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

Commercial lithium-ion batteries (LIBs) use carbonate-based liquid electrolytes (LEs), which have safety issues, including risks of fire and explosion. To address safety issues, solid-state electrolytes are extensively researched to replace LEs. Among various solid-state electrolytes, polymer electrolytes receive much attention due to various advantages such as flexibility and ease of processing. However, solid polymer electrolytes suffer from low ionic conductivity at room temperature and reduced stability at elevated temperatures compared to other solid-state electrolytes. In this work, ionic liquid-based composite polymer electrolytes (CPEs) with outstanding thermal stability and ionic conductivity are developed using in-situ polymerization, which allows excellent interfacial contact with the electrode. The fabricated CPEs achieve a high ionic conductivity of 1.38 mS cm^–1 and have a wide electrochemical stability of 5 V, which makes them suitable for high-voltage cell operation. In addition, CPEs demonstrate exceptional thermal stability that withstands thermal decomposition up to 350 °C and retains nearly the same initial discharge capacity even after being stored at elevated temperatures above 120 °C. NCM 811/CPE/Li cell shows substantial discharge capacity of 210 mAh g^–1 and capacity retention of 66% after 100 cycles at 4.5 V in high-voltage condition. As a result, the developed CPEs exhibit promising thermal stability and cell performance for future applications in LIBs.