Flexible wearable energy storage devices: Materials, structures, and applications

Wearable electronics are expected to be light, durable, flexible, and comfortable. Many fibrous, planar, and tridimensional structures have been designed to realize flexible devices that can sustain geometrical deformations, such as bending, twisting, folding, and stretching normally under the premise of relatively good electrochemical performance and mechanical stability. As a flexible electrode for batteries or other devices, it possesses favorable mechanical strength and large specific capacity and preserves efficient ionic and electronic conductivity with a certain shape, structure, and function. To fulfill flexible energy‐storage devices, much effort has been devoted to the design of structures and materials with mechanical characteristics. This review attempts to critically review the state of the art with respect to materials of electrodes and electrolyte, the device structure, and the corresponding fabrication techniques as well as applications of the flexible energy storage devices. Finally, the limitations of materials and preparation methods, the functions, and the working conditions of devices in the future were discussed and presented. Flexible and reliable power sources with high energy density, long cycle life, and excellent rate capability are increasingly required. Meanwhile, safety and cost should be taken into consideration as well in the practical application, such as the manufactory cost. To fulfill the requirements of electronic equipment in the future, it is necessary to fabricate flexible batteries with multiple functions such as optical transparency and tensile and mechanical wear resistance.