Phosphonate- and Phosphonic Acid-Functionalized Polycyclooctenes Enabling High Ionic Conductivity and Intrinsic Flame...

Polymer electrolytes are considered promising materials for next-generation solid-state lithium-ion batteries due to their enhanced safety, flexibility, and electrochemical stability compared to conventional liquid electrolytes. Nevertheless, major bottlenecks including low room-temperature ionic conductivity, poor electrode/electrolyte interfacial stability, and limited flame-retardant properties still hinder their practical applications. In the pursuit of safer and high-performance lithium-ion batteries (LIBs), polymer electrolytes with improved ionic conductivity, thermal stability, and flame retardancy are highly desirable. In this study, a novel phosphonate-functionalized polycyclooctene (PPCO) was synthesized and characterized as a solid polymer electrolyte for LIBs. The synthetic route began with a thiol-ene click reaction between cyclooctadiene and mercaptoethanol to afford a hydroxyl-functionalized monomer, which was subsequently reacted with a chlorophosphate reagent to introduce phosphonate groups. Polymerization using Grubbs' third-generation catalyst yielded a polymer bearing pendant phosphonate functionalities, PolyCOD_phosphonate. Phosphonic acid derivatives, PolyCOD_{phosphonic acid}, were further prepared via treatment with trimethylsilyl bromide. These polymers were blended with polyvinylidene difluoride (PVDF) and lithium bis-(trifluoromethanesulfonyl)-imide (LiTFSI). Thermal analysis confirmed successful functionalization and high thermal stability (>300 °C). Microcone calorimetry revealed that PolyCOD_{phosphonic acid} exhibited superior heat release rate (HRR) reduction compared to PolyCOD_phosphonate. Electrochemical impedance spectroscopy demonstrated enhanced lithium-ion conductivity in the presence of LiTFSI, attributed to the strong solvation ability of the phosphonate moieties. A maximum conductivity of 0.63 × 10^-3 S cm^-1 at ambient temperature was achieved. The incorporation of phosphonate functionalities not only improves ionic transport but also imparts flame-retardant characteristics, establishing these materials as promising candidates for next-generation solid-state LIB electrolytes. Future work will include Li⁺ transference number determination, extended electrochemical stability analysis, and coin cell testing to evaluate cycling performance and interfacial stability, alongside further optimization of polymer structure for enhanced ionic conductivity.