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The energy density of a Li-ion battery or an electrochemical capacitor can be significantly improved by utilizing a lithiated negative electrode.Pre-lithiation opens up a new avenue of using high capacity Li insertion carbon anodes which were otherwise not feasible due to the high irreversible capacity to build SEI layer in the formation cycles.During lithiation of the negative carbon electrode, the electrolyte reacts with the electrode surface and undergoes decomposition to form a solid electrolyte interphase (SEI) layer that passivates the surface of the carbon electrode from further reactions between Li and the electrolyte.The reduction reactions that the solvent undergoes also form insoluble and gaseous by-products.In this work, those gaseous by-products generated by reductive decomposition of a carbonate-based electrolyte, 1.2 M LiPF6 in EC/PC/DEC (3∶1∶4), were determined at different stages during the lithiation process of an amorphous carbon electrode.The stages in the generation of gaseous by-products were determined to come as a result of two, 1-electron reduction steps of the cyclic carbonate components of the electrolyte.Electrochemical impedance spectroscopy was also used to investigate the two distinct electrochemical reactions and the development of the two phases of the SEI structure.