Novel and highly effective polyethylene oxide (PEO) based composite electrolytes were prepared by combining the graphene nanoplatelets (GR) and multiwall carbon nanotubes (MWCNT) for the application as solid polymer electrolyte. MWCNT and GR were used as reinforcing filler and as electrical conductivity enhancement agent. Dispersions in N,N-dimethylformamide (DMF) of MWCNT and GR within the PEO matrix were prepared. DMF are featured by high electron-pair donor number and low hydrogen-bonding parameters, therefore DMF is considered a standard for liquid-phase exfoliation of MWCNT and GR. In our study, the MWCNT and GR solutions were tip sonicated using an ultrasonic processor, operated at 80% amplitude. A pulse-mode (cycle of 0.5 s) sonication was used because of the system relaxation role for the off phase, allowing a higher cavitation intensity and lower heat generation to be reached. Subsequent heat pressing was applied to obtain thin solid PEO composite electrolytes. Analyses of the experimental and theoretical density of prepared solid PEO composite electrolytes are calculated and discussed. GR and MWCNT functionalization effect on void content of polymer composites is evaluated. FTIR analysis was carried out to further investigate the effect of fillers content. The SEM results showed that surface of electrolyte film became rougher after the addition of MWCNT and GR. It is concluded, that the higher is filler fraction, the lower is void content and greater is composite density.