Synthesis And Electrochemical Properties Of Na_xFe[Fe（CN）₆]（1<x<2） As Cathode Material For Aqueous Sodium-ion Batteries

In recent years,sodium-ion batteries have become a new-generation potential hot topic in energy storage area,because of the same energy-storage mechanism as lithium-ion batteries,abundant reverses on the earth and low cost.Compared with the organic electrolytes,aqueous electrolytes have attracted more interests with the advantages of safety,economy,faster ions diffusion kinetics and more flexible operation.At present,electrode materials,play an important role for developing aqueous sodium-ion batteries（ASIBs）,especially cathode materials.Prussian blue analogues（PBAs）materials are considered as a promising candidate for ASIBs because of the open three-dimensional frame structure,high theoretical capacity,facile synthesis process and low cost.However,the decomposition of crystal water,low electric conductivity and poor cyclic stability restrain the large-scale application for PBAs materials.This paper is aimed to improve the electrochemical performance for PBAs materials in ASIBs through synthesis-method optimization,modification with doped Ni²⁺and multiwall carbon nanotubes（MWCNTs）.（1）Na_xFe[Fe（CN）₆]nanocubes with the uniform size of about 50 nm were successfully synthesized by sodium citrate assisted coprecipitation method.The morphology and sodium-ion storage properties in 1 M NaNO₃ electrolyte were compared with and without sodium citrate.The results indicated that the addition of sodium citrate can effectively reduce nucleation rate,restrain defects and finally enhance electrochemical properties.It showed 124.9 mAh g^-1 first discharge capacity at the current density of 100 mA g^-1 and 65 mAh g^-11 after 300 cycles at the current density of 200 mA g^-1.（2）Ni²⁺was doped into Na_xFe[Fe（CN）₆]by sodium citrate-assisted coprecipitation method to form Na_xNi_y Fe[Fe（CN）₆]（y=0.15,0.20,0.23,0.25）with different Ni²⁺contents and the influence between different Ni²⁺contents and electrochemical performance.According to XRD,SEM,TEM,FT-IR and electrochemical tests,we found that Na_xNi_y Fe[Fe（CN）₆]with 23%Ni²⁺showed the best electrochemical properties.It exhibited 105.9 mAh g^-1 specific capacity at the current density of 200 mA g^-1,55.5 mAh g^-1 specific capacity at a high current density of 2000 mA g^-1 and 73.1%capacity retention after 1000cycles at the current density of 1000 mA g^-1.The introduction of Ni²⁺displayed as a skeletal supporter,which restrained the collapse of crystalline structure and improved the cyclic stability and rate performance for Na_xNi_y Fe[Fe（CN）₆].（3）Na_xFe[Fe（CN）₆]/MWCNTs composite was synthesized by in situ growth method.By XRD,SEM and electrochemical tests,the composite exhibited a good electrochemical performance in ASIBs.It provided 114.5 mAh g^-1 specific capacity at the current density of 1000 mA g^-1 with stable capacity of 88.3 mAh g^-1 after 1000 cycles and a good specific capcity of 83.8 mAh g^-1 at a high current density of 3000 mA g^-1.The addition of MWCNTs strengthened electric conductivity and enhanced the fast transportation of sodium ions and electrons in electrode material.Meanwhile,it effectively restrained the side resctions in electrolytes and improved the rate performance and cyclic stability.