Lithium-metal batteries (LMBs) offer higher energy density than lithium-ion batteries (LIBs) but suffer from dendrite growth, low coulombic efficiency, and safety concerns. This study introduces a deep eutectic electrolyte (DEE) composed of lithium nitrate (LiNO3) as lithium salt and lithium difluoro(oxalato)borate (LiDFOB) as functional additives, and sulfolane (SL) mixed with ethylene carbonate (EC) as hybrid solvent. The optimized electrolyte composition, LSEC-B4, achieves moderate viscosity (27.5 cP) and the highest conductivity (1.15 mS/cm). Thermal analyses confirm its superior thermal stability and resistance to crystallization, attributed to the synergistic roles of EC and LiDFOB. Spectroscopic studies reveal that LSEC-B4 tailors Li+ solvation by regulating interactions with NO3-, SL (S
O), and EC (C
O), forming a stable coordination environment. This enhances ion transport and stabilizes the solid electrolyte interphase (SEI). LSEC-B4 exhibits outstanding performance in Li//Cu cells, delivering 98.27 % average coulombic efficiency with low overpotential, ensuring facile and high reversibility of Li deposition and stripping. In Li//Li cells, it sustains over 400 h of stable cycling with minimal voltage fluctuations, confirming long-term interfacial stability and suppressed dendrite growth. Paired with LiMn2O4 (LMO) cathode, Li//LMO cells achieve 84 % capacity retention (85 mAh g−1) after 500 cycles at 300 mAg−1 and ∼99 % average coulombic efficiency. Flammability tests highlight remarkable safety, unlike commercial electrolytes, LSEC-B4 resists ignition, benefiting from the flame-retardant nature of LiNO3 and SL, while LiDFOB reinforces SEI stability and mitigates thermal runaway. Overall, LSEC-B4 combines conductivity, stability, safety, and cathode compatibility, providing a promising pathway toward practical, safe, and efficient LMBs.
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