Ion-containing block copolymers continue to attract significant interest as conducting membranes in energy storage devices. Reversible addition-fragmentation chain transfer (RAFT) polymerization enables the synthesis of well-defined ionomeric A-BC-A triblock copolymers, featuring a microphase-separated morphology and a combination of excellent mechanical properties and high ion transport. The soft central "BC" block is composed of poly(4-styrenesulfonyl-(trifluoromethylsulfonyl)imide) (poly(Sty-Tf2N)) with -SO2-N--SO2-CF3 anionic groups associated with,a mobile lithium cation and low-T-g di(ethylene glycol)methyl ether methacrylate (DEGMEMA) units. External polystyrene A blocks provide mechanical strength with nanoscale morphology even at high ion content. Electrochemical impedance spectroscopy (EIS) and pulse-field-gradient (PFG) NMR spectroscopy have clarified the ion transport properties of these ionomeric A-BC-A triblock copolymers. Results confirmed that well-defined ionomeric A-BC-A triblock copolymers combine improved ion-transport properties with mechanical stability with significant potential for application in energy storage devices.