Effect of graphite crystal structure on lithium electrochemical intercalation

The electrochemical intercalation of lithium into various graphite materials has been examined in LIPF6 solutions of ethylene carbonate (EC)/dimethyl carbonate (DMC) or EC/DMC/propylene carbonate (PC). When the graphite powder samples contained at least 30% of the rhombohedral form, no exfoliation was observed, even with electrolytes containing a large amount (80%) of PC. However, when the graphites were heat-treated at temperatures above 1000 degrees C, the faradaic losses due to the exfoliation reappeared, even though the rhombohedral phase content was unchanged. From Raman spectroscopy measurements, a correlation was found between the irreversible capacity due to the exfoliation and the ratio, R, of the integrated intensity of the disorder-induced line at 1350 cm(-1) to the Raman-allowed line at 1580 cm(-1). This suggests that structure defects, probably localized in grain boundaries between rhombohedral and hexagonal domains, hinder the layer opening necessary for the intercalation of solvated lithium species at the beginning of the first electrochemical cycle. (C) 1999 The Electrochemical Society. S0013-4651(99)04-044-6. All rights reserved.