Stoichiometric, morphological, and electrochemical impact of the phase stability of LixCoO2

The effect of stoichiometry, heat-treatment, and resulting bulk and surface properties on the electrochemical cycling stability of native LixCoO2 under a 35% depth-of-discharge protocol was investigated. The materials were fabricated from mixtures of Li2CO3 and Co3O4 with Li/Co ratios spanning from 0.95 to 1.20. The single-phase stoichiometric sample exhibited the highest electrochemical performance under the applied protocol. A combination of X-ray diffraction, Fourier transform infrared, transmission electron microscopy, and thermogravimetric (TGA) analyses revealed residual Co3O4 and Li2CO3 existed in the materials fabricated from all the nonstoichiometric mixtures with Li/Co< 1 and Li/Co> 1, respectively. The use of TGA was found to be by far the most effective and sensitive tool for the detection and quantification of Li2CO3. Using various surface characterization techniques, we showed at least part of the residual Li2CO3 phase forms a layer at the surface of the LiCoO2 particles fabricated from lithium excess mixtures. The Li1-xCoO2 samples, which cycle poorly at room temperature and exhibit bulk crystallographic properties that differ from stoichiometric LiCoO2, were found to cycle well after Li2CO3 removal. The surface phase Li2CO3 is the root of the poor room temperature cycling for the overstoichiometric Li1+xCoO2 samples. (C) 2004 The Electrochemical Society.