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 1-2 μm and~50 nm, respectively. The free space in the flowers and the thin feature of Ni₃S₂ buffered the volume changes and relieved mechanical strain during repeated 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, demonstrating the good cycling stability as a result of the unique microstructure of this electrode material.