UBC Theses and Dissertations
Application of electrospun carbon nanofibres for batteries and supercapacitors Lee, Nicole Ying-Ju
With the great demand for energy storage devices with much higher energy density, better power performance and longer cycle life, researchers are looking into nano-structured battery and supercapacitor electrodes due to the higher accessibility of ions to electrodes, improved specific capacitance, and reduced chance of mechanical degradation compared to bulk materials. In this study, a composite system of conductive carbon nanofibres with active materials (polypyrrole and silicon nanoparticles) are fabricated for supercapacitor and lithium ion battery electrode applications. The aim is to use carbon as a strong mechanical and electrical support and the high energy storage capability of active materials to develop new generation electrode systems. Carbon nanofibres with 3.9±0.5×10² nm in diameter are fabricated from a copolymer precursor, poly(acrylonitrile-co-acrylamide), through electrospinning and carbonization. The mild exothermic heat reaction of this copolymer and the enhanced heat flow into nano-scaled fibres during stabilization permits fabrication of high quality and large-scale carbon nanofibre mats with reasonable conductivity (18±1 S/cm), high porosity, and high accessible surface area. Carbon nanofibres are subsequently deposited on electrochemically with polypyrrole for 4 and 8 hours for use as a supercapacitor electrode. Capacity is compared with that of a bulk polypyrrole film. Both systems possess a gravimetric capacity of ca. 150 F/g, but an enhanced volumetric capacity in the nanofibrous electrode (9±1×10⁷ F/m³ for nanofibres vs. 6.0±0.5×10⁷ F/m³ for film) at greater amount of polypyrole deposition (8 hour deposition) is observed. The porous nanofibrous system also reduces the ionic resistance from that of the pure polypyrrole film, which is at least 1.9±0.8×10²Ω, to just 2 – 3 Ω at the highly reduced state. In lithium ion battery applications, a core-shell electrospinning method is used to fabricate carbon nanofibres containing silicon nanoparticles in the core. The core-shell structural advantage over non-core-shell structure observed to be in the prevention of silicon particle fusion and reduced breakage after interacting with lithium ions. At 30 wt% nanoparticle loading, both systems can reach over 1000 mAh/g initial capacity at 100 – 600 mA/g cycle rates. After 20 cycles, the capacity retention of Si in most core-shell systems are significantly higher than that of the non-core-shell system by ca. 10 – 20 %. Further optimization is required to improve the cycle life and electrode stability.
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