The formation mechanism of the passivation film on the surface of carbon-based electrodes during the first electroreduction was studied by chronopotentiometry and impedance spectroscopy. Two types of electrolytes are compared: ethylene carbonate-based liquid and poly( ethylene oxide)-based solid state electrolytes. The differences in stability towards reduction and in mobility of the solvent molecules in these electrolytes suggest two different mechanisms of reduction: (i) decomposition of the solvent in the liquid electrolyte system, and () formation of an immobilized dense layer in the solid electrolyte system. The chemical diffusion coefficient was also determined by impedance spectroscopy at increased amounts of Lithium in the carbon electrode. The film formation and the diffusion are correlated.