Lithium poly(4-styrenesulfonyl(trifluoromethylsulfonyl)imide) (PSTFSI) polyelectrolytes made up of SO2 N− SO2 CF3 anionic groups associated with a lithium cation and attached to a polystyrene chain have been prepared. In these polymers the transference number of the lithium cation is expected to be unity while the ionic conductivity is kept high. Two different single-ion polymers with a similar final structure were synthesized by two ways, monomer free radical polymerization and polymer modification (PSTFSI (a) and PSTFSI (b), respectively). These polyelectrolytes were mixed with poly(ethylene oxide) PEO into a composite membrane to form solid electrolytes for lithium batteries. These polymer alloys were studied by scanning electron microscopy (SEM), thermal analysis and impedance spectroscopy to measure the ionic transport properties. Interestingly, ionic conductivity is about ten times higher for a membrane with PSTFSI (a) (≈10−5 Scm−1) compared to a membrane with lithium poly(styrene sulfonate) or with PSTFSI (b) above PEO melting temperature.