Porous NiS2 nanosheets anchored on reduced graphene oxide as high-rate and long-life anode materials for sodium-ion batteries

In this work, graphene oxide is used as a substrate for the nucleation and growth of Ni(OH)(OCH3) during solvothermal synthesis. As a result, 2D Ni(OH)(OCH3) nanosheets are vertically anchored on 2D reduced graphene oxide (rGO) nanosheets, forming a Ni(OH)(OCH3)@rGO composite. Afterward, a NiS2@rGO hybrid is created by sulfurizing the Ni(OH)(OCH3)@rGO intermediate, which presents the same morphology as the intermediate but a porous structure of NiS2 nanosheets. The characteristic 2D porous structure of NiS2 nanosheets offers abundant active sites for charge storage, accelerates ion/electron transport, and buffers volume expansion. Meanwhile, rGO nanosheets can suppress the shuttle of polysulfides while also enhancing electrical conductivity. These factors in conjunction with the thin and robust ether-derived SEI are favorable to mechanical stability and rapid sodiation kinetics in the NiS2@rGO electrode in the ether-based electrolyte, thereby endowing it with exceptional rate capability (324 mA h g-1 at 5 A g-1) and a long lifespan (378 mA h g-1 over 1800 cycles at 1 A g-1). However, in the ester-based electrolyte, NiS2@rGO shows poor rate capability and cyclability. The kinetic analysis unveils the mechanism underlying the different electrochemical behavior in NiS2@rGO in the two electrolytes. More importantly, the NiS2@rGO electrode outperforms most NiS2-based materials reported in terms of rate and cycling performance owing to its unique hierarchical structure, making it a viable contender for anode materials in sodium-ion batteries. We also propose a novel M(OH)(OCH3)@rGO intermediate for the fabrication of transition-metal oxides/sulfides@rGO composites for energy storage and catalysis.