NOVELTY - Electrolyte comprises a non-aqueous organic solvent, a lithium salt and at least one aromatic phosphate compound (I).    USE - The electrolyte is useful in a lithium ion secondary battery (claimed).    ADVANTAGE - The electrolyte provides a lithium ion secondary battery in which side reactions and exothermic reactions are inhibited upon overcharge or during high-temperature storage to prevent an increase in the temperature of the battery, resulting in an improvement in safety, and the battery exhibits good swelling stability during high-temperature storage as well as improved cycle life characteristics. The electrolyte: protects the battery from the danger of fire or explosion; absorbs the internal heat of the battery to contribute to the prevention of fire and/or combustion; and provides smooth conduction of the ions.    DETAILED DESCRIPTION - Electrolyte comprises a non-aqueous organic solvent, a lithium salt, and at least one aromatic phosphate compound of formula (I).    n = 1-10; and    R1, R2 = 1-5C alkyl.    INDEPENDENT CLAIMS are included for:    (1) a lithium ion secondary battery (10) comprising: a positive electrode (13) comprising a positive electrode active material capable of reversibly inserting or removing lithium; a negative electrode (15) comprising a negative electrode active material capable of reversibly inserting or removing lithium; and the electrolyte; and    (2) a method of manufacturing a lithium ion secondary battery comprising: mixing the positive electrode active material, a binder and a conductive material in a solvent to prepare a positive active electrode slurry; coating the positive active electrode slurry on aluminum foil; drying and rolling the positive active electrode slurry and foil composition together; mixing the negative electrode active material, a binder and a thickener in water to prepare a negative active electrode slurry; coating the negative active electrode slurry on copper foil; drying and rolling the negative active electrode slurry and foil composition together; interposing a separator (14) between the electrodes; winding and pressing the electrodes and separator laminate together; inserting the electrode and separator laminate into a battery can; injecting a mixture of a lithium salt, non-aqueous organic solvent and aromatic phosphate compounds into the can (11); and capping the can.    DESCRIPTION OF DRAWING(S) - The figure shows the rectangular type lithium ion secondary battery.    Lithium ion secondary battery (10)    Can (11)    Positive electrode plate (13)    Separator (14)    Negative electrode plate (15)