The solid electrolyte interphase (SEI) layer is essential for battery performance and safety due to its electron insulation and Li-ion conduction. However, issues such as ongoing electrolyte decomposition and Li dendrite growth often arise. The most common strategy for improving the SEI is using electrolyte additives. However, the growth mechanism of the SEI with additives remains unclear. In this study, we use operando electrochemical liquid cell scanning transmission electron microscopy (ec-LC-STEM) to monitor in real time the nanoscale processes at the electrode–electrolyte interface during battery operation. We investigate how the additive fluoroethylene carbonate (FEC) influences the formation and properties of the SEI, as well as the growth and dissolution of Li dendrites. Our study shows that FEC decomposes early, allowing the nucleation and growth of LiF nanoparticles (NPs) that create a dense, uniform, and thin SEI layer. Interestingly, our analysis reveals that these NPs have structural defects that could influence ionic and electronic conductivity. The real-time observations show that the FEC-based SEI facilitates the formation of dense and short Li metals, whereas the FEC-free SEI leads to the growth of long Li whiskers with thinner roots than tips. This structural difference influences their dissolution mechanism: in FEC-rich electrolytes, the strong contact between Li metal and the electrode ensures complete dissolution, while in FEC-free electrolytes, partial dissolution occurs, leaving behind inactive Li metal. These findings emphasize the crucial role of additives in shaping the growth mechanism and the local structure of the SEI, thereby regulating the growth and dissolution of Li metal.