Study On The Storage Mechanism And Properties Of Transition Metal Chalcogenides

As the global environmental issues and energy shortage have led to continuous demand for efficient green energy,it is extremely urgent to develop rechargeable batteries that can enable the harvested energy to be released on demand to smooth out the power variations.In the past 30 years,LIBs have become the indispensable tools for the human beings and changed our everyday lives.However,lithium is too scarce to meet the increasing demand for energy storage.Therefore,it is particularly necessary to develop new secondary battery systems.Sodium and potassium ion batteries shares the similar physical and chemical properties with lithium,and show great advantage for cost and natural.So SIBs and PIBs can effectively make up for the inadequacy of lithium batteries and show great development prospect.And developing new anode materials is one of the most methods to solve the slow dynamics caused by large radius of Na⁺and K⁺.In this thesis,we selected three kind of Transition metal chalcogenides（TMCs）with different band gap as the research objects and proposed a series of materials design strategies for TMCs based on the conductivity of materials.On this basis,we obtained several kinds of new anode materials with satisfied enenrgy storage performance and studied the structure-property relationship of anode materials.The main research conclusions are listed as follows:First,we chose Co9S8 with wide band gap as the research objects and synthesized1D Co9S8@carbon nanofibers through a facile electrospinning process and studied its electrochemical performance as anode of potassium ion batteries.The unique one-dimensional nanostructures can shorten the transmission path of electron/ions and raise the electrical conductivity of the composite materials.In addition,the coated carbon fiber can help to alleviate strain induced by volumetric expansion.The material showed a discharge specific capacity of 515 m Ah g^-1 after 50 cycles under the current density of 100 m A g^-1 and good rate performance.The results of DSCV showed that the nano-sized Co9S8 fibers have obvious pseudo-capacitance storage characteristics,which ensures the fast and efficient storage of sodium ions.The results show that the Co9S8@C nanofiber is an ideal anode material of potassium ion batteries.Next,we turn to the semiconductor phase transition metal selenide with smaller band gap,whose intrinsic conductivity is higher than that of sulfide.And selenide drives more kinetically favorable bond breaking process during the conversion reaction.We synthesized Fe Se2 microspheres with hierarchical structure through a simple solvothermal method and studied its electrochemical performance in ether base electrolyte.In SIBs,a discharge specific capacity of 617 m Ah g^-1 can be obtained under the current density of 100 m A g^-1,the capacity retention rate is 98.8%after 100 cycles.At the high current density of 20 A g^-1,the material still showed a high discharge specific capacity of 525 m Ah g^-1.In PIBs,a discharge specific capacity of 470 m Ah g^-1 can be obtained under the current density of 400 m A g^-1,the capacity retention rate is93.8%after 80 cycles.At the high current density of 5 A g^-1,the material still showed a high discharge specific capacity of 323 m Ah g^-1.The hierarchical structure can provide short ion-diffusion channel and effectively restrain the volume expansion in the cycling process.At the same time,ether-based electrolyte can effectively reduce the charge transfer resistance of Fe Se2 electrode and prevent the loss of active material during repetitive（de）sodiation reactions.Finally,based on the result above,we pay attention to metallic Nb S2 with band gap of 0 and further studied the relationship of band gap and electrochemical performance of transition metal chalcogenide.By taking advantage of the metallic conductivity of Nb S2,we remove the conductive additives from the electrode complex,and increase the utilization of active material from 70%to 96.7%,the highest value in the sodium-based electrode materials.The carbon-free electrode showed a reversible specific capacity of 300 m Ah g^-1 at 100 m A g^-1 and 229 m Ah g^-1 at 10 A g^-1.Then we extended the design strategy and established carbon-free electrode in potassium ion batteries for the first time achieving a high reversible specific capacity of 195 m Ah g^-1at 200 m A g^-1 and 102 m Ah g^-1 at 5 A g^-1.In sum,based on the intrinsic properties of transition metal chalcogenide,we use targeted optimization strategy boosting electrochemical performance of materials with different band gap.We gradually improve the ratio of active material in electrode with the conductivity of materials increasing,and finally build carbon-free electrodes based on metallic Nb S2 with high conductivity.This methodology opens a new way to design new anode materials of SIBs and KIBs in future.