The reactivity of metallic lithium toward solvents of relevance to energy storage: A surface science approach

The reactivity of symmetric and asymmetric alkyl linear carbonates of the type ROCO₂R′, where R and R′ represent methyl or ethyl groups, propylene carbonate (PC), γ-butyrolactone (γ-BL), dioxolane (DIOX) and tetrahydrofuran (THF) toward metallic Li was studied by infrared reflection absorption (IRAS), X-ray photoelectron (XPS) and Auger electron (AES) spectroscopies in ultrahigh vacuum (UHV). An identification of the products of these reactions may provide much needed insight into the physicochemical properties of passive films formed on Li electrodes, which are believed to be primarily responsible for controlling the performance of rechargeable Li batteries. Comparison of the IRAS and C(1s) and O(1s) XPS spectra of clean Li surfaces exposed to dimethyl (DMC), diethyl (DEC) and ethylmethyl carbonates (EMC) with those obtained in very similar experiments involving judiciously selected alcohols provided unambiguous evidence that the products of such reactions are primarily Li alkoxides (ROLi and RγOLi). Spectral evidence was also found for the presence of Li ethyl carbonate for experiments involving DEC and EMC, as well as Li oxide as judged by the occurrence of an O(1s) XPS peak at 530 eV.; In the case of PC the IRAS and XPS spectra of PC/Li was found to be identical to that obtained by exposing Li to 1,2-propanediol vapors under otherwise identical conditions, and may thus be attributed to the corresponding alkoxide derivative. Unlike all other alkyl carbonates examined, the presence of a small amount of Li carbonate as a Li surface impurity led to the formation of detectable amounts of the Li alkyl carbonate derivative following exposure to PC. Similar experiments involving γ-BL yielded IRAS and XPS features consistent with Li butyrate and a derivative of a cyclic β-keto ester anion of Li as the main reaction products. The conclusions emerging from this study support by and large the mechanism proposed by Aurbach et al. for reactions between Li and condensed phase linear and cyclic alkyl carbonates, and γ-BL. Also discussed are implications of these results to the interpretation of data obtained in situ for Li electrodes in electrochemical environments.