Controllable Synthesis,Electrochemical Reactions And Enhanced Performances Of Transitional Metal Sulfides/Selenides For K Storage

Lithium-ion batteries(LIBs),as one of high-efficiency energy storage systems,are widely used in various energy storage devices.However,the lack and uneven distribution of resources in the earth's crust have led to the continuous increase in the cost,and therefore cannot meet the increasing demand for large-scale,high-energy-density devices.Due to the abundant reserves of potassium in the earth's crust,potassium ion batteries(PIBs)have attracted widespread attention in recent years.Among the various anode materials of PIBs,transition metal chalcogenides(TMCs)have the advantages in theoretical specific capacity,common use,and ease of preparation.In this thesis,a variety of TMCs with excellent electrochemical performances are prepared,and their potassium-storage mechanisms are analyzed and explored in depth by combining mature electrochemical techniques and advanced characterization techniques.The details are as follows(1)Using ZIF-67 as a self-sacrificial template,Co0.85Se nanoparticles were coated in a nitrogen-doped carbon framework.In this composite,N-doped carbon enhances the electron conductivity,cushions the volume change,and inhibits the aggregation of Co0.85Se upon cycling.Meanwhile,the mesoporous structure shortens the diffusion distance of potassium ions,as well as increases the contact between electrolyte and electrode.These features endow this composite superior performance.It delivers a specific capacity of 114.7 mAh g-1 at 1000 mA g-1 after 250 cycles.At 2.0 A g-1,the specific capacity still maintains at 110.7 mAh g-1.(2)Carbon-coated mesoporous Co9S8 nanoparticles supported on reduced graphene oxide(rGO)were successfully synthesized by a simple chemical synthesis.This composite makes full use of the protection of the carbon layer on the surface,the good conductivity and three-dimensional(3D)structure of rGO,the mesoporous structure and nanoscale size of Co9S8,thereby presenting the excellent electrochemical performances in PIBs.This material exhibits high specific capacity(407.9 mAh g-1 after 100 cycles at 0.2 A g-1),high-rate capability(215.1 mAh g-1 at 5 A g-1),and remarkable cycling stability(210.8 mAh g-1 after 1200 cycles at 1.0 A g-1).The redox reactions for potassium storage are revealed by ex-situ transmission electron microscope(TEM)/high-resolution transmission electron microscope(HRTEM),selected area electron diffraction(SAED),and X-ray photoelectron spectroscopy(XPS).(3)MoS2 nanosheets grown on N and P co-doped reduced graphene oxide are demonstrated as the anode material for potassium-ion batteries.This composite delivers a reversible capacity of 462.7 mAh g-1 at 100 mA g-1 over 200 cycles,an outstanding rate capability with a capacity of 224.9 mAh g-1 at 20 A g-1,and remarkable cycling stability of 236.6 mAh g-1 at a high current of 2 A g-1 after 7000 cycles.Meanwhile,the electrochemical reactions of MoS2/N,P-rGO as the anode material for PIBs are successfully revealed through a series of ex-situ and in-situ characterizations.(4)MoS2/C nanotubes assembled by ultrathin nanosheets were synthesized as a model of TMCs to illustrate the electrolyte salt chemistry of KFSI vs.KPF6.Compared to the case of KPF6,the electrochemical performances using KFSI as the electrolyte salt are greatly improved,-275 mAh g-1 after 15000 cycles at 1 A g-1,or?172 mAh g-1 even at 40 A g-1.The enhanced performances originate from the FSI-induced changes in the solvated structures,i.e.a large solvation energy,a high-energy lowest unoccupied molecule orbital(LUMO),and a small bonding dissociation energy of S-F.In this case,the solid-electrolyte-interphase(SEI)film contains more KF and less organics,leading to superior mechanical properties.And the Young's modulus of the electrode material are significantly improved,which helps to suppress the rupture of the SEI film and the dissolution of K2S,thereby maintaining the cycling stability.When KPF6 is used as the electrolyte salt,the SEI film is basically organics so that it is prone to cracking and damage upon cycling,leading to particle agglomeration and sulphur dissolution.This electrolyte salt chemistry also works for other sulfides,as demonstrated by NiS,V3S4,and Co9S8.