Solid polymer electrolytes for Lithium batteries applications are commonly prepared by dissolving a lithium salt in poly(ethylene oxide) (PEO)-based materials. Their performance is strongly related to the structure of the polymer network. In this article, a new salt-in-polymer electrolyte prepared by the fast and easy radical photopolymerization of PEO acrylate oligomers is studied. Here, a difunctional monomer used as the polymer backbone is copolymerized with monofunctional monomers of different length and concentration. Thus, the crosslinking density and conductivity are changed. These systems are investigated by a detailed NMR study yielding local dynamics and mass transport by temperature-dependent spin-lattice relaxation time and PFG-NMR diffusion measurements for different nuclei (7Li and 19F). The results indicate that a sufficiently long monofunctional oligoether improves the properties, since it provides a lower crosslinking density as well as more coordinating oxygens for the Li ions. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1571-1580