Bimetal-organic frameworks derived ternary metal sulphide nanoparticles embedded in porous carbon spheres/carbon nanotubes as high-performance lithium storage materials

Designing advanced structures to ameliorate the deficiencies of conversion anodes in terms of poor conductivity and sever volume change over cycling is still the attracting area for fundamental research but challenging, toward which more efforts are needed. Here, we demonstrate the synthesis of a hierarchical composite of CoNi2S4 from sulfurizing the elaborately designed Co/Ni bimetal-organic frameworks (MOF-74) precursors with well-defined sphere morphology. In this composite, the CoNi2S4 nanoparticles are evenly embedded in the carbon nanosheets which are in situ carbonized from organic ligand and self-assembled into porous carbon spheres with short carbon nanotubes (CNTs) deeply rooted in the surface (CoNi2S4@PCS/CNTs). The porous structure provides favourable paths for electrolyte penetration and spaces for volume changing upon cycling, while the outwardly grew short CNTs and the carbon skeleton of the spheres guarantee fast electron transfer. Beneficial from the robust structure plus the synergetic effect from two different metal elements, the obtained CoNi2S4@PCS/CNTs demonstrates excellent lithium storage performance in terms of reversible capacity (1250 mAh g at 500 mA g(-1)), cycle stability (843 mAh g(-1) at 1 A g(-1) after 600 cycles) and rate capability (608 mAh g at 4 A g(-1)). The kinetic analysis reveals that the CoNi2S4@PCS/CNTs is benefit from capacitive contribution to the reversible lithium storage capacity under a faradaic surface pseudocapacitive mechanism, especially at high current densities. This work demonstrates an effective strategy that combines the synergetic effect from two metals upon lithium insertion and advantages from structure design to enhance the lithium storage performance of conversion materials.