NEW ELECTROLYTE SYSTEM FOR LI-ION BATTERY

Lithium ions reversibly intercalate into graphite anodes in electrolyt es containing sulfur dioxide (SO2). The electrolyte solutions were com posed of LiAsF6 1 M and similar to 20% by weight SO2 in methyl formate , propylene carbonate, diethyl carbonate, or dimethyl carbonate. A hig h degree of Li-ion intercalation into graphite and stable cycle life w ere obtained in cells containing these electrolyte systems, which were previously thought to be incompatible with graphite negative electrod es because the graphite structure was destroyed by interaction with th e solvent molecules. The use of SO2 as an additive to the organic solu tion offers the advantage of forming fully developed passive films on graphite at much higher potential (2.7 V vs. Li/Li+) than that of elec trolyte 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 compoun ds, are primarily responsible for the improved characteristics of Li-i on cells containing these new electrolyte systems.