Two polymer hosts have been used to prepare composite electrolytes, i.e. PEO (M W=1×106) and a triblock copolymer [poly(propylene glycol–ethylene glycol–propylene glycol)] with M W=2000. The effect of the LiAl5O8 addition on the properties of electrolytes prepared with polyether–LiI was studied. The aluminate ceramic powder was prepared by coprecipitation and calcined at 1150°C. The main goal of the present investigation is to compare the two classes of composite electrolytes with polyether–LiI with a composition fixed at n=molar ratio of [O]/[Li]=20. LiAl5O8 powder was dispersed by ultrasound in the electrolyte matrix in a range of concentrations up to 20 wt%. Differential scanning calorimetry curves indicated that all materials are semi-crystalline, and the triblock composites show a smaller glass transition (T g=−28°C at 20 wt% of ceramic) than that of PEO electrolytes (−16°C at 20 wt% of ceramic). Thermomechanical analyses were performed using penetration mode and the composite electrolytes exhibited a significant increase in mechanical stability. The ionic conductivity of both systems (PEO and triblock–LiI–LiAl5O8) is 5×10−4 S/cm at 80°C. The triblock composite shows a conductivity of one order of magnitude higher than that of the PEO material near room temperature as a consequence of the decrease in crystallinity after a first heating. This result raises the interest in the development of composites based on a triblock polyether.