Sulfolane-ethylene carbonate hybrid LiNO3 electrolyte with LiDFOB as functional additive for safe and stable high-voltage lithium metal batteries

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 (LiNO 3 ) 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 NO 3 - , 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 hours of stable cycling with minimal voltage fluctuations, confirming long-term interfacial stability and suppressed dendrite growth. Paired with LiMn 2 O 4 (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 LiNO 3 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. • EC and LiDFOB tailor Li + solvation, effectively enhance the interfacial stability • LSEC-B4 achieves an average Li plating/stripping Coulombic efficiency of 95.4% • Li//LMO with LSEC-B4 retains 84% capacity (85 mAh g -1 ) and 99% CE after 500 cycles • Li 3 N, LiF and B-O-rich SEI provides balanced conductivity and mechanical stability • LSEC-B4 exhibits excellent flame-retardant and thermal stability for LMBs