Lithiated perfluorosulfonate ionomer has been used as the binder for LiMn2O4 cathodes. Casted membranes of the lithiated ionomer exhibit ionic conductivity of 1.4×10−4 S/cm. Composite cathodes composed of LiMn2O4, carbon black and the ionomer binder have been fabricated. All components of the cathodes are well bound and dispersed as characterized by scanning electron microscope and energy dispersive spectroscope. The cathodes using the conventional poly-vinylidene fluoride binder have also been prepared for comparison. Under high rate (5 C-20C) and high temperature (60°C) operation, the LiMn2O4 cathodes with the ionomer binder exhibit higher capacity and improved cycling stability. As indicated by the electrochemical impedance spectra, the ionomer binder forms ion-conducting interface layers on the LiMn2O4 particles and results in lower interface resistance. It enables the cells utilizing the ionomer binder to achieve higher capacity and enhanced cycling stability even under harsh conditions.