Yolk–shell-structured (Fe_0.5Ni_0.5)₉S₈ solid-solution powders: Synthesis and application as anode materials for Na-ion batteries

Multicomponent metal sulfide materials with a yolk–shell structure and a single phase were studied for the first time as anode materials for sodium-ion batteries. Yolk–shell-structured Fe–Ni–O powders with a molar ratio of iron and nickel components of 1/1 were prepared via one-pot spray pyrolysis. The prepared Fe–Ni–O powders were transformed into yolk–shell-structured (Fe_0.5Ni_0.5)₉S₈ solid-solution powders via a sulfidation process. The initial discharge and charge capacities of the (Fe_0.5Ni_0.5)₉S₈ powders at a current density of 1 A·g⁻¹ were 601 and 504 mA·h·g⁻¹, respectively. The discharge capacities of the (Fe_0.5Ni_0.5)₉S₈ powders for the 2^nd and 100^th cycle were 530 and 527 mA·h·g⁻¹, respectively, and their corresponding capacity retention measured from the 2^nd cycle was 99%. The (Fe_0.5Ni_0.5)₉S₈ powders had high initial discharge and charge capacities at a low current density of 0.1 A·g⁻¹, and the reversible discharge capacity decreased slightly from 568 to 465 mA·h·g⁻¹ as the current density increased from 0.1 to 5.0 A·g⁻¹. The synergetic effect of the novel yolk–shell structure and the multicomponent sulfide composition of the (Fe_0.5Ni_0.5)₉S₈ powders resulted in excellent sodium-ion storage performance.