| dc.description.abstract | The work embodied in this dissertation may be divided into five parts. (A): oxalic acid as a chelating agent for the sol-gel synthesis of LiNi0.8Co0.2O2. The various synthesis parameters such as calcination temperature, duration of heat treatment, lithium stoichiometry and dopant ion (Sr2+) concentration were optimized in order to obtain the best-performing cathode material. The structural and morphological characterizations of the products were done by XRD and SEM, respectively. The lithium intercalation properties were studied by galvanostatic charge-discharge cycling. (B): This part is similar to Part (A) except that the chelating agent was maleic acid. The effects of solvent and the acid-to-total cation ratio (R) were investigated. (C): Having identified the optimal calcination conditions (800°C and 12 h), the effect of the carbon number of the dicarboxylic acids (defined as the number of –CH2– groups in the molecule) on the sol-gel synthesis of LiNi0.8Co0.2O2 was investigated.
(D): A solid-state procedure was adopted for the synthesis of Zn-doped LiNi0.8Co0.2O2 with the aim of unraveling the role of the size-invariant Zn ions towards the stabilization and, consequently, the cyclability of the cathode material. Cyclic voltammetry and electrochemical impedance measurements were made to understand the electrochemical features of the samples. DSC experiments were carried out to study the thermal stability of the doped materials. (E): The enhancement in the electrochemical and thermal characteristics of solid-state prepared LiM0.05Ti0.05Ni0.70Co0.20O2 (M = Mg, Al, Zn) were investigated by cyclic voltammetry, DSC and galvanostatic charge-discharge studies.
(A) Oxalic acid as a chelating agent for the sol-gel preparation of LiNi0.8Co0.2O2
The best synthesis condition was a calcination treatment at 800°C for 12 hours. A product synthesized under this condition gave a first discharge capacity of 163 mAh/g, which faded to 158 mAh/g in the tenth cycle, registering charge retention of 96.4%. In order to improve the cathodic performance, doping with Sr was attempted. The amount of the dopant was such that the Sr2+/Li+ ratio was between 10-8 and 10-4. At a Sr2+/Li+ ratio of 10-6, the first and the hundredth cycle capacities of the material were 173 and 138 mAh/g, respectively, with charge retention of 80.1%. Among the lithium-rich phases studied (Li stoichiometries: 1.00 to 1.15), the most desirable results were obtained at a lithium stoichiometry of 1.10, with a first discharge capacity of 182mAh/g. The fiftieth cycle capacity of this material was 153 mAh/g, corresponding to charge retention of 84.1%. The synthesized sample was compared to a commercial sample obtained from the Foote Mineral Corporation (FMC). Although the charge retention value after ten cycles for the FMC sample was an impressive 100%, its first-cycle discharge capacity (166 mAh/g) was inferior to that of our samples.
(B) Maleic acid as a chelating agent for the synthesis of LiNi0.8Co0.2O2
The various synthesis parameters such as calcination temperature, duration of heat treatment, solvent, acid-to-total cation ratio (R) and lithium stoichiometry were optimized in order to obtain a cathode material with desirable electrochemical properties. The ideal conditions were a heat treatment protocol of 800°C for 12 hours in flowing oxygen, with ethanol as the solvent, at an R value of 1 and a lithium stoichiometry of 1.00. A product synthesized under these conditions yielded a first-cycle capacity of 190 mAh/g at a discharge rate of 0.1 C between 3.0 and 4.2 V. The capacity of the material in the tenth cycle was 183 mAh/g.
(C) Dicarboxylic acids as chelating agents for the sol-gel synthesis of LiNi0.8Co0.2O2
Six dicarboxylic acids (oxalic acid to sebacic acid, representing carbon numbers 0 to 8) were used as chelating agents for the synthesis of LiNi0.8Co0.2O2. The best results were obtained with adipic acid, which has a carbon number of 4. The first and tenth cycle capacities for the products obtained with this acid were 178 and 166 mAh/g, respectively. The charge retention after ten cycles was 93%. The pH of the as-prepared precursor (0.05 to1.58) was found to increase linearly with the carbon number (0 to 8).
(D) Zn-doped lithium-nickel-cobalt oxides, Li1.05ZnyNi0.8-yCo0.2O2 (y = 0.0000 to 0.0100)
Zn-doped Li1.05ZnyNi0.8-yCo0.2O2 compositions were synthesized by a conventional solid-state method. The products were characterized by XRD, galvanostatic cycling, cyclic voltammetry, electrochemical impedance spectroscopy and thermal analysis. For the Li1.05Zn0.0025Ni0.7975Co0.2O2 sample cycled between 3.0 and 4.2 V, the discharge capacities in the first and hundredth cycles were 170 and 138 mAh/g, respectively, registering charge retention of 81.0%. The corresponding values for the undoped material were 158 and 97 mAh/g, with charge retention of 61.4%. The improved electrochemical properties of the doped system were attributed to the structural stability derived from incorporating the size-invariant Zn2+ ions. The Zn-doped system also showed improved capacity and cyclability when the cycling was performed in a wider voltage window (2.5 to 4.4 V) as well as at an elevated temperature (55°C).
(E) Electroanalytical and thermal stability studies of multi-doped lithium-nickel-cobalt oxides
A solid-state fusion method was employed for the synthesis of LiM0.05Ti0.05Ni0.70Co0.20O2 (M = Mg, Al, Zn). Al as a co-dopant yielded a first-cycle capacity of 153mAh/g. The charge retention rates after ten and one hundred cycles were 98.0 and 84.3%, respectively. Although the first-cycle capacity for the Mg-doped material was 145 mAh/g, the charge retentions in the tenth and hundredth cycles were 100 and 91.0%, respectively. Zn as a co-dopant gave a first-cycle capacity of 140 mAh/g. In this case, the capacity retention after ten cycles was 98.0% and after 100 cycles it was 82.1%. DSC data revealed improved thermal stability for the Mg co-doped system. No improvement in the thermal stability of the Zn-doped system was noticed. | en_US |