Effects of coating with gold on the performance of nanosized LiNi0.5Mn1.5O4 for lithium batteries

The electrochemical behavior of an Au-coated LiNi0.5Mn1.5O4 nanospinel in lithium cells was studied at two different temperatures (25 and 50 degrees C) and at five charge/discharge rates (C/6, C/4, 2C, 4C, and 8C). Two different coating methods were tested and the resulting products characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. One method, which involved treatment with HAuCl4 in the presence of HCOH as reductant, resulted in poorer cell performance irrespective of the particular charge/discharge regime used, probably by effect of the attack on the material degrading its surface during the coating process. The other coating method involved evaporating Au on the spinel, the effect of coating on cell performance being dependent on both temperature and the charge/discharge rate. Thus, low rates (C/6, C/4) increased the capacity delivered by the cell at 50 degrees C by ca. 20% relative to the bare spinel. This beneficial effect can be ascribed to the coating layer altering the electrolyte decomposition and the spinel particles being protected from attack by the species formed in the electrolyte decomposition. At high rates (2C, 4C), however, electrolyte decomposition played a minor role and crossing of the gold layer by lithium ions raised an energy barrier to be overcome. Under these conditions, the capacity delivered by the cell was markedly degraded irrespective of the temperature used. (c) 2007 The Electrochemical Society.