Polypyrrole-based Nanofibrous Membrane Separator for Lithium-ion Battery
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Date
2021-12-22
Authors
Li, Yifu
Journal Title
Journal ISSN
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Publisher
University of Waterloo
Abstract
A battery separator is one of the key components of a Lithium-ion battery (LIB). It serves as an insulator between the electrodes to prevent the internal short circuit. More importantly, the battery separator retains liquid electrolyte within its porous structure, allowing the migration of lithium ions during battery cycling. The fast-growing demand for high-performance LIBs in various applications requires the development of superior separators.
Electrospun nanofibrous separator receives considerable attention among all the progresses of battery separator research. Generally, electrospun nanofibrous separator offers appealing features including large pore size (typically above 500 nm), high porosity (typically above 70%) and interconnected porous structure. This improves ions transportation efficiency and battery cycling performance. However, most studies are focused on electrochemical inert material as battery separator, which is incapable of contributing any battery capacity to the LIB cells.
This thesis study develops a redox-active separator based on electrospun polypyrrole (PPy) composite nanofibers to enhance battery capacity. The proposed separator is fabricated by in-situ polymerization of PPy onto electrospun polyacrylonitrile (PAN) nanofibers followed by subsequent electrospinning to form a bilayer membrane.
This thesis starts with understanding the separator’s effects on the battery performances to provide insights and guidance to the separator design. A two-dimensional electrochemical-thermal coupled model is developed for a 38120-type LiFePO4 LIB. The model results show that separator thickness strongly impacts battery energy density. In addition, the mass transfer resistance of the separator increases with decreasing separator porosity, which results in increased electrolyte concentration gradient. However, the correlation between separator porosity and electrolyte concentration gradient indicates that a separator porosity of 80% or greater contributes little to the resistance to mass transfer.
After that, a detailed study of the kinetics on the in-situ polymerization of PPy with electrospun fibrous membrane as the template is carried out to better understand the mechanisms behind the fabrication of the proposed separator. The in-situ polymerizations of PPy are produced on electrospun fibrous PAN templates at temperatures ranging from 273 to 285 K. The experimental results show that the overall reaction rate of the in-situ polymerization process in the presence of electrospun fibrous template is faster than that without template. Further investigation confirms that the increase in the overall reaction rate results from the enhanced reactions between oxidized pyrrole oligomers and neutral pyrrole monomers
Then, the proposed separator with expected properties is fabricated and characterized. The produced separator exhibits a bi-layer structure, including a layer of PAN@PPy core-shell structured fibers and another layer of PAN fibers. The porosity and electrolyte uptake of the redox-active separator (79.3±7.1% and 294.6±31.5%) are much higher than that of a commercial PP separator (41% and 81.5±17.4%). In addition, the redox-active separator is thermally stable up to 250 ℃ and capable of maintaining its dimensions at 160 ℃. Moreover, the redox-active separator exhibits superb mechanical properties than the electrospun PAN separator dose.
Finally, the separators are assembled into separate LIB cells for performance evaluation. The battery cell containing redox-active separator exhibits the highest discharge capacity of 158.7-227.0 mAh∙g-1 at different current rates of 2-0.2 C. The enhanced battery capacity stems from the redox-activity of the PPy polymer contained in the redox-active separator. In addition, the battery cell with redox-active separator achieves the highest gravimetric energy density of 103.0 mAh∙g-1, which is 56.1% higher than that with the commercial PP separator. These results suggest a promising strategy to enhance the capacity of LIBs by merely modifying the conventional separators into nanofibrous redox-active separators.
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Keywords
lithium-ion battery, battery separator, nanofiber