|Fig. 1: Schematic of conventional electrode components in LIBs. (After Zhou et al. )|
Innovation in flexible devices, such as roll-up displays, touch screens, wearable sensors, implantable medical devices, artificial skins, and flexible electronics is driving the rapid development of flexible electronics.  However, these ubiquitous requirement cannot be realized unless relevant flexible energy storage systems and power sources are developed. Rechargeable battery systems, e.g. lithium-ion batteries (LIBs), are the main power sources that dominate the portable device market due to their high energy density, long cycle life, and environmental benefits.  Before used for wiring up flexible electronics, compatible prototype LIBs are still facing great challenges and under exploration.
The key issue to realize flexible LIBs with various sizes, shapes, and mechanical properties lies in the rational design and fabrication of suitable materials with high energy/power density, good cycling stability, desirable conductivity, and robust flexibility to construct light, thin, flexible, small units.  Besides, high durability of flexible LIBs to sustain high performance under mechanical deformation and safety risk should also be considered. In a conventional LIB, conductive carbon additives commonly account for 10 wt % of the total electrode mass to provide necessary electrical contact between active materials and current collectors; polymeric binders and current collectors in form of metal foil or mesh are always employed to guarantee both mechanical and electrical connection to external circuits (Fig. 1). However, all components above are electrochemical inactive. And the use of heavy metal and insulating polymer binders significantly reduces the overall energy density of the electrodes. Moreover, parasitic reactions between binder, current collector, and electrolyte may further lower the cycling stability and induce self-discharge. Therefore, building a light and flexible electrode without employing binder and metal current collector is the prerequisite to design prototype LIBs for flexible devices.
The most effective and efficient strategy to develop flexible electrode materials is to integrate active phase with light and conductive low-dimensional nanostructure materials such as carbon nanotubes (CNTs), graphene, carbon cloth, conductive paper and textiles.  For example, Wang et al. employed super-aligned CNT films as light current collectors for graphite anodes of LIBs. Such innovative current collectors exhibited largely improved mechanical properties and affinity to active materials than conventional Cu foils.  Li et al. synthesized LiFePO4 and Li4Ti5O12 electrode materials directly grown on a unique three-dimensional graphene foam and assembled them into a flexible battery.  Extraordinary high-rate performance of a capacity of 135 mAh g-1 with very short discharge time of 18 s and cycling stability of over 500 cycles even under deformation were achieved. Hu et al fabricated series of conductive paper by immersing commercial cellulose paper into CNT ink. The as obtained paper serves as excellent current collector for LIBs with high capacity and long cycle life.  This method is highly scalable and economically beneficial. In general, the use of low-dimensional nanostructured materials to construct flexible electrode could considerably improve the overall energy density and meet the requirement of rapid electron/ion pathway and mechanical robustness.
Low-dimensional nanostructure materials especially nanocarbon play an unreplaceable role in constructing flexible electrodes owing to their inherent properties of extraordinary electrical conductivity, superior mechanical flexibility, high chemical stability, and affordable costs. However, there are still some crucial issues impeding the commercial extension of flexible LIBs: (1) relatively high weight ratio of inactive components; (2) low loading amount of active phase; (3) reduced volumetric energy density due to the use of low-density materials. Therefore, new chemistry involving multi-electron-transfer process beyond LIBs such as Li-S battery and Li-air battery should be further exploited to meet the ever increasing demand for flexible power sources. Besides, multiple functionalities such as optical transparency, stretchability, wearability and biological compatibility should also be taken into account to fulfill different functions of various types of flexible devices.
© Xinyan Liu. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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