Study on the decomposition mechanism of alkyl carbonate on lithium metal by pyrolysis-gas chromatography-mass spectroscopy

摘要

The surface films formed on deposited lithium in electrolyte solutions based on ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) were analyzed by pyrolysis-gas chromatography-mass spectroscopy (Py-GC-MS). In 1 M LiClO_4/EC, the main component of the surface film was easily hydrolyzed to give ethylene glycol after exposure to air, and hence was considered to have a chemical structure of ROCH_2CH_2OR′, of which -OR and -OR′ are -OLi or -OCO_2Li. Ethylene oxide, acetaldehyde, and 1,4-dioxane were detected in decomposition products, and they were considered to have been formed by pyrolysis of ROCH_2CH_2OR′ in the pyrolyzer. The presence of ethanol in decomposition products confirmed that ring cleavage at the CH_2-O bonds of EC occurs by one electron reduction. In addition, the presence of methanol implied the cleavage of the C-C bond of EC upon reduction. From the surface films formed in 1 M LiClO_4/DEC and /DMC, ethanol and methanol, respectively, were detected, which suggested that corresponding lithium alkoxides and/or lithium alkyl carbonates were the main components. In 1 M LiClO_4/EC + DEC (1:1), EC dominantly decomposed to form the surface film. The surface film formed in 1 M LiPF_6/EC + DEC (1:1) contained a much smaller amount of organic compounds.