Polymer-Ionic Liquid-TiO2 Composite Electrolyte for Safe and High-Voltage Lithium Metal Batteries

Abstract Lithium metal batteries (LMBs) are promising next-generation energy storage systems. However, their practical application remains limited by safety concerns and poor compatibility with high-voltage cathodes, arising from interfacial instability, dendrite growth, and uneven ion transport at the electrode–electrolyte interface. Solid polymer electrolytes (SPEs) improve safety but still suffer from low Li+ transference numbers and insufficient interfacial stability. Herein, a synergistic polymer, ionic liquid, and inorganic composite electrolyte (FALET) is developed to regulate ion transport and interfacial chemistry. The polymer/ionic liquid matrix facilitates Li+ transport, while EMIMTFSI also participates in interfacial electrochemical processes during Li plating/stripping. TiO2 nanofibers further regulate TFSI– anions and promote lithium salt dissociation through Lewis acid interactions. The resulting cooperative regulation of ion transport and interfacial chemistry homogenizes Li+ flux, promotes the formation of a dense inorganic rich SEI, and suppresses dendrite growth. The FALET electrolyte exhibits an ionic conductivity of 6.52 × 10–4 S cm–1, a Li+ transference number of 0.31, and a stability window of 5.01 V. It enables stable Li||Li cycling over 2700 h and a Coulombic efficiency of 96% in Li||Cu cells. Full cells further demonstrate stable cycling, high-voltage compatibility, and enhanced safety with improved flame retardancy and thermal stability. This work provides an effective strategy for developing safe and high-voltage-compatible solid polymer electrolytes.