The solid-electrolyte interphase (SEI) formed through the reductive decomposition of solvent molecules plays a crucial role in the stability and durability of lithium-ion batteries. Here, we investigate the initial process of SEI formation through reactive force field-molecular dynamics (ReaxFF-MD) simulations and density functional theory (DFT) calculations. ReaxFF-MD is used as a simulation protocol to predict the evolution of SEI components, and products are obtained in good agreement with the experimental results. DFT calculations are then used to model the reaction center. We find that one-electron reduction induces the similar breaking of the C-O bond in solvent ethylene carbonate (EC) and additive fluoroethylene carbonate (FEC). When another electron is added, EC decomposition produces gas CO + alkylcarbonate or ethylene (C2H4) + carbonate (CO32-), whereas FEC decomposition generates lithium fluoride (LiF) and vinylene carbonate (VC) in addition to CO + alkylcarbonate. LiF and VC could also be regarded as important electrolyte additives to improve battery performance. The reduction on FEC moiety/molecule is more energetically favorable than that on the corresponding EC moiety/molecule. This knowledge on the decomposition products at the atomic scale well correlate with available experiments, and theory provides useful guidelines and structural motifs for interpretations of future SEI-related experiments.