High voltage spinel LiMn1.5Ni0.5O4 has been synthesized by a modified Pechini sol-gel method and has been characterized by transmission electron microscopy, X-ray diffraction (XRD), and electrochemical methods. The synthesized materials are porous structures of nanosized crystallites ranging in size from 21 to over 400 nm depending on the sintering temperature used. The XRD patterns of the materials were assigned to the disordered spinel structure of the space group Fd3m. The Li-ion batteries assembled using the synthesized cathode materials showed significant capacity fade for samples sintered at 500 degrees C, while for those sintered at 800 degrees C the capacity fade was low. Impedance spectroscopy, Fourier transform IR spectroscopy, and X-ray photoelectron spectroscopy were used to determine the compositions of the cathode electrolyte interphase (CEI). Impedance spectroscopy confirmed the spontaneous formation of the CEI on LiMn1.5Ni0.5O4 and that its thickness grows on cycling. After more than 100 cycles, it is found that the CEI film is composed of polycarbonates, polyether, LiF, and LixPOyFz salts. The composition of the organic layer was the same regardless of the capacity fade.