Long-cycling lithium metal batteries enabled by cooperative lithium trifluoromethanesulfonate/lithium bis(oxalate)borate dual-salt electrolyte with nonflammable trimethyl phosphate solvent

The practical application of lithium metal batteries (LMBs) is still severely limited by safety concerns such as lithium dendrite growth and the flammability of conventional electrolytes. To address these critical issues, this study proposes cooperative dual-salt electrolytes utilizing lithium trifluoromethanesulfonate (LiOTf) and lithium bis(oxalato)borate (LiBOB) in a non-flammable trimethyl phosphate (TMP) solvent for high-stability and safety LMBs. The optimized Li[OTf/BOB]-55 electrolyte significantly reconstructs the Li⁺ primary solvation shell. This unique structural modification promotes the preferential reductive decomposition of LiBOB at higher potentials and operates synergistically with LiOTf decomposition products, thereby forming a robust dual-layer inorganic–organic composite solid electrolyte interphase (SEI) on the lithium metal anode. The inner layer is enriched with inorganic species such as Li2CO3 and LixBOy that facilitate ionic conduction, while the outer elastic organic polymeric layer imparts mechanical toughness. This highly stable and elastic SEI effectively suppresses the continuous consumption of the TMP solvent and blocks lithium dendrite propagation at the microscopic level. The optimally proportioned electrolyte endows the batteries with outstanding electrochemical performance: Li//Li symmetric cells achieve an ultra-long stable cycle life of over 1700 hours and tolerate a critical current density of up to 1.25 mA cm-2, while Li//Cu cells deliver a high average coulombic efficiency of 91.83%. In application evaluations, Li//LiMn2O4 cells utilizing this electrolyte realize a remarkably long lifespan of 1600 cycles at a high rate of 500 mA g-1. Furthermore, the system exhibits excellent temperature adaptability and interfacial stability across a wide temperature range from -20 °C to 60 °C. The proposed non-flammable dual-salt electrolyte design, enabled by solvation structure engineering, offers a highly practical and promising strategy for the development of next-generation safe LMBs.