Recent progress in flexible energy storage materials for lithium-ion batteries and electrochemical capacitors: A review
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With the advent of flexible, wearable and portable electronic products, flexible lithium-ion batteries (LIBs) and electrochemical capacitors (ECs), which are able to withstand repeated deformation or bending, have attracted considerable attention as one type of energy-storage device. However, the fabrication of these flexible electrodes is the main bottleneck in the practical utilization and application of these energy-storage devices. Up to now, enormous efforts have been made in addressing the shortcomings and remarkable improvements have also been achieved. So a systematic review of the status and progresses is highly required. In this review, we first make a short introduction about the challenges faced in the conventional batteries and capacitors. Then, we summarize the recent improvements in flexible and wearable LIBs and ECs with a focus on the flexible active materials and substrates. Finally, we discuss the prospects and challenges towards the practical applications of the flexible electrodes in the future.
I. INTRODUCTION
In recent years, flexible or bendable energy storage and conversion systems, which are designed to be portable, lightweight, bendable and even wearable, have attracted tremendous attention due to their mechanical flexibility and high energy density.1–3 Among various energy-storage devices, lithium-ion batteries (LIBs) and electrochemical capacitors (ECs) are two of the most promising candidates because of their high energy density, large power density and excellent stability.4 Due to the excellent properties of flexible devices, LIBs and ECs, a combination of flexible electrodes with LIBs and ECs has attracted much more interest as an important branch of bendable electronic systems. However, the bottlenecks of transforming the traditional batteries and capacitors into flexible energy storage devices lie in the preparation methods, assembly process, and the selection of proper electrolytes. Conventional LIBs and ECs usually consist of a carbon-based anode, a transition metal oxides-based cathode, a polymer separator and organic liquid electrolyte. However, there are several disadvantages for the traditional methods. In these batteries, relatively heavy metal foils are always used as both conductive substrate and structural support with slurry of active materials, binders and conductive additives coated on the surface of it. Due to the smooth surface of the metal foil, the active materials can easily detach Contributing Editor: Xiaobo Chen a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.195 1648
J. Mater. Res., Vol. 31, No. 12, Jun 28, 2016
http://journals.cambridge.org
Downloaded: 28 Jun 2016
from the current collector. In addition, the relatively poor chemical stability of these metal foils also results in increased internal impedance 5 and passivation of active materials, 6 because metal foils are susceptible to the corrosion of some electrodes and active materials. Carbon black, carbon nanotubes (CNTs) and graphite are always used as conductive
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