In situ Raman studies of graphite surface structures during lithium electrochemical intercalation

Surface structural changes were investigated using in situ Raman microspectroscopy during lithium intercalation into Lonza KS-44 and KS-6 graphite materials in LiClO4, solutions of ethylene carbonate (EC)/dimethyl carbonate (DMC) or EC/1,2-dimethoxyethane (DME). In the 1 M LiClO4 EC/DMC solution, the Raman spectral changes indicate that graphite undergoes a series of transitions during the electrochemical lithium intercalation process from initial formation of the dilute stage 1 GIC (graphite intercalation compound) to a stage 4 GIG, then through a stage 3 to stage 2 and finally to a stage 1 GIG. The reversibility of the Raman spectral changes indicates that the top layers of the graphite are not damaged during the first charge and discharge cycle. Intercalation-induced strain causes the E-2g2,,, (i) mode of the unbound graphite layers to shift toward lower frequency in the stage 4 and stage 3 GICs. In the 1 M LiClO4, EC/DME solution, the Raman spectra changed dramatically in the high-potential range (0.9-0.5 V) during the discharge and charge processes, indicating that the graphite surface structure is altered even at the high potentials where major lithium intercalation is not expected to occur. Irreversible spectral changes in the E-2g2 band shoulders suggest that the surface structural changes are also irreversible. Raman spectral changes of the E-2g2 band in the potential range 0.9-0.5 V are attributed to extensive surface graphite exfoliation caused by solvent cointercalation with lithium ions. This is in accordance with the large irreversible capacity consumption in the potential range 0.9-0.5 V observed during the first discharge process. In the low-potential range, the Raman spectral changes of the graphite in LiClO4, EC/DME solutions are similar to those in LiClO4, EC/DMC solution.