Lithium-iodine (Li-I) batteries have attracted tremendous attention due to their high energy and power densities as well as the low cost of iodine. However, the severe shuttle effect of iodine species and the uncontrollable lithium dendrite growth have strongly hindered their practical applications. Here we successfully develop a quasi-solid-state Li-I battery enabled by a MXene-based iodine cathode and a composite polymer electrolyte (CPE) containing NaNO3 particles dispersing in a pentaerythritol-tetraacrylate-based (PETEA-based) gel polymer electrolyte. As verified by experimental characterizations and first-principle calculations, the abundant functional groups on the surface of MXene sheets provide strong chemical binding to iodine species, and therefore immobilize their shuttling. The PETEA-based polymer matrix simultaneously suppresses the diffusion of iodine species and stabilizes the Li anode/CPE interface against dendrite growth. The NaNO3 particles act as an effective catalyst to facilitate the transformation kinetics of LiI3 on the cathode. Owing to such synergistic optimization, the as-developed Li-I batteries deliver high energy/power density with long cycling stability and good flexibility. This work opens up a new avenue to improve the performance of Li-I batteries.