Micro/nano Regulation And Electrochemical Properties Of Carbon-based Metal Selenides Anodes For Sodium Ion Batteries

The rapid development of renewable energy devices has stimulated the pursuit of electrochemical energy storage devices with high-cost performance and high performance.As a new generation of green energy devices,lithium-ion batteries have been widely used in mobile phones,computers,electric vehicles and other fields,but the lack of lithium resources seriously limits the long-term development of lithium-ion batteries.Because sodium is abundant,low cost and works similarly,sodium-ion batteries have become a good alternative to lithium-ion batteries among many promising technologies.However,the energy density and cycling stability of sodium-ion batteries are relatively poor compared to lithium-ion batteries,mainly due to the large volume strain created by the insertion of larger sodium ions during the sodiation/desodiation processes.This phenomenon is more prominent in the conversion reaction type electrode.These serious problems encourage us to find suitable high energy density,long-term stability of cathode materials,and to carry out reasonable structural design.Transition metal selenides,with their easily controlled morphology and high theoretical specific capacity,make them also popular for sodium ion battery applications and very competitive with other negative electrode materials.However,the capacity of transition metal selenides is seriously affected by the structural damage caused by the huge volume change in the process of charge and discharge and the low electron（ion）transport efficiency caused by low conductivity and low ion diffusion coefficient.Therefore,it has become a hot topic in the research of transition metal selenide anode materials for sodium ion batteries to conduct reasonable structural regulation and interface design.In this paper,Cu₂Se nanoarray electrodes with different degrees of openness and Co Se₂@NC electrode materials with different degrees of load were prepared through the construction of nanoarray,material morphology regulation,porous carbon modification and other strategies.Meanwhile,the basic morphology,basic structure and electrochemical properties of Cu₂Se nanoarray electrodes were characterized,and the mechanism of sodium storage was studied.The results are as follows:（1）Cu₂Se with three-dimensional array structure was prepared by liquid phase selenization method on the basis of copper foam matrix.Cu₂Se electrodes with different morphology and porosity can be obtained by prepressing copper foamed precursor under different pressures,which provides a good opportunity for us to study the effect of array morphology on the performance of electrode for storing sodium.The results show that,when the pressure is controlled at 1 MPa,the petal-like array structure with proper opening Angle and uniform distribution can be grown on the copper foam matrix,and there are linear defects in the Cu₂Se lamellar.Surprisingly,the Cu₂Se nanoarray as sodium ion battery anode has a high capacity of679 m Ah/g and significant rate capability:85%capacity retention when current density is increased from 0.1 A/g to 2 A/g.Specifically,after 600 cycles at A high current of 5 A/g,the volume is still as high as 368 m Ah/g.Finally,the assembled Cu₂Se-1//Na₃V₂（PO₄）₃ full battery demonstrated A stable storage capacity of 270 m Ah/g at 0.1 A/g.This provides a good example for improving the performance of electrode materials through array morphology control.（2）Using NaCl as a template,glucose as a carbon source and urea as a nitrogen source,a three-dimensional nitrogen-doped carbon network was prepared and used as a highly conductive substrate.Co Se₂ was uniformly anchored to the carbon network by a simple low temperature impregnation method,and the effect of microstructure on the electrochemical performance of the electrode was studied.The interfacial interaction of C-O-Co bonds between Co Se₂ nanoparticles prepared by ice bath impregnation method and the carbon network was observed,which firmly and orderly fixed Co Se₂ to the carbon conductive network（Co Se₂@NC）.Due to the uniform dispersion of nanocrystals and the high specific surface area of NC conductive network,the contact stress between nanoparticles is effectively reduced and the ion/electron diffusion channel is shortened.In addition,excellent anchoring structure enables Co Se₂@NC to form a stable SEI film,reduce the amount of dead sodium and show high ICE（89.2%）.Finally,simple impregnation methods for highly conductive substrates are not limited to the type of MOF,do not require the introduction of additional carbon layers,can effectively control the load of transition metal selenides on the carbon substrate,and can also be controlled by changing the impregnation temperature to control particle size.This provides a general and effective method for carbon modification of other transition metal selenides.