The Synthesis Of Copper And Cobalt Based Selenides And Their Electrochemical Properties

Lithium ion battery is one of the most applied mobile energy device.While the commercial lithium ion battery anode material,graphite,shows a low theoretical capacity.Transition metal selenides are one of the potential anode materials for lithium ion battery.Transition metal selenides not only have high theoretical capacities,but sometimes present a higher conductivity.Therefore,the rate properties of transition metal selenides present more superior.However,transition metal selenides also have some degree of volume expansion during the lithiation and delithiation process.In this paper,copper selenide and cobalt selenide are selected as the research target,and the performance of lithium ion battery is improved by constructing special grading structure.Moreover,the reasons of improved stability of the cycle performance are carefully investigated.The main contents of this paper are as follows:Self-assembled hierarchical Cu_2-x-x Se nanosheets were prepared via solvothermal process with the mixed solvent of EG and DI water.According to the time-dependent experiments,the growth mechanism of Cu_2-x-x Se nanosheets was investigated.Moreover,the hierarchical Cu_2-x-x Se nanosheets presents good rate performance.The cycling performance is still poor,after 10 cycles at 50 m A g^-1,the capacity quickly decades to about 150 m A h g^-1.Moreover,flower-like hierarchical Cu_2-x-x Se was prepared with sodium tartrate as surfactant.This flower-like Cu_2-xSe demonstrated enhanced cycling stability.After 200 cycles at 1 A g^-1,about 199 m A h g^-1can be obtained.The superior performance of flower-like hierarchical Cu_2-x-x Se can be attributed to its unique morphology.Firstly,the distance between the nanosheets which are the building blocks of flower-like Cu_2-xSe are quite changeless.Thus,the nanosheets will not easily stack to each other and the electrodes will not easily break during charging and discharging process.At the same time,the pores between the nanosheets are benefit for the contact between electrolytes and electrodes.Moreover,the transport path of lithium ion can be shorten.Sphere ZIF template was prepared via solvothermal method using citric acid as surfactant and methanol as solvent.After annealing under Ar environment,hollow sphere Zn Co₂O₄ was obtained.This hollow sphere presented good lithium ion battery performance,after cycling for 400 cycles at a current density of 1 A g^-1,the capacity of 511 m A h g^-1can be reached.After annealed with Se powder,sphere Zn doped Co Se₂ was obtained.After charging and discharging for 500 cycles at 1 A g^-1,the cycling capacity of Zn doped Co Se₂ can reach 335 m A h g^-1,about 91.8%of the reversible capacity.The hollow structure is beneficial to shorten the diffusion path of lithium ion.The hollow sphere morphology can also suffer the volume changes during charging and discharging.Therefore,hollow sphere Zn Co₂O₄ presents good cycling stability.Cu doped ZIF materials have been prepared via a solvothermal route using methanol as solvent without surfactant.Moreover,porous Cu doped Co Se₂ samples composed with nanoparticles were prepared by using Cu doped ZIF as template.The everage diameter of nanoparticles is about 80 nm.Pores was formed after the generation of crosslinking structure.The everage pore size is around 36 nm.Over1000 m A h g^-1can be obtained after 450 cycles at 1 A g^-1.Even improved current density to 5 A g^-1,the capacity of 747 m A h g^-1can still maintain.Electrochemical analysis demonstrations that the porous structure has a more obvious pseudocapacitance effect.Surface pseudocapacitors can hold up to 84%of the total capacity.The pseudocapacitance is also an important reason for the excellent performance of the porous structure.By carving the electrodes after cycling tests,better mechanical properties of the porous Cu doped Co Se₂ can be proved.The outstanding rate and cycling performances are ascribed to the porous morphology.Firstly,the network structure formed by the interconnection of nanoparticles can effectively form the transmission path of the electron and the transmission of electrons can be facilitated.Secondly,the porous structure can provide abundant surface area,which can store more lithium ion.Moreover,the contact between electrode and electrolyte can be also improved.The rate performance can be enhanced by shortening the diffusion path of lithium ion.Finally,the porous structure can efficiently accommodate the volume changes during charging and discharging.Moreover,the porous structure can be kept.