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

Abstract

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, (Co_0.5Ni_0.5)₉S₈ solid-solution combined with in situ N-doped carbon ((Co_0.5Ni_0.5)₉S₈/N–C) is demonstrated to balance several aspects to deliver a superior electrochemical performance. (Co_0.5Ni_0.5)₉S₈/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.