Highly conductive solid polymer electrolytes were generated by blending linear poly(ethyleneimine)-graftpoly(ethylene glycol) with linear poly(ethyleneimine) bearing lithium N-propylsulfonate groups as the lithium source. The effect of polymer backbone structure on Li + conductivity was determined by comparing a series of blends made from the PEI-based materials with those from polymethacrylate backbone analogues. The use of PEI backbones promoted ion-pair dissociation, stabilized the macromolecular mix and generated blends with ionic conductivities up to 2 orders of magnitude higher than those of the polymethacrylate-based systems. Blends containing the PEI-bound lithium sulfonates exhibited lithium conductivities higher than those measured for PEG doped with lithium bis(trifluoromethyl)sulfonimide. Shifts in the ν s (SO 3 ) IR absorption band suggest that the solvation environment for the lithium sulfonates changes with polymer structure. The PEI-based blends are thermally stable up to 200°C, electrochemically stable in the ±5 V range, and showed unparalleled ionic conductivities (0.4 mS/cm at room temperature and 5 mS/cm at 80°C) for solvent-free systems with polymer-bound anions.