Tetra-Arm Poly(methyl acrylate) Gels with a Sulfolane-Based Concentrated Electrolyte and High Li-Ion Transference Numbers

For the practical use of polymer gel electrolytes in advanced battery systems, simultaneous improvement of mechanical strength, ionic conductivity, and Li-ion transport properties remains challenging. In this study, a mechanically reliable and ionically conductive gel electrolyte was developed by cross-linking tetra-arm poly(methyl acrylate) (tetra-PMA), a weakly coordinating polymer to Li+, in a sulfolane (SL)-based highly concentrated electrolyte (Li(SL)3TFSA) using a copper-free click reaction. Tetra-PMA was synthesized via atom transfer radical polymerization and converted to azide-terminated precursors, enabling efficient gelation with an electron-deficient dialkyne. The resulting tetra-PMA gel formed a uniform, defect-minimized network structure, allowing membrane formation at a low polymer content of 10 wt %, where conventional nonuniform PMA gels failed to form self-standing membranes. Mechanical testing revealed that the tetra-PMA gel exhibited a relatively high Young’s modulus and fracture energy, comparable to those of tetra-arm poly(ethylene glycol) (tetra-PEG) gels. Raman spectroscopy confirmed that the tetra-PMA gel preserved the Li+ solvation structure of the parent SL-based concentrated electrolyte, in contrast to PEG-based gels, which disrupted Li+ coordination owing to the strong coordinating ability of the ether chains. Electrochemical measurements demonstrated that the tetra-PMA gel retained a high Li-ion transference number (tLiabc = 0.59), comparable to that of the SL-based concentrated electrolyte, and exhibited sufficient oxidative stability up to 4.5 V vs Li/Li+. In Li||LiCoO2 cells, the gel enabled stable cycling and delivered discharge capacities exceeding 120 mA h g–1 at 2 C, attributable to suppressed concentration polarization resulting from the high tLiabc. These results demonstrate the effectiveness of a weakly coordinating, homogeneous polymer network in achieving both mechanical robustness and favorable Li-ion transport properties in gel electrolytes, offering a promising platform for next-generation high-rate lithium-ion polymer batteries.