Here, we introduce a fluorine-free and flame-retardant dual-salt electrolyte formulation based on a new electron-deficient aromatic and Hückel-type lithium 4,6-diethoxy-2-oxo-2H-1,3,5-triazin-5-ide (LiDET) salt in combination with lithium bis(oxalato)borate (LiBOB) salt, both dissolved in a triethyl phosphate (TEP) solvent with vinylene carbonate (VC) as an additive. The interactions between BOB and DET anions promote a loosely coordinated Li+ solvation sheath, which results in enhanced interfacial stability of electrodes. The Li||Li symmetric cells maintain stable overpotentials during prolonged cycling, while the full cells demonstrate a capacity retention of 91% in Li||NMC811 cell after 500 cycles and 94% in Li||LTO cell after 200 cycles. The dual-salt electrolyte facilitates the formation of stable cathode electrolyte interphase (CEI) and solid electrolyte interphase (SEI) layers as confirmed with X-ray photoelectron spectroscopy (XPS), where the presence of species such as Li3N, LiNxOy, B–N, and Li–B–O is evident. Overall, this study paves the way for development of fluorine-free and flame-retardant electrolytes for lithium metal batteries (LMBs), especially for high-voltage cathodes like NMC811.