Reactivity of propylene carbonate toward metallic lithium: Temperature programmed desorption study

The reactivity of perdeuterated propylene carbonate (PC-d₆) 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-d₆ at a temperature (T) of ca. 120 K yielded three distinct types of features: a broad mle = 44 peak in the range 230 < T < 510 K; a series of mle fragments consistent with (albeit not unique to) ethylene oxide in the range 550 < T < 650 K; and two minor mle = 44 peaks centered at 750 and 860 K attributed to the thermal decomposition of Li₂CO₃. A similar broad mle = 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 (BuCO₃Li). 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 BuLiCO₃ 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-d₆ was intentionally mixed with CO₂, O₂, 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-d₆/Li thermal decomposition pathway.