A series of poly(ethylene oxide) (PEO)/lithium trifluoromethanesulfonate(LiTf)/ethylene carbonate/sepiolite composite electrolytes have been prepared by melt compounding. Neat sepiolite, sepiolite coated with polyethylene glycol and with D-α-tocopherol polyethylene glycol 1000 succinate have been used as fillers, and nanocomposites with different ratio of the components have been prepared. The concentration of EC and LiTf has been progressively increased from low to high values and in this way, electrolytes with conductivities from 2 × 10−6 to 3 × 10−4 S cm−1 at 30 °C have been prepared. Together with conductivity, viscoelasticity has also been studied, evidencing a complex rheological behaviour which depends on the type of filler introduced in the blend. Some of the thermoplastic electrolytes are seen to display a liquid-like conductivity together with solid-like mechanical properties over the melting point of PEO. The solid-like performance is featured by shear moduli crossover G’ = G” at 75 °C at very low frequencies. This combination of properties makes them appealing starting points for the development of solid polymer electrolytes.
