Metastable Marcasite-FeS₂ as a New Anode Material for Lithium Ion Batteries: CNFs-Improved Lithiation/Delithiation Reversibility and Li-Storage Properties

Marcasite (m-FeS 2 ) exhibits higher electronic conductivity than that of pyrite (p-FeS 2 ) because of its lower semiconducting gap (0.4 vs 0.7 eV). Meanwhile, as demonstrates stronger Fe–S bonds and less S–S interactions, the m-FeS 2 seems to be a better choice for electrode materials compared to p-FeS 2 . However, the m-FeS 2 has been seldom studied due to its sophisticated synthetic methods until now. Herein, a hierarchical m-FeS 2 and carbon nanofibers composite (m-FeS 2 /CNFs) with grape-cluster structure was designed and successfully prepared by a straightforward hydrothermal method. When evaluated as an electrode material for lithium ion batteries, the m-FeS 2 /CNFs exhibited superior lithium storage properties with a high reversible capacity of 1399.5 mAh g –1 after 100 cycles at 100 mA g –1 and good rate capability of 782.2 mAh g –1 up to 10 A g –1 . The Li-storage mechanism for the lithiation/delithiation processes of m-FeS 2 /CNFs was systematically investigated by ex situ powder X-ray diffraction patterns and scanning electron microscopy. Interestingly, the hierarchical m-FeS 2 microspheres assembled by small FeS 2 nanoparticles in the m-FeS 2 /CNFs composite converted into a mimosa with leaves open shape during Li + insertion process and vice versa. Accordingly, a “CNFs accelerated decrystallization–recrystallization” mechanism was proposed to explain such morphology variations and the decent electrochemical performance of m-FeS 2 /CNFs.