Influence of Fe (II) Species in Electrolyte on Performance of Graphite Anode for Lithium-Ion Batteries

Iron dissolution into an electrolyte from the olivine LiFePO4 in the lithium-ion cell has been recently investigated. In order to study the influence of the dissolved Fe2+ species in electrolyte on the lithium intercalation/deintercalation into an artificial graphite electrode, the electrochemical behavior of graphite was investigated in 1 mol L−1 LiPF6 electrolyte solution containing Fe2+ species. The cells containing Fe2+ species showed larger reversible capacity loss than the Fe-free cells. Electrochemical impedance spectroscopy and ex-situ techniques, such as scanning electron microscopy (SEM), Raman spectroscopy, Fourier Transform Infrared Spectrometer (FTIR) were employed to investigate the aging mechanism. The Fe2+ species were reduced on the graphite anode and formed iron deposits. The iron deposits affected the performance of graphite on two approaches. On the one hand, the deposits hindered intercalation of Li ions into graphite. On the other hand, the deposits accelerated decomposition of electrolyte and aggravated surface structural disorder in graphite, resulting in thicker solid-electrolyte interface SEI layer. Both effects caused capacity fading upon cycling.