On the capacity fading of LiCoO2 intercalation electrodes:: the effect of cycling, storage, temperature, and surface film forming additives

The electrochemical behavior and surface chemistry of LiCoO2 intercalation cathodes as a function of cycling and storage at 25, 45, and 60 degreesC was studied. The standard solutions for this work comprised ethylene carbonate (EC), ethyl-methyl carbonate (EMC), (1:2) and I M LiPF6. The effect of two surface film-forming additives, vinylene carbonate (VC) and an organo-borate complex (denoted as Merck's AD25) in solutions was also explored. We analyzed temperature-dependent processes of surface film formation on the cathodes, which increase their impedance upon cycling and storage, thus making their electrochemical kinetics sluggish. We also analyzed cobalt dissolution from the cathodes at 25, 45 and 60 degreesC. The apparent capacity fading of the LiCoO2 electrodes is attributed mostly to changes on their surface, rather than to bulk degradation. There are signs that the presence of HF in solutions may play a major negative role. Hence, as the electrode's surface/solution volume ratio is higher, the capacity fading of the LiCoO2 electrodes should be lower. The main tools for this study were cyclic voltammetry, chronopotentiometry, impedance spectroscopy, electrochemical quartz crystal microbalance (EQCM), IR-spectroscopy, XRD, XPS, and SEM. (C) 2002 Elsevier Science Ltd. All rights reserved.