Fabrication Defects And Electrochemical Performance Investigation For TiS₂

In recent years,potassium ion batteries have been extensively studied as an alternative for lithium ion batteries,due to excellent theoretical performance and abundant potassium resources.Thus developing electrode materials is important for the advancement of potassium ion batteries.Transition metal dichalcogenides（TMDs）are regarded as a potential electrode materials on account of the unique layer structure.It is similar to graphite,there are weak Van der Waals interactions between the individual layers,which provide greater spacing for the transport of ions.However,TMDs materials are often hybridized with other kinds of materials,when applied in energy storage devices.Therefore,the reasonable design of TMDs is important for its developments and application in the field of energy storage.In this work,TiS₂ material was selected as the research object and was systematically studied at its atomic defects,including the conditions for creating defects,the control of species,the impact of electrochemical performance and so forth.Firstly,the controllable conditions for the formation of anion defect（S vacancy）and the effect of defect on electrochemical performance were investigated.A series of concentrations of sulfur vacancies are introduced in TiS₂ by annealing methods.The XRD,XPS,EPR and TG measurements exhibit that defect engineering can be controlled via annealing temperature and Ti powder weight,including species and concentration.Benefiting from the superior properties of sulfur vacancies,the electrochemical characteristics of TiS₂ are greatly optimized,including cycle ability,rate performance and dynamic characteristics.Meanwhile,the improvement of electrochemical performance is closely related to the concentration of defects.As an anode material for potassium ion batteries,300+5Ti possesses the best cycle ability,outstanding rate performance and lowest R_ct value in all TiS_2-x materials.The capacity retention of 300+5Ti is three times as much as pristine TiS₂ after 500 cycles at 50 m A g^-1.In addition,The R_ct value of pristine TiS₂ also decreased from 262.5Ωto 105.5Ω（300+5Ti）.Moreover,the R_ct value of 300+5Ti is still the lowest after 10 cycles.Secondly,cation defect（Ti vacancy）was generated on the surface of TiS₂ via thermal annealing method,replacing Ti powder with S powder.In this part,not only the influence of Ti vacancy on electrochemical performance,but also the mechanism of energy storage was explored.The XRD,XPS and EPR results of Ti vacancy showed that its chemical environment was opposite to that of S vacancy.The effect of Ti vacancy on electrochemical performance is studied in alkali metal ion batteries.Introduced Ti vacancy effectively improves cycle ability and enhances kinetic performance.The microscopic stress and strain can be weakened due to the existence of defects,demonstrating that defects can alleviate volume expansion during the ion intercalation process to maintain structural stability,thus obtaining excellent cycle ability.Meanwhile,Ti vacancy also can regulate alkali metal ions insertion sites to stabilize crystal structure.In addition,Ti vacancies could be conducive to the improvement of kinetics,including charge transfer resistance and ions transport.Compared with S vacancy,TiS₂ with Ti vacancy obtain better electrochemical performance.The in situ XRD techique was employed to study energy storage mechanism of TiS₂with Ti vacancy.These features are also verified by density functional theory calculations.