Design Strategy for Concurrently Enhancing the Ionic Conductivity and Mechanical Integrity of Sulfonated Polymer Electrolytes

Achieving high ionic conductivity in solid polymer electrolytes remains challenging because of the strong coupling between ion transport and polymer segmental dynamics. In this study, a molecular design strategy simultaneously enhances the ionic conductivity and mechanical strength in fully sulfonated polystyrenes. The meta-substitution of −SO3H groups (PS3S), unlike the conventional para-substituted analog (PS4S), facilitated intrachain hydrogen bonding and resulted in a 3-fold increase in the ionization degree to ∼71%. Unexpectedly, PS3S exhibited a glass transition temperature (Tg) 100 °C lower than that of conventional PS4S. Incorporating an additional −OH group at the para-position adjacent to −SO3H yielded a bifunctional polymer capable of coplanar intramonomer hydrogen bonding, which raised the Tg by approximately 50 °C and further increased the ionization degree to about 86%. Upon doping with ionic liquids, the system achieved 0.2 mS cm–1 at 250 MPa (25 °C) and 9.2 mS cm–1 at 0.8 MPa (120 °C), shifting the conductivity-modulus relationship toward that of superionic inorganic conductors.