Preface: Flexible Energy‐Storage Systems

This special issue of Small features a collection on Flexible Energy-Storage Systems. Energy systems that are bendable, foldable, portable, and potentially wearable have immense potential in flexible smartphones and electronic papers among many other devices. In recent years, this type of energy storage technology has not only attracted the attention of the scientific community, but is also strongly backed by the desire of the general consumer. This special issue showcases to our readership articles that are representative of the current scenario and illustrate the future of flexible energy storage systems. Specifically, the 15 articles (5 communications, 7 full papers, and 3 reviews) will present topics spanning from flexible supercapacitors to batteries and barriers to practical applications. As described in the review article presented by Shen et al. (10.1002/smll.201702829), supercapacitors are especially attractive for flexible energy storage systems due to their high cyclability and power density. In this issue, Choi et al. (10.1002/smll.201702145) present a carbon-nanosheet based capacitor that can be operated under extreme structural deformation (bending, stretching, folding, and twisting) while Guan et al. (10.1002/smll.201702641) present a 2D metal-organic framework based flexible supercapacitor with a demonstrated energy density of 0.1248 mWh cm−2 and a power density of 2.55 mW cm−2. Beyond pure performance based metrics, Peng et al. (10.1002/smll.201702052) have developed a supercapacitor with 10000 cycles of stable cycling as well as an intriguing fluorescent capability. This special feature allows the supercapacitor to glow in the dark with a variety of colors, which can present itself as a useful feature for future wearable electronic clothing/accessories fashion designers. This special issue also features a review article by Dai et al. (10.1002/smll.201800879), which discusses the materials and structural designs of flexible and wearable supercapacitors with graphene-based nanomaterials. Sodium-ion and lithium-ion batteries are another type of electrochemical systems that have drawn immense research interest for flexible electronic application. Sodium-ion batteries have the advantage of being more cost effective than lithium. Yushin et al. (10.1002/smll.201703425) have developed an iron phosphate coated carbon nanotube based fabric with high flexibility. This material was used as a cathode in a sodium-ion battery, demonstrating stable cycling for up to 450 times. Wu et al. (10.1002/smll.201702864) presented an electrospun macroporous and flexible network of wrapped large-sized Na3V2(PO4)3 used as a cathode for a sodium-ion battery. A highly reversible capacity of 116 mAh g−1 was obtained at 0.1C with 86% of its capacity retained after 150 cycles when cycled at 0.5C. Furthermore, the macroporous nature of this electrode design allows for fast electrolyte wetting and Na-ion transfer. Being a commercialized and more mature technology, lithium-ion batteries are being heavily considered for flexible electronic systems. Yoon et al. (10.1002/smll.201702985) present a systematic study on the effect of the uniformity of ordered mesoporous nanostructures on the lithium storage behavior. This special issue also focuses on the practicality of lithium-ion based flexible electronics. A review article presented by Cho et al. (10.1002/smll.201702989) discusses the hurdles against the practical application of flexible lithium-ion battery systems. Choi et al. (10.1002/smll.201703028) report a solution processing technique for the scalable formation of metal coated nonwoven fabrics for flexible lithium-ion batteries. When tested in a full cell configuration, this design was able to offer stable cycling with high capacity. Reaching high areal active material mass loading is often a major testing parameter that separates fundamental research from practical research. Lee et al. (10.1002/smll.201703418) presents a high areal capacity flexible lithium-ion battery. A high 1.2 mAh cm−2 is achieved with a lithium titanate anode and lithium iron phosphate cathode both composited with reduced graphene oxide. The active material free standing was roll pressed on to a carbon fabric material. This configuration was able to maintain a stable open circuit voltage and capacity after being subjected to an impressive repeated flexing for 1000 times. An article is also featured in this special issue that discusses the design of a flexible integrated circuit for battery systems. Wang et al. (10.1002/smll.201800938) reports the fabrication of a flexible hexagonal battery circuit printed with an anisotropic conductive paste that can be used as a battery pack assembly for flexible electronics. The circuitry also included sensors and data transmission functions in addition to the battery system. Flexible metal-air batteries are also presented in this special issue. Zhong et al. (10.1002/smll.201702987) report an atomic layer of Co3O4 nanosheets as a catalyst layer for a flexible Zn-air battery. Zhang et al. (10.1002/smll.201800590) report a Co-N-C composite as an efficient cathode for Li-O2 batteries. A high 11 512.4 mAh g−1 specific capacity (normalized versus total cathode weight) with a long cycle life (130 cycles) is presented. The ultrahigh capacity and energy density of metal-air battery systems present a very attractive future direction for flexible energy storage systems. J.L. gratefully acknowledges support from the U. S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office. Argonne National Laboratory is operated for DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357. Jun Lu is a chemist at Argonne National Laboratory. His research interests focus on the electrochemical energy storage and conversion technology, with a main focus on beyond Li-ion battery technology. Jun Lu earned his bachelor degree in Chemistry Physics from University of Science and Technology of China (USTC) in 2000. He completed his Ph.D. from the Department of Metallurgical Engineering at University of Utah in 2009 with the major research focusing on metal hydrides for reversible hydrogen storage application.