New electrolyte system for Li-ion battery

Lithium ions reversibly intercalate into graphite anodes in electrolytes containing sulfur dioxide (SO2). The electrolyte solutions were composed of LiAsF6 1 M and similar to 20% by weight SO2 in methyl formate, propylene carbonate, diethyl carbonate, or dimethyl carbonate. A high degree of Li-ion intercalation into graphite and stable cycle life were obtained in cells containing these electrolyte systems, which were previously thought to be incompatible with graphite negative electrodes because the graphite structure was destroyed by interaction with the solvent molecules. The use of SO2 as an additive to the organic solution offers the advantage of forming fully developed passive films on graphite at much higher potential (2.7 V vs. Li/Li+) than that of electrolyte reduction (<2 V vs. Li/Li+) and the intercalation stages (0.3 to 0 V vs. Li/Li+). These carbon surface films, composed of a mixture of SO2 reduction products including Li2S and lithium oxysulfur compounds, are primarily responsible for the improved characteristics of Li-ion cells containing these new electrolyte systems.