| Energy Crisis and Environmental Pollution are the two main problems human beings are facing today. Current concerns over the limited availability of energy resources have sparked the need to use renewable energies on large scale. As a result, energy storage is a vital issue to be addressed within this scenario and batteries are playing the key role. Among the existing energy storage devices, lithium-ion batteries are now considered to be the most promising technology due to their high electric storage densities. Existing rechargeable batteries are mostly based on transition-metal cathodes, which have limited capacity with serious cost. A vast variety of organic compounds and polymers have been explored for battery applications. Among them, p-doped conducting polymers seem to be the most promising cathodes because of their high electronic conductivity, high reversible theoretically, redox capacity and environment friendly. However, their realizable capacity is usually very low and decreases gradually upon charge-discharge cycling, possibly because of a low p-doping level of counter anions in the polymeric matrixes and a diffusive dissolution of the anions to the electrolyte. This PhD. work focuses on developing possible strategy to overcome the problems and increase the realized capacity of the conducting polymers.1. Development of redox-active doped polymers as cathode materials for Li-ion batteries. In this work, polypyrrole were doped with electrochemically active Fe(CN)64' anions that have its redox potential closely located at the Fermi levels of PPy. The experimental results showed that the pure PPy exhibited a reversible capacity of only-35mAh g-1, while the Fe(CN)e4-doped polymers demonstrated a greatly enhanced redox capacity of>140mAh g-1and then remained at? 115mAh g-1after100cycles, corresponding to a capacity retention of80%over100cycles. When the current density was increased to a high value of400mA g-1, the reversible discharge capacity of the polymer electrode could still reach? 110mAh g"1at successive cycling. The reason why and how the Fe(CN)64-doping can cause such a great enhancement in the reversible capacity and cycling stability of the PPy electrode could be explained that the doped anions can act as a counterion dopant to enhance the electronic conductivity of the polymer chains and function as redox-active sites to add their capacity to the doped polymer electrode. Even more, the large size of the Fe(CN)64-anions change the redox mechanism of the polymer materials from conventional p-doping/dedoping reactions of larger anions to insertion/extraction reactions of small lithium cations, so as to improve the electrochemical utilization and cycling stability of the polymers.2. To further study the mechanism of the redox-active doping, different redox-active dopants, conducting polymers and electrolyte were used. First, we used polypyrrole as a model polymer, various redox-active anions were used as doping anions, such as coordination compound anions, oxometallate anions, organic redox-active anions. All doped polymers demonstrated greatly enhanced redox capacity compared to the pristine PPy. As an example, the diphenylamine-4-sulfonate doped polymers (PPy/DS) delivered a reversible capacity of?140mAh g-1at the current density of50mA g-1and remained a reversible capacity of87%over100cycles. When the current density was increased to a high value of1600mA g-1, the discharge capacity of the polymer electrode could still reach?50mAh g-1. Also, to test the applicability of this method for other electroactive polymers, we extended the Fe(CN)64-doping to PAns and PThs The experiment results showed that all the doped polymers exhibited a great enhancement in the electrochemical activities compared to the pure polymer. Finally, we used the Fe(CN)64-doped polypyrrole PPy/FC and diphenylamine-4-sulfonate (DS)-doped polypyrrole PPy/DS as cathode materials of sodium-ion batteries. The PPy/FC were found to have a redox capacity of135mAh g-1with a strong capacity retention of85%over100cycles and an excellent rate capability at1600mA g-1of75mAh g-1. Meanwhile, PPy/DS exhibited a quite high redox capacity of115mAh g-1, excellent rate capability and cycling stability.3. In the above work, the capacity contributed by the redox-active doped polymers is limited due to the low doping level of the redox-active anions. To overcome this problem, a polyanine matrix polymer with carbonyl groups (poly (1,5-diaminoanthraquinone)) was synthesized by a chemical oxidation method and ball-milled with VGCF to construct a composite PDAQ/C. The PDAQ/C composite displayed a reversible capacity of285mAh g-1and remained at160mAh g-1after200cycles. When the current density was increased to a high value of800mA g-1, the reversible discharge capacity of the polymer electrode could still reach?110mAh g-1. This superior reversible capacity is partly contributed by the p-doping/dedoping reactions of the polyamine matrix and the other from the electrochemical redox of the carbonyl groups.4. The electrochemical utilization of the conducting polymers is always small, possibly due to the low doping level of the large anions into the polymer chains. To solve this problem, a self-doped polymer-poly (diphenylaminesulfonic acid sodium)(PDS) with a doping level of100%was synthesized. This Na-riched self-doped polymer were used as cathode-active materials for the sodium-ion battery applications. PDS demonstrated an average discharge voltage of3.6V, a redox capacity of99mAh g-1, which was closely to its theoretical value and cycled50cycles with slight decay. When the current density was increased to a value of400mA g-1, the reversible discharge capacity of the polymer electrode could still reach?45mAh g-1. The experiment results showed that the mechanism of the charging and discharging of the self-doped polymer was the insertion-deinsertion of Na+. |
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