Combining nickel-rich layered metal oxide, particularly LiNi0.8Mn0.1Co0.1O2 (NMC811), with a silicon-based anode while simultaneously increasing the upper cutoff voltage (> 4.3 V vs. Li|Li+) represents a significant advancement in lithium-ion battery technology due to its exceptional energy density. However, the current electrolytes’ instability poses a notable challenge for the NMC811||SiOx-Gr application, leading to sudden cell failure. This challenge primarily arises from lithium metal plating on SiOx-Gr, which hinders lithium transport due to the formation of a thicker solid electrolyte interface (SEI), resulting in a catastrophic failure. In this study, we introduce a new multifunctional electrolyte additive designed to protect fluoroethylene carbonate (FEC) from degradation caused by hydrofluoric acid (HF) and PF6-, thereby facilitating the formation of a stable SEI on SiOx-Gr. The electrolyte formulation comprises 1M lithium hexafluorophosphate (LiPF6) dissolved in a blend of ethyl carbonate (EC) and ethyl methyl carbonate (EMC) (1 M LiPF6 in EC:EMC 3:7 by weight), with 5 wt.% FEC and 1.5% X. This tailored electrolyte demonstrates high stability against oxidation potential (around 5.6 V), aids in SEI formation, and protects FEC from acidic degradation, allowing its involvement in SEI formation. The research investigates how safeguarding FEC with scavenging co-additives enables the high-voltage application of NMC811||SiOx-Gr lithium-ion batteries (LiBs).