Synergistic Bifunctional Design Toward Highly Ion-Conductive Polyamide Composites Ion-Solvating Membranes for Lithium Batteries

Achieving high room-temperature ionic conductivity without sacrificing mechanical and thermal stability remains a central challenge for polyamide-based ion-solvating membranes (ISMs). Herein, we develop a rigid−flexible coordination strategy by incorporating controlled amounts of flexible aromatic ether linkages into a rigid fluorinated aromatic polyamide backbone. The amide-rich aromatic framework provides mechanical and thermal robustness, whereas the ether-containing segments regulate chain packing, Lewis-basic coordination-site density, and LiTFSI dissociation. Systematic comparison of PA-TFDB, PA-6FODA, and PA-6FAPB demonstrates that Li+ transport does not increase monotonically with ether content but instead reaches a maximum at an intermediate rigid−flexible balance. The optimized PA-6FODA-40-ISM exhibits an ionic conductivity of 9.18 × 10−4 S cm−1, representing a 2.4-fold enhancement over PA-TFDB-40-ISM, together with a Li+ transference number of 0.72, an apparent migration activation energy of 0.0402 eV, and oxidative stability up to 4.95 V. Raman analysis reveals a free-TFSI– fraction of 64% in PA-6FODA-40-ISM, compared with 46% in PA-6FAPB-40-ISM and 37% in PA-TFDB-40-ISM. Combined X-ray diffraction, spectroscopic, and density functional theory analyses indicate that the balanced rigid−flexible architecture produces less organized chain packing and a heterogeneous Li+-binding-energy landscape, thereby promoting salt dissociation and facilitating Li+ exchange between polymer and solvent coordination environments. In contrast, insufficient ether coordination favors contact ion pairs, whereas excessive ether-site density is accompanied by stronger coordination and increased ion aggregation. PA-6FODA-40-ISM simultaneously retains a tensile strength of 43 MPa, an elongation at break of 112%, and dimensional stability at 160 °C. The Li∣PA-6FODA-40-ISM∣Li symmetric cell maintained stable cycling for 250 h at 0.1 mA cm−2 and 0.1 mAh cm−2, while LFP∥Li cells retain 83% of their initial capacity after 300 cycles at 0.5 C. These findings establish rigid−flexible coordination as an effective molecular design principle for balancing ion transport, structural robustness, and interfacial stability in aromatic polyamide ISMs.