Decoupling ion size from electrochemistry: cation-size-independent accommodation of Li+ to Cs+ in an amorphous sulfonamide coordination polymer

Abstract

Understanding how cation identity governs charge storage is critical for next-generation batteries beyond lithium. Here we show that the amorphous Ca–Zn–PTtSA coordination polymer functions as a universal host for reversible electrochemical storage of all alkali-metal cations from Li⁺ to Cs⁺, including the rare case of reversible Rb⁺ and Cs⁺ electrochemical cycling in a positive electrode material. Despite the large variation in ionic radius, all cations yield nearly identical redox potentials, full material utilization (∼95 mAh g⁻¹), and low hysteresis. Elemental and spectroscopic analyses confirm a cation storage mechanism without solvent co-intercalation. This behavior originates from the framework's amorphous flexibility and the delocalized electronic structure of the conjugated sulfonamide ligand, which together enable weak, reversible metal–ligand interactions and fast cation transport (D ≈ 10⁻⁹ cm² s⁻¹). Consequently, M₂–Zn–PTtSA delivers high-rate capability and long-term cycling stability across the entire alkali-metal series, providing a platform that decouples ion size from electrochemical performance and supports “cation-of-choice” battery chemistries.