Aqueous Rechargeable Zinc Batteries Based On Small Molecular Organic Cathode Materials

With the mushroom growth of electric vehicles and smart power grids,traditional lithium ion batteries are facing many challenges including limited resources,high costs and unsatisfactory safety.On the other hand,aqueous rechargeable zinc batteries（ARZBs）,which combine aqueous electrolytes and zinc anodes,have been attracting extensive attention in recent years because of their inherent safety,low costs,ease of fabrication,and environmental friendliness.Besides,organic electrode materials have been viewed as promising candidates for the next generation of sustainable and versatile energy storage devices considering their merits in terms of competitive performance,structural diversity,and potential resource sustainability.Up to now,a variety of organic electrode materials for ARZBs have been reported,but there are still many challenges such as unaffordable synthesis,unsatisfactory performance,and unclear mechanisms in this field.To promote the practical application of aqueous zinc-organic batteries,it is still necessary to explore for better materials and gain deeper insights into the mechanisms as well.Herein,we report two small organic molecules,phenazine（PNZ）and 5,7,12,14-tetraaza-6,13-pentacenequinone（TAPQ）as readily-available and high-performance organic cathode materials for ARZBs.Meanwhile,their underlying energy storage mechanisms were also systematically investigated by a series of ex-situ characterizations,electrochemical tests,and DFT calculations.PNZ has been used as a raw material in drug and dye industry for a long time and thus can be commercially purchased.In the common Zn SO₄ electrolyte,PNZ cathode could achieve a high specific capacity of 289 m Ah g^–1（97%of the theoretical value）,a high energy density of 205 Wh kg^–1(289 m Ah g^–1×0.71 V),the good rate performance(185 m Ah g^–1 at 5000 m A g^–1),and the stable cycle performance(73.3%after 3000cycles at 5000 m A g^–1).Besides,in contrast to the intuitive view of Zn²⁺insertion,the complete H⁺insertion mechanism of PNZ cathode was also confirmed by the above methods.To obtain higher capacity and better durability,we designed the TAPQ cathode with a novel quinone/pyrazine hybrid structure.It was easily synthesized from relatively low-cost 2,5-dihydroxy-1,4-benzoquinone（DHBQ）and o-phenylenediamine（o PDA）through a two-step route under mild conditions.Benefitting from the multiple electroactive C=O and C=N bonds,TAPQ possessed a theoretical capacity of 515 m Ah g^–1（based on a six-electron reaction）and a practically reversible capacity of 443 m Ah g^–1 within 0.1–1.6 V vs.Zn²⁺/Zn,both of which set new records for organic cathodes in ARZBs.The H⁺/Zn²⁺co-insertion mechanism involving H⁺as the predominant participant was confirmed by the above methods as well.Based on the clear mechanism understanding,a modified voltage range of 0.5–1.6 V was adopted to simultaneously achieve a high energy density(270 m Ah g^–1×0.84 V=227 Wh kg^–1)and the excellent cycling stability(capacity retention of 92%after 250 cycles under 50 m A g^–1,with an average Coulombic efficiency of 99.96%).Furthermore,the evolution mechanism of TAPQ electrode structure during cycling was also carefully studied to reveal the origin of capacity decline.We believe the above easily-synthesized and high-performance materials and the deep understanding of their energy storage mechanisms can provide researchers important insights into the further development of aqueous zinc-organic batteries toward practical applications.