The electrical properties of solvent-free, PEO-LiTFSI solid polymer electrolytes (SPEs), incorporating different N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, PYR1ATFSI, ionic liquids (ILs), are reported. For this purpose, PYR1ATFSI materials containing side alkyl groups with different chain-length and branching, i.e., n-propyl, sec-propyl, n-butyl, iso-butyl, sec-butyl and n-pentyl, were properly synthesized and homogeneously incorporated into the SPE samples without phase separation. The addition of ILs to PEO-LiTFSI electrolytes results in a large increase of the conductivity and in a decrease of the interfacial resistance with the lithium metal anode. Most of the PEO-LiTFSI-PYR1ATFSI samples showed similar ionic conductivities (>10(-4) S cm(-1) at 20 degrees C and stable interfacial resistance values (400 Omega cm(2) at 40 degrees C and 3000 Omega cm(2) at 20 degrees C) upon several months of storage. Preliminary battery tests have shown that Li/P(EO)(10)LiTFSI + 0.96 PYR1ATFSI/LiFePO4 solid-state cells are capable to deliver a capacity of 125 mAh g(-1) and 100 mAh g(-1) at 30 degrees C and 25 degrees C, respectively. (C) 2007 Elsevier B.V. All rights reserved.