A Self-Standing and Flexible Electrode of Yolk-Shell CoS2 Spheres Encapsulated with Nitrogen-Doped Graphene for High-Performance Lithium-Ion Batteries

Flexible lithium-ion batteries (LIBs) have recently attracted increasing attention with the fast development of bendable electronic systems. Herein, a facile and template-free solvothermal method is presented for the fabrication of hybrid yolk-shell CoS2 and nitrogen-doped graphene (NG) sheets. The yolk-shell architecture of CoS2 encapsulated with NG coating is designed for the dual protection of CoS2 to address the structural and interfacial stability concerns facing the CoS2 anode. The as-prepared composite can be assembled into a film, which can be used as a binder-free and flexible electrode for LIBs that does not require any carbon black conducting additives or current collectors. When evaluating lithium-storage properties, such a flexible electrode exhibits a high specific capacity of 992 mAh g(-1) in the first reversible discharge capacity at a current rate of 100 mA g(-1) and high reversible capacity of 882 mAh g(-1) after 150 cycles with excellent capacity retention of 89.91 %. Furthermore, a reversible capacity as high as 655 mAh g(-1) is still achieved after 50 cycles even at a high rate of 5 C due to the yolk-shell structure and NG coating, which not only provide short Li-ion and electron pathways, but also accommodate large volume variation.