TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Aromatic Ether: The aromatic ether is benzyl ether.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The lithium secondary battery further comprises a separator made of multi-porous films.Preferred Composition: 0.1-5.0 weight percent (wt.%) of aromatic ether is contained with respect to total weight of non-aqueous organic solvent and lithium salt. SPECIFIC MATERIALS - The lithium salt is lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium trifluoromethanesulfonate and lithium bistrifluoromethanesulfonylamide.    The aromatic ether is diphenyl ether. The non-aqueous organic solvent is propylene carbonate, ethylene carbonate, ethylmethyl carbonate, methyl acetate, gamma-butyrolactone, 1,3-dioxolane, dimethoxy ethane, dimethyl carbonate, diethyl carbonate, tetrahydrofuran, dimethyl sulfoxide or polyethylene glycol dimethyl ether.    EXAMPLE - An electrode structural body was manufactured by inserting separator made of polyethylene resin between cathode comprising lithium cobalt oxide and anode comprising carbon. The structural body was then inserted into a cylindrical can of battery case and an electrolyte containing 1.0 M lithium hexafluorophosphate and 1.0 wt.% of diphenyl ether dissolved in mixed solvent of ethylene carbonate and dimethyl carbonate was added to form lithium secondary batteries. After charging the manufactured batteries with current of 320 mA and charging voltage of 4.2 V, they were left for one hour. Then they were discharged to 2.5 V with 320 mA and left for one hour. The charging and discharging process were repeated for three times. The batteries were left at high temperature chamber for four days. The cylinder lithium secondary batteries to which aromatic ether was added, the current interrupt device did not operate even when the batteries were left at 85degreesC for more than four days. The lifetime characteristics of the batteries lowered a little. The thickness of the battery before charging was 4.25 mm. The increase in the thickness of the battery after one day and two days were 5.98 mm and 6.34 mm, respectively.