Graphene-based materials for flexible energy storage devices

The booming developments in portable and wearable electronics promote the design of flexible energy storage systems. Flexible supercapacitors and batteries as promising energy storage devices have attracted tremendous attention. As the key component of both supercapacitors and batteries, electrode materials with excellent flexibility should be considered to match with highly flexible energy storage devices. Owing to large surface area, good thermal and chemical stability, high conductivity and mechanical flexibility,graphene-based materials have been widely employed to serve as promising electrodes of flexible energy storage devices. Considerable efforts have been devoted to the fabrication of flexible graphene-based electrodes through a variety of strategies. Moreover, different configurations of energy storage devices based on these active materials are designed. This review highlights flexible graphene-based two-dimensional film and one-dimensional fiber supercapacitors and various batteries including lithium-ion, lithium–sulfur and other batteries. The challenges and promising perspectives of the graphene-based materials for flexible energy storage devices are also discussed.

Sulfur nanoparticles encapsulated in reduced graphene oxide nanotubes for flexible lithium-sulfur batteries

Rapid development of flexible electronic devices is promoting the design of flexible energy-storage devices. Lithium-sulfur （Li-S） batteries are considered as promising candidates for high energy density energy-storage devices. Therefore, flexible Li-S batteries are desired. In this study, we fabricated composite films of freestanding reduced graphene oxide nanotubes wrapped sulfur nanoparticles （RGONTs@S） by pressing RGONTs@S composite foams, which were synthesized by combining cold quenching with freeze-drying and a subsequent reduction process. These RGONTs@S composite films can serve as self-supporting cathodes for Li-S batteries without additional binders and conductive agents. Their interconnected tubular structure allows easy electron transport throughout the network and helps to confine the polysulfides produced during the charge/ discharge process. As a result, the RGONTs@S composite films exhibited a high initial specific capacity, remarkable cycling stabilit36 and excellent rate capability. More importantly, the RGONTs@S composite films can serve as electrodes in flexible Li-S batteries. As a proof of concept, soft-packaged Li-S batteries were assembled using these electrodes and they displayed stable electrochemical performance at different bending states.