Study on the decomposition mechanism of alkyl carbonate on lithium metal by pyrolysis-gas chromatography-mass spectroscopy Ryo Mogi a,b , Minoru Inaba c,* , Yasutoshi Iriyama a , Takeshi Abe a , Zempachi Ogumi a a Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan b Shibukawa Laboratory, Kanto Denka Kogyo Co. Ltd., 1497 Shibukawa, Gunma 377-8513, Japan c Department of Molecular Science and Technology, Faculty of Engineering, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan Abstract The surface films formed on deposited lithium in electrolyte solutions based on ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) were analyzed by pyrolysis-gas chromatography-mass spectroscopy (Py-GC-MS). In 1 M LiClO 4 /EC, the main component of the surface film was easily hydrolyzed to give ethylene glycol after exposure to air, and hence was considered to have a chemical structure of ROCH 2 CH 2 OR 0 , of which –OR and –OR 0 are –OLi or –OCO 2 Li. Ethylene oxide, acetaldehyde, and 1,4-dioxane were detected in decomposition products, and they were considered to have been formed by pyrolysis of ROCH 2 CH 2 OR 0 in the pyrolyzer. The presence of ethanol in decomposition products confirmed that ring cleavage at the CH 2 –O bonds of EC occurs by one electron reduction. In addition, the presence of methanol implied the cleavage of the C–C bond of EC upon reduction. From the surface films formed in 1 M LiClO 4 /DEC and /DMC, ethanol and methanol, respectively, were detected, which suggested that corresponding lithium alkoxides and/or lithium alkyl carbonates were the main components. In 1 M LiClO 4 =EC þ DEC (1:1), EC dominantly decomposed to form the surface film. The surface film formed in 1 M LiPF 6 =EC þ DEC (1:1) contained a much smaller amount of organic compounds. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Alkyl carbonate; Decomposition mechanism; Lithium metal; Pyrolysis-gas chromatography-mass spectroscopy 1. Introduction Lithium metal has a high energy density, and is expected to be used as a negative electrode in rechargeable lithium batteries [1]. However, lithium metal negative electrode shows a low cycling efficiency, which is mainly caused by unfavorable morphology of deposited lithium, called dendrites [1–3]. It is widely recognized that a protective surface film, called the solid electrolyte interface (SEI) [4], is formed on lithium surface by reductive decomposition of electrolyte solution, and that this surface film greatly affects the morphology of deposited lithium. The compositions of the surface films formed on lithium in non-aqueous electrolyte solutions have been extensively investigated with a variety of analytical tools, e.g. Fourier transform-infrared spectroscopy (FT-IR) [5–12], X-ray photoelectron spectroscopy (XPS) [5,9,11,13–18], electro- chemical quartz crystal microbalance (EQCM) [15,19–21], pyrolysis-gas chromatography-mass spectroscopy (Py- GC-MS) [10,22], etc. The results in these studies have shown that the surface film on lithium consists of decom- position products of solvent molecules, lithium salts, con- taminants such as H 2 O, CO 2 and O 2 , and additives such as HF. In the present study, the Py-GC-MS technique was applied to the surface films formed on lithium in electrolyte solu- tions based on ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC). The mechanisms for reductive decomposition of these alkyl carbonates on lithium metal were discussed. 2. Experimental The electrolyte solutions used in the present study were 1 mol dm 3 (M) lithium perchlorate (LiClO 4 ) dissolved in Journal of Power Sources 119–121 (2003) 597–603 * Corresponding author. Tel.: þ81-774-65-6591; fax: þ81-774-65-6803. E-mail address: minaba@mail.doshisha.ac.jp (M. Inaba). 0378-7753/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-7753(03)00302-1