Preparation And Electrochemical Properties Of Cobalt Compounds And Composites

The need for high energy density batteries becomes increasingly important for the development of new and clean energy technologies, such as electric vehicles and electrical storage from wind and solar power. In principle, the rechargeable battery system based on electrode-active materials with multi-electron reaction can achieve higher energy density as compared to a battery system built on conventional electrode materials with a single-electron redox. Therefore, it is critical to explore electrode-active materials with multi-electron reaction and construct novel battery-systems with dramatically increased energy density for the development of battery technology. As a two-electron reaction process between metallic Co and Co2+ ions is involved in the electrochemcial reaction, multi-electron transfer can been realized in Co-based compounds or composites. In this thesis, the electrochemical properties of Co-based compounds and composites as negative-active materials for alkaline rechargeable battery were investigated in detail. Furthermore, a new rechargeable battery system was constructed based on the explorement of the new Co-based negative materials. The main contents and results presented in this thesis are shown as following:In this work, Co-Si3N4 composites were synthesized by direct ball-milling metallic Co and Si3N4 powders. The microstructure, morphology and chemical state of the ball-milled Co-Si3N4 composites were characterized by X-ray diffraction(XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The electrochemical performance of Co-Si3N4 composites used as negative materials in alkaline solution was investigated by galvanostatic charge-discharge process and cyclic voltammetry. It is demonstrated that Co exists as metallic state in the bulk phase of the composites except a trace of CoO on the surface. The metallic Co nanoparticles are highly dispersed on the amorphous inactive Si3N4 matrix. The maximum discharge capacity (403 mAh/g) of the Co-Si3N4 composites is obtained at the molar ratio of Co/Si=2/1 based on the reversible faradic reaction between Co and Co(Ⅱ). The Co-Si3N4 composite with the molar ratio of Co/Si=2/1 presents good cycle stability. Moreover, it is shown that the addition of Co-Si3N4 composites to the hydrogen storage PrMg12-Ni composite can notably enhance the initial discharge capacity of the hydrogen storage composite based on both the hydrogen electrochemical oxidation and Co electrochemical redox reaction.Next, the electrochemical performance of CoO and Co3O4 used as negative-active materials for rechargeable alkaline battery was also investigated. CoO and Co3O4 were obtained by decomposition of the a-Co(OH)2 precursor at 450℃in Ar and air atmosphere, respectively. XRD and SEM were used to characterize their microstructure and morphology. The discharge capacity and reversible faradic reaction involved in the material were elucidated through galvanostatic charge-discharge process and cycle voltammetry (CV) technique. It is shown that CoO has the highest capacity of up to 507 mAh/g at the discharge current density of 60 mA/g. The discharge potential plateau is around-0.8 V, consistent with Co redox potential in alkaline solution. The electrochemical reaction process of Co3O4 is similar to that of CoO. The maximum discharge capacity of Co3O4 is 391 mAh/g, indicating a low utilization of active materials due to the partially irreversible conversion between Co and Co3O4. It is concluded that CoO and Co3O4 electrodes exhibit a good capacity retention at the large discharge current density (400 mA/g), and enhanced capacity at the low discharge current density (60 mA/g).Finally, a-Co(OH)2 andβ-Co(OH)2 were synthesized controllably via homogeneous precipitation, their microstructure as well as morphology were examined in detail by XRD and SEM. The high-rate discharge ability of cobalt hydroxides using as negative-active material in alkaline solution was investigated. It is shown thatβ-Co(OH)2 exhibits improved discharge capacity and excellent electrochemical durability as compared toα-Co(OH)2. The maximum discharge capacities of 455 and 338mAh/g are achieved for theβ-Co(OH)2 electrode at 1 and 10 C rate, respectively. In particular, on average,1.58 electrons are involved in the practical electrochemical process of theβ-Co(OH)2 electrode. Meanwhile, a new rechargeable battery system, consisting of a-Ni(OH)2 as the positive-active material andβ-Co(OH)2 as the negative-active material, is proposed on the basis of multi-electron reactions. The output energy density of Ni/Co prototype cell reaches 165 Wh/kg (based on the weights of both a-Ni(OH)2 and (3-Co(OH)2 active materials) and stabilizes at 158.8 Wh/kg after 50 cycles, revealing an energy retention of 96.2%. The strategy adapted in the present study could be helpful to explore and develop new power sources with a high energy density.