Electrodeposition of metallic lithium has been investigated on a tungsten electrode in 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfone)imide (BMPTFSI) room-temperature molten salt. It was suggested that a passivation film forms on the electrode surface as a result of the reductive decomposition of the organic cation, BMP+. The film obtained in the absence of LiTFSI seems to have no conductivity and reduced further at more negative potential. On the other hand, the film obtained in BMPTFSI containing 1 M LiTFSI seems to have Li+-conductivity and enable the deposition and dissolution of metallic lithium electrochemically. The addition of ethylene carbonate reduced the amount of electric charge required for the formation of the passivation film.
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