The Study of Li-Graphite Intercalation Processes in Several Electrolyte Systems Using In Situ X-Ray Diffraction

The behavior of graphite electrodes in various electrolyte solutions was explored using in situ x-ray diffraction in conjunction with chronopotentiometry. The solvent systems studied included ethylene and diethyl carbonate mixtures (EC-DEC), propylene carbonate (PC), tetrahydrofuran (THF), and dimethyl carbonate (DMC). These studies revealed that the above systems can be divided into three classes. EC-DEC and water contaminated DMC are solvent systems in which highly passivating and protective surface films are precipitated on the carbon at potentials much higher than the intercalation potentials. Therefore, graphite electrodes behave reversibly in these solutions and are stable on Li intercalation-deintercalation cycling. An opposite case occurs with PC and THF, where the carbon is destroyed before or during the intercalation processes, and therefore graphite anodes behave totally irreversibly in these systems. In an intermediate case (dry DMC is a good example), a passivating layer is formed on the carbon at a potential higher than where Li intercalation occurs, but it is not sufficient to protect the carbon totally and therefore the electrode is slowly destroyed by cycling.