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 w as explored using in situ x-ray diffraction in conjunction with chrono potentiometry. The solvent systems studied included ethylene and dieth yl carbonate mixtures (EC-DEC), propylene carbonate (PC), tetrahydrofu ran (THF), and dimethyl carbonate (DMC). These studies revealed that t he above systems can be divided into three classes. EC-DEC and water c ontaminated DMC are solvent systems in which th highly passivating and protective surface films are precipitated on the carbon at potentials much higher than the intercalation potentials. Therefore, graphite el ectrodes behave reversibly in these solutions and are stable on Li int ercalation-deintercalation cycling. An opposite case occurs with PC an d THF, where the carbon is destroyed before or during the intercalatio n 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 th an where Li intercalation occurs, but it is not sufficient to protect the carbon totally and therefore the electrode is slowly destroyed by cycling.