Chemical Syntheses And Application Of Prussian Blue And Analogues As Positive Electrodes For High-Rate Aqueous Secondary Batteries

With the depletion of fossil fuel resources,the use and transformation of secondary energy has become the most important part of the sustainable development strategy.Electrochemical energy storage devices,such as secondary batteries,capacitors,etc.,are the most direct,cleanest,and most widely used carriers for efficient energy conversion,storage,and transportation.However,at present,the most important electrochemical energy storage devices,lithium batteries,face problems such as shortage of lithium resources,low lithium cyclic utilization,and some inherent defects of organic electrolyte such as low ion mobility,flammability and explosion.Therefore,using aqueous solution as an electrolyte and an aqueous secondary battery and Na+ as a charge carrier can solve the above problems.The ionic conductivity of the aqueous electrolyte is several orders of magnitude higher than that of the organic electrolyte,which can effectively reduce the internal resistance of batteries,and the higher specific heat capacity of the water also improves the safety.The distribution of sodium metal resources is wider than that of lithium,and the use of Na+ as the deintercalation ion can effectively reduce the cost of batteries.Therefore,to explore a material that can be stably and quickly de-intercalated with sodium ions is a perfect way to actually push the aqueous sodium ion battery to a practical stage.Electrode materials of aqueous batteries have very few choices to choose from because the electrochemical window of water is very narrow and many materials are not stable in water.Due to its unique three-dimensional structure,Prussian blue and its analogues allow ions to be de-intercalated from three axes,which can ensure the rapid reaction kinetic of electrochemical reaction.At the same time,Prussian blue material has attracted many researchers’ attention because of its simple synthesis steps,fast synthesis rate,high yield,and easy control of morphology.However,High crystallization rate is also accompanied by high defect concentration,and the electronic conductivity of Prussian blue itself is not high as a result.Therefore,the collapse of the material structure is very likely to occur at high current density,resulting in serious capacity attenuation.But previous work always use water as the reaction solvent,the presence of water usally causes the Fe(CN)6 site occupied by water molecules during the synthesis of Prussian blue,resulting in a large amount of water-occupied defects in the structure,which ultimately reduces the electrochemical stability of the material.In this paper,Prussian blue analogues(NiHCF,CoHCF,CuHCF)were used as research objects to investigate the relationship of material defect concentration and the existence of ligand during synthesis process,and discuss the positive effect of low defect concentration and carbon materials for Prussian blue materials.Meanwhile,we also changed the synthetic environment of Prussian blue,removed the contribution of water molecules to the formation of defects during the synthesis process,explored the morphology changes and electrochemical properties of the products under different synthetic environments.The details are as follows:(1)An NiHCF covered by carbon nanopaticle nanocomposite(eNiHCF/C)is successfully synthesized using a simple process at room temperature,using en molecules as a ligand.It is believed that en molecules could effectively reduce the concentration of free Ni2+ ions,thus benefiting the surface deposition on carbon powders and inhibiting the formation of structure defects.The results greatly enhance the charge-transfer kinetics and the reversibility of Na+ intercalation/deintercalation.With Pt as the counter electrode,eNiHCF/C shows a capacity retention of 94%after 900 cycles at 0.5 A g-1.At 2 A g-1,the capacity could be kept at 93%of that at 0.1 A g-1.Similar results are also obtained in eNiHCF/C//rGo,where the good match between them makes the stability still astonishing.This device could keep 94%of the initial capacity after 5000 cycles at 2 A g-1,or almost 100%of that at 0.1 A g-1 when tested at 5Ag-1.(2)we successfully prepared truncated CoHCF nanocubes threaded by carbon nanotubes(CoHCF-Cit/CNT),where citrate and glycol greatly reduce the reaction rate and lower the structure vacancies in CoHCF.The unique structure and reduced structure vacancies effectively enhance the electrochemical kinetics and activate the redox couple of FeⅡ/FeⅢ in CoHCF,leading to a high-capacity and fast-rate cathode material for aqueous sodium ion batteries.This composite could reach a capacity of 107 2 mAh g-1 at 0.1 A g-1.Even at 5.0 A g-1,87.3%of the capacity at 0.1 A g-1 was kept in rate capability.After 400 cycles at 0.5 A g-1,there is still a capacity of 78 mAh g-1.CoHCF-Cit/CNT was coupled with Zn to build a dual-ion aqueous rechargeable battery.This full cell delivers a capacity of 92.9 mAh g-1,corresponding to an energy density of 145.9 Wh kg-1.At 7.87 kW kg-1,the energy density is still 107.1 Wh kg-1 The easy preparation and excellent rate capability makes CoHCF-Cit/CNT promising as a cathode material for aqueous batteries.(3)A kind of CuHCF material with low water content has been prepared by using the alcohol heating method.Non-aqueous environment greatly reduces the possibility of introducing water molecules into the material structure.Meanwhile,there are some pores at the surface of the materials,which can increase the contact area of the material with the electrolyte,improve the electrochemical performance of the material.As a result,CuHCF-A is a perfect cathode material of aqueous sodium ion battery with high rate performance.The composite has a capacity of 66.1 mAh g-1 at 0.1 A g-1.Even at 5.0 A g-1,the capacity retention rate still remains 71.6%.After 1000 cycles at 0.5 A g-1,it still had a capacity of 41.4 mAh g-1 and a capacity retention of 74%.An aqueous Na-Zn dual ion secondary battery was constructed by using CuHCF-A with a Zn negative electrode.The full battery can provide a capacity of 62.8 mAh g-1 on behalf of 88.3 Wh kg-1 energy density.At the same time,we also used a salt bridge to isolate the positive and negative electrolytes,and tested the cycle performance of full cell.After 600 cycles,the capacity retention rate was still 71.2%,reflecting high stability.