The invention provides a novel lithium-ion battery electrolyte which is produced by adding an aluminate ester compound as a plasticizer in a solid polymer electrolyte. The lithium-ion battery solid polymer electrolyte of the present invention comprises a lithium ion source, an organic polymer compound, and one or more kinds of aluminate ester compounds represented by the general formula ##STR00001## where R.sub.1, R.sub.2, and R.sub.3 each independently represent a straight-chain or branched-chain lower alkyl group having one to eight carbon atoms, an alkenyl group having three to six carbon atoms, acryloyl, or methacryloyl; OA.sub.1, OA.sub.2, and OA.sub.3 each independently represent an oxyalkylene group having two to four carbon atoms; l represents a number from 1 to 100; and m and n each independently represent a number from 0 to 100.