Ternary phase diagram of prepolymer mixtures containing poly(ethylene glycol diacrylate) (PEGDA), succinonitrile (SCN) and lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) has been established in order to provide guidance for producing completely amorphous self-standing solid electrolyte membranes. The phase diagram of the precursor mixtures, thus obtained, consisted of a wide isotropic amorphous region, crystal + amorphous coexistence region at high lithium salt concentrations and plastic crystal + amorphous coexistence region at succinonitrile-rich compositions. Of particular interest is that upon photo-polymerization of isotropic compositions, the resulting networks remained completely amorphous without undergoing polymerization-induced phase separation or polymerization-induced crystallization. The ionic conductivity was mapped out as a function of composition within the isotropic region of the starting ternary phase diagram in conjunction with room temperature conductivities showing high ionic conductivities on the order of 10(-3) S cm(-1) at several compositions. These solid state polymer electrolyte membranes are transparent, light weight, and flexible, which can be shaped in any forms with space saving attributes. More importantly, no solvent is required in the membrane fabrication as well as in the battery operation, thereby improving safety and cost effectiveness. (C) 2013 Elsevier B.V. All rights reserved.