A novel lithium composite solid polymer electrolyte based on a polymer, poly(ethylene oxide) (PEO), and a rare earth oxide, Sm2O3, is reported. The incorporation of lithium salt and samarium oxide to PEO shows a dramatic modification of surface morphology. Scanning electron microscopy (SEM) shows that at a low Sm2O3 content, the polymer film displays smooth morphology. This indicates that satisfactory interaction takes place between the oxides and the polymer in the presence of the salt. Differential scanning calorimetry (DSC) results corroborate the SEM micrographs, where the crystallinity decreases further to 10% when compared with a pure PEO/Li salt polymer electrolyte. 7 Li magic angle spinning (MAS) NMR spectra suggest two major lithium species attributed to the Li+ ions associated with amorphous PEO and Sm2O3. The conductivity in the composite electrolyte is one order of magnitude higher compared with pure PEO/Li electrolyte. The oxide–salt complex functions as cross-linking centres for the PEO segments and establishes additional pathways to conduct ions through the filler surface, thus complements ion movement. The ionic conductivity decreases, with increasing content of Sm2O3 to over 10wt.% where an aggregated Sm2O3:Li-rich domain is identified.