Addition of filler increases the ionic conductivity and mechanical stability of polymer electrolytes by inhibiting the recrystallization of polymer chains and providing Li+ conducting pathways at the filler surface through Lewis acid-base interaction among different species in the electrolyte. The ion movement is obstructed by the crystalline region present in polymer electrolyte while blocking the paths of the ions. The amorphous region favors the conduction of Li+ due to its greater free volume.

Ref: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4124230/