NOVELTY - Crosslinking agent comprises a compound with polymethyl siloxane polymer backbone having alkylene oxide branches and 2-4 acrylic functional groups at both ends.    USE - For crosslinkable solid polymer electrolyte composition that has high ionic conductivity at room temperature and can be readily formed into a film for large-size lithium-polymer secondary batteries (claimed) for electric cars, power storage devices for load leveling, as well as in small-size lithium-polymer secondary batteries applicable to video cameras, portable data terminals, i.e. cellular phones and notebook computers.    ADVANTAGE - The inventive cross-linkable agent produces a solid polymer electrolyte that exhibits enhanced chemical and mechanical properties, and electrochemical stabilities due to its three dimensional network structure. The produced solid polymer electrolyte can be readily formed into a film with high ionic conductivity at room temperature and has excellent adhesive property to electrodes of secondary batteries.    DETAILED DESCRIPTION - Crosslinking agent comprises a compound with polymethyl siloxane polymer backbone having alkylene oxide branches and 2-4 acrylic functional groups of formula (I) at both ends.    R1, R2, R3 = H or methyl;    R4 = single bond or formula (II);    X = -O- or -O(O)CO-;    m, n = 0-1000;    p, q = 0-20; and    r = 1-6.    INDEPENDENT CLAIMS are included for:    (1) a solid polymer electrolyte composition that comprises (wt.%) crosslinking agent (0.1-80), plasticizer, (0.1-80) comprising polyalkylene glycol dialkyl ethers of formula (III) and/or non-aqueous polar solvents, lithium salt (3-30) and curing initiator (0.5-5);    (2) a solid polymer electrolyte thin film prepared using the solid polymer electrolyte composition; and    (3) a lithium polymer secondary battery prepared using the solid polymer electrolyte composition.    R1, R2, R3, R4 = as defined above;    R5, R6 = optionally branched 1-10C alkyl;    R7, R8, R9 = H or methyl; and    p, q, l = 0-20.    DESCRIPTION OF DRAWING(S) - The figure shows a graph showing the ionic conductivity of the solid polymer electrolyte composition.