Higher energy density, greater reliability, and better safety are the main goals of lithium-ion batteries (LIBs) research and development. Electrolytes with improved stability and non-flammability for realizing high-energy density and high-safety LIBs have been actively pursued. The electrolyte is thus a critical component that determines the performance of LIBs.

This study proposes a high-entropy electrolyte that is co-intercalation free, stable at high voltage, and non-flammable. First, the effects of Li salts (namely LiPF6, LiTFSI, LiFSI, and LiDFOB) are examined. Then, FEC and TTE additives are introduced to modify the properties of the PMP–TFSI ionic liquid (IL)/DME composite electrolyte. This is the first study to explore the functions of these dual additives in an IL electrolyte. Operando XRD data confirm that the additives ensure reversible Li+ intercalation/deintercalation at the graphite anode without DME solvent and IL cation co-intercalation. The synergistic FEC and TTE interaction optimizes the Li+ coordination structure in the electrolyte and alters the SEI composition, leading to superior charge-discharge performance (including reversible capacity, rate capability, and cyclability) of graphite and silicon-based anodes. In addition, the proposed electrolyte is compatible with Ni-rich NMC-811, mitigating cathode surface reactivity and capacity decay upon cycling. The high-entropy 1 M LiDFOB PMP–TFSI/DME electrolyte with FEC and TTE dual additives, which is thermally stable and non-flammable, is used for a 4.5-V NMC-811//graphite full cell to validate its practical application.

An electrolyte composed of Li+, PMP+, TFSI–, DFOB–, DME, FEC, and TTE is proposed. This IL-based high-entropy electrolyte demonstrates low flammability, high thermal stability, minimal corrosiveness, and a wide potential window without requiring a high Li salt concentration. Additionally, the long-standing issue of PMP+ and DME co-intercalation into the graphite lattice is eliminated, as verified by the operando XRD study. The significant shifts in the 7Li NMR peaks indicate a pronounced deshielding effect on Li+, implying that the interaction between Li+ and solvent molecules is weakened. As a result, anions are more likely to participate in the coordination sheath within the high-entropy PT/DME-FT electrolyte. This unique Li+ coordination structure, confirmed by NMR and Raman analyses, leads to a facile Li+ desolvation process and the formation of a LiF-, Li3N-, BxOy-, and SO22–-rich robust SEI. As a result, the graphite and SiOx anodes show excellent rate capability and cycling stability. The proposed electrolyte is highly compatible with the high-Ni NCM-811 cathode, forming a stable CEI that reduces transition metal dissolution. A high-performance 4.5-V NMC-811//graphite full cell is validated. The results demonstrate that the proposed high-entropy electrolyte is promising for high-energy-density and high-safety LIB applications.