In the context of energy transition, irradiation is a powerful tool to mimic quickly the modification of electrode materials upon charge/discharge cycles in lithium-ion batteries. In this study, the evolution of the surface of silicon nanoparticles upon irradiation in two electrolytes, containing or not fluoroethylene carbonate (FEC), was studied. In the presence of FEC, irradiation leads to the formation of a homogeneous layer of a few nanometers thick, covering the whole surface of the nanoparticles. The formation of an artificial solid electrolyte interphase (SEI) layer through radiolysis is thus achieved. Without FEC, only patches of degradation products are formed on the nanoparticle surfaces for the same irradiation dose. In the absence of FEC, Li$_x$PF$_y$O$_z$ salts are formed. In the presence of FEC, Li$_x$PO$_y$, LiF, and Si–F bonds are generated. In both cases, the interphase contains Li$_2$CO$_3$ and a polymer containing ethylene carbonate units. Slightly different polymers are formed at the surface of nanoparticles in the presence or absence of FEC, i.e., more cross-linked in the former case. The elastomeric properties of the polymer formed in the presence of FEC are thought to be responsible for the formation of the homogeneous layer on the Si surfaces, leading to the generation of an artificial solid SEI through the radiolysis process. This SEI, however, prevents the efficient transfer of Li$^+$ ions, and more work is required to optimize its intrinsic (electro)chemical properties.