Investigation On The Electrochemical Properties And Reaction Mechanisms Of Bimetallic Selenides

With the increasing attention to energy and environmental issues,various new technologies for energy storage have emerged.Among them,lithium-ion batteries(LIBs)are regarded as the most promising energy storage technology due to its high energy density and environmental friendliness.However,the shortage of lithium resources severely limit the ever-growing demand of modern society.Metal sodium and potassium,which in the same main group as metal lithium,are abundant in the earth’s crust and exhibit the same physiochemical properites.Therefore,sodium ion batteries(SIBs)and potassium ion batteries(PIBs)have been attracted the numerous attentions as the alternates of LIBs.In recent years,transition metal selenides have stood out from the anode materials due to their high theoretical specific capacities,good electrical conductivity,and high electrochemical activity.However,the large volume change during the repeated charge and discharge processes leads to the structural collapse,loosing connection with the current collector,and thus resulting in the deterioration of the electrochemical performance.Therefore,it is urgent to find suitable nanostructures to serve as the anode materials,which can accommodate the large volume variation caused by the insertion/extraction of sodium/potassium ions.This thesis mainly focuses on the synthesis and properties of the high-performance electrode materials through reasonable structure designs,and the main contents are as follows:(1)A hierarchical material of CoSe2-MoSe2 tubes anchored on rGO network(CoSe2-MoSe2/rGO)was designed and prepared as the SIBs and PIBs anode material via a hydrothermal approach.Benefiting from the synergistic effects between CoSe2 and MoSe2,unique hierarchical structure,and effective reduced graphene oxide coating,the CoSe2-MoSe2/rGO exhibited improved reaction kinetics and structural stability,and thus good electrochemical properties.As the anode material for SIBs,CoSe2MoSe2/rGO exhibited a capacity value of 533.5 mA h g-1 at the current density of 0.1 A g-1 after the 200th cycle.For PIBs,a high discharge capacity of 498.4 mA h g-1 was retained at the current density of 0.1 A g-1 after 60 cycles.In addition,a combination mechanism of intercalation and conversion of CoSe2-MoSe2/rGO by forming NaxCoSe2 and Mo15Se19 as intermediate states was put forward on the basis of in-situ and ex-situ XRD analyses.(2)The heterostructured CoSe2-Cu2Se nanospheres covered by nitrogen-doped carbon(NC)layer(CoSe2-Cu2Se@NC)was fabricated using a facile hydrothermal method followed by an annealing process.The as obtained CoSe2-Cu2Se@NC composite featured unique heterogeneous structure,uniform carbon coating,large specific surface area and synergistic effects between CoSe2 and Cu2Se,which could ultimately accelerate charge transfer,restrain the volume expansion resulting from the ion intercalation,increase active sites,and enhance the electrochemical reaction activity of the CoSe2-Cu2Se@NC composite as an anode material for SIBs.As expected,CoSe2Cu2Se@NC nanospheres displayed outstanding capacity and durability.A high capacity value of 759.6 mA h g-1 was maintained after 500 cycles at the current density of 2 A g-1.And at the high current density of 20 A g-1,the CoSe2-Cu2Se@NC delivered a high specific capacity of 367 mA h g-1 after 2000 cycles.In addition,the reaction mechanism of CoSe2-Cu2Se@NC was explored by in-situ and ex-situ XRD analyses.