|Title:||Advanced polymeric materials for the enhanced performance of energy storage devices|
|Advisors:||Chen, Guohua (ME)|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Mechanical Engineering|
|Pages:||xiii, 130 pages : color illustrations|
|Abstract:||This thesis reports the utilization of polymeric materials in the improvement of electrochemical devices. A sodium-ion conducting PVA-based gel electrolyte was first developed for the application of electric double-layer capacitors (EDLCs). Sodium triflate (NaTf) was selected as the electrolytic salt and NMP was employed as the solvent of PVA and NaTf. The composite (PVA with 30% NaTf and 10% EMITf) showed a high ionic conductivity of 3.8×10⁻³ S cm⁻¹ and good thermal stability up to 150 °C. It was thus employed in the fabrication of EDLCs by using a spinning-casting method, serving as both ion-conducting electrolyte and separator. The capacitor showed almost 100% coulombic efficiency and stable charge-discharge cyclic property with almost 100% capacity retention after 1000 cycles, when charging up to 1.6 and 2.0 V with a capacity of 103.7 and 127.8 F g⁻¹, respectively.|
The PEDOT-PDMS co-polymer was then synthesized to have good electrical conductivity from PEDOT and flexibility because of the amorphous nature and conjugated structure. The synthesis was made through the hydrosilylation and dehydrocoupling reaction occurred between the h2PDMS and the EDOTmonomer. The Young's modulus, tensile stress, and the strain of the polymer film at the rupture are 1.17±0.10 MPa, 22.4±2.1%, and 0.24±0.03 MPa, respectively. The PEDOT-PDMS co-polymer was coated onto the prepared high-voltage NaLiFePO4F electrode to protect the electrode from hydrofluoric (HF) acid attack and prevent the loss of active material. The resulting PEDOT-PDMS coated NaLiFePO₄F electrode showed a specific capacity of 88.1 mAh g⁻¹ at 0.5 C after 500 cycles, and good cycling stability, with about 100% retention of the initial discharge capacity. As for the NaLiFePO₄F control electrode, its specific capacity recorded was 59.5 mAh g⁻¹ only under the same conditions, which is 78% retention of the initial discharge capacity. The coated electrode shows high chemical diffusion coefficient of Li+ (1.89×10⁻⁹ and 1.20×10⁻⁹ cm2 s⁻¹ during charging and discharging) compared to NaLiFePO4F control electrode (7.17×10⁻¹⁰ and 5.29×10⁻¹⁰ cm² s⁻¹ during charging and discharging). Furthermore, PEDOT-PDMS was employed as a protection coating layer on the surface of commercially available Si nanoparticles to provide conducting pathways and suppress the volume change in cycling. The coated Si was found to show superior performance as anode of Li-ion battery with a capacity of 1512 mAh g⁻¹ obtained after 1000 cycles, which is 69.8% retention of the highest specific capacity around the 160th cycle (~2166 mAh g⁻¹) at 0.5 C. As for the Si control electrode, its specific capacity only was 605 mAh g⁻¹ after the 500th, which is 29% retention of the highest/initial discharge capacity. Also, the coated Si electrode shows higher Li-ion diffusion coefficient (2.47×10⁻¹¹ cm² s⁻¹) and lower internal resistance than that of the Si control electrode (5.07×10⁻¹³ cm² s⁻¹). The in-situ TEM results directly confirm that the polymer coating layer provides conducting pathways and buffers the stress induced by the lithiation, leading to less retardation effect and less self-limiting lithiation. Hence, the conducting-flexible polymer has a positive impact on the electrochemical performances of Si nanoparticles anode.
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