Disclosed is an electrolyte of a lithium secondary battery comprising a lithium salt, an organic solvent, and at least one additive compound selected from the group consisting of compounds represented by the following formula (1) and derivatives thereof: ##STR00001## where R.sub.1 is selected from the group consisting of hydrogen radicals, alkyls, aryls, cycloalkyls, alkenyls, alkynyls, ester radicals, and aliphatic carbonate radicals. The electrolyte improves both swelling inhibition properties at high temperature and capacity characteristics of a lithium secondary battery.