The thermal behavior of the fully lithiated nano-silicon electrode in contact with an 1 M LiPF6 in EC/DEC electrolyte is investigated by differential scanning calorimetry (DSC) in the absence and presence of fluoroethylene carbonate (FEC) and vinylene carbonate (VC) electrolyte additives. In the additive-free electrolyte, intense exothermic reactions commence at approximately 153 °C when in contact with Li x Si. At contrast, the onset temperature of the thermal runaway is shifted to 200 °C and 214 °C when the charged Li x Si is heated in the presence of FEC and VC in the electrolytes, respectively. The mechanism of this thermal stability improvement by FEC and VC is studied using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and by specific measures to design the SEI on the surface of the nano-Si particles. The additives form a robust “primary SEI” on the surface of the Li x Si particles during the first charge that is rich in polycarbonate species and stable at elevated temperatures. In addition, the thermal degradation of FEC and VC in the electrolyte solution yields a highly resistive “secondary SEI” on the electrode surface that covers the lithiated silicon electrode (together with the “primary SEI”) and protects it from thermal runaway up to 200 °C.