The study of the anodic stability of alkyl carbonate solutions by in situ FTIR spectroscopy, EQCM, NMR and MS

The anodic stability of alkyl carbonate solutions that are important for lithium and Li-ion batteries was studied using Au, Pt and Al electrodes. The systems studied included dimethyl carbonate, DMC + EC (ethylene carbonate) and EC + DEC (diethylcarbonate) solutions containing Li salts, such as LiClO₄, LiAsF₆, LiPF₆, LiC(SO₂CF₃)₃ and LiN(SO₂CF₃)₂. In situ Fourier transform infrared reflectance (FTIR) spectroscopy (internal reflectance mode) and an electrochemical quartz crystal microbalance (EQCM) were used in conjunction with linear sweep voltammetry in order to analyze the onset potentials of the oxidation of the above solutions and their oxidation products. A few selected solutions were electrolyzed and the oxidation products were studied by ¹H, ¹³C, ³¹P and ¹⁹F nuclear magnetic resonance (NMR), mass spectroscopy (MS) and gas chromatographic mass spectroscopy (GCMS). The study found that the onset potential for the electro-oxidation of these solutions on noble metals or aluminum is usually > 3.5V (Li @Z Li⁺). Oxidation products containing species with carbonyl groups (e.g. carboxyl and aldehydes), CO₂ and CO, were detected by in situ FTIR spectroscopy. The bulk analytical techniques (NMR, MS) also indicate the formation of oligomers of alkyl carbonates. The EQCM studies support the assumption that the oxidation reactions produce bulk products, and there is no film precipitation on the electrodes during the oxidation of these solutions (even at potentials > 5V vs. Li @Z Li⁺). The oxidation processes of these solutions mostly involve the solvents and not the salts. Furthermore, it is assumed that in practical Li batteries, where the positive electrodes are composite electrodes containing transition metal oxides, the oxidation processes are inhibited due to passivation phenomena that occur on the cathodes. Possible oxidation routes for the alkyl carbonate solvents are suggested.