Synthesis And In Situ Study Of Core-Shell Sulfide And Phosphide As Anode Materials For Sodium-Ion Batteries

With the ever-increasing demand for large-scale energy storage,considerable progress has been made in discovering new advanced energy storage materials.Sodium-ion batteries(SIBs)have attracted much attention in recent years due to their use of abundant sodium resources and their comparable electrochemical capacity to lithium-ion batteries(LIBs).Much research has been devoted to pursuing the anode materials for SIBs with high-energy-density and long-cycle-life.In this thesis,transition metal sulfide(the representative of conversion-type anode materials)and transition metal phosphide(the representative of alloy-type anode materials)are selected as the two study objects of this paper.The above two study objects were investigated according to the ideas of material synthesis,physical characterization,electrochemical testing and in situ mechanism analysis.The main conclusions of this thesis are as follows:(1)In the third chapter,a kind of core-shell Ni-Co bimetallic sulfide anode material((Ni0.5Co0.5)9S8@N-C)was designed and successfully prepared from the point of view of nano-structure-modification,aiming at solving the volume expansion problem of conversion-type materials.The material showed excellent capacity and outstanding cycle stability in electrochemical performance in SIBs,which because the novel hollow core-shell structure can effectively adapt the volume expansion.The discharge specific capacity was maintained at 636 mA g-1 after 100 cycles under the current density of 100 mA·g-1.Furthermore,in situ XRD was used to observe the reaction of the anode material during the first charge-discharge process to understand the mechanism of the conversion reaction,which would be helpful to the follow-up research.(2)In the fourth chapter,we developed a kind of novel hollow core-shell Ni-Co bimetallic phosphide nanocubes with N-doped carbon coatings(Ni1.2Co0.8P@N-C)as the anode material for SIBs.When used as the SIBs anode material,Ni1.2Co0.8P@N-C exhibited excellent electrochemical cycling stability and demonstrated an especially high coulombic efficiency of 99%,even after 200 cycles with a current density of 100 mA·g-1.Such good electrochemical performance could aid with the novel design of a Ni1.2Co0.8P@N-C composite material.First,N-doped carbon can effectively enhance the conductivity of the phosphorus-based material.Second,the excellent mechanical property of N-doped carbon shell and the inner hollow space can mitigate expansion and large volume changes during cycling.Furthermore,in situ Raman spectroscopy was used to explore the electrode material in order to understand the electrochemical processes in the N-doped carbon shell of Ni1.2Co0.8P@N-C.Raman results showed that the intercalation and de-intercalation behavior of sodium ions in the N-doped carbon shell was almost reversible,providing valuable information about the charge and discharge processes in these novel SIBs.