| Benefiting from the inexhaustible natural sources and the similar physical and chemical properties of lithium,sodium-ion batteries(SIBs)have been regarded as the prime candidates to replace lithium-ion batteries(LIBs).The past few decades have witnessed the tremendous development of anode technology,and now has high capacities.Nevertheless,the performance capabilities of the corresponding metal oxide as the cathode materials are much worse,and the practical capacities is only 100–150 mAh/g.Since the anode and cathode capacities are usually matched in actual batteries,the full advantages of higher capacity currently available in the anode are unachievable.The most straight forward solution to these problems is to use organic materials,more precisely redox-active polymers,as cathodes.In recent years,organic carbonyl compounds have been studied intensively as the most promising class of organic cathode material,given the reversible redox reactions between Na atoms and carbonyl groups in the organic electrodes.Quinones and their derivates were also applied as active cathode material due to their comparably high redox potential and theoretical capacity.However,it suffers from low practical capacity because of quinone compound dissolution in common electrolyte.It indicates polymer quinone cathodes have not yet reached capacities similar to high performance monomers restricted by the proportion of quinone groups.Thus,we provide quinone-rich polydopamine cathode active material by rational manipulation of oxidant and conductive substrate to improve the electrochemical performance of polydopamine for charge storage applications.The research contents were illustrated in detail as follows:(1)The polydopamine coatings were synthesized by a spontaneous polymerization and coating process of dopamine on different carbon substrates.Due to the lower specific surface area of the reduced graphite oxide and carbon nanotubes,in addition to forming a polydopamine coating on the substrate,isolated polydopamine particles were also formed in the solution by the self-nucleation reaction.The rich pore structure of the graphene/carbon nanotube composite material provides a relatively high specific surface area for polydopamine coating and obtaining a uniform polydopamine coating.(2)The polydopamine coatings were synthesized via using different oxidants induced polymerization and coating process of dopamine on graphene/carbon nanotube composites.The content of quinone functional groups in polydopamine coatings induced by ammonium persulfate is much higher than that of polydopamine coatings formed in oxygen,and the prepared polydopamine coatings are more uniformly distributed on the substrate and the polymerization is more thorough.(3)The cathode materials of sodium ion batteries were assembled into button batteries for analysis of electrochemical performance.As is expected,the as-prepared cathode of G/CNT-PDA-APS delivered an ultra-high specific capacity of 322 mAh/g at 0.1 A/g in NIBs due to the quinone-rich polydopamine.Even at a very high rate of 10 A/g,a considerable capacity of 102 mAh/g can still be remained,much higher than those of previously reported values for organic and polymer-based electrodes.These electrodes also showed excellent rate-performance owing to their unique graphene/carbon nanotube composite structure which can reinforce the structural stability of the electrode and realize ultrafast surface redox reactions of polydopamine.Moreover,the as-prepared cathode exhibits an excellent cycling stability up to 500 cycles.In addition,the energy storage mechanism of this cathode material was analyzed and explained in detail through DFT calculation and ex-situ FT-IR and XPS.Our results not only give a better understanding of Na~+storage mechanisms in polydopamine cathodes but also facilitate the development of high-performance organic sodium-ion electrode materials. |
PDF Full Text Request