Study of the reactions of Li with tetrahydrofuran and propylene carbonate by photoemission spectroscopy

The reaction of Li with two organic solvents of technical importance in Li batteries, tetrahydrofuran (THF) and propylene carbonate (PC), were studied in ultrahigh vacuum by photoemission spectroscopy. The organic condensate layers were formed by dosing thin (6-10 nm) films of Li at 120-135 K, with the reactions monitored by x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy upon subsequent warming of the sample. Activation of the first layer of THF by Li starts at a temperature as low as 120 It. Polymerization of tetrahydrofuran (THF) (forming poly-THF) occurs upon melting near 180 K, but is accompanied by chain-terminating reactions that form lithium alkoxide(s) and hydrocarbon gas(es), such as ethylene and/or propylene. Between 180 and 320 K, there is progressively greater conversion of poly-THF to alkoxide such that at 320 K, the surface film is almost entirely composed of alkoxide. At or near its bulk melting temperature of 220 K, essentially all of the PC remaining on the surface has reacted with Li to form an alkyl carbonate. With increasing temperature, part (25%-33%) of the alkyl carbonate decomposes to form an alkoxide. The alkyl group in the organo-li compounds derived from PC are most probably propylene. There is no evidence of the formation of any gaseous products containing carbon or oxygen at temperatures below 320 K under the conditions of these experiments. Of particular relevance to battery technology, however, is that in both cases the organo-li layers that have formed at 270-320 K were formed in the presence of excess unreacted Li, which is the usual circumstance in a real battery, and that no evidence was found of inorganic Li carbonate as a product of the reaction with PC. (C) 1998 American Vacuum Society. [S0734-2101 (98)00405-9].