The role of graphite surface group chemistry on graphite exfoliation during electrochemical lithium insertion

Abstract Heat treatment of highly crystalline graphite TIMREX® SLX50 at temperatures above 1200 °C in an inert gas atmosphere resulted in a significant increase of the irreversible capacity, which is observed during the first electrochemical lithium insertion using a 1 M LiPF6 ethylene carbonate/dimethyl carbonate 1:1 (w/w) electrolyte mixture. An additional potential plateau could be observed at about 0.45 V versus Li/Li+ during the galvanostatic insertion of lithium into these heat-treated graphite materials. Post-mortem scanning electron microscope studies of negative electrodes containing such heat-treated graphite materials indicated the exfoliation of the graphite structure as reason for the additional potential plateau and the significant increase of the irreversible capacity in the first electrochemical lithium insertion. The graphite exfoliation during the first electrochemical lithium insertion disappeared when the heat-treated graphite materials were aged in a humid air atmosphere at room temperature for several months. This aging process turned out to be reversible. When regenerating the aged graphite material in a dry flux of argon gas at room temperature, the electrochemical graphite exfoliation could be observed again. Structure characterization of the graphite materials by X-ray diffraction (XRD) as well as surface investigations using Raman spectroscopy and gas adsorption measurements indicated that the exfoliation tendency of polycrystalline, highly crystallized graphite in ethylene carbonate containing electrolytes is controlled neither by the rhombohedral stacking faults in the graphite structure, the surface defects, nor by the superficial disordered carbon but only by the surface group chemistry. The amount of acidic surface oxide groups especially seems to be the key factor that influences graphite exfoliation during electrochemical lithium insertion in ethylene carbonate containing electrolytes.