Graphene-immobilized flower-like Ni3S2 nanoflakes as a stable binder-free anode material for sodium-ion batteries

A binder-free Ni3S2 electrode was prepared directly on a graphene-coated Ni foam (G/Ni) substrate through surface sulfiding of substrate using thiourea as the sulfur source in this work. The Ni3S2 showed a flower-like morphology and was uniformly distributed on the G/Ni surface. The flower-like Ni3S2 was composed of cross-arrayed nanoflakes with a diameter and a thickness of 1-2μm and~50 nm, re-spectively. The free space in the flowers and the thin feature of Ni3S2 buffered the volume changes and relieved mechanical strain during re-peated cycling. The intimate contact with the Ni substrate and the fixing effect of graphene maintained the structural stability of the Ni3S2 electrode during cycling. The G/Ni-supported Ni3S2 maintained a reversible capacity of 250 mAh·g-1 after 100 cycles at 50 mA·g-1, demon-strating the good cycling stability as a result of the unique microstructure of this electrode material.