| The depletion of non-renewable energy resources and their accompanying environmental concerns after over usage make people pay more attention to the development of renewable energy utilization.So far,new forms of power generation such as wind and photovoltaic technologies,are being developed with a fast growing scale.On the other hand,electrochemical energy storage and conversion devices represented by secondary batteries have played an important role in this transition of energy using.Though lithium ion batteries show great advantages in energy density and currently account for most of the market share,there are still many problems need to be solved,such as the increasing cost due to the shortage of lithium resources,and safety issues caused by flammable organic electrolyte.Hence,aqueous batteries which possess high safety have attracted much attention.At the same time,conjugated polymers have high structural tunability and abundant electroactive group selection.Additionally,diverse synthesis routes and flexible access to raw materials significantly reduce their preparation cost.These factors endow them with great potential as electrochemical energy storage electrode materials in aqueous batteries system.Hence,in this research series,under targeted molecular engineering,we designed and synthesized redox conjugated polymer electrodes used in aqueous batteries,then conducted systematic characterization,electrochemical tests and theoretical calculation to explore influence of their structural characteristics on electrochemical performance.The main research contents of this research series are as follows:Firstly,dithieno[3,2-b:2’,3’-d]pyrrol（DTP）molecules that have fused-ring structures are used to replace conventional conductive polymer monomers（thiophene,pyrrole etc.）,and their N sites are coupled with naphthoquinone（NQ）and anthraquinone（AQ）units by Buchwald-Hartwig reaction,then these monomers are polymerized to obtain PDTP-NQ and PDTP-AQ,respectively.The interaction between electron donors（DTP units）and electron acceptors（NQ/AQ units）promotes the formation of narrowed energy level band gap,which improves the internal charge transfer and electronic conductivity.Moreover,poly DTP possesses stronger conjugation and higher structural rigidity,allowing PDTP-NQ and PDTP-AQ to keep electroactivity under high current density.PDTP-NQ and PDTP-AQ are assembled into an aqueous proton battery as cathode and anode,respectively.This all-organic aqueous proton battery exhibits specific capacity of 78 m Ah g-1 at current density of 0.5 A g-1,and maintain 50%of capacity retention when current density increases to 20 A g-1.Next,in order to further improve the energy density of organic aqueous battery,on the basis of poly DTP main polymeric chains,we adopt Buchwald-Hartwig reaction,oxidative polymerization,boron tribromide demethylation and electrochemical oxidation,respective,to couple para benzoquinone（p BQ）and ortho benzoquinone（o BQ）as redox pendant,which possess higher redox potential and theoretical specific capacities.Moreover,due to the low redox potential of zinc anode,the obtained PDp BQ and PDo BQ polymers are used as cathodes in aqueous zinc-ion battery to attain higher voltage output and higher energy density.As results,PDp BQ//Zn battery reveals high specific capacity of 120 m Ah g-1 at current density of 0.1 A g-1(corresponding energy density:139 Wh kg-1),and shows capacity retention of 79%after 500GCD cycles.Comparing with PDo BQ,PDp BQ only reacted with Zn2+in aqueous zinc-ion battery,currently show faster charge transfer and higher redox reversibility due to the excellent molecular planarity and narrower energy level band gap during the reaction process.Subsequently,in order to improve the rate performance of redox polymer cathode in aqueous zinc-ion battery,we optimize the selection of redox unit and polymeric form,and try to synthesize redox-site embedded conjugated polymers with extended conjugated systems.Specifically,We synthesize diquinoxalino[2,3-a:2’,3’-c]phenazine（HATN）and dipyrazino[2,3-f:2’,3’-h]quinoxaline（HAT）molecules with three pairs of ortho cyanogroups,respectively.Then,these cyanogroups are trimerized into triazine rings by melting catalysis of Zn Cl2 to obtain PHATN-T and PHAT-T polymers,which are assembled into aqueous zinc-ion batteries as cathodes.This multi-site cyanogroup trimerization endow PHATN-T and PHAT-T with high structural stability and strong conjugation,making them show fast reaction kinetics,excellent rate capability and cycle performance.PHATN-T//Zn and PHAT-T//Zn batteries attain 190 m Ah g-1 and 171 m Ah g-1,respectively at 0.3 A g-1,and keep 132 m Ah g-1 and 121 m Ah g-1 at 10 A g-1,even when the current density increases to 20 A g-1,PHATN-T//Zn and PHAT-T//Zn batteries still maintain 58%and 56%of the initial capacity,respectively.Besides,after 1000GCD cycles at 3 A g-1,PHATN-T//Zn and PHAT-T//Zn batteries show high capacity retention of 86.8%and 77.6%,respectively.Finally,in order to further increase the specific capacity output of redox conjugated polymer in aqueous zinc-ion batteries,on the basis of last work,we reduce the proportion of polymerized triazine rings and improve the molecular planarity.Specifically,diquinoxalino[2,3-a:2’,3’-c]phenazine（3Q）are still used as the redox unit,and it is coupled with three cyanogroups（3Q-CN）,then trimerized to obtain P3Q-t by melted Zn Cl2 catalysis.Meanwhile,the P3Q polymer with 3Q molecules as basic units is synthesized by solid-phase Maillard reaction.As results,P3Q-t//Zn battery releases high specific capacity of 237 m Ah g-1(corresponding energy density:155 Wh kg-1)at current density of 0.3 A g-1,and show capacity retention of 45%at 10 A g-1.It also maintains average capacity retention rate of 80%after 1500GCD cycles.Comparing to P3Q,P3Q-t has excellent molecular planarity,which is beneficial to the intermolecular charge transfer and the interfacial cation coordination,thus attaining higher specific capacity.In addition,with the increase of the number of captured zinc ions,the3Q unit shows narrowed energy level band gap and accelerated interfacial cation motion,which improve electrochemical performance when combined with the outstanding conjugation planarity of P3Q-t.In conclusion,based on the molecular difference,this thesis explores the influence of redox polymer as electrode material on the electrochemical performance of aqueous secondary batteries,which provides an important reference for the molecular design of polymer materials applied in aqueous energy storage and fulfills the blank in relevant subdivision field. |
