In an effort to develop free-standing lithium battery membrane, binary and ternary phase diagrams of poly(ethylene oxide) (PEO), bis(trifluoromethane)sulfonimide (LiTFSI), and succinonitrile (SCN) (i.e., solid plasticizer) mixtures have been established by means of differential scanning calorimetry and polarized optical microscopy. The occurrence of hydrogen bonds and/or coordination bonds in each binary pair (PEO/SCN, SCN/LiTFSI, and PEO/LiTFSI) was examined using Fourier transform infrared spectroscopy. The binary PEO/LiTFSI mixture exhibits a eutectic phase diagram with the liquid + crystal coexistence region having various crystal forms of the lithium salt, whereas the SCN/LiTFSI blend shows a wide noncrystalline region, which is highly desirable for organic solvent-free battery applications. The PEO/SCN blend shows a typical eutectic behavior, which is explicable in the framework of the Flory−Huggins theory in conjunction with the phase field theory of crystal solidification. Various coexistence regions of the PEO/SCN/LiTFSI mixtures have been mapped out using polarized optical microscopy and wide-angle X-ray diffraction. The ionic conductivity was determined at various coexistence regions such as isotropic noncrystalline liquid, crystal + liquid, liquid + plastic crystal regions using ac impedance spectroscopy. Of particular interest is that the conductivity in the isotropic liquid region is higher than those of the crystal (or plastic crystal) + liquid coexistence regions.