Latest Advances in Flexible Symmetric Supercapacitors: From Material Engineering to Wearable Applications

Flexible symmetric supercapacitors (FSSs) have received enormous attention in energy storage and conversion areas by virtue of their superior flexibility, high power density, and good cycling stability. FSS devices are typically composed of one solid electrolyte layer laminated by two electrode layers, which can realize energy storage, response to electrical stimulus, and even detect external stress or strain change based on various working mechanisms. The as-mentioned multifunctions of FSS devices are expected to play many critical roles in practical applications of wearable power supply and in artificial intelligence. This realization strongly associates with the rapid development of materials science and engineering, especially nanomaterials and smart structure design, and the multifunctions are results of rational designs of critical materials, optimization of device dimensions, and selectivity of active ion species.This Account showcases the latest advances in FSS devices concerning several critical aspects from fundamental material engineering to practical wearable applications. We first describe advanced functional materials utilized in flexible solid electrolytes and electrodes of FSS systems. Several highly ion-conductive hydrogel and ionogel electrolytes with excellent mechanical properties have been designed for the fast and stable ionic migration kinetics in devices. Some high-performance electrode materials with high charge storage capacity, efficient electromechanical conversion, and sensitive ionic response are presented for realizing multifunctions of FSS devices. After that, analysis of interfaces in devices on their performances is provided, and the construction strategies of robust interface are displayed as well. We then summarize flexible and wearable applications of FSS devices, including high-energy density power sources, flexible and electroactive actuators, and wearable and sensitive sensors. These multifunctions are realized by optimization of device dimensions, control of ion migration kinetics, and development of advanced materials, and the corresponding working mechanisms of the devices are presented in detail. The long-term development and future research directions of FSS devices are also envisioned.At present, the rise of nanomaterials and nanoscience is providing great opportunity to further improve performances of FSS devices and finally realize their wearable applications. These wearable FSS devices with smart multifunctions will significantly promote the development of next-generation flexible electronics for artificial intelligence. It is expected that this Account can promote tremendous efforts toward fundamental clarification of FSS devices, and the design mentality will accelerate the development of other flexible and wearable electrochemical energy devices.