ELECTROCHEMICAL OXIDATION PROCESSES ON NI ELECTRODES IN PROPYLENE CARBONATE CONTAINING VARIOUS ELECTROLYTE SALTS

Electrochemical oxidation processes on Ni electrodes in propylene carb onate electrolytes were investigated by using cyclic voltammetry, x-ra y photoelectron spectroscopy, and in situ Fourier transform infrared s pectroscopy. The results of these analyses suggest that Ni electrodes, electrolyte salts, and solvent are oxidized at a greater anodic poten tial than 4.2 V vs. Li/Li+. When propylene carbonate (PC) electrolyte containing LiAsF6, LiBF4, or LiPF6 was used, a large amount of Ni fluo rides and oxides formed on the Ni electrodes and became inactive in re sponse to Ni oxidation. The Fourier transform infrared measurement sho wed that the oxidation of PC in these electrolytes is enhanced by the formation of the above-mentioned Ni compounds in the first scan. On th e ether hand, inactivation was not observed for PC electrolytes contai ning LiCF3SO3. Correspondingly, the oxidation of PC in this electrolyt e was more suppressed than that in the other three electrolytes. When PC containing LiClO4 was used as an electrolyte, the formation of Ni o xides was observed as well as the active oxidation of PC. This result indicates that Ni oxides are actively involved in the electrochemical oxidation of PC. Thus, electrochemical oxidation processes on Ni elect rodes in various PC electrolytes can be explained by the oxidation of Ni. electrodes in association with anion decomposition, which determin es a type of products formed on Ni electrodes.