| dc.description.abstract | Pollution, climate change and the rapid consumption of fossil fuel resources are driving the development and adoption of clean and renewable energy sources, including hydro, wind and solar power, as well as non-fossil fuel powered products such as electric vehicles. Therefore, there is an urgent and growing demand for corresponding high-efficiency, high-density, and low-cost energy storage systems. Among them, Li, Na, K, Mg, Zn, and Al-ion batteries and other types of rechargeable batteries have the characteristics of high efficiency and reversibility, light weight, environmental friendliness, and low cost, and have been widely studied in academia and industries. While extensive research over the past decades has led to the highly successful commercialization of lithium-ion batteries in portable electronics and electric vehicles, there have been many efforts to develop next-generation batteries with higher energy density, longer cycle life, and lower cost, such as batteries based on sulfur and organic materials.
This thesis has explored the use of functional polymers to improve the specific capacity, discharge voltage, cycling, and rate performance of sulfur and organic cathodes. Through a combination of characterization techniques including thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), cyclic voltammetry (CV), chronopotentiometry, electrochemical impedance spectroscopy (EIS), galvanostatic cycling, UV-Vis, and peel test, new insights into the strategies for tackling the dissolution issue of sulfur and organic cathodes, conductivity change of conductive polymers in the battery, capacity fading mechanism, and charge storage mechanism have been gained. These findings may inspire the development of novel cathode materials with higher energy density and lower cost than conventional intercalation cathode materials.
Firstly, a multi-functional PEDOT:PSS-Mg2+ binder formed by cross-linking PEDOT:PSS with Mg2+ was developed for the sulfur cathode in Li–S batteries. This new binder has a robust 3-D network structure achieved by the cross-linking of PSS- anions with Mg2+ ions, and a strong binding ability toward lithium polysulfides due to the strong interaction between the oxygen atoms in PEDOT and lithium polysulfides. These functionalities can increase the charge transfer reactions, cushion the drastic volume change during discharge/charge cycling, and trap the soluble lithium polysulfides in the cathode. The Li–S battery with a cathode using this new binder exhibited an initial capacity of 1097 mA h g-1 and capacity retention of 74% over 250 cycles at 0.5C, which are significant improvements compared with the Li–S battery using a conventional PVDF binder. Moreover, the preparation of the cathode slurry and the subsequent cathode fabrication using the PEDOT:PSS-Mg2+ binder uses water present in the PEDOT:PSS dispersion as the only dispersing solvent, which eliminates the use of any organic solvent, making the fabrication of Li–S batteries more environmentally friendly. Therefore, this study demonstrated that the cross-linked PEDOT:PSS-Mg2+ is a very promising new binder for high-performance Li–S batteries.
Secondly, an innovative facile in-cell electrochemical polymerization method has been developed to incorporate a conductive polymer PEDOT into the sulfur cathode, which enables an intimate contact between PEDOT and other components in the cathode, leading to enhanced electron transport and effective trapping of soluble polysulfides. As a result, the sulfur cathode with the in-cell formed PEDOT shows substantially improved capacity, cycling stability, and rate performance compared with that using the commercial PEDOT. Furthermore, it has been found that the conductivity of PEDOT changes drastically during the battery cycling process, which affects the battery performance. Finally, the in-situ synthesis of PEDOT has been applied to LiFeO4 cathode, and a notable improvement in the specific capacity has been observed.
Thirdly, a series of one-dimensional coordination polymers using 2,5-dihydroxy-1,4-benzoquinone (DHBQ) as the ligand and divalent metal ions (Ni, Co, Mn, Zn, and Cu) as the metal center have been synthesized and their electrochemical properties have been compared. It has been found that the coordination polymers using Ni, Co, Mn, and Zn (M-DHBQ·2H2O) exhibit the redox activities of both metal and ligand in the potential range of 0.5~3 V vs. Li+/Li, while the coordination polymer using Cu (Cu-DHBQ) only exhibits the redox activity of the ligand in the same potential range. In the potential range of 1.3~3 V vs. Li+/Li where only the DHBQ ligand is redox active, Cu-DHBQ exhibits the highest utilization of the quinone groups among the as-synthesized coordination polymers. Moreover, the capacity fading mechanism of Cu-DHBQ cathode is identified as the dissolution of the discharged product or intermediate in the electrolyte by UV-Vis analysis. By using the alginate binder (25 wt% in the cathode), which can strongly bind the electrode film and effectively trap the soluble species, the Cu-DHBQ cathode exhibits a high capacity of 261 mA h g-1 (98.1% of the theoretical capacity) at the current rate of 20 mA g-1, and can maintain a capacity of 194 mA h g-1 after 200 cycles at 100 mA g-1 with a capacity retention of 91.5%. Furthermore, our coordination approach is very versatile and can be extended to other ligand such as 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (DHBQ-Cl) which has a higher discharge voltage than that of DHBQ. The Cu-DHBQ-Cl cathode shows a fast capacity fading, which might be caused by the collapse of the crystal structure after Li+ insertion. Nevertheless, our approach opens up a new avenue for the application of coordination polymers in energy storage. Finally, the stabilization of organic cathode through acid-base interaction with polymer binders has also been studied. It has been found that the binder approach for improving the cycling stability of organic cathode is only an auxiliary approach, whereas the polymerization approach, which includes the formation of conventional polymers, macrostructures, coordination polymers, covalent organic frameworks (COFs) and metal organic frameworks (MOFs), is be considered as the primary approach.
Finally, two diketopyrrolopyrrole (DPP) based conjugated polymers, namely diketopyrrolopyrrole-quaterthiophene copolymer (PDQT) and diketopyrrolopyrrole-bithiophene polymer (PDBT) have been explored as the cathode materials for Li-ion storage. The PDQT cathode shows a p-type charge storge mechanism with a theoretical capacity of 52.4 mA h g-1 and an experimental capacity of 44.4 mA h g-1 (corresponding to a high doping level of 42%), while the PDBT cathode shows a bipolar charge storage mechanism with a theoretical capacity of 93.6 mA h g-1 and an experimental capacity of 17.1 mA h g-1. The experimental average discharge voltages of PDQT and PDBT cathodes are ~3.8 and ~2.95 V, respectively, which are one of the highest among organic cathodes. Further optimization of the testing condition (e.g. nanocomposite formation with porous carbon, better electrolyte solvent which is stable over a broad potential range so that both the p- and n-doping reaction can occur) is needed to increase the experimental capacity of PDBT. | en |
