Synthesis and characterization of nanostructured materials for Li-ion secondary batteries

Rechargeable lithium batteries have already revolutionized the market of portable electronic devices. Potentially, in the next future, they are going to become the technology of choice for the huge market of electric vehicles (EV), hybrid-electric vehicles (HEV), and plug-in hybrid-electric vehicles (PHEV), where low cost, low environmental impact, as well as high specific performance batteries are needed. Even so, intensive efforts are still under way to further improve this relatively young systems. Actually, for the next generation of rechargeable lithium-ion batteries, further breakthroughs, especially in modifying and improving already known materials, are essential. In this context, considering the challenges and expectations for the energy storage devices in the near future, the present Ph. D. thesis offers an overview of the most promising electrode material for secondary Li-ion cells. Several materials were developed and thoroughly investigated, from the synthesis to the structural-morphological characterization and the evaluation of their electrochemical performance. The introductory section of the thesis (chapters I to III) provides a brief survey of the necessary background in the field of Li-ion batteries and battery materials. Chapter IV focuses on the several characterization techniques and methods used to analyse the synthesized samples. The following three chapters (chapters V to VII) focuses on negative electrodes. Three different kinds of materials were taken into account, each of them presenting a different reaction mechanisms towards lithium. To overcome the specific problems related to each material different synthesis approaches were considered and structural-morphological aspects were modified to optimise the final electrochemical performance. The first material considered was titanium oxide (TiO2), able to intercalate lithium ions. TiO2 powder is employed in a wide range of application, thus a great number of publications are present in the literature on this subject. Aim of my work was to probe different syntheses and evaluate potential differences among them, both from structural-morphological and electrochemical point of view. Two different TiO2 polymorphs were synthesized by means of different synthesis strategies. TiO2-B, usually considered the most promising polymorph, was firstly prepared. Although it showed appreciable characteristics, the performances of prepared samples were only modest, with respect to the following samples prepared. Nanostructured anatase, the second TiO2 polymorph considered, was successfully prepared by three different synthesis techniques: an evaporation-induced self-assembly (EISA) process, a sol-gel synthesis and a hydrolytic process. When the synthetic processes was optimized, the electrochemical performances of all these anatase samples were found to be superior with respect to the previous TiO2-B. Despite of the different synthesis methods adopted, the obtained results were similar: Coulombic efficiency, cyclability and specific capacity were highly valuable, also at the highest value of current regime applied, that is 10C (i.e. 3350 mA g−1). Some results are particularly interesting in view of a possible practical application. Actually they join good overall performances with a synthesis procedure extremely quick and easy to perform: this is greatly appreciable bearing in mind the increasing attention to low cost and environment friendly materials. The second anode material considered was iron oxide (particularly, α-Fe2O3), that presents interesting features, such as abundance, low cost and environmental friendliness. Moreover, its theoretical specific capacity is extremely high, more than 1000 mAh g-1, compared to 370 mAh g-1 of graphite, the standard Li-ion battery anode. Its mechanism of reactivity towards lithium (conversion reaction) involves the formation and decomposition of Li2O, accompanying the reduction and oxidation of the metal. As these reactions are possible and reversible only if the material has nanometric dimensions/large interfacial surface, several nanostructured α-Fe2O3 samples were prepared by nanocasting strategy, using different kinds of mesoporous silica (i.e. SBA 15, MCM 41, MCM 48) as hard templates. The selected approach allowed to easily tune the characteristics of the final products. Indeed, opportunely selecting the template significant improvements in cycling stability were obtained. Another interesting path, that needs further in-depth examination, was the substitution of the traditional liquid electrolyte solution with a quasi-solid polymer electrolyte membrane. This new configuration demonstrated good cycling stability and capacity retention, directly related to the polymerization process used (i.e., UV photo-polymerisation) in which the polymeric network is directly formed in situ at the interface with the electrode film. The last material considered for application as anode was tin oxide (SnO2), able to alloy with lithium and deliver the interesting reversible theoretical specific capacity value of 780 mAh g-1. As for α-Fe2O3, its main issue is the cycling stability after prolonged cycling and two different strategies have been explored in order to overcome this problem: particle nano-structuration and use of a carbonaceous buffer matrix. The first path allowed to obtain an interesting material, with very high specific capacity and good overall cycling efficiency. However, the problem of cyclability was only minimized but not completely solved. On the contrary, the second approach demonstrated to be completely successful, as the cyclability problem was completely solved and the obtained material showed elevated and stable electrochemical performances and was able to undergo prolonged galvanostatic discharge/charge cycles. The last chapter of the thesis concerns on the development of a material for the positive electrode, that is lithium iron phosphate (LiFePO4). It has received great attention during the last years because of its low toxicity and low cost. Aim of my work was the optimisation of a previously developed synthetic route (i.e., mild hydrothermal process in the presence of an organic surfactant), in view of a possible practical application of the resulting material in commercial lithium-based batteries. The presence of the cationic surfactant CTAB confirmed to be a distinguishing feature of this synthesis. Actually, the most significant improvements were obtained modifying the physico-chemical properties of the CTAB water solution. In particular, the most significant results were obtained by the addition of a co-solvent during synthesis, which led to significant differences on the micellization process of the surfactant, thus resulting in marked differences on the structural-morphological characteristics and electrochemical performances of the resulting samples. Two most evident effects were evidenced and both were related to the use of the co-solvent: the drastic change in morphology and the improved characteristics of the carbon layer. The changed morphology enhanced the ionic conductivity of the material, due to the promotion of the growth of the electrochemical active crystalline faces of LiFePO4. In addition, the improved characteristics of the carbon layer led to an increase of the final electronic conductivity of the active material particles, as the carbon layer surrounding the particles presented higher homogeneity and degree of graphitization. When the choice of the reaction conditions was carefully controlled, the resulting material showed outstanding electrochemical results: the increased performance, particularly registered at extremely high discharge rates (even as high as 100C), led to the publication of an international patent in 2011. This confirmed the feasibility for this material to be commercially applied in several areas, the most important one being batteries conceived for automotive and transportation.