Mesoporous materials and nanostructured LiFePO4 as cathodes for secondary Li-ion batteries: synthesis and characterisation

Energy, environmental concerns and information technology (IT) have become thrust areas for the 21st century, as they are closely linked to the technological development. The search for energy sources to provide comfort and a smooth lifestyle has taken place since the beginning of civilization; however, the present energy needs are very much dependent on nuclear and fossil fuel (oil). Currently, as the internal combustion engine is a major user of fossil fuel, consuming about 1/3 of the annual total demand for energy, concern over global warming and air pollution has become evident. Consequently, there is a high worldwide incentive to find more efficient, convenient, pollution-free and safe power sources; examples of advanced techniques include fuel cells and solar cells. Reliable methods for storing energy are just as important and secondary lithium-ion batteries provide an attractive solution. The lithium-based battery technology of today out-performs many other conventional systems, such as the lead-acid, nickel-cadmium and nickel-metal hydride batteries, because of its high energy and power density, and design flexibility, in combination with the use of environmentally acceptable constituents. Li-ion battery is a compact, lightweight, rechargeable power source stable to over 500 cycles. It can be fabricated in size ranging from a few microns to a large-scale battery capable of providing power for computer memory chips, communication equipments, colour motion pictures and, potentially, for the huge market of electric vehicles (EV) and hybrid-electric vehicles (HEV), where low cost, low environmental impact, as well as high specific performance batteries are needed. Li-based battery chemistry is, however, relatively young. Thus, the constant demand for higher energy density, thinner, lighter and even more mechanically flexible batteries has motivated research into new cell configurations and new battery chemistries and electrode materials. The scope of this thesis is the development of new cathode materials for secondary Li-ion batteries and the assessment of their structural-morphological characteristics and electrochemical performance. Chapter I deals with the basic concepts for cells and batteries and with a brief description of the history of the battery and of the general characteristics of the mature portable power-source technologies (i.e., lead-acid, Ni-Cd and Ni-MH). Chapter II discusses the historical developments, present status and future trends in Li-based batteries research. Their characteristics, working principles and components are also discussed. The energy density of lithium batteries has not increased in the last 20 years, and the specific capacity (Ah/kg) has actually decreased. Thus, much effort is being directed at finding new materials, both electrode materials and electrolytes, that can provide higher capacities, lower costs, and are environmentally benign. This is shown in Chapter III, that presents the materials and components relevant to the Li-ion battery technology during recent years and for the next future. The cathode is particularly critical in determining the capacity of a Li-based battery, as it is the heaviest component, and it has the greatest potential for improvement. For this component, further basic requirements for the potentially wide market of EVs and HEVs are the availability, low cost (high Wh/€) and low environmental impact, as well as high power capability. The experimental part of this thesis deals with the research work carried out on ordered modified mesoporous materials and nano-structured phospho-olivine LiFePO4 as cathode materials for Li-ion cells. In Chapter IV the characterization techniques and methods used to analyse the synthesized samples, either from the structural-morphological or electrochemical point of view, are briefly described. In Chapter V, the experimental results are shown regarding the three different strategies adopted in order to produce transition-metal containing mesoporous materials, suitable as cathodes for Li-ion cells. In this connection, the first attempt was to produce siliceous (MCM-41 type) and alumino-phosphate (AlPO-type) mesoporous materials, trying to substitute the largest amount of Si and/or Al with Mn ions, by direct hydrothermal synthesis. As their electrochemical results in Li cells were not satisfying, we synthesized mesoporous MCM-48 silica supported with transition-metal oxides (both MnO2 and Fe2O3) nanoparticles, by the wet impregnation technique. Finally, the experimental results are reported regarding the strategy adopted in order to support nanoparticles of FePO4, which is a well-known and extensively studied material for Li-ion batteries, into the channels of an ordered mesoporous SBA-15 silica. Phospho-olivine lithium iron phosphate (LiFePO4) is a potential cathode material for the next generation of secondary Li-ion batteries, as it fulfils the requirements of high theoretical specific capacity, low toxicity, low cost and availability. In Chapter VI, the development of a new, easy and low cost hydrothermal synthetic route to produce high surface area and high performance nano-structured LiFePO4 powders is reported. The aim was to investigate the influence of the synthetic route and preparation conditions (presence of a surfactant during synthesis) on the chemical-physical properties of the LiFePO4/C composites and, as a consequence, on their electrochemical performances. CTAB has been chosen as the surfactant for its high dispersing activity. Moreover, during the firing step in inert atmosphere, CTAB favours the preparation and the homogeneity of the LiFePO4/C composites and the obtaining of a positive influence on their electrochemical performances.