TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Electrode: The electrolyte (A1) further comprises vinylene carbonate, propene sultone, and/or ethylene carbonate group compound of formula (II) (preferably fluoroethylene carbonate).R1 and R2=H, halo, cyano (CN), nitro (NO2), vinyl or 1-5C fluorinated alkyl.Provided that any one of R1 and R2 is not H.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The lithium salt is selected from LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiCF3SO3, LiC4F9SO3, LiAlO4, LiAlCl4, LiN(CpF2p+1SO2)(CqF2q+1SO2), lithium chloride (LiCl) and/or lithium iodide (LiI).p and q=natural numbers.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The phosphonitrile fluoride trimer is present in the electrolyte, in an amount of 0.1-5 parts by weight (pbw). The non-aqueous organic solvent is selected from carbonate, ester, ether and/or ketone. The carbonate is selected from dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, ethylmethyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate and/or pentylene carbonate. The ester is selected from n-methyl acetate, n-ethyl acetate, n-propyl acetate, butyl acetate, methyl propionate, ethyl propionate, gamma -butyrolactone, decanolide, valerolactone, mevalonolactone and/or caprolactone. The ether is selected from dibutyl ether, tetraglyme, diglyme, dimethoxy ethane, 2-methyltetrahydrofuran and/or tetrahydrofuran. The ketone is selected from polymethylvinyl ketone and/or cyclohexanone. EXAMPLE - An electrolyte was prepared by adding 1M hexaflurophosphate (LiPF6) in a mixture solvent of ethylene carbonate, ethylmethyl carbonate and diethyl carbonate having volume ratio of 1:1:1 and adding phosphonitrile fluoride trimer (1 part by weight). Cathode active material slurry was prepared by dispersing lithium cobalt oxide (LiCoO2) as a cathode active material, polyvinylidene fluoride (PVdF) as a binder, and carbon as a conductive material in N-methyl-2-pyrrolidone (NMP) solvent by a weight ratio of 92:4:4. Then a cathode was made by coating the cathode active material slurry on an aluminum foil having a thickness of 15 mu m, drying and rolling it. The anode active material slurry was prepared by mixing artificial graphite as an anode active material, styrene-butadiene rubber as a binder, and carboxymethylcelluose as a thickener by a weight ratio of 96:2:2, then dispersing it in the water. Then an anode was made by coating the slurry on a copper foil having a thickness of 10 mu m, drying and rolling it. Next, after a polyethylene separator having a thickness of 10 mu m was interposed between the electrodes, it was wound, and pressurized. Then, it was inserted into a rectangular can. Then, electrolyte was injected into the can, to produce a lithium secondary battery (test). Comparative lithium secondary battery was prepared in the same manner test except that the phosphonitrile fluoride trimer was not added. The test/comparative batteries were left for 7 days at 60 degrees C, and then thickness increase rates were measured. The test/comparative battery showed thickness increase rate of 9.5/17%. The result showed that due to the thickness increase rate was reduce in test battery, it showed improved thermal stability.