| With the continuous consumption of fossil energy,the development and utilization of efficient,clean and economic energy storage,Lithium-ion batteries(LIBs)own the most probable prospect as the alternate candidate in hybrid vehicles and portable electronics because of their high energy,the high power density and the superior cycling stability.Graphite,as the traditional anode,exhibits an excellent cycling stability,but its low theoretical capacity(372 mAh g-1)and layer spacing(0.335 nm)limit the application in high power batteries.Although the materials,such as Ge,Sb and Sn have higher theoretical capacities of 480,660 and 994 mAh g-1,respectively,the dramatic volume variation during the charge-discharge process that caused by the alloying reaction,leading to the inferior cycling stability and rate capability.ZnSe,as a superior semiconductor material,is widely used in the field of blue-green diode lasers,light-emitting diode lasers and tunable mid-IR laser sources.Due to its higher theoretical capacity of 557 mAh g-1,ZnSe can be considered as the alternate anode in LIBs.Metal-Organic Frameworks(MOFs)as the original and multifunctional materials,which possess the characteristics of high specific surface area,porosity and diverse structure,are widely used in the field of gas adsorption and desorption,catalytic application and electrode.In this work,developing cycle-stable and high capacity ZnSe-based anodes it's the research objective,this work takes ZnSe hollowsphere anodes,ZnSe/C composites and ZnSe/C nanocubes as the study objects.The electrochemical performance and the reaction theory of them as anodes in LIBs are also investigated.The main research contents are as follows.Firstly,the ZnSe hollowspheres were synthesized by using hydrothermal method.Adjusting the ratio of reactant.the reaction conditions as well as calcining the precursor in argon/dihydrogen,the uniform and conductive carbon frameworks were synthesized at the surface of ZnSe hollowspheres.In conclusion,the ZnSe/C electrode exhibits a superior discharge capacity of 573.5 mAh g-1 at a current density of 1.0 A g-1 between 0.01 and 3.0 V after 800 cycles.Even at the high current density of 10.0 A g-1,ZnSe/C electrode still shows the discharge capacity of 318.8 mAh g-1 after 5000 cycles,delivers an excellent cycling stability and rate capability.The electrochemical impedance spectroscopy certified that the enhanced electrochemical performances of ZnSe/C electrode is caused by the inducted carbon frameworks which may enhance the transfer of Li-ion and electron.Secondly,a series of ZIF-8 with different structure were synthesized through the chemical precipitation method with surfactant CTAB in solvent.In conclusion,the best molar ratio of 2-methyl imidazole and Zn(NO3)2 is 55.3/1,and the best precipitation time is 24 h.We confirmed that adjusting the content of surfactant CTAB and the time of precipitation can change the structure and size of ZIF-8.The structural stability of ZnSe/C composites from the calcination of ZIF-8 precursor mixed with Se powders in different calcined temperatures(450,500 and 550?)were also researched.After that,we also investigate the electrochemical performances of ZnSe/C composites as andoe electrodes in LIBs.After calcine at 450?,it exhibits the best electrochemical performances than others,inclouding a high initial columbic efficiency of 65.2%.Besides,the reversible capacity of it can achieve to 617 mAh g-1 after 500 cycles between the voltage ranges from 0.01 to 3.0 V at the current density of 1.0 A g-1.It is confirmed that the existence of faradaic pseudocapacitance in ZnSe/C composites provides additional capacity and leads to an increased behavior of capacity during cycling,which was investigated by changing the sweep speed from 0.1 to 10.0 mV s-1 in cyclic voltammetry test.Lastly,the N-doped ZnSe/C nanocubes were composited by two-step calcination process from the mixture of ZIF-8 nanocubes and Se powders.The XPS analysis showed the existence of C-N bond and N-N bond,which certified the N-doped carbon.The BET analysis showed that the N-doped ZnSe nanocubes calcined at 600?process a high specific surface area of 493.45 m2 g-1,containing a large number of micropores and a small amount of mesopores,which provides transfer channel and storage sites for Li-ion and electron that providing additional reversible capacity and improving its electrochemical properties.Morevoer,it also delivers a high initial discharge capacity of 1099 mAh g-1 and the initial columbic efficiency of 49.9%in LIBs,which can recover up to 597.8 mAh g-1 after 100 cycles at the current density of 0.1 Ag-1. |
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