The Micro-nano Structural Design And Lithium/Sodium Ion Storage Mechanisms Of Metal Selenides Composites

Lithium-ion batteries(LIBs)have been considered to be one of the most important energy storage devices and extensively applied in application of portable electronics and large-scale equipment owing to the high energy density,long-term life and pollution-free environment.The continuous consumption of lithium makes its cost increase and availability decrease.Sodium ion batteries(SIBs)have been widely investigated in the past decades and considered to be a promising alternative to LIBs owing to their outstanding advantages such as low cost,abundant sodium resources,and high redox potentials.As a commercial battery anode material,the graphite shows a relatively satisfied cycling capacity and long cycle life.However,the lithium and sodium dendrites of graphite under low potential can lead to the safety issues.Moreover,the specific capacity is not high enough to meet the requirements for the large-scale energy storage.Thus,it is necessary and great significance to seek new electrode materials with large capacity,high safety,long-cycle life and high energy density for LIBs and SIBs.Metal chalcogenides,including metal oxides,metal sulphides and metal selenides,have attracted much attention for high-energy-density LIBs and SIBs due to their unique structures and high theoretical capacity through conversion and alloying reactions.In comparison with metal oxides and metal sulphides,metal selenides have the comparable theoretical volumetric capacity density and better electronic conductivity.In this thesis,we select the metal selenides as the research object,and adopt different methods to synthesize the metal selenides with different morphologies for the anode materials in the LIBs and SIBs and further improve their electrochemical performance.The reaction mechanism and kinetics of metal selenides are also studied.The specific contents are as follows:(1)Graphene has excellent electronic conductivity and good mechanical properties,and is widely used to improve the electrochemical properties of electrode materials.FeSe2 nanoparticles with graphene modification are synthesized by hydrothermal method.The FeSe2@rGO(reduced graphene oxide)composite shows higher capacity and better rate capability than the pristine FeSe2,due to the fact that RGO not only improves electron conductivity,but also can prevent the pulverization active materials,and protect the structural stability in the long-term cycles.The capacity for LIBs of FeSe2@rGO can reach 945.8 mAh g-1 after 100 cycles at the current density of 0.1 A g-1,which is much higher than some reported metal selenide materials.Graphene modified CdSe nanoparticles are prepared by the similar method.The introduction of graphene has greatly enhanced the electrochemical activity of CdSe nanoparticles,resulting in greater ionic diffusion coefficient and lower transfer impedance of CdSe@rGO composite.We also find that the lithium ion storage mechanism of CdSe is composed of insertion,conversion and alloying reactions.(2)In order to prevent the side reaction between the active substance and the electrolyte,Carbon coating strategy is adopted to form stable heterogeneous interfaces on the surface of ZnSe nanoparticles.By adjusting the content of carbon coating,the electrochemical activity of electrode material can be brought into full play.Carbon coating strategy can reduce the volume change of electrode material while improving the conductivity of particles.This method is simple and efficient,and has good potential applications.(3)For the poor conductivity and large volume change of metal selenides,we design a double-carbon conductive network structure,which can not only improve the internal conductivity of FeSe2 nanoparticles,but also provide a continuous conductive channel between the nanoparticles.The carbon matrix and flexible RGO serve as an elastic buffer space to sustain the structural strain and accommodate the volume expansion and contraction of electrodes.(4)One-dimensional SnSe@C composites with core-shell structure are designed by electrospinning and annealing processes.The core-shell structure formed by carbon fiber and SnSe can effectively improve the conductivity of the electrode material,prevent the active material from falling off the collector fluid,and improve the structural and cycling stability.Ex-situ testing proves that the sodium storage process of SnSe crystal structure can be divided into three processes:insertion,conversion and alloying.We also synthesize MnSe@C composite materials with self-supporting structure by adjusting the spinning composition,and explore the effect of temperature on the electrode materials.The carbon matrix with self-supporting structure can not only provide three-dimensional conductive network for particles and accelerate ion transport,but also inhibit the volume change of electrode material and prevent particle degradation.This method is beneficial to the mass production of metal selenide composites.