Research progress of fiber-shaped electrochemical energy storage devices

Abstract

During the past two decades, wearable devices have been broadly used for a variety of fields such as biomedical system, communication and microelectronics. The power system such as lithium-ion batteries is essential to the operation of wearable devices, which should adapt to irregular substrates and sustain complex deformations. A promising strategy is to fabricate high-performance energy storage devices in a fiber shape, e.g., fiber lithium-ion batteries (LIBs). These fiber LIBs with diameters ranging from tens to hundreds of micrometers can be readily integrated with human body and work stably under constant body motions. They can also be woven into breathable fabrics to satisfy the needs of wearable electronics. The key challenges facing fiber LIBs include effective loading of active materials on fiber electrodes, efficient charge transfer along fiber electrodes and interface stability of fiber electrodes under operation and deformation. Fiber electrodes are thus designed with hierarchical and aligned channels to effectively load active materials, so an effective electric field can be established between the fiber cathode and anode. Aligned carbon nanotubes (CNTs) can be made with conductive additives and active materials into fiber electrodes by a continuous spinning process. Conventional metal current collectors, binders and conductive agents are not required, and active materials may reach 90% of the electrode by weight. The resulting hierarchically helical CNT fiber electrode shows high flexibility, high strength, and high electrical conductivity. The flexural rigidity is as low as 10−7 nN/m2, which is several orders of magnitude lower than those of ordinary synthetic fibers. The specific strength reaches 107 Nm/kg, which is higher than those of ordinary synthetic fibers. The conductivity reaches 104 S/cm, which can meet the needs of batteries. The aligned CNT composite fiber permits efficient electron transport, where the electrons hop among neighboring CNTs. Besides, due to the capillary force, the electrolyte can penetrate throughout the entire fiber via micrometer-sized and nanometer-sized channels, benefiting ion transport. The twisted CNT bundles firmly bind active materials and prevent their significant volume variations during charge and discharge processes. Therefore, the efficient conductive pathways can be maintained to deliver both high rate and cyclic stability. Fiber LIBs could be fabricated by assembling two fiber electrodes in parallel, twisting or coaxial structure. Gel electrolyte avoids the potential safety risks stemmed from liquid electrolytes. The continuously fiber LIBs show both high flexibility and energy storage capacity. After bending for 100000 cycles, the capacities have been well maintained, and the energy densities exceed 100 Wh/kg. Based on the similar strategy, a series of fiber energy storage devices such as supercapacitors, lithium-sulfur batteries, lithium-air batteries, zinc-ion batteries, zinc-air batteries and aluminum-air batteries, have been also produced. To summarize, fiber energy storage devices can be woven into flexible fabrics or integrated with energy harvesting devices to satisfy the next-generation electronics. Such a new strategy establishes an efficient avenue to particularly accelerate the development of wearable electronics in the future.