Synthesis And Electrochemical Properties Of Se/C As Cathode Material For Novel Secondary Batteries

The selenium cathode material has been attracted considerable attention due to the high volume specific capacity(3253 mAh cm^-3).The lithium selenium battery with lithium foil as the anode electrode has high energy density?3-5?times of the traditional lithium-ion battery),low cost and friendly environment.However,it is still face many obstacles,such as rapid capacity decline,poor cycle stability and low coulombic efficiency due to the dissolution and shuttling effect of polyselenide,and the volume expansion of the electrode during the discharge/charge process.In order to solve these problems,the selenium cathode material needs to be modified.In this paper,selenium/carbon composite cathodes are obtained by combining the hierarchical porous carbon and heteroatom doped carbon with the active materials of selenium,respectively.And the electrochemical prosperities of modified lithium selenium battery are further studied.The detailed research contents are shown below:Firstly,Zn and Mg powder were fully ground and mixed at a certain mass ratio.Then,CO₂ was calcined and reduced to C by metal thermal reduction reaction under a high purity CO₂ atmosphere.Next,the hierarchical porous carbon?HPC?was obtained by further washing off the oxide impurities,which has excellent specific surface area.Finally,Se/HPC composite cathode was prepared by combining HPC and Se with the method of melt-diffusion.XRD,Raman,SEM and HRTEM tests have shown that Se is uniformly distributed in the pores of HPC,which can effectively prevent side reactions between Se and carbonate-based electrolyte,and ensure the stability of electrochemical performance.The electrochemical performance test of Se/HPC composite cathode showed that the initial charge capacity of 509 mAh g^-11 is achieved at a current density of 0.5 C,and maintains a high charge specific capacity of 416 mAh g^-11 after 300 cycles with a capacity retention of 82%,and the average capacity decay is only 0.061%per cycle.Even at a high current density of 10 C,the Se/HPC composite cathode still can deliver a high reversible capacity of 270 mAh g^-1.The excellent cycle reversibility and rate performance of the Se/HPC composite cathode are determined by the large specific surface area of the HPC and hierarchical porous structure,which greatly improve the whole electrochemical performance of the battery system.Then,the N-doped 3D nano-carbon flower?3D-NDCN?was also prepared by using SiO₂ as template and N/C source.Se₅₀/3D-NDCN composite cathode material was obtained after Se melting diffusion into carbon flower.XRD,SEM and Raman analysis indicate that Se uniformly disperses in the pores of 3D-NDCN without obvious agglomeration.The XPS survey spectrum confirms the abundant presence of the N-containing functional groups in 3D-NDCN,which has chemical adsorption to polyselenide.The electrochemical analysis presents that at a current density of 0.5 C,the initial charge capacity of 494 mAh g^-11 is achieved.After 100 cycles,a reversible capacity of 454 mAh g^-11 is still maintained with a capacity retention of 91.7%.And a high reversible capacity of 270 mAh g^-11 is obtained,even at a high current density of10 C.As demonstrated above,the 3D-NDCN can promote electrolyte infiltration,supply facile transport channels for Li⁺and increase electron conductivity due to the interconnected 3D network structure with rich mesopores.Meanwhile,the heteroatom N can increase active sites.The dual functional synergic effect of physical and chemical adsorption is used to immobilize Se,which effectively improves the electrochemical performance of the Li-Se battery.In conclusion,the electrochemical performance of Li-Se batteries is improved caused by the carbon and Se composite cathode with different structural characteristics,which can effectively inhibit the shuttling effect of polyselenide during discharge/charge process and improve battery kinetic performance.The above experimental results can provide a useful theoretical and experimental basis for the future research work of cathode materials for new secondary Li-Se batteries.