Electrochemical intercalation of PF6 into graphite

Nonaqueous electrochemical cells using carbon as both the negative and positive electrodes have been proposed in the literature. In such "dual graphite" cells, lithium intercalates into the negative electrode, and the anion intercalates into the positive electrode when the cells are charged, depleting the electrolyte of salt. Here, the origin of the cell potential is considered first. Then, using Li/graphite test cells, PF6 intercalation into graphite is studied using electrochemical methods and bt situ X-ray diffraction. We prove that PF6 intercalated graphite exists in a series of staged phases. Differential capacity vs. voltage measurements are used to determine the voltages of the staging transitions and the compositions of the staged phases. The cell potential during anion intercalation rises to over 5V, and we have learned that cells with electrolytes using ethyl methyl sulfone solvent: can give reliable results up to 5.5 V under these highly oxidizing conditions. By contrast, ethylene carbonate based electrolytes are strongly oxidized above about 5.2 V, preventing complete loading of graphite with PF6. (C) 2000 The Electrochemical Society. S0013-4651(99)10-063-6. Ail rights reserved.