Behavior of graphite electrodes in solutions based on ionic liquids in in situ Raman studies

In this work, the behavior of composite graphite electrodes comprising synthetic graphite flakes in solutions based on a 1-methyl-1-propylpiperidinium [bis(trifluoromethylsulfonyl)] imide (MPPp TFSI) ionic liquid (IL) was investigated, using in situ Raman spectroscopy with microscopic lateral resolution, in conjunction with cyclic voltammetry. Both pure IL and IL solutions containing a LiN (S O2 C F3) 2 (LiTFSI) salt were studied. Upon cathodic polarization, the IL cations (MPPp+) are intercalated. This process is irreversible in a pure IL solution. When the solution comprises both IL and a Li salt (LiTFSI), the graphite electrodes can intercalate simultaneously the IL cations MPPp+ and the Li cations at potentials ∼0.5 V and below 0.3 V vs Li Li+, respectively. The graphite electrodes become passivated due to the presence of the Li salt by the formation of surface films, which are Li-ion conducting, but electronically insulating. Hence, upon consecutive voltammetric cycling, the IL cation-intercalation is suppressed, while reversible Li intercalation becomes the dominant process. Raman spectroscopy enables one to distinguish among the various processes in these systems.