| dc.description.abstract | The first part of this dissertation deals with the sol-gel synthesis of LixNi0.8Co0.2O2 with malonic acid as a chelating agent. The effects of the temperature and duration of calcination, pH of the precursor solution, nature of the solvent used, and excess lithium stoichiometry on the characteristics of the products were investigated. The second part covers the enhanced cyclability of oxide-coated commercial LiCoO2 and LiNi0.8Co0.2O2 materials. The coatings were electro-inactive oxides such as MgO, Al2O3, MgAl2O4 applied by a sol-gel process.
(A) The malonic acid-assisted synthesis of LixNi0.8Co0.2O2
Polycrystalline LiNi0.8Co0.2O2 was synthesized by a solution combustion method, with malonic acid as the chelating agent. A 12-h calcination at 800°C was found to be optimal for the production of phase-pure LiNi0.8Co0.2O2 powders with high specific capacities. The electrochemical performance of the products was correlated with the crystallographic parameters of the products obtained under different heat-treatment protocols. The first- and tenth-cycle capacities of the product obtained by a 12-h calcination at 800°C were 173 and 169 mAh/g, respectively. The initial discharge capacity was about 4–7 mAh/g higher than the capacities of commercial FMC LiNi0.8Co0.2O2 and EIC LiNi0.8Co0.2O2 samples. However, the cycle lives of the three materials were similar. The improved performance of the products obtained from ethanolic solutions, compared to those from aqueous solutions, is attributed to an easier combustion process aided by coordinated ethanol molecules in the precursors. Moreover, at a pH of 7 in aqueous solutions, the coordination of the cations by the dissociated malonic acid is complete and free from competition from other nucleophiles such as hydroxyl ions. Therefore, it was ideal for the synthesis of LiNi0.8Co0.2O2 with good characteristics. An excess lithium stoichiometry of 1.05 was detrimental to the capacity of the cathode material.
(B) MgAl2O4-coated cathode materials
A semi-alkoxy sol-gel method was employed for coating MgAl2O4 spinel oxide on commercial LiCoO2. XRD studies suggested the formation of a substitutional compound of the type LixMyCo1–yO2 (M = Al/Mg) on the surface of the cathode particles. Electron microscopic images of the coated particles indicated that the spinel existed as a loosely-held kernel. ESCA depth profiles showed that Al3+ and Mg2+ diffused into the bulk of the cathode material during the calcination process. Cycling studies showed that the cathode with a 1.0 wt.% coating had the maximum improvement in cyclability, a fact supported by its lowest R-factor. This study showed that at a 1.0 wt.% coating level, the cyclability of LiCoO2 improved five-fold: from 26 for the bare LiCoO2 to 131 for the spinel-coated LiCoO2. However, spinel coatings on commercial LiNi0.8Co0.2O2 did not register such dramatic improvements. The sol-gel approach was adopted for coating with MgO and Al2O3 too, which at a 1.0 wt.% coating level gave materials that could sustain only 34 and 28 cycles, respectively. Thus, coating with a mixed oxide was found to bestow desirable improvements in the cycling behavior of LiCoO2. | en_US |