Electrochemical performance of amorphous carbon coated α-Fe₂O₃/expanded natural graphite composites as anode active materials for lithium ion batteries

An amorphous carbon-coated α-Fe₂O₃/expanded natural graphite (ENG) composite as an anode active material for lithium-ion batteries was successfully synthesized by a simple hydrothermal method. The ENG anode material used as a matrix showed better cycling performance but a larger irreversible capacity than pristine NG anode material due to the large amount of solid electrolyte interface film. The reversible capacity of the ENG anode material increased by approximately 26% through impregnation of α-Fe₂O₃ nanoparticles. Nevertheless, the capacity retention significantly decreased from 98.9 to 74.9% after 63 cycles due to volume expansion and microstructural change caused by aggregation of Fe and Li₂O during the charge-discharge process. This degradation of cycling performance was overcome by the surface coating of amorphous carbons. The reversible capacity of an amorphous carbon-coated α-Fe₂O₃/ENG composite increased with cycling and reached 576 mAh/g at the 95^th cycle and 524 mAh/g at the 350^th cycle, showing 99% capacity retention. The favorable cycling performance was attributed to suppression of nanoparticle aggregation, a fast charge transfer, and lowering of the contact resistance by the conductive amorphous carbon coating.