Synthesis, Characterization and Electrochemical Studies of Transition Metal Fluorides Nanostructures by Microemulsion Method

Abstract

The growing energy demand is one of the critical issues to be addressed through viable research. The existing energy resources, majority based on fossil fuels, are either depleting very quickly or causing serious environmental problems. Therefore, it is highly desirable to explore alternative energy resources which are renewable on one hand, and on the other hand environmentally responsible. Solar and wind power are the ultimate choices as alternative energy resources, if high energy density storage system have been materialized successfully. One of the commercial energy storage system largely used today i.e., the lithium ion batteries (LIBs) is suffered by low energy density owing to its inherited storage mechanism, the intercalation, of Lithium (Li) insertion/deinsertion, involving only 0.5 electrons per redox site. In this context, LIBs are desired having the ability to allow two or more than two transferable electrons per redox activity so that the specific capacities of LIBs are enhanced manifold. This purpose is well served by the electrode (particularly cathode) materials undergoing conversion reaction during lithium insertion/deinsertion. The conversion based LIBs involve transfer of two or more than two electrons per redox reaction and thus possess very high specific capacities i.e., manifold higher than the intercalation based LIBs. Among the conversion-type cathode materials, transition metal fluorides (TMFs) are the most exciting owing to their lower cost, environmental benignity and of course excellent specific capacities. However, their commercial applications are restricted by their poor conductivity owing to the highly ionic metal-fluorine bond, large band gap and capacity fading during charge/discharge cycling. To overcome some of the critical issues of conversion-type transition metal fluoride cathode materials, we bring in a solution through viable engineering of nanoarchitectures. This dissertation comprises of the research work in which we have prepared different types of TMFs nanoparticles (NPs) by an easy and inexpensive reverse micro-emulsion method. First, cobalt fluoride (CoF2) nanoparticles were prepared by reverse micro-emulsion method, to the best of our knowledge for the first time, employing cetyltri methyl ammonium bromide (CTAB) as surfactant, 2-octanol as continuous phase and water as disperse phase. A series of CoF2 nanoparticles was achieved by varying the different parameters of the water-in-oil micro-emulsion synthesis x method. The as synthesized CoF2nanoparticles were hydrated and therefore, sintered at 400ºC in a vacuum chamber operated at 5.0 millbar (mbar) pressure for 3 hours under inert atmosphere. This led to the achievement of anhydrous CoF2 nanoparticles. The synthesis protocol was further extended to the preparation of hydrated and anhydrous nickel fluoride (NiF2) nanoparticles. Additionally, we have also prepared ammonium manganese tri-fluoride based nanoparticles by employing the same synthesis method. To know the impact and/or importance of the synthesis method upon the characteristics of the resulting transition metal fluoride based nanomaterials, transition metal fluorides i.e.,(CoF2, NiF2 and NH4MnF3) were also prepared by simple precipitation method involving none of the structure controlling agents e.g., surfactants. The results (presented in results and discussion chapter) showed that the transition metal fluoride nanostructures prepared by reverse micro-emulsion method are better electrode materials in terms of their electrochemical activity as compared to those prepared by precipitation method. This study demonstrated that the prepared nanostructures of transition metal fluoride are promising cathode materials for LIBs i.e., high storage capacity secondary batteries. All the as prepared nanomaterials i.e., CoF2, NiF2and NH4MnF3 and their dehydrated progenies were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), UV-Vis spectroscopy (UV-Vis), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Scanning electron microscopy (SEM), and Energy-dispersive X-ray spectroscopy (EDX) and Cyclic voltammetry has been used to measure the electrochemical activity of as-prepared and subsequently dehydrated transition metal fluoride nanoparticles. The results showed that the electrochemical performance i.e.,reversibility andoxidation / reduction potential of the transition metal fluoride based nanomaterials are significantly improved upon their dehydration. For example, the calcined CoF2 nanomaterial having particle size in the range of ~20-50 nm with uniform composition have shown better electrochemical performances (0.079mA) as compared to its parent nanomaterials i.e., the hydrated CoF2 nanoparticles