Design of Machine-Washable and Wearable Supercapacitors Using Composite Laminates

Abstract

Flexible and wearable supercapacitors, as a rechargeable energy storage device, are promising power sources for next-generation wearable electronics and electronic textiles in many consumer, medical, and military applications, as they offer safety, mechanical flexibility, stability, and durability that are lacking in current batteries. However, flexible supercapacitor prototypes reported so far have not demonstrated sufficient mechanical properties that could withstand critical situations, such as folding, twisting, and machine washing, which are commonly happening for real-world applications. Here we present the design of flexible supercapacitors that are foldable, twistable, and machine-washable due to exceptional mechanical properties of the materials. The devices are constructed using thin composite laminates as electrodes and ion-conducting separators. Each layer of the laminates is composed of randomly distributed activated carbon fibers or glass fibers in polymer electrolyte matrix. Calculations and simulations show that such laminates with proper fiber length and volume fraction of fibers can provide both high strength and high flexibility that enable them to survive critical mechanical deformations, while traditional particle-based electrodes cannot. The electrochemical and mechanical tests demonstrate that the activation of the carbon fiber surface does not affect much of the mechanical properties of the laminates, but greatly improves the surface area of electrodes, which leads to two orders increase in their electrical double-layer capacitance. The prototypes made of electrode/separator/electrode laminates show adjustable areal capacitances of over 10 mF/cm2. The electrochemical performance has remained unchanged after repeated folding and twisting tests. Moreover, we sewed the devices onto fabrics and washed them together with daily garments in a washing machine, after which we observed no mechanical damage or capacitance degradation. In conclusion, we believe that the excellent electrochemical performance, flexibility and mechanical durability make the composite laminates-based flexible supercapacitors promising to be integrated with daily garments to power various wearable electronics. The design rules and results are also applicable to developing other flexible functional devices that require extraordinary mechanical properties without sacrificing another function.