Dithiocarbonic anhydride (CS2) was evaluated as an additive to electrolytes in Li-ion rechargeable batteries. Graphite electrodes, polarized versus Li metal, can undergo reversible intercalation–deintercalation processes in diethyl carbonate (DEC)+1 M LiPF6 electrolytes containing CS2 as an additive. It was found that the use of CS2 as a passive film formation agent on the graphite anode surface is more powerful than the use of carbon dioxide (CO2) as an additive in DEC electrolytes. On the other hand, we found that CS2 is less dominant, as a surface film formation agent, than SO2. Cyclic voltammetry and chronopotentiometric methods were used in order to understand the reaction mechanism better. A methodology employing DEC is proposed to evaluate new electrolyte formulations for Li and Li-ion batteries quickly and conveniently.