Exfoliation of Graphite during Electrochemical Lithium Insertion in Ethylene Carbonate-Containing Electrolytes

Post mortem scanning electron microscopy, X-ray diffraction analysis, and Raman spectroscopy were applied to study the exfoliation tendency of a high temperature-treated graphite negative electrode material during the first electrochemical lithium insertion in various carbonate electrolyte systems. Exfoliation of the heat-treated graphite electrode material was observed in propylene carbonate (PC)- and ethylene carbonate (EC)-containing electrolytes. Using acyclic carbonates and 1-fluoro ethylene carbonate, exfoliation of the graphite structure could be avoided. LiPF6 used as a conducting salt in the EC-based electrolyte increased the exfoliation tendency of the graphite material. Differential electrochemical mass spectrometry was performed to study the passivation of the untreated and heat-treated graphite surface during the first electrochemical Li+ insertion. The heat-treated graphite surface showed a reduced reactivity towards EC, which hindered the graphite surface passivation in EC-based electrolyte systems and led to the exfoliation of the graphite structure so far known only for PC-containing electrolytes.