Quasi-Solid Electrolytes with Flexible Branches and Rigid Skeletons for High-Temperature Li Metal Batteries

Quasi-solid electrolytes are poised to revolutionize the next generation of high-energy-density lithium metal batteries. However, they face considerable challenges in operating at high temperatures due to severe side reactions, particularly with high-voltage cathodes. Here, we fabricated a nonflammable quasi-solid electrolyte (QSE) using an in situ copolymerization method. This electrolyte features a copolymer host with both flexible branches and a rigid skeleton, along with fire-retardant fluorine- and phosphorus-based plasticizers. The flexible branches reduce crystallinity and facilitate lithium-ion migration, while the rigid skeleton provides excellent high-temperature stability. The optimized fluorine–phosphorus mixed plasticizer provides effective sequestration of highly reactive oxygen/hydroxide radicals, along with a wide electrochemical window (up to 4.7 V vs Li⁺/Li), thereby ensuring excellent interface stability and safety characteristics. Consequently, the resulting quasi-solid electrolyte demonstrates remarkable physicochemical and electrochemical properties, enabling highly reversible lithium plating-stripping processes and good compatibility with cathodes. Using the nonflammable QSE, a high capacity retention of 82.3% is achieved for LiFePO₄ after 1000 cycles at a high current density of 5 C and 80 °C. The good compatibility with high-voltage cathodes was demonstrated by the stable cycling of LiCoO₂ over 400 cycles at a high cutoff voltage of 4.5 V and an elevated temperature of 60 °C.