Rational design of three-dimensional graphene encapsulated core-shell FeS@carbon nanocomposite as a flexible high-performance anode for sodium-ion batteries

The development of high-performance electrochemical energy storage systems is highly dependent on the synergistical structural design of electrode materials and whole electrodes with appropriate compositions. Here we create a novel flexible three-dimensional graphene (3DG) hybrid electrode with a core-shell FeS@carbon (FeS@C) nanocomposite encapsulated within 3DG by one-step thermal transformation of the deliberately designed 3DG wrapped metal-organic framework (3DG/MOF) composite based on the newly disclosed spatially confined phase separation of the metal and organic moieties of MOFs and the following in situ composition transformation mechanism. Benefitting from effective ion/charge transport in the whole electrode and the robust structural stability of FeS during electrochemical processes guaranteed by the highly interpenetrated porous conductive network of 3DG and the carbon protective layer with N- and S-codoping, the free-standing 3DG/FeS@C electrode delivers an ultrahigh specific capacity of 632 mA h g-1 after 80 cycles at 100 mA g-1, and excellent rate capacities of 363.3 and 152.5 mA h g-1 at 1 and 6 A g-1 with unprecedented cycling stability with a capacity retention of 97.9% after 300 cycles at 1 A g-1, which is the best ever reported result for FeS-based anode materials for sodium ion batteries. This study opens up a new MOF-based phase separation avenue to construct sophisticated 3DG wrapped core@shell nanocomposites and represents an important step to the structural design of high-performance electrodes for electrochemical energy storage.