This study investigates the electrochemical performance, stability, and decomposition mechanisms of fluorine-based electrolytes in large-scale cylindrical Ni-rich lithium-ion batteries (LIBs) under high-voltage conditions (up to 4.8 V). We examine fluoroethylene carbonate (FEC) and di-fluoroethylene carbonate (DFEC) in electrolyte formulations and their effects on battery longevity, gas evolution, and solvation dynamics. While FEC is known for improving the solid electrolyte interphase (SEI), DFEC remains underexplored. Using molecular dynamics (MD) simulations, density functional theory (DFT) calculations, and electrochemical analysis, we identify key solvation properties, ion transport characteristics (tLi+, CIP%), and electronic structures influencing electrolyte stability. The 1.2 M LiPF6 in DMC/FEC/DFEC (4:0.5:0.5% v/v) electrolyte achieves the highest capacity retention (85.11% after 1,000 cycles), with DFEC reducing solvation shell binding energy and stabilizing electrolyte performance. Differential electrochemical mass spectrometry (DEMS) and nuclear magnetic resonance (NMR) spectroscopy reveal that FEC leads to higher CO2 production via ring-opening and de-fluorination to vinylene carbonate (VC), while DFEC reduces gas evolution. These insights provide a holistic framework for optimizing high-energy electrolyte formulations, supporting the development of safer, more efficient LIBs for electric vehicles and energy storage applications.