage capacity.

The polymer cathode-based battery embedded with the ‘pi (π) bond spacer’ enables fast charging by providing space for ion movement compared to cases without the spacer. The zinc ion battery showed a discharge capacity of 110 mAh/g, expanding storage capacity by more than 40% compared to previous versions, and the color change performance improved by 30%, rapidly shifting from navy blue to transparent during charging and discharging. Furthermore, applying the transparent flexible battery technology to smart windows allows the window to darken during daytime solar energy absorption, functioning as a curtain that blocks ultraviolet rays and dazzling sunlight, making it a futuristic energy storage technology.

Professor Kim said, "We have succeeded in developing a smart zinc ion battery with excellent color change efficiency and high energy capacity," adding, "We expect this to be utilized as a future energy storage system that accelerates innovation in smart batteries and wearable technologies, going beyond the conventional concept of batteries that only perform energy storage."

The research results were published as an Inside Cover article in the international journal Advanced Materials on August 3rd (Vol. 35, No. 31). (Paper title: A π-Bridge Spacer Embedded Electron Donor?Acceptor Polymer for Flexible Electrochromic Zn-Ion Batteries)