UC Berkeley

Flexible energy storage components are indispensable in the field of flexible electronics. Supercapacitors featuring high power density, fast charge/discharge rate, long cycle life, and safe operation are desirable devices. Micro/nanofiber-based electrode materials, fabrication processes, and device configurations are investigated towards flexible supercapacitors applications.

Micro flexible supercapacitors (FSCs) prepared using direct-write nanofibers is demonstrated for flexible integrated microelectronics. The micro electrodes with porous network are fabricated by the near-field electrospinning process of polypyrrole (PPy) nanofibers on a patterned metal layer on flexible substrate. Such conductive nanofiber network with a pseudocapacitance effect greatly increases the capacitance and facilitates ion transport in the electrodes. The prototype micro FSCs are experimentally proved to be highly flexible with excellent electrochemical performance and cycling stability. The approach is simple, versatile, and compatible with different substrates for the direct integration of energy storage devices in flexible microsystems.

An ultra-thin micro coaxial fiber supercapacitor (μCFSC) with high energy and power densities, and excellent mechanical properties is demonstrated. The prototype with the smallest reported overall diameter of 13 μm is fabricated by successive coating of functional layers onto a single micro carbon fiber via a scalable process. Combining the simulation results via the electrochemical model, the high performance is attributed to the well-controlled thin coatings that make full use of the electrode materials and minimize the ion transport path between electrodes. Moreover, the μCFSC features high bending flexibility and large tensile strength (more than 1 GPa), which make it promising as a building block for various flexible energy storage applications.

Free-standing woven supercapacitor fabrics that can store high electrical energy and sustain large mechanical loads are demonstrated as flexible power sources. The prototype with reduced package weight/volume provides an impressive overall energy density of 2.58 mWh g-1 or 3.6 mWh cm-3, high tensile strength of over 1000 MPa, and bearable pressure of over 100 MPa. The nanoporous thread electrodes are prepared by the activation of commercial carbon fibers to have three-orders of magnitude increase in the specific surface area and 86% retention of the original mechanical strength. The novel device configuration woven by solid electrolyte-coated threads shows excellent flexibility and stability during repeated mechanical bending tests. Such scalable energy storage fabrics can be woven into daily graments as flexible power sources for a variety of wearable electronics.