Improved Na-storage cycling of amorphous-carbon-sheathed Ni₃S₂ arrays and investigation by in situ TEM characterization

The past few years have witnessed increasing attention of sodium ion batteries due to the concerns on the shortage of lithium resources. Great challenge, however, remains to develop anode materials with high capacity and long cycle life. In this work, a binder-free array-type electrode, constructed of nickel sulfide (Ni₃S₂) nanoparticles encapsulated in amorphous carbon sheath (ACS), was fabricated by a facile templating method. This array-type Ni₃S₂/ACS electrode can yield a high initial reversible capacity of 772 mAh g⁻¹ and a long cycle life with a reversible capacity of 440 mAh g⁻¹ retained after 100 cycles. Both ex situ and in situ characterizations reveal that the excellent electrochemical performance of Ni₃S₂/ACS electrode originates from the unique array-type structure, in which high activity of Ni₃S₂ can be obtained by the thin-layered structure and the sodiation-induced volume expansion of Ni₃S₂ can be effectively accommodated by the carbon sheath and inner space, resulting in a high mechanical stability of the Ni₃S₂/ACS rods during cycling. Our results reveal the electrochemical performance and fundamental reaction mechanism of Ni₃S₂/ACS during sodiation-desodiation cycles, shedding lights onto the design of novel sulfide-based anodes for sodium ion batteries.