XPS characterization of the reactions of Li with tetrahydrofuran and propylene carbonate

The reaction of tetrahydrofuran (THF) and propylene carbonate with thin (6-10 nm) films of Li vapor deposited an Al(111) al 120-135 K was studied in UHV by C 1s and O 1s core-level photoemission. THF and PC condensate layers were formed by dosing the Li overlayer surfaces at 120-135 K, with the reaction observed by subsequent warming of the sample. Activation of the first layer of THF molecules by Li, probably as ring-opening, starts at a temperature as low as 120 K. Polymerization of THF (forming poly-THF) occurs upon melting near 180 K, but is accompanied by chain-terminating reactions that form Li 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. Only a few monolayers of the Li film react with the THF under these conditions. Al 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, and in the temperature interval of 270-320 K there may be a small amount (<5% of the carbon present) of Li carbonate formed. The alkyl group in the organo-Li compounds derived from PC is most probably propylene. There is no evidence of the formation of any gaseous products at temperatures below 320 K under the conditions of these experiments. As with THF, only a few monolayers of the Li film have reacted with the PC. Of particular relevance to battery technology, however, is the fact that in both cases the organo-Li layers that have formed at 270 K were formed in the presence of excess (unreacted) Li. (C) 1997 Elsevier Science B.V.