Design of nanostructured materials for advanced lithium ion batteries
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Among currently available rechargeable battery systems, Lithium-ion (Li-ion) batteries feature high energy density and operating voltage, and long cycle life. Since the 1990s, the development of Li-ion battery technology has been ongoing, owing to the ever-growing demand in portable electronics, electric vehicles (EV), hybrid electric vehicles (HEV) and stationary energy storage devices. Although the improvements in battery technology rely on achievements in electrode materials, separators, electrolytes and external management systems, electrode materials play the most important role as they control the electrochemical reaction and determine the properties of a battery system. Based on extensive literature reviews, the electrochemical performances of electrode materials are size- and morphology- dependent. In this study, several nanostructured materials with specifically designed morphologies were synthesized, characterized, and used as electrode materials for Li-ion battery applications. First, spherical over-lithiated transition metal oxide was chosen as the high capacity cathode material. Through a modified co-precipitation method, this material exhibited relatively low irreversible capacity loss, high specific capacity, satisfactory cyclability and rate capability. These are suitable for large-scale application. Secondly, two different carbon coating techniques were designed and applied in the synthesis of LiFePO₄ and Li₂FeSiO₄ cathode materials through in-situ polymerization and modified ball-milling methods respectively. Carbon-coated LiFePO₄ consists of primary particles (40-50 nm) and agglomerated secondary particles (100-110 nm). Each particle is evenly coated with an amorphous carbon layer, which has a thickness around 3-5 nm. Meanwhile, Li₂FeSiO₄/C nanoparticles were coated with an amorphous carbon layer, owing to the carbonization of glycolic acid. Both materials exhibited much higher specific capacity, better capacity retention, and better rate capability than their pristine counterparts. After that, a hydrothermal method was chosen and applied to synthesize hollow-structured CoFe₂O₄ nanospheres and CoFe₂O₄/mutiwalled carbon nanotubes (MWCNTs) hybrid material. The significant improvements in the electrochemical performances of these two materials, including high capacities, excellent capacity retentions and satisfactory rate capabilities could benefit from their unique nano architectures. CoFe₂O₄ demonstrated uniform hollow nanosphere architecture, with an outer diameter of 200-300 nm and the wall thickness of about 100 nm. Hybrid material resembled wintersweet flower “buds on branches”, in which CoFe₂O₄ nanoclusters, consisting of nanocrystals with a size of 5-10 nm, were anchored along carbon nanotubes. Both materials could be a promising high capacity anode material for lithium ion batteries.
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