Finding new ways to power the future by making safer energy without creating extra CO2 in the atmosphere has become a crucial. The growing importance of environmental issues and security of supply has accelerated energy-related materials research efforts.
As an alternative for solving the environmen...
Finding new ways to power the future by making safer energy without creating extra CO2 in the atmosphere has become a crucial. The growing importance of environmental issues and security of supply has accelerated energy-related materials research efforts.
As an alternative for solving the environmental problems, in recent, rechargeable lithium ion batteries have intensively been attracted because they have become the power source of choice for portable electronic devices due to higher energy density compared to other rechargeable battery systems. They are also intensely pursued for hybrid electric vehicle (HEV) and plug-in hybrid electric vehicle (PHEV) applications. However, high cost, safety concerns, and limited energy and power densities of currently used materials demand the development of alternative materials.
Olivine LiMPO4 (M=Fe, Mn, Ni, Co) has been identified as an excellent candidate for cathodes in rechargeable Li-ion batteries when compared to its counterparts as LiCoO2, LiNiO2 and LiMn2O4 because of its environmental benignity, relatively high capacity of 170 mAh/g, cost-effectiveness, non-toxicity, electrochemical and thermal stability. In particular, LiFePO4 has a strong thermal stability at high temperatures, because of its robust structure in which there are strong covalent bonds between the oxygen and P5+ ions forming the (PO4)3- units, resulting in higher structure stabilization. In chapter 3, olivine structure LiMPO4 (M=Fe, Mn, Co) cathode materials have been synthesized by solvothermal routes using polyol medium as a solvent at low temperatures. Specifically, the synthesis of LiFePO4 nanoparticles as various polyols such as EG, DEG, TEG, and TTEG and its morphology variations of plate, rod, and multi-morphous depending on the different preparation of solutions have been investigated. Further, during the solvothermal reaction, the crystal formation mechanism of LiFePO4 nanocrystals based on the results as reaction times has been suggested and high crystalline LiFePO4 nanoparticles were synthesized using low-cost Fe3+ precursor because the cost of the materials such as the precursors is the most important consideration in terms of the cost of Li-ion batteries. In last, LiFePO4/graphene nanocomposite, in which LiFePO4 nanoparticles were well-dispersed on the graphene sheets, was prepared by one-step microwave-assisted polyol process and its improved electrochemical properties have been investigated by galvanostatic measurements.
As a new chemical lithiation method, polyol process has been introduced in chaper 4, since the electrochemical reaction of the polyol can be favored as the temperature increases due to decreasing the oxidation potential of the polyol. The new cathode material of “Spinel+Layered” nanocomposite was synthesized by chemical lithiation method in polyol medium of tetraethylene glycol, showing that the new nanocomposite can be produced by direct lithium insertion with Mn reduction during polyol reaction. The lithiated sample has showed pulverized nanoparticles having spherical shapes and it consisted of two phases, namely, “Spinel LiMn2O4+Layered Li1.5Mn0.97O2” nanocomposite.
In chaper 5, layered a Li-excess manganese-nickel-cobalt oxide nanocomposite, formulated as 0.3Li[Li0.33Mn0.67]O2•0.7Li[Ni0.5Co0.2Mn0.3]O2 and then Mo doped designated as 0.3Li[Li0.33Mn0.67]O2•0.7Li[Ni0.486Co0.20Mn0.286Mo0.028]O2 was prepared using the co-precipitation method. From the observed results, it was evident that partial substitution of Ni and Mn with Mo can significantly improve the structural stability and hence affect the electrochemical performance.
Li[Li1/3Mn2/3]O2 is electrochemically inactive since the entire Mn ions exist in the 4+ oxidation state so that no lithium can be deintercalated from the Li[Li1/3Mn2/3]O2 phase. However, the Li[Li1/3Mn2/3]O2 tends to become electrochemically activated above ~4.5 V and further, it can be fully activated as the particle sizes decrease. In chapter 6, synthesis of fully activated Li2MnO3 nanoparticles by chemical based oxidation reaction which is useful for approaching nanoscale materials has been introduced. In particular, when the molar ratio of Li:Mn is 6:5, multi-phases lithium manganese oxide with high capacity of almost 250 mAh/g was synthesized and it was suggested by ex-situ synchrotron XRD experiment that the multi-phases lithium manganese oxide is composed of “Li4+xMn5O12+Li2MnO3”.
Finding new ways to power the future by making safer energy without creating extra CO2 in the atmosphere has become a crucial. The growing importance of environmental issues and security of supply has accelerated energy-related materials research efforts.
As an alternative for solving the environmental problems, in recent, rechargeable lithium ion batteries have intensively been attracted because they have become the power source of choice for portable electronic devices due to higher energy density compared to other rechargeable battery systems. They are also intensely pursued for hybrid electric vehicle (HEV) and plug-in hybrid electric vehicle (PHEV) applications. However, high cost, safety concerns, and limited energy and power densities of currently used materials demand the development of alternative materials.
Olivine LiMPO4 (M=Fe, Mn, Ni, Co) has been identified as an excellent candidate for cathodes in rechargeable Li-ion batteries when compared to its counterparts as LiCoO2, LiNiO2 and LiMn2O4 because of its environmental benignity, relatively high capacity of 170 mAh/g, cost-effectiveness, non-toxicity, electrochemical and thermal stability. In particular, LiFePO4 has a strong thermal stability at high temperatures, because of its robust structure in which there are strong covalent bonds between the oxygen and P5+ ions forming the (PO4)3- units, resulting in higher structure stabilization. In chapter 3, olivine structure LiMPO4 (M=Fe, Mn, Co) cathode materials have been synthesized by solvothermal routes using polyol medium as a solvent at low temperatures. Specifically, the synthesis of LiFePO4 nanoparticles as various polyols such as EG, DEG, TEG, and TTEG and its morphology variations of plate, rod, and multi-morphous depending on the different preparation of solutions have been investigated. Further, during the solvothermal reaction, the crystal formation mechanism of LiFePO4 nanocrystals based on the results as reaction times has been suggested and high crystalline LiFePO4 nanoparticles were synthesized using low-cost Fe3+ precursor because the cost of the materials such as the precursors is the most important consideration in terms of the cost of Li-ion batteries. In last, LiFePO4/graphene nanocomposite, in which LiFePO4 nanoparticles were well-dispersed on the graphene sheets, was prepared by one-step microwave-assisted polyol process and its improved electrochemical properties have been investigated by galvanostatic measurements.
As a new chemical lithiation method, polyol process has been introduced in chaper 4, since the electrochemical reaction of the polyol can be favored as the temperature increases due to decreasing the oxidation potential of the polyol. The new cathode material of “Spinel+Layered” nanocomposite was synthesized by chemical lithiation method in polyol medium of tetraethylene glycol, showing that the new nanocomposite can be produced by direct lithium insertion with Mn reduction during polyol reaction. The lithiated sample has showed pulverized nanoparticles having spherical shapes and it consisted of two phases, namely, “Spinel LiMn2O4+Layered Li1.5Mn0.97O2” nanocomposite.
In chaper 5, layered a Li-excess manganese-nickel-cobalt oxide nanocomposite, formulated as 0.3Li[Li0.33Mn0.67]O2•0.7Li[Ni0.5Co0.2Mn0.3]O2 and then Mo doped designated as 0.3Li[Li0.33Mn0.67]O2•0.7Li[Ni0.486Co0.20Mn0.286Mo0.028]O2 was prepared using the co-precipitation method. From the observed results, it was evident that partial substitution of Ni and Mn with Mo can significantly improve the structural stability and hence affect the electrochemical performance.
Li[Li1/3Mn2/3]O2 is electrochemically inactive since the entire Mn ions exist in the 4+ oxidation state so that no lithium can be deintercalated from the Li[Li1/3Mn2/3]O2 phase. However, the Li[Li1/3Mn2/3]O2 tends to become electrochemically activated above ~4.5 V and further, it can be fully activated as the particle sizes decrease. In chapter 6, synthesis of fully activated Li2MnO3 nanoparticles by chemical based oxidation reaction which is useful for approaching nanoscale materials has been introduced. In particular, when the molar ratio of Li:Mn is 6:5, multi-phases lithium manganese oxide with high capacity of almost 250 mAh/g was synthesized and it was suggested by ex-situ synchrotron XRD experiment that the multi-phases lithium manganese oxide is composed of “Li4+xMn5O12+Li2MnO3”.
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