In situ investigation of electrochemical lithium intercalation into graphite powder

The reaction kinetics for the electrochemical insertion–extraction of lithium into and out of graphite and the electrode intrinsic resistance have been studied using galvanostatic intermittent titration with applied microcurrent pulses (GIT), electrochemical impedance spectroscopy (EIS) and in situ intrinsic (i.e. physical) resistance measurements. The formation of a so-called solid electrolyte interphase (SEI) or ‘passivation layer’ on the graphite surface at about +0.8 V versus Li+/Li increases the total intrinsic resistance within the electrode on the first cycle. The gradual growth (i.e. change in volume) of the SEI film during repeated lithium insertion–extraction results in a continued increase in intrinsic resistance. The good agreement between EIS and in situ intrinsic resistance measurements indicates that the latter is a simple and powerful method for investigating the change in this parameter for graphite electrodes. GIT showed a gradual increase in kinetic rate in an initial single-phase region, followed by a decrease in a two-phase transformation region, which was in turn followed by an increase in kinetic rate in a second single-phase region. This is explained by a shrinking unreacted core model. The controlling step of lithium insertion–extraction during the two-phase transformation region was investigated by EIS and GIT.