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 i n Li batteries, tetrahydrofuran (THF) and propylene carbonate (PC), we re studied in ultrahigh vacuum by photoemission spectroscopy. The orga nic 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 spe ctroscopy and ultraviolet photoemission spectroscopy upon subsequent w arming of the sample. Activation of the first layer of THF by Li start s at a temperature as low as 120 It. Polymerization of tetrahydrofuran (THF) (forming poly-THF) occurs upon melting near 180 K, but is accom panied by chain-terminating reactions that form lithium alkoxide(s) an d hydrocarbon gas(es), such as ethylene and/or propylene. Between 180 and 320 K, there is progressively greater conversion of poly-THF to al koxide such that at 320 K, the surface film is almost entirely compose d of alkoxide. At or near its bulk melting temperature of 220 K, essen tially all of the PC remaining on the surface has reacted with Li to f orm an alkyl carbonate. With increasing temperature, part (25%-33%) of the alkyl carbonate decomposes to form an alkoxide. The alkyl group i n the organo-li compounds derived from PC are most probably propylene. There is no evidence of the formation of any gaseous products contain ing carbon or oxygen at temperatures below 320 K under the conditions of these experiments. Of particular relevance to battery technology, h owever, 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 i s the usual circumstance in a real battery, and that no evidence was f ound of inorganic Li carbonate as a product of the reaction with PC. ( C) 1998 American Vacuum Society. [S0734-2101 (98)00405-9].