Short and ‘fuzzy’ poly(ethylene) glycol chains with different molecule weights have been successfully grafted on to a pristine SiO2 nanoparticle surface using toluene 2,4-diisocyanate as the bridging molecule. Solvent-free composite electrolytes based on poly(ethylene oxide), LiBF4 and SiO2 or modified SiO2 particles have been prepared and compared. Composite electrolytes with modified SiO2 show a noticeably smoother surface texture under scanning electron microscopy. This is attributed to improved compatibility between the ceramic particles and polymer. The increased amorphization of the polymer leads to increase in room-temperature ionic conductivity as more ion-conduction channels are created in close proximity to the modified silica particles. On the other hand, a lower transference number is the result of weakened Lewis acid–base interactions between the polymer backbone and a smaller number of OH groups on the silica surface.