In situ N-doped carbon modified (Co0.5Ni0.5)9S8 solid-solution hollow spheres as high-capacity anodes for sodium-ion batteries

Sodium ion batteries (SIBs) have been widely considered as a competitive alternative to construct low-cost and large-scale energy storage system. Unfortunately, low energy density and poor cycle life are main issues that need prompt solutions. Herein, an effective and controllable strategy, using metal-organic framework (MOF) precursors, was utilized to fabricate in situ N-doped carbon modified bimetallic sulfide hollow spheres with different metal ratios. When used as the anode material for SIBs, (Co0.5Ni0.5)(9)S-8 solid-solution combined with in situ N-doped carbon ((Co0.5Ni0.5)(9)S-8/N-C) is demonstrated to balance several aspects to deliver a superior electrochemical performance. (Co0.5Ni0.5)(9)S-8/N-C exhibits the high specific capacity of 723.7 mA h g(-1) at the 100th cycle at the current of 1.0 A g(-1) and a high rate performance with the capacity of 569.1 mA h g(-1) at 10 A g(-1), yielding the high capacity retention of 60.2% compared with the capacity of 0.1 A g(-1) (945.1 mA h g(-1)). The fast sodium storage capability is promoted by the capacitive contribution. Meanwhile, ex situ XRD is used to study the phase transition of the solid solution anode during the Na-storage process. The excellent performance can be attributed to the incorporation of Ni into the Co9S8 phase, in situ N-C coating and a stable hollow structure.