Electrochemical impedance study of initial lithium ion intercalation into graphite powders

A Johnson Matthey 287 (JM 287) graphite powder disks sandwiched between two nickel screens was used as working electrodes in 1.0 M lithium hexafluorophosphate in mixed carbonate ester electrolyte. Their intrinsic impedance and lithium intercalation kinetics were determined using several different electrochemical impedance spectroscopy (EIS) protocols. The thermal stability of the electrolyte was similarly studied using a floating palladium wire working electrode. Studies at 25 degreesC and 65 degreesC show that a first high frequency depressed semicircle, a second depressed semicircle, and a sloping line in the low frequency range are respectively related to the initial formation of a solid electrolyte interface (SEI) film, the charge-transfer reaction, and lithium diffusion in graphite. The intrinsic resistance is in series with the reaction impedance and effectively increases the latter. The charge-transfer semicircle at the open-circuit potential moves to the low frequency range, whereas the semicircle associated with the freshly formed SEI film moves to high frequency range as lithium insertion proceeds. The passivating SEI film formed at 25 degreesC slows down co-insertion of the electrolyte into graphite when the electrode is cycled at 65 degreesC, which increases the cycle life of graphite at elevated temperature. (C) 2001 Elsevier Science Ltd. All rights reserved.