SEI film formation on highly crystalline graphitic materials in lithium-ion batteries

In situ differential electrochemical mass spectrometry (DEMS) was used to study the SEI film formation on highly crystalline TIMREX® SLX50 graphite negative electrodes during the first electrochemical lithium insertion using either 1 M LiPF6 in ethylene carbonate (EC) with either dimethyl carbonate (DMC) or propylene carbonate (PC) as co-solvent. In the case of the propylene and ethylene carbonate containing electrolyte, DEMS measurements indicate strong formation of ethylene and propylene gas below 0.75 V versus Li/Li+, which does not decrease at lower cell potential and in the subsequent charge/discharge cycles. Whereas for the dimethyl carbonate containing electrolyte, ethylene gas formation could be observed already above 1 V versus Li/Li+. Post mortem scanning electron microscopy (SEM) studies of the electrodes show strong exfoliation of the graphite electrode when they are discharged in the ethylene/propylene carbonate electrolyte, indicating the formation of an unstable SEI layer. The addition of vinylene carbonate (VC) as a film forming additive significantly decreases the gas formation at the graphite electrode in the propylene carbonate containing electrolyte. The exfoliation was suppressed by the vinylene carbonate additive. We show that the combination of different in situ and ex situ methods can provide new useful information about the passivation process of graphite, as well as the solid electrolyte interphase layer formed, during the first electrochemical insertion of lithium into graphite negative electrode materials.