Zeolitic imidazole frameworks (ZIFs) have emerged as potential conductive materials for Li ion-transport in polymer solid state electrolytes. However, developing ZIFs with high Li ionic conductivity is rather limited due to their flexible frameworks allowing dual ion conduction. Herein, we have used a mixed ligand strategy for fine-tuning the aperture and enhancing the rigidity of ZIF-8, which restricts the passage of large size anions. Poly(ethylene oxide)-based quasi-solid state electrolytes utilizing mixed ligand ZIF-7-8 frameworks as passive fillers show a continuous enhancement in Li ion-conductivity exclusively attributed to modifications in the flexibility and pore architecture of ZIF-8 as confirmed through broadband dielectric spectroscopy and positron annihilation spectroscopy. This study shows that polymer segmental relaxation and conductivity relaxation processes are decoupled in these electrolytes. Consequently, our proposed approach provides a new strategy for manufacturing a polymer-based electrolyte with enhanced ionic conductivity.