Synthesis, XRD characterization and electrochemical performance of overlithiated LiNiO2

The synthesis and XRD characterization of overlithiated LiNiO2 samples is briefly described. The effect of the extraction of the residual LiOH from the samples after their synthesis is studied. The extraction in water even at a low liquid/solid ratio leads to a considerable chemical delithiation accompanied by the loss of oxygen from the crystal lattice of LiNiO2. It is supposed that oxygen vacancies are formed in part of the NiO6 octahedra, The defective octahedra cannot release electrons whereby the number of Li+ which can be deintercalated during the electrochemical delithiation of the cathodes prepared with chemically delithiated LiNiO2 samples are reduced. Although the fast XRD characterization here employed shows negligible change in the criterial parameters, the cycling test reveals a considerable decrease in the charge capacity and consequently in the next discharge capacity. The change in the crystal structure of the water extracted samples is more clearly observed in the VA-grams showing a considerable shift of the first R-1/M peaks in anodic direction and a strong reduction in the areas of the R-2/R-3 peaks. Ethanol as a nonaqueous solvent was found to extract the residual LiOH without causing any chemical delithiation of the LiNiO2 samples, thus, preserving their good electrochemical characteristics. The effect of storage of the LiNiO2 powdered samples in the ambient atmosphere was investigated. The results revealed that the ethanol extracted samples are more resistant to chemical delithiation by the water vapours in the air than those containing residual LiOH. A beneficial effect on the cycling characteristics of LiNiO2 cathodes containing 10% of the newly developed UFC (ultra-fine carbon) conductive material is established. The cycling by potentiostatic charging to 4.20 V with initial Limited current of 60 mA/g was found to contribute substantially to the discharge capacity and cycling stability of the LiNiO2 cathodes, yielding more than 170 mA h/g after 50 cycles between 3.00 and 4.20 V. (C) 1999 Elsevier Science S.A. All rights reserved.