Synthesis And Electrochemical Properties Of An Anthraquinone Derivative As Cathode For Na-ion Batteries

Sodium-ion（Na-ion）batteries（SIBs）have attracted a great deal of attention because sodium is high content in the earth’s crust and low cost.Recently,SIBs based on inorganic electrodes with（de）intercalation mechanism have achieved advancements.However,advanced inorganic electrodes in particular with cathodes have been mainly limited by the large-radius Na ion（1.02（?）vs.Li:0.76（?））.Alternatively,redox-active organic electrodes are capable of storing Na⁺reversibly for their intrinsic large void space.Small-molecule organic electrode materials are generally easily dissolved in liquid electrolytes.This paper has studied a new insoluble organic cathode material in SIBs,namely sodium2,6-dihydroxyanthraquinone（AQ26ONa）with a theoretical capacity of 189 m Ah g^-1.There are two strong O-Na bonds within the anthraquinone skeleton and thus the organic framework exhibits a-2 valency.AQ26ONa can be called as a polyanionic small-molecule organic compound.In this paper,the first part was that the synthesis of AQ26ONa was introduced in detail and AQ26ONa cathode was fully characterized.Organic electrolytes are electrically neutral.On the basis of the“like dissolves like”principle,the polyanionic AQ26ONa is hardly soluble in ether-electrolytes.Afterwards,in this paper,the effects of different binders,conductive additives and electrolytes on the performance of Na-ion half cells are studied.The experiment results show that it is helpful to use PVDF as the binder and use THF as the dispersing agent.It can prevent the electrode material from dissolving and then precipitating out in the grinding process.Using Ketjen Black with CNTs（2:1）as conductive additives can improve the capacity and the stability of batteries effectively.The Na-ion half cells exhibited the best performance in the electrolyte system of 1 M Na PF₆/DME.Under these optimal conditions,the AQ26ONa cathode delivers a discharge capacity of 240 m Ah g^-1at 50 m A g^-1 in the potential range of 3.3 V-0.2 V.It basically reaches the theoretical specific capacity with removing the capacity from conductive additives(52 m Ah g^-1).Because the AQ26ONa electrode is insoluble in DME,the battery shows a highly stable long cycle performance.After 1000 cycles,the long-lifespan AQ26ONa electrode releases a reversible average capacity of 199 m Ah g^-1 while the capacity retention ratio is63%for 6 months at least.At a high current(500 m A g^-1),the average specific capacity is 128 m Ah g^-1 after 2500cycles with the capacity retention rate of 66%.Finally,rocking-chair full organic Na-ion cells have been configurated which AQ26ONa is used to be the cathode while a reduced state（Na4TP）of sodium terephthalate is used to be the anode.Notably,1 M Na PF₆/DME is still the optimized electrolyte in AQ26ONa||Na4TP full cells.In the working voltage window of 0.01–3.0 V,the full cells can reach a reversible average discharge capacity of 150 m Ah g^-1 at 50 m A g^-1 for 50 cycles and an average capacity of 66 m Ah g^-1 at 500 m A g^-1 for 1000 cycles.Presently,the combination property of AQ26ONa||Na4TP full cells achieves one of the best rocking-chair full organic Na-ion cells.