Flexible Co0.85Se nanosheets/graphene composite film as binder-free anode with high Li- and Na-Ion storage performance

A free-standing Co0.85Se nanosheets/graphene (Co0.85Se NSs/G) composite film was developed as a high performance anode material for both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) by a simple vacuum filtration and thermal reduction process. The Co0.85Se NSs/G film demonstrates good flexibility, and the Co0.85Se NSs are uniformly anchored on the graphene to form “sheet-on-sheet” nanostructures with strong interfacial interactions. The hybrid lamellar film demonstrated excellent electrochemical performances when used as a binder-free anode for LIBs and SIBs. As an anode material for LIBs, the Co0.85Se NSs/G film exhibited a high initial reversible capacity of 680 mA h g−1 at 50 mA g−1 and an excellent cycling stability and rate performance. The specific capacity of the Co0.85Se NSs/G film shows almost no fading from 0.05 to 0.5 A g−1, and the capacities were maintained at 613.3 and 522.7 mA h g −1 at 1 and 2 A g−1, respectively. When used as an anode for SIBs, the reversible capacity of the Co0.85Se NSs/G film reached up to 388 mA h g−1 with an initial coulombic efficiency as high as 82.5% at 50 mA g−1. Even at the higher current densities of 1 and 2 A g−1, the reversible capacities remained at 137.5 and 112.4 mA h g−1 after 500 cycles, respectively. The excellent electrochemical performances should be attributed to the special structure of the Co0.85Se NSs/G film. On one hand, graphene not only provides an efficient electrically conductive network, but also acts as an elastic layer to buffer the volumetric expansion of the Co0.85Se NSs during the discharge/charge process in LIBs/SIBs, which is helpful for enhancing the cycling stability of the Co0.85Se NSs/G film. On the other hand, strong interfacial interactions can further enhance electron transfer between Co0.85Se NSs and graphene, which is beneficial for improving the cycling stability and rate-performance of the Co0.85Se NSs/G film.