Coulomb interactions in charged block copolymers have known to play a key role in determining the self-assembly behavior, which is largely deviated from conventional phase diagrams of block copolymers. In this study, self-assembled morphologies of acid-tethered block copolymers were investigated by introducing a range of ionic liquids, aiming at controlling the electrostatic interactions within. For this, two types of acid-tethered block copolymers were prepared and nonstoichiometric ionic liquids were employed, leading to the evolution of various self-assembled morphologies. Especially, we observed reentrant phase transition between lamellae and A15 structures upon a small change in the nonstoichiometry of ionic liquids, without altering the volume fraction of ionic domains. Computational simulations suggested the formation of thin ionic shell layers at the micellar interfaces, which was key to stabilizing the A15 structures. The A15 structures having three-dimensionally interconnected ionic domains showed enhanced ionic conductivity, compared with lamellar-forming counterparts.