| dc.description.abstract | This dissertation covers the synthesis and lithium-intercalating properties of LiCoO2 prepared by a combustion process with triethanolamine (TEA) as a complexant and sucrose as a fuel-cum-complexing agent. The synthesis parameters – TEA: sucrose mole ratio, and temperature and duration of calcination – as well as lithium stoichiometry were optimized in order to obtain products with the best electrochemical activity. Structural properties of the products were investigated by x-ray diffraction, surface morphology by scanning electron microscopy and transmission electron microscopy, and surface area by the BET method. Lithium intercalation properties were studied by galvanostatic charge-discharge studies at different rates and voltage windows. The various redox regions and phase changes occurring during the charge-discharge processes were studied by cyclic voltammetry.
The precursors for the synthesis of LiCoO2 were metal nitrates dissolved in an aqueous solution of TEA and sucrose in various mole ratios: 1:1, 1:2, 1:4, 1:8 and 1:16. Although phase-pure products could be obtained by a 10-h calcination at 600°C, the crystallinity of the product improved with the duration and temperature of calcination. The optimal synthesis conditions were found to be a 10-h calcination at 800°C. The electrochemical properties of the products were correlated with their surface area and R-parameter. Sucrose was first hydrolyzed to glucose and fructose, and subsequently oxidized to gluconic or polyhydroxy acids, which coordinated with the cations and cross-linked with the TEA. TEA complexes with cations and immobilizes them in a carbonaceous matrix formed from sucrose. Thus, upon decomposition of the precursor, the cations find themselves dispersed uniformly in a carbonaceous matrix. Sucrose also acts as a fuel, providing the energy for product formation and sintering. However, a large amount of sucrose in the precursor can also reduce the partial pressure of oxygen in the reaction zone, adversely affecting the product characteristics. At the same time, at low concentrations of TEA, less chelation of the cations means less distribution. The product formation is discussed in terms of the TEA:sucrose ratios.
At a 0.1 C rate between 3.0 and 4.3 V, the 10-h 800°C product gave a first-cycle discharge capacity of 156 mAh/g, which faded to 153 mAh/g in fifth cycle, with charge retention of 98%. A subsequent cycling between 3.0 and 4.4 V at a 0.1 C rate gave a discharge capacity of 167 mAh/g in the sixth cycle, fading to 165 mAh/g in tenth cycle, registering a charge retention of 98%. The superior performance of the material compared to the commercial LiCoO2 sample was also demonstrated. For example, at a 0.2 C rate between 3.0 and 4.2 V, not only was the initial capacity of our material higher (137 mAh/g) than that of the commercial sample (132 mAh/g), its cyclability was also higher: 100 cycles versus 68 for the commercial product for an 80% charge-retention cut-off value.
Lithium-rich LixCoO2 (where x = 1.05~1.15) phases were also studied. The excess lithium stoichiometric phases were synthesized to compensate for any lithium that might be lost during heat treatment. The first-cycle discharge capacities of these products were 157, 154 and 155 mAh/g, respectively, for x = 1.05, 1.10 and 1.15 at a charge-discharge rate of 0.1 C between 3.0 and 4.3 V. When the voltage window was 3.0~4.4 V in the sixth cycle, the corresponding capacities were 166, 165 and 166 mAh/g, fading to 162, 163 and 164 mAh/g in the tenth cycle. | en_US |