Changing of SEI Film and Electrochemical Properties about MCMB Electrodes during Long-Term Charge/Discharge Cycles

The reasons for the capacity loss of mesocarbon microbead (MCMB) in lithium ion cells during long-term charge-discharge cycles were evaluated in this paper. The MCMB electrodes were charged and discharged for 2000, 2500 and 3000 cycles after activation. Solid electrolyte interphase (SEI) film was deposited on the surface of MCMB particles after activation. The contents of O and F in SEI film tended to increase continuously. Li2O was generated firstly, then LiF was produced later and the content of LiF increased with the cycles proceeding. During the cycles, the voltage differences between charge and discharge platforms increased gradually and the specific capacity decreased. The cell resistances increased continuously and the peak currents of intercalation/de-intercalation reactions in cyclic voltammetry curves were reduced. The decline of electrochemical properties was related to both the growth of SEI films and the consumption of electrolyte salt LiPF6.