First-principles Study Of Ionic Doping Effect In FeF₃ With Structure Of Hexagonal-tungsten-bronze Type

FeF3 with hexagonal-tungsten-bronze(HTB-FeF3)structure is one promising next generation of cathode material as lithium-ion battery due to its advantages of high voltage and high theoretical specific capacity.However,the poor conductivty and structural stability of HTB-FeF3 have prevented its practical application.Although ion doping is an effective method to improve material conductivity,the problems of complextion and random in selection of doping cation still exist,moreover,its lacks systemic theoretical guidance.Therefore,in this paper,first-principles method is used to discuss the mechanism of doping of HTB-FeF3 with anions and cations at the electronic and atomic scale.It breaks.through the bottleneck of poor electronic conductivity and structural stability,designing HTB-FeF3 with excellent properties.Our calculated results will provide an important theoretical guidance for promoting its application in lithium-ion batteries.The concrete content can be concluded as follows:(1)The effect of Ti ion doping on the crystal structure and electronic structure of HTB-FeF3 has been studied.First,all types of defective structure of Ti ion(Ti2+、Ti3+、Ti4+)doped HTB-FeF3 system with different valence states was discussed.Then the most stable Ti-doped structure was obtained by calculating the formation energy of each ionic doping system.Then the electronic structure such as band structure,density of states,differential charge density and Bader charge were further analyzed.The results show that Ti3+ is the easiest to dope into HTB-FeF3.With Ti3+ doping,the Fe-F bond length increases,resulting in the formation of F vacancies.This can broaden the one-dimensional channel of HTB-FeF3,which can help to facilitate the transport of lithium ions in HTB-FeF3.Moreover,Ti doping can reduce the band gap of the HTB-FeF3,increasing its electronic conductivity.(2)The effect of Co ion doping on the crystal structure and electronic structure of HTB-FeF3 has been studied.First,the HTB-FeF3 is doped by Co ion with different valence(Co2+ and Co3+),considering the F vacancy defective structure which perhaps to be generated.Then,by calculating formation energy of dafective structure,it is shown that Co2+ is the easiest to dope into HTB-FeF3,moreover,its band structure,density of states,differential charge density,and Bader charge are analyzed.It is found that Co2+doping can reduce the band gap of HTB-FeF3.(3)The effects of OH-ion doping on the crystal structure,mechanical properties and electronic structure of HTB-FeF3 have been studied.The formation energy results show that OH-can be doped under usual conditions.With the increase of OH-doping concentration,the structural stability of HTB-FeF3-x(OH),and its band gap value gradually decreases,and it has the lowest band gap of 1.443 eV.In addition,the calculated results of the elastic constants of HTB-FeF3 and FeF2.167OH0.833 indicate that OH-doping can effectively improve the mechanical properties of HTB-FeF3,which helps to enhance the structural stability.(4)The effects of oxygen ion doping on the crystal structure and electronic structure of HTB-FeF3 has been studied.First,the defective structures with fluorine vacancies or interstitial iron ion are investigated in HTB-FeF3.By calculating formation energy of defective structure,it is noticed that O2-doping has the smallest formation energy and FeF2.986O0.014 2-has the most stable crystal structure.Next,the band structure,density of states,differential charge density and Bader charge of FeF2.986O0.014 2-were further analyzed.It is found that O2-doping can effectively improve conductivity of HTB-FeF3.