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.