The possible application of a novel plasticized polymer electrolyte in lithium battery based on substituted spinel cathode was investigated. The polymer electrolyte was prepared by dissolving LiClO4 in the host polymer, poly (ethylene glycol) methacrylates (PEGMs) plasticized by both propylene carbonate (PC) and poly (ethylene glycol)-borate ester (PEG-BE) of various compositions. The ionic conductivity sigma of the polymer is enhanced with the addition of PC, while the lithium ion transference number t(Li+) reaches an optimum in a mixture of 49% PEG-BE and 21% of PC in the polymer weight. PEG-BE has been recognized to interact with anion as a Lewis acid, which leads to high lithium ion conductivity. The observed trend of sigma and t(Li)+ with decreasing PEG-BE is suggested as being due to enhancement of anion conductivity in the medium as a result of low viscous and high dielectric properties of PC coupled with suppressed Lewis acid interaction of PEG-BE. The cycle performance and the storage stability of the Li/polymer electrolyte/LiMn1.8Co0.2O4 cell comprising the above electrolyte were found to be quite acceptable for practical utility. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.1946387] All rights reserved.