Addressing the critical need for enhanced safety and performance in lithium-ion batteries (LIBs), this work presents a comprehensive evaluation of a novel flame-retardant electrolyte additive, 2,2,2-trifluoroethoxy(pentafluoro)cyclo-triphosphazene (CF₃PFPN), in combination with vinylene carbonate (VC) and 2-phenoxy-1,3,2-dioxaphospholane (PhEPi) for high-energy NMC811||Si-graphite (20 % Si) cells. This synergistic additive mixture not only demonstrates superior flame-retardant properties compared to a non-fluorinated analogue but also yields improvements in discharge capacity. Detailed investigation of the solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) formation reveals the beneficial contributions of each component, leading to reduced interfacial resistance and enhanced electrochemical performance. Furthermore, gas chromatography-mass spectrometry (GC-MS) analysis confirms the effective suppression of electrolyte degradation. These findings highlight the substantial potential of tailored electrolyte additive combinations, particularly incorporating fluorinated phosphazenes, to simultaneously advance the safety and energy density of LIBs utilizing silicon-based anodes.