A star-shaped siloxane acrylate with a different number of repeating units of oligo(ethylene oxide) (EO) was synthesized as a cross-linker of solid polymer electrolytes. The cross-linked solid polymer electrolytes blended with the ionic conducting plasticizers, such as low molecular weight poly(ethylene oxide)dimethyl ether (PEGDME) were prepared by the in situ thermal curing of the star-shaped siloxane acrylate. Different morphologies of the cross-linked polymer electrolytes were observed according to the number of repeating units of EO (n) in the cross-linker. A micro-phase separated solid polymer electrolyte was obtained when the n of cross-linker was 1. When the n of cross-linker was larger than 1, homogeneously blended solid polymer electrolytes were prepared. The ionic conductivity was measured to be 6.3 to 7.8×10−4 Scm−1 with 80wt.% PEGDME at 30°C. The ionic conductivity of the micro-phase separated solid polymer electrolyte was slightly higher than that of the homogeneously blended solid polymer electrolyte. The electrochemical stability window of the resulting solid polymer electrolyte could be extended to up to 4.8V versus Li/Li+ reference electrode.