REACTIVITY OF PROPYLENE CARBONATE TOWARD METALLIC LITHIUM - TEMPERATURE-PROGRAMMED DESORPTION STUDY

The reactivity of perdeuterated propylene carbonate (PC-d(6)) toward metallic lithium has been examined in ultrahigh vacuum (UHV) using temperature programmed desorption (TPD). The TPD spectra of clean Li layers evaporated on polycrystalline (poly) Au or Ag foils and later exposed to PC-d(6) at a temperature (T) of ca. 120 K yielded three distinct types of features: a broad m/e = 44 peak in the range 230 < T < 510 K; a series of m/e fragments consistent with (albeit not unique to) ethylene oxide in the range 550 < T < 650 K; and two minor m/e = 44 peaks centered at 750 and 860 K attributed to the thermal decomposition of Li2CO3. A similar broad m/e = 44 TPD peak was observed in the range 150 < T < 450 K in experiments in which Li/Au(poly) surfaces were exposed in sequence to 1-butanol (BuOH) and CO2 at 120 K, to generate, respectively, lithium butoxide (BuOLi) and lithium butylcarbonate (BuCO(3)Li). In addition, a series of peaks having the same shape and temperature. including mle = 57, 43, 44, 42, and 41, were observed in the TPD of BuOLi prepared either by adsorption of BuOH on Li/Au(poly) or by the thermal decomposition of layers of BuLiCO(3) formed on the same surface, in the range 510 < T < 610 K. These findings suggest that the most likely product of the reaction between PC and metallic Li is a lithium alkylcarbonate. The same type of species has been proposed to be the major constituent of the passive film on Li in PC in electrochemical environments based on in situ Fourier transform infrared spectroscopic studies. Experiments in which PC-d(6) was intentionally mixed with CO2, O-2, or H2O showed that in each case the impurity reacts with Li to form the expected products without affecting, other than the net amount, the PC-d(6)/Li thermal decomposition pathway.