The advancement of rechargeable lithium-metal batteries incorporating Ni-rich layered LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathodes provides a promising avenue for enhancing energy density in practical applications. Nevertheless, a significant obstacle remains in the form of inadequate electrolytes capable of maintaining robust stability at the interface between lithium-metal anodes and highly reactive cathodes under high-voltage conditions. This study explores an innovative high-voltage sulfolane-based electrolyte, engineered to form a highly fluorinated electrode/electrolyte interphase (EEI) through the synergistic action of fluorinated anions (TFSI⁻) and fluorinated co-solvents (FEC). This formulation facilitates stable lithium-metal anode cycling without dendrite formation, demonstrating exceptional performance with a coulombic efficiency (CE) of approximately 98.82% and superior capacity retention (exceeding 91% after 400 cycles) in NCM811||Li batteries. Analytical characterization and theoretical calculations reveal that the developed electrolyte promotes a high fluorine content in the lithium-ion solvation sheath. This results in the formation of stable interphase films, rich in organic fluorides and LiF, on both the NCM811 cathode and lithium-metal anode surfaces. In comparison to fluorinated solvents and high-concentration lithium salts, the synergistically derived fluorinated EEI exhibits superior performance in suppressing interfacial side reactions, reducing interfacial impedance, and minimizing charge transfer resistance. Consequently, this leads to a substantial enhancement in overall electrochemical performance.