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

Marcasite (m-FeS2) exhibits higher electronic conductivity than that of pyrite (p-FeS2) 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-FeS2 seems to be a better choice for electrode materials compared to p-FeS2. However, the m-FeS2 has been seldom studied due to its sophisticated synthetic methods until now. Herein, a hierarchical m-FeS2 and carbon nanofibers composite (m-FeS2/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-FeS2/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-FeS2/CNFs was systematically investigated by ex situ powder X-ray diffraction patterns and scanning electron microscopy. Interestingly, the hierarchical m-FeS2 microspheres assembled by small FeS2 nanoparticles in the m-FeS2/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-FeS2/CNFs.