In Situ Raman Study on Reversible Structural Changes of Graphite Negative-Electrodes at High Potentials in LiPF6-Based Electrolyte Solution

Abstract

Changes in the surface crystallinity of graphite negative-electrodes were investigated in a high potential region between 1.5 and 3 V vs. Li+/Li in LiPF6- and LiClO4-based electrolyte solution by in situ Raman spectroscopy using highly oriented pyrolytic graphite (HOPG) basal plane as a model electrode. The variation of Raman spectra suggested that some intercalation reactions and a decrease in the surface crystallinity of graphite occurred at a potential as high as 1.5 V vs. Li+/Li where Li+ is not intercalated into graphite. The Raman bands restored to their original states after the electrode potential was returned to 3.0 V. These changes repeatedly occurred in an almost reversible manner in the high potential region between 1.5 and 3 V. We focused on POF2OH, which was contained in the conventional LiPF6-based electrolyte solution as an impurity, and investigated the effects on the above structural changes to clarify the reaction mechanism. On the other hand, irreversible structural degradation of the graphite surface occurred at potentials below 1.0 V, where Li+ is intercalated into graphite, specifically in LiPF6-based electrolyte solution. These results indicate that PF6−-derived compounds should be deeply involved in the structural deterioration of graphite and surface film formation on it.