Development of Thermally Stable Ionic Liquid-Based Composite Polymer Electrolytes Enabled by In Situ Polymerization for Lithium-Ion Rechargeable Batteries

Commercial lithium-ion batteries using organic solvent-based liquid electrolytes (LEs) face safety issues, including risks of fire and explosion. As a safer alternative, solid-state electrolytes are being extensively explored to replace these organic solvent-based LEs. Among various solid electrolyte options, polymer electrolytes offer advantages such as flexibility and ease of processing. However, they present challenges like low ionic conductivity at room temperature and reduced stability at elevated temperatures compared with other solid electrolytes. In this study, ionic liquid-based composite polymer electrolytes (CPEs) with exceptional thermal stability are synthesized via in situ polymerization. The in situ polymerized CPEs exhibit high ionic conductivity, reaching up to 1.38 mS cm^-1 at 25 °C, and show improved interfacial contact with the electrode. These CPEs demonstrate robust thermal stability, withstanding thermal decomposition at 350 °C, and maintaining nearly the same initial cell capacity, even after being stored at elevated temperatures above 120 °C. The cell using CPEs also had a broad electrochemical stability window of 5 V, making it suitable for high-voltage electrode applications. Additionally, CPEs showed a high lithium-ion transference number (t _Li+ = 0.5), and the NCM 811/CPE/Li cell showed a substantial discharge capacity of 210 mA h g^-1 at 0.1 C at 25 °C. Furthermore, the battery cell retained a 66% capacity after 100 cycles at a 0.3 C-rate compared to the initial value. The results suggest that the developed CPE has promising thermal stability, cell performance, and cycle life for future applications in lithium-ion rechargeable batteries.