Enabling Stable Cycling of 4.6 V High-Voltage LiCoO₂ with an In Situ-Modified PEGDA-Based Quasi-Solid Electrolyte

Gel polymer electrolytes (GPEs) are expected to solve both safety concerns in liquid electrolytes and high interface impedance in solid electrolytes. However, the electrochemical window of GPEs is narrow with poor tolerance to high voltage. In this work, a poly(ethylene glycol) diacrylate (PEGDA)-based quasi-solid-state GPE is modified by incorporating a functional imidazolium-based ionic liquid with cyano and vinyl groups into a PEGDA matrix through an in situ thermal polymerization. The electrochemical stability window of PEGDA-based GPEs is effectively extended to approximately 5 V by introducing the ionic liquid with a strong electron-withdrawing cyano group, which possesses good resistance to anodic oxidation. The GPE takes advantage of the PEGDA matrix and the functional ionic liquid, exhibiting a high ionic conductivity (4.97 × 10–3 S cm–1 at ambient temperature), a lithium-ion migration number of up to 0.69, and excellent resistance to anodic oxidation at high voltage. It also features a distinguishing function of flame retardancy, enhancing the safety performance of pristine PEGDA-based GPEs. The 4.6 V high-voltage LiCoO2|Li cells with the as-prepared GPE display excellent cycling properties and superior rate capability at room temperature. Thus, the modified PEGDA-based in situ gel polymer electrolyte is a promising electrolyte candidate for high-voltage polymer Li-ion batteries.