Characterization of surface films formed prior to bulk reduction of lithium in rigorously dried propylene carbonate solutions

Surface films formed prior to bulk reduction of lithium have been studied a t gold, platinum, and copper electrodes in rigorously dried propylene carbo nate solutions using electrochemical quartz crystal microbalance (EQCM) and secondary ion mass spectrometry experiments. The results indicate that the passive film formation takes place at a potential as positive as about 2.0 V vs. Li/Li+, and the passive film thus formed in this potential region is thicker than a monolayer. Quantitative analysis of the EQCM results indica tes that electrogenerated lithium reacts with solvent molecules to produce a passive film consisting of lithium carbonate and other compounds of large r molecular weights. The presence of lithium carbonate is verified by SIMS, whereas the lithium compounds of low molecular weights, including lithium hydroxide and oxide, are not detected. Further lithium reduction takes plac e underneath the passive film at potentials lower than 1.2 V with a voltamm etric current peak at about 0.6 V.