A Study Of All-Organic Rechargeable Batteries Based On Redox-active Polymers

The development of advanced energy storage is the key for effective use of renewable energies. Currently, lithium-ion batteries are considered to be the ideal choice as energy storage devices for next-generation electric vehicles and renewable power stations due to its high energy density. However, it is very difficult to enhance the energy density of Li-ion battery due to the low utilization of the present materials; moreover, the Li-ion battery has intrinsic problems of, cost, safety and natural shortage of lithium resources. Therefore, it is of great importance to develop new battery systems for large scale energy storage. The redox-active polymers seem to be an attractive candidate for such new batteries, because of their advantages of structural diversity, rich resources and environment-friendlyness. In this thesis, we were aimed at exploring new anode and cathode materials for all-organic rechargeable batteries with higher energy storage density, higher power density and better environmental compatibility. The main results and new findings in this work are summarized as follows:1. Development of Polytriphenylamine (PTPAn)-based cathode materials for Li-ion batteries. Because of their conductive PPP backbone and electroactive PAn unit, PTPAn and its derivatives have not only superior high power capability but also high energy density. In this work, poly (4-cyano) triphenylamine and poly (4-nitro) triphenylamine bearing electron-drawing groups were chemically synthesized enhance their redox potential of the polymer and specific energy. The experimental results show that the charging and discharging process of the polymer is accomplished by doping-dedoping of the anions. It was found that poly (4-cyano) triphenylamine cathodes display areversible capacity of83mAh g-1at an average discharge voltage of3.9V, with slight capacity decay over hundreds of charge-discharge cycles. And also, this material shows a storng rate capability with55mAh g-1delivered at a very high rate of4C. The poly (4-nitro) triphenylamine cathode displays a redox capacity of70mAh g-1at an average discharge voltage of3.9V, and remains64.3mAh g-1,91.8%of initial capacity, after hundreds of charge-discharge cycles. Even at a rate of320mA g-1, the material can still delivers a capability of50mAh g-1. Overall, these polymers show excellent performance with sufficient cycleability and quit a high-rate capacity, capable to be a low cost and environmentally benign cathode material for next generation of organic batteries.2. Development of polymers as lithium storage anode materials. In this thesis, a polybithiophene-carbon (PBT/C) composite was synthesized by ball-milling chemically polymerized polybithiophene with carbon nanofibiers and tested as anode materials for Li-ion and Na-ion batteries. The experimental results show that the charging and discharging process of the polymer is accomplished by doping-dedoping of the cations. It was found that PBT-C composite displays a anodic capacity of～850mAh g-1at two potential plateaus of～2.25V and～1.25V and remains its initial capacity after hundreds of charge-discharge cycles. This material also has a superior high rate capability with280mAh g-1delivered at a very high rate of4C. Overall; the polymer composite showed excellent performance with sufficient cycleability and quit a high-rate capacity. Also, the polymer composite can be cycled in Na+-electrolyte, delivering a reversible capacity of500mAh g-1at a potential plateau of～1.7V, and remained～400mAh g-1after40cycles, exhibiting a considerable cyclability as Na-storage anode material.3. In the search for suitable anode material for all-organic rechargeable batteries, we synthesized n-dopable poly (3,4-dihexylthiophene)(PDHT) with longer conjugated, highly regular and coplanar chains by a Ni-catalyzed oxidative coupling reaction and also synthesized a novel polythiophene/carbon composite by in-situ chemically polymerization on carbon nanofibers. Then we tested the n-type redox behavior of PDTH and constructed an all-organic battery to test its possible battery applications as organic anodes. The PDHT-C composite displays a reversible capacity of～300mAh g-1at the potential below0.6V and remains its initial capacity after hundreds of charge-discharge cycles. Even at a very high rate of1280mA g-1this composite can still deliver200mAh g-1, showing a superior high rate capability. In addition, the all-organic PTPAn-PDHT/C cells could be charged and then discharged at～3.0V and can realize a reversible capacity of～250mAh g-1. These results suggest that these polymers may be used to construct sustainable and high efficiency organic batteries, which are greatly needed for electric storage.4. Most of the polymer can only deliver a very small capacity when used as a cathode-active material, possibly due to the frustrated doping and dedoping of the larger anions into/from the polymer chains. To solve this problem, we synthetized poly{pyrrole-co-[3-(pyrrol-1-yl) propanesulphonate]}(PP-PS) and examined its p-type redox reversibility of the self-doped polymer as a cathode-active material for Na-ion battery applications. The experimental results show that the charging and discharging process of the polymer is accomplished by doping-dedoping of the Na+It was found that the self-doped PP-PS polymer displays a capacity of90mAh g-1at an average discharge voltage of3.6V and remains its initial capacity after hundreds of charge-discharge cycles, showing a great promise for Na-storage anode material.5. An all-organic battery is build up with the same polymer as both cathodic and anodic material. In this thesis, we found that polyparaphenylene (PPP) is an attractive candidate for such new battery systems because of its p-and n-doping properties and a large potential gap between its n-and p-type reactions. Because the polymer molecule has a highly conductive backbone and potentially high redox capacity, it is expected for the polymer to have superior high power capability and high energy density. The experimental results show that PPP can be either p-doped with a reversible redox capacity of80mAh g-1at a high potential>3.9V or n-doped with a huge reversible redox capacity of600mAh g-1at quite low potential<1.5V and all of those can maintain quite steadily during100successive cycles. Such a PPP-based organic battery can work at3.0V with considerably high capacity of150mAh g-1and cycling stability, providing a possible alternative to the widely used, transition metals-based conventional batteries.