In situ Characterization of a Graphite Electrode in a Secondary Lithium-Ion Battery Using Raman Microscopy

Abstract

A Raman microscopy study of lithium intercalation into the graphite electrode of a lithium-ion battery is presented. An in situ spectro-electrochemical cell was designed for direct observation of the electrode/electrolyte interface. The performance of this cell is discussed in terms of the results of a calibration experiment performed at a single defined point on the electrode surface. The electrode was made of TIMREX SFG 44 synthetic graphite with polyvinylidene fluoride binder. The electrolyte was lithium perchlorate, LiClO4, dissolved in a mixture of ethylene carbonate and dimethyl carbonate. In the region of the carbonyl stretching vibrational modes of the electrolyte components, changes in the band profile have been observed. At electrode potentials negative to 180 mV vs. Li/Li+, a new band evolved at about 1850 cm−1. This band has tentatively been assigned to a complex between lithium ions and decomposition products of the ethylene carbonate electrolyte component. The maximum intensity of this new band is observed at 5 mV vs. Li/Li+; its intensity decreases with increasing potential upon lithium de-intercalation. Raman mapping of the graphite electrode under potentiostatic conditions indicates that lithium intercalation does not proceed homogeneously over the graphite electrode surface at a potential of 200 mV vs. Li/Li+. An additional Raman mapping study was performed under galvanostatic conditions. With this method, the presence of “blind spots” on the electrode surface can be detected. These points lag behind in the process of lithium intercalation. Furthermore, information on changes in the carbon component of the electrode can be inferred from these measurements.