Nanomaterials for energy storage
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Nanotubes (inner diameter of 8nm and outer diameter of 10nm with a length of up to several hundred nm) and nanowires (diameter 20 – 50nm and up to several μm in length) of TiO₂-B have been synthesised and characterised for the first time. These exhibit excellent properties as a host for lithium intercalation and are able to accommodate lithium up to a composition of Li₀.₉₈TiO₂-B for the nanotubes and Li₀.₈₉TiO₂-B for the nanowires. Following some irreversible capacity on the first cycle, which could be reduced to 4% for the nanowires, capacity retention for the nanowires is 99.9% and for the nanotubes is 99.5% per cycle. In both cases, the cycling occurs at ~1.6V versus lithium. The cycling performance was compared with other forms of bulk and nano-TiO₂, all of which were able to intercalate less lithium. Nanowires of VO₂-B (50 – 100nm in diameter and up to several μm in length) were synthesised by a hydrothermal reaction and characterised. By reducing the pressure inside the hydrothermal bomb, narrower VO₂-B nanowires with a diameter of 2 – 5nm and length of up to several hundred nm were created - some of the narrowest nanowires ever made by a hydrothermal reaction. These materials are isostructural with TiO₂-B and were also found to perform well in rechargeable lithium ion batteries, being able to intercalate 0.84Li for the ultra-thin nanowires and 0.57Li for the standard nanowires. The standard VO₂-B nanowires have a capacity retention of 99.8% and the ultra-thin nanowires have 98.4% per cycle after some irreversible capacity on the first cycle. This was found to improve markedly when different electrolytes were used. Macroporous Co₃O₄ (pore size 400nm with a surface area of 208m²/g) was prepared and cycled in rechargeable lithium cells with capacities of 1500mAh/g being achieved. The structure was found to break down on the first cycle and after this the material behaved in the manner of Co₃O₄ nanoparticles. Finally a new candidate for next generation rechargeable lithium batteries was examined; Li/O₂ cells. The cathode is composed of porous carbon in which Li⁺, e⁻ and O₂ meet to form Li₂O₂ on discharge. The reaction is reversible on charge. Capacities of 2800mAh/g can be achieved when 5%mole of αMnO₂ nanowires catalyst is used. Fade is high at 3.4% per cycle meaning that there is much work to do to develop these into a commercial prospect.
Thesis, PhD Doctor of Philosophy
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