Cobalt-vanadium sulfide yolk-shell nanocages from surface etching and ion-exchange of ZIF-67 for ultra-high rate-capability sodium ion battery.

Development of high-performance metal sulfides anode materials is a great challenge for sodium-ion batteries (SIBs). In this work, a cobalt-based imidazolate framework (ZIF-67) were firstly synthesized and applied as precursor. After the successive surface etching, ion exchange and sulfidation processes, the final cobalt-vanadium sulfide yolk-shell nanocages were obtained (CoS/VS@NC) with VS shell and CoS yolk encapsulated into nitrogen doped carbon frameworks. This yolk-shell nanocage structure effectively increases the specific surface area and provides enough space for inhibiting the volume change during charge/discharge processes. Besides, the nitrogen doped carbon skeleton greatly improves the ionic conductivity and facilitates ion transport. When used as the anode materials for SIBs, the yolk-shell nanocages of CoS/VS@NC electrode exhibits excellent rate capability and stable cycle performance. Notably, it displays a long-term cycling stability with excellent capacity of 417.28 mA h g after 700 cycles at a high current density of 5 A/g. This developed approach here provides a new route for the design and synthesis of various yolk-shell nanocages nanomaterials from enormous MOFs with multitudinous compositions and morphologies and can be extended to the application into other secondary batteries and energy storage fields.