Ultrathin carbon-coated Fe7S8 core/shell nanosheets towards superb Na storage in both ether and ester electrolyte systems

Carbon-coated Fe7S8 composite nanosheets with a two-dimensional (2-D) core/shell structure were synthesized by a facile pyrolysis method based on an oriented attachment growth mechanism. The composite nanosheets have an ultrathin thickness of only ca. 5 nm. When used as anode materials for SIBs, the composite nanosheets exhibited excellent sodium ion storage performances in both ether- and ester-based electrolytes. At 100 mA g−1, the composites delivered a high reversible specific capacity of 540.7 mA h g−1 with an outstanding initial coulombic efficiency (ICE) of over 90% in the ether electrolyte, and a reversible capacity of 398.6 mA h g−1 with an ICE of 65.8% in the ester electrolyte. Even at 5 A g−1, the composites still delivered capacities of 346.2 mA h g−1 in the ether electrolyte and 272 mA h g−1 in the ester electrolyte. Remarkably, the composites exhibited ultralong cycling stability at high current density in the ether electrolyte. The reversible capacities of the composite nanosheets could be maintained at 310.2 mA h g−1 at 5 A g−1 after 2000 cycles, 243 mA h g−1 at 10 A g−1 after 4600 cycles and 170.0 mA h g−1 at 20 A g−1 after ultralong cycling for 8000 cycles, respectively. The excellent electrochemical performances should be attributed to the unique carbon-coating 2-D core/shell structure of the sample. The method for preparation of the unique structure is also expected to be extended to synthesize other high-performance metal sulfide electrode materials for Na-ion storage.