Antimony pentoxide nanocrystals with controlled sizes (4–400 nm) and shapes (spheres, sheets and cubes) have been synthesized hydrothermally. The growth process of the crystals observed by transmission electron microscopy (TEM) obeys Ostwald ripening theory. Upon annealing, the composites of the Sb₂O₅ nanoparticles with PEO–LiClO₄ polymer electrolyte exhibit significantly enhanced ionic conductivities and these enhanced conductivities have long-term stability at ambient temperature. The conductivity variation after special thermal treatment and the impedance evolution with time of the PEO–Sb₂O₅–LiClO₄ films have direct relationships with the sizes and shapes of the Sb₂O₅ fillers. Such performance of the composite solid polymer electrolytes can be interpreted by dipole–dipole interactions between the PEO macromolecules, lithium salt and the Sb₂O₅ dopants.