Preparation And Electrochemical Performance Of Transition Metal Selenide Based Composites

Due to the advantages of low cost,abundant storage,and high specific capacity,sodium ion batteries have gradually replaced lithium ion batteries and become a hot secondary battery system.However,due to the larger radius of sodium ions,it is easy to cause serious damage to the material structure in the sodiation/disodiation process,resulting in the decline of cyclic performance.Metal selenides have become a potential alternative material for the new generation of sodium ion battery anode due to their large layer spacing and high theoretical capacity.However,the huge volume expansion of metal selenides during the process of cycling leads to poor cycle stability,which limits its development.In view of these problems,different kinds of metal selenides composite materials were prepared by doping heteroatom,compounding with carbon materials and changing the spatial structure.Meanwhile,their electrochemical properties were measured.(1)Three-dimensional structure of crosslinked graphene can effectively alleviate the volume expansion of metal selenides in the circulation process and enhance their electronic conductivity.Zinc selenide/three-dimensional graphene composites(denoted as ZSG)were synthesized by a one-step hydrothermal method.As-obtained ZnSe nanoparticles with a diameter of about 50 nm were uniformly dispersed on the graphene sheet.When the graphene addition was adjusted to 2 mg mL-1(denoted as ZSG-2),the obtained ZSG-2 had the best performance.The capacity could remain 276.6 mA h g-1 after 100 cycles at current density of 100 mA g-1.Even under the large current density of 10 A g-1,the capacity still maintained 119.4 mA h g-1.The good electrochemical Na storage performance of ZSG-2 benefits from its high specific surface area and enhanced electronic conductivity,which can effectively relieve the volume expansion of the composite.(2)In order to further enhance the cyclic stability of metal selenide materials,zinc selenide quantum dots/three-dimensional graphene composites(denoted as ZSQG)were further synthesized by using a low temperature water bath method.Their sodium storage performance was studied.Based on the graphene support,quantum dots can further increase the specific surface area of materials and the contact area with electrolyte.When the amount of graphene is 1 mg mL-1(denoted as ZSQG-1),the composite reached the most stable performance,which remained 208.5 mA h g-1 after 200 cycles at a current density of 100 mA g-1,and still maintained 148.6 mA h g-1 after 450 cycles at a current density of 500 mA g-1,showing preferable rate performance.(3)Metal organic framework(MOF)materials with large specific surface area and rich mesoporous structure have significant advantages in sodium ion battery anode materials.This section used Zn(NO3)2·6H2O and 2-methylimidazole as raw materials and CTAB as a blocking agent,respectively.ZIF-8 cubic was synthesized by a simple hydrothermal method.Hydrothermal duration and the selenization method were both adjusted to better control the structural stability of the material.Finally,the final product denoted as ZnSe/HNC with a nitrogen-doped hollow cubic structure was obtained.When the hydrothermal duration was 10 h,the composite material had a capacity of 335.7 mA h g-1 after 50 cycles at 100 mA g-1.When the current density increased to 10 A g-1,it still maintained a stable reversible capacity of 115.4 mA h g-1,and had good stability at a large current density.(4)Although the traditional single metal selenides electrodes already have good rate performance,cyclic performance still needs to be improved.By introducing another metal into metal selenides,the bimetal selenides can provide richer redox reactions.On the basis of ZIF-8,by adding Co(NO3)2·6H2O as raw material,ZIF-8/67 dodecahedron precursor was prepared and the morphology was adjusted by surfactant,the final product labeled as ZnSe/CoSe@NPC composite was obtained through a selenization process.As an anode material for sodium ion batteries,when the dosage of surfactant is 0.8 g,after 200 cycles under 100 mA g-1,the capacity of the composite could still maintain 417.6 mA h g-1.Meanwhile,the composite had excellent rate performance.It still maintained 236.4 mA h g-1 under a current density of 10 A g-1.Furthermore,it retained a reversible capacity of 303.9 mA h g-1 after 900 cycles at 1 A g-1.As anode materials of lithium ion battery,it delivered the high reversible capacity of 1213.4 mA h g-1 after 80 cycles.(5)So as to further improve the electroconductivity and the initial coulombic efficiency of the material,a hollow NiCoSe2@C composite material was obtained through the improved one-step hydrothermal method and subsequent dopamine-derived nitrogen-doped carbon coating process and used as the anode material for sodium ion batteries for the first time.Under a current density of 100 mA g-1,the reversible capacity still maintained at 464.7 mA h g-1 even after 200 cycles,and the initial coulombic efficiency could be increased to 79.4%,showing good electrochemical sodium storage performance.Meanwhile,the rate performance of the hollow NiCoSe2@C composite was fairly good.At a current density of 5 A g-1,a reversible capacity of 337.5 mA h g-1 could be achieved.Furthermore,the electrode showed a high and stable discharge capacity of 338.3 mA h g-1 at a relatively high current density of 500 mA g-1 after 250 cycles.When used in lithium ion battery,it could still provide a reversible capacity of 1344.1 mA h g-1 after 100 cycles.In addition,the kinetic analysis of the electrochemical Na/Li+storage performance of the hollow NiCoSe2@C composite shows that the pseudocapacitive contributed to achieve excellent rate performance and good long-term cycle life.