THE ELECTROCHEMICAL INTERCALATION OF LI INTO GRAPHITE IN LI POLYMER ELECTROLYTE GRAPHITE CELLS

The electrochemical intercalation of Li into graphite has been studied in Li/polymer electrolyte/graphite cells using an in situ x-ray diffr action (XRD) technique. In cells containing an electrolyte of PAN(poly acrylonitrile)-EC(ethylene carbonate)-LiPF6, a minor irreversible redu ction of the electrolyte is observed only during the first discharge. In these cells, Li is reversibly intercalated into graphite to form Li 1.0C6, principally at potentials between 0.2 and 0.0 V vs. Li+/Li. No evidence for the cointercalation of EC was obtained. In cells containi ng PAN-EC/PC(propylene carbonate)-LiPF6-based electrolyte, a massive r eduction of electrolyte occurs during the first discharge at similar t o 0.8 V vs. Li+/Li, which precludes Li intercalation into graphite. In situ XRD data are consistent with the absence of the intercalation of PC or Li+(PC)(2) solvates into the graphite lattice, either prior to or during the solvent reduction process. The latter appears to be a su rface-catalyzed process, the extent of which is determined by a combin ation of thermodynamic and kinetic factors including the reduction pot ential of the electrolyte, and the passivating films which form on the graphite surface as a result of electrolyte reduction.