Synthesis, Structure And Performance Of Nickel-based Cathode Materials For Green Chemical Power Sources

It is well known that the availability of high performance and low cost batteries is an essential factorfor the widespread diffusion of electric vehicles in the mobility system, with important environmentalbenefits. Ni–MH batteries is considered as one of the most promising devices for electric vehicle (EV) andhybrid electric vehicle (HEV) applications because of its high specific energy power and specific energydensity, fast charge and discharge capabilities, environment-friendly characteristics, long cyclic stabilityand good security. Although Ni–MH batteries are commercially available, further research is still requiredto improve their power performance for applications in electric vehicles and hydride vehicles. It is alsonecessary to reduce the cost of Ni–MH batteries in order to further expand their application fields, such asa replacement for nickel–cadmium (Ni–Cd) batteries in power tools. As a result of excellentelectrochemical properties, nickel-based electrode materials have been used in many importantapplications. For example, Ni(OH)2has been intensively studied and used in commercial alkalinerechargeable batteries. However, the common spherical Ni(OH)2can not well meet the demand in electricvehicle battery such as high temperature and current charge and discharge, etc.In recent years, with increasing type and amount of digital products, a new type of alkalineZn-NiOOH battery, which contains NiOOH as the positive electrode material, has attracted worldwideresearch interest because of its high practical specific energy, excellent specific power, high workingvoltage and low toxicity. As a result, there is substantial scientific and technological interest in the study ofNiOOH synthesis due to its extended application in the positive electrodes of alkaline batteries.Conventionally, NiOOH can be easily prepared by a chemical synthetic method. However, the chemicaloxidization method contains many reaction steps, consumes a great deal of reagent, and generates a largeamount of waste effluents. In addition, the products are inconvenient to filtrate and launder due to the useof the strong alkali solution. Hence, it is necessary to improve the synthetic method and performanceimprovement of NiOOH, which are keys for the production of this new type of battery.This thesis is in order to solve the above problems and the results are following:1) Non-spherical Ni(OH)2as positive electrode materials for alkaline Ni-MH batteries Positive electrode active materials of non-spherical nickel hydroxide powders with a high tap-densityfor alkaline Ni-MH batteries have been successfully synthesized using a polyacrylamide (PAM) assistedtwo-step drying method. In this work, we have also studied the effect of tap-density on the electrochemicalperformance of non-spherical Ni(OH)2electrodes. The tap-density of the powders reaches2.32g cm3,which is significantly higher than that of nickel hydroxide powders obtained by the conventionalco-precipitation method. X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electronmicroscopy (SEM), Thermogravimetric/differential thermal analysis (TG-DTA), Brunauer–Emmett–Teller(BET) testing, laser particle size analysis, tap-density testing, cyclic voltammetry (CV), electrochemicalimpedance spectroscopy (EIS), and a charge-discharge test were used to characterize the physical andelectrochemical properties of the synthesized material. The results show that the as-prepared nickelhydroxide materials have an irregular tabular shape, a high density of structural disorder, and a highspecific surface area. The charge-discharge tests indicate that nickel hydroxide powders synthesized by thenew method have better electrochemical performance than those obtained by the conventionalco-precipitation method. This performance improvement could be attributable to a more compact electrodemicrostructure, a lower amount of intercalated anions, better reaction reversibility, a higher protondiffusion coefficient, and lower electrochemical impedance. The results clearly show that betterelectrochemical activity can be achieved using nickel hydroxide that has a higher tap-density.Moreover, we compare the behavior of non-spherical and spherical β-Ni(OH)2as cathode materialsfor Ni–MH batteries in an attempt to explore the effect of microstructure and surface properties ofβ-Ni(OH)2on their electrochemical performances. Non-spherical β-Ni(OH)2powders with a high-densitywere synthesized using a simple polyacrylamide (PAM) assisted two-step drying method. The resultsshow that the non-spherical β-Ni(OH)2materials exhibit an irregular tabular shape and a dense solidstructure, which contains many overlapped sheet nano crystalline grains, and have a high density ofstructural disorder and a large specific surface area. Compared with the spherical β-Ni(OH)2, thenon-spherical β-Ni(OH)2materials have an enhanced discharge capacity, higher discharge potentialplateau and superior cycle stability. This performance improvement could be attributable to a higherproton diffusion coefficient (4.26×109cm2s1), better reaction reversibility, and lower electrochemicalimpedance of the synthesized material. Therefore, it is believed that the non-spherical Ni(OH)2 synthesized by the new method is a promising positive electrode active material for Ni–MH batteries.2)NiOOH as positive electrode materials for alkaline Zn-NiOOH batteriesWe propose a novel electrolysis method to prepare NiOOH by oxidizing spherical Ni(OH)2in adilute alkaline solution (pH=8-13). The effects of preparation conditions, including KCl concentration, PH,constant voltage, temperature and oxidation time, on the synthesis of NiOOH were systematicallyinvestigated. The results show that the electrolytic efficiency and reaction rate are superior and theelectrolyte can be conveniently re-utilized. The results also show that the NiOOH prepared by thiselectrolysis method is with a pure β phase, even if the electrolysis duration is fully prolonged. Thespherical NiOOH sample not only possesses excellent electrochemical activities and provides a dischargecapacity of256.5and199.1mAh g1, and a volume capacity of625.86and485.80mAh cm3at rates of0.2C and3C, respectively, but also has a high tap density of2.44g cm3.Following upon our previous work, nickel oxyhydroxide was synthesized by electrolysis oxidationof different Ni(OH)2precursors, which were prepared by three methods: polyacrylamide (PAM) assistedtwo-step drying (PTSD), conventional co-precipitation (CCP), and "controlled crystallization"(CC). Theeffects of different precursors on the microstructure and electrochemical properties of NiOOH arediscussed in detail. The results demonstrate that the physical and electrochemical properties of NiOOH arestrongly dependent on the properties of the Ni(OH)2precursor, such as its morphology, microstructure, tapdensity, and specific surface area. The results of the electrochemical studies also show that the sampleprepared by the PTSD method is superior to the others in electrochemical performance. The as-prepared,high-density, non-spherical NiOOH is a promising active material for the positive electrode in Zn-NiOOHbatteries.3)Regulation the discharge reservoir of negative electrodes for Ni–MH batteriesA novel strategy to regulate the discharge reservoir of negative electrodes in Ni–MH batteries isinvented by using Ni(OH)x(x=2.10) and γ-CoOOH. The electrochemical measurements of these batteriesdemonstrate that the use of Ni(OH)x(x=2.10) and γ-CoOOH can not only successfully regulate thedischarge reservoir of negative electrodes in Ni–MH batteries to an adequate quantity, but also effectivelyimprove the electrochemical performance of the batteries. Compared with normal batteries, the in-houseprepared batteries with a lower discharge reservoir exhibit an enhanced discharge capacity, improved high-rate discharge ability, higher discharge potential plateau and superior cycle stability. The effect ofNi(OH)x(x=2.10) and γ-CoOOH on the electrochemical performance of nickel electrode is alsoinvestigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The resultssuggest that the new method is simple and effective for cost reduction of Ni–MH batteries with improvedelectrochemical performance.