Study of the decomposition of propylene carbonate on lithium metal surface by pyrolysis-gas chromatography-mass spectroscopy

The compositions of surface films formed on lithium metal in different propylene carbonate (PC)-based solutions and the decomposition processes in the surface film formation were investigated by pyrolysis-gas chromatography-mass spectroscopy (Py-GC-MS). It was found that the main component in the surface film formed in 1 mol dm(-3) (M) LiClO4/PC has a chemical structure of ROCH(CH3)CH2OR', of which -OR and -OR' can be -OLi or -OCO2Li. A general scheme for reactions that took place on the lithium electrode in 1 M LiClO4/PC and in the pyrolyzer was elucidated from the results by Py-GC-MS analysis. In 1 M LiClO4/PC containing 5 wt % fluoroethylene carbonate (FEC) as an additive, FEC was dominantly reduced to form an active species, Li+-OCO2., Which further reacted with a PC molecule to form the same main component, ROCH(CH3)CH2OR'. This mechanism suggested that the same active species also might be formed from PC even in the absence of FEC. Another possible route was suggested, in which decomposition of PC is initiated by a nucleophilic attack of an alkoxide. The chemical composition of surface film was greatly affected by the kind of lithium salts. Decomposition products of lithium bis(perfluoroethylsulfonyl)imide (LiBETI) were detected from the surface film formed in 1 M LiBETI/PC. In contrast, the surface film formed in 1 M LiPF6/PC consisted mainly of inorganic compounds with a much smaller amount of organic compounds.