Nowadays, low carbon economy becomes a significant topic over the world. Due to the decreasing amount of fossil energy source and the worsening environmental pollution, traditional energy sources should be transferred to renewable energy sources. A transition to renewable energy will require radical changes to systems and technologies for energy storage. Lithium ion (Li-ion) batteries are now considered as the most important electrochemical energy source for portable devices, electrical vehicles and expected to be used in grid electrical energy storage. Beside on Li-ion batteries, sodium ion (Na-ion) batteries are another promising energy source, which have the advantages in cost, safety and environmental factors, and they could be used for stationary energy storage systems and large vehicles. Tin-based nanocomposites are promising to replace the traditional graphite for Li-ion batteries to achieve a higher battery performance. In 2005, Sony Corporation launched the first Sn-based anode Li-ion batteries (Nexelion) to obtain a 50% increase in volumetric capacity over the conventional battery, which marked Li-ion batteries to enter into a new cutting edge. However, Sn-based materials faced with challenges. The battery performance was limited by a low cycling life and low rate performance, and methods should be devised to overcome these shortcomings. In this thesis, SnO2-based nanocomposites, including the graphene-SnO2, the carbon-coated graphene-SnO2 and the carbon-coated nanostructured SnO2 have been prepared and investigated as anodes for Li-ion and Na-ion batteries. The microstructure, electrochemical performances and even the degradation mechanisms have been investigated as the effects for different composite materials. Chapter 4 reports an amorphous carbon coated graphene-SnO2 composite which exhibited an enhanced cycling stability. In previous researches, the performance enhancements of that type of materials were commonly attributed to the carbon coating enhancing the electronic conductivity. However, it is found that the carbon coating deeply relates to the microstructure stability of the active materials, the performance enhancement can be attributed to the enhancement of structural stability. Chapter 5 reports same composites with various graphene to amorphous carbon mass ratios. In this chapter, we try to find out the optimized composition and understanding the different roles of graphene and amorphous carbon in that type of composites. It is found that an optimised graphene to carbon mass ratio can effectively enhance the structural stability and the electrode conductivity. Chapter 6 reports a carbon-coated flower-like nanostructured SnO2 for Na-ion battery application, which has been demonstrated to have a high reversible capacity and high rate performance. The carbon coating is found to help in the formation of a high quality solid electrolyte interface (SEI) layer on the surface of the active materials. These researches focus on modifying SnO2 and SnO2-based materials by carbon coating technologies, which aim to develop novel electrode materials to obtain a better battery performance for Li-ion and Na-ion batteries.

An alternate family of “high” voltage (where the equilibrium voltage lies between 3.6 V and 4.2 V) polyanion cathode materials is reported in this thesis with the objective of improving specific energy density (Wh/kg) and developing a better understanding of polyanion electrochemistry. The electrochemical properties, synthesis and the structure of novel fluorosulfate materials crystallizing in the tavorite and the triplite type mineral structures are described. These materials display highest discharge voltages reported for any Fe2+/Fe3+ redox couple. LiFeSO4F was prepared in both the tavorite and the triplite polymorphs using inexpensive and scalable methods. Complete structural characterization was performed using X-ray and neutron based diffraction methods. A rapid synthesis of fluorosulfates can be achieved by using microwave heating. The local rapid heating created by the microwaves generates nanocrystalline LiFeSO4F tavorite with defects that induce significant microstrain. To date, this is unique to the microwave synthesis method. Phase transformation to the more stable triplite framework, facilitated by the lattice defects which include hydroxyl groups, is therefore easily triggered. The formation of nanocrystalline tavorite leads to nanocrystalline triplite, which greatly favors its electrochemical performance because of the inherently disordered nature of the triplite structure. Direct synthesis of the electrochemically active triplite type compound can be carried out either by extending the duration of the solvothermal reactions or by the partial substitution of Fe by Mn to produce LiFe1-xMnxSO4F. This study, overall, has led to a better understanding of the transformation of tavorite to the triplite phase. To examine Li and the Na ion conduction and their correlation with the electrochemical performance of 3-D, 2-D and 1-D ion conductors, atomistic scale simulations have been used to investigate tavorite type LiFeSO4F, NaFeSO4F, olivine type NaMPO4 (M= Fe, Mn, Fe0.5Mn0.5) and layered Na2FePO4F. These calculations predict high mobility of the Li-ion in the tavorite type LiFeSO4F but sluggish Na-ion transport in iso-structural NaFeSO4F. High mobility of the Na-ion is predicted for phosphate layered and olivine structures. Finally, the synthesis and structural details of NaMSO4F (M=Fe, Mn) and NH4MSO4F (M=Fe, Mn) are presented in the last chapter to show the structural diversity present in the fluorosulfate family.

An alternate family of “high” voltage (where the equilibrium voltage lies between 3.6 V and 4.2 V) polyanion cathode materials is reported in this thesis with the objective of improving specific energy density (Wh/kg) and developing a better understanding of polyanion electrochemistry. The electrochemical properties, synthesis and the structure of novel fluorosulfate materials crystallizing in the tavorite and the triplite type mineral structures are described. These materials display highest discharge voltages reported for any Fe2+/Fe3+ redox couple. LiFeSO4F was prepared in both the tavorite and the triplite polymorphs using inexpensive and scalable methods. Complete structural characterization was performed using X-ray and neutron based diffraction methods. A rapid synthesis of fluorosulfates can be achieved by using microwave heating. The local rapid heating created by the microwaves generates nanocrystalline LiFeSO4F tavorite with defects that induce significant microstrain. To date, this is unique to the microwave synthesis method. Phase transformation to the more stable triplite framework, facilitated by the lattice defects which include hydroxyl groups, is therefore easily triggered. The formation of nanocrystalline tavorite leads to nanocrystalline triplite, which greatly favors its electrochemical performance because of the inherently disordered nature of the triplite structure. Direct synthesis of the electrochemically active triplite type compound can be carried out either by extending the duration of the solvothermal reactions or by the partial substitution of Fe by Mn to produce LiFe1-xMnxSO4F. This study, overall, has led to a better understanding of the transformation of tavorite to the triplite phase. To examine Li and the Na ion conduction and their correlation with the electrochemical performance of 3-D, 2-D and 1-D ion conductors, atomistic scale simulations have been used to investigate tavorite type LiFeSO4F, NaFeSO4F, olivine type NaMPO4 (M= Fe, Mn, Fe0.5Mn0.5) and layered Na2FePO4F. These calculations predict high mobility of the Li-ion in the tavorite type LiFeSO4F but sluggish Na-ion transport in iso-structural NaFeSO4F. High mobility of the Na-ion is predicted for phosphate layered and olivine structures. Finally, the synthesis and structural details of NaMSO4F (M=Fe, Mn) and NH4MSO4F (M=Fe, Mn) are presented in the last chapter to show the structural diversity present in the fluorosulfate family.

This thesis deals with ionic liquids and use of ionic liquids as electrolytes in lithium-ion batteries. Thesis describes basic characteristics of secondary electrochemical cells and characteristics of ionic liquids. Thesis also describes modifications and measurement of ionic liquids. Thesis also presents the results of measurement conductivity and potential window of modified ionic liquids.