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

The reactivity of perdeuterated propylene carbonate (PC-d(6)) toward m etallic lithium has been examined in ultrahigh vacuum (UHV) using temp erature programmed desorption (TPD). The TPD spectra of clean Li layer s evaporated on polycrystalline (poly) Au or Ag foils and later expose d 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) ethy lene 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 L i2CO3. 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, respec tively, lithium butoxide (BuOLi) and lithium butylcarbonate (BuCO(3)Li ). In addition, a series of peaks having the same shape and temperatur e. 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 surfac e, 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 e nvironments 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 amoun t, the PC-d(6)/Li thermal decomposition pathway.