In Situ Copolymerized Zwitterionic-Oligoether Gel Electrolytes: Balancing Li Transport versus Li-Mediated Cross-Links

Gel polymer electrolytes (GPEs) are promising candidates for the improvement of safety and electrochemical performance of lithium-ion batteries; however, achieving a simultaneous balance between high ionic conductivity and mechanical robustness remains a major challenge. Here, we report GPEs based on Li salt in dimethylformamide (DMF) formed via in situ copolymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) and poly-(ethylene glycol) methyl ether methacrylate (PEGMA), designed to balance ion transport and structural integrity through different functional units. Ethylene oxide (EO) segments facilitate Li⁺ coordination and transport, while zwitterionic MPC units introduce physical cross-links, which enhance mechanical strength and favor salt dissociation. By systematically varying the salt concentration (discussed as the EO/Li ratio) in poly-(MPC-co-PEGMA) GPEs, we observe a transition from a transport-optimized to a coordination-dominated regime. Increasing salt content strengthens Li⁺-polymer interactions due to competition for coordination sites, leading to reduced ionic diffusivity but an increased apparent Li⁺ transference number (up to ∼0.42 at EO/Li ≈ 3-2.5). Ionic conductivity exhibits a maximum (∼3.2 mS cm^-1 at EO/Li ≈ 9), reflecting an optimal balance between charge carrier density and mobility. Raman spectroscopy indicates MPC-induced reduced Li-anion coordination, which favorably contributes to enhanced charge carrier density. Investigating several compositional series, we identify a compositional regime combining optimized Li transport and Li-mediated cross-linking for mechanical strength. At high salt loadings, further increasing MPC content elevates the gel modulus via enhanced cross-link density while suppressing Li-anion aggregation, suggesting a dual role of MPC in mechanical reinforcement and ion dissociation. These findings highlight the importance of balancing ion coordination and zwitterionic cross-linking in designing mechanically robust, yet highly conductive gel polymer electrolytes for advanced lithium battery systems.