NOVELTY - An electrolyte (2) comprises: (A) a compound having methylene group(s) adjacent to an oxygen atom in the molecule; and (B) a perfluoroisopropenyl ester substituted compound (I) and an electrolytic salt. The compounds (A) and (B) are polymerized with each other.    USE - For lithium ion secondary battery (claimed) used as power supply for integrated circuit cards, personal computers, large-sized computers, notebook type personal computers, pen-inputting personal computers, notebook type word processors, portable telephones, pocket cards, wrist watches, cameras, electric shavers, cordless telephones, facsimiles, videos, video cameras, electronic pocket goods, desk-top computers, electronic pocket goods provided with an unit for communication, portable copiers, televisions provided with liquid crystal display, electromotive tools, cleaners, game-playing machine provided with a function of virtual reality, toys, electromotive bicycles, medical aids for medical cad use, wheelchairs, mobile beds, escalators, elevators, forklifts , golf carts, power supplies provided against emergencies, road conditioners and electric power storing systems, for munition use and for space use.    ADVANTAGE - The electrolyte has high ionic conductivity, and the lithium ion secondary battery using the electrolyte has superior charge and discharge characteristics.    DETAILED DESCRIPTION - An electrolyte (2) comprises:    (A) a compound having methylene group(s) adjacent to an oxygen atom in the molecule; and    (B) a perfluoroisopropenyl ester substituted compound of formula (I) and an electrolytic salt.    The compounds (A) and (B) are polymerized with each other.    R = phenyl group, cyclohexyl or -CnH2n-, where n is 2 or more.    An INDEPENDENT CLAIM is included for lithium ion secondary battery which has an anode (1) and cathode (3) capable of occluding and discharging lithium ion and the electrolyte.    DESCRIPTION OF DRAWING(S) - The figure shows a structural view of the anode, cathode and electrolyte film.    Anode (1)    Electrolyte (2)    Cathode (3)