The synthesis and the properties of nanocomposite polymer electrolytes based on low-particle size fillers having a high concentration of surface states are reported. These electrolytes that are formed by dispersing Al2O3 in a P(EO)(20)LiCF3SO3 [PEO is poly(ethylene oxide)] matrix, show higher conductivity and, particularly, much higher lithium transference number than those of common, ceramic-free P(EO)(20)LiCF3SO3 counterparts. These unique properties make the nanocomposite polymer electrolytes of particular interest as improved separators in novel types of lithium rechargeable batteries. This paper reports the performance of a battery using a P(EO)(20)LiCF3SO3 + 10 wt % Al2O3 as the polymer electrolyte and LiMn3O6 as the cathode.