Research On Electrochemical Properties Of Low Dimensional Electrode Materials For Secondary Ion Batteries

Lithium ion batteries have been widely used in various fields,but they still can not meet the needs of large scale energy storage.The development of new electode materials and new secondary battery technology have important scientific significance and practical value.Two-dimensional materials have large specific surface area,high electron mobility,high thermal stability and chemical stability,and they have broad application prospects in the field of secondary ion batteries.In this thesis,the effect of defects and doping on ion adsorption and diffusion in two-dimensional materials is systematically studied using density functional theory.It is proposed that the defect enhanced ion adsorption and diffusion on two-dimensional semiconductor materials.The mechanism of electrochemically induced structural phase transition is studied,and several new secondary battery electrode materials are predicted.1.Using first principles calculations,the structural stability of 27 two-dimensional monolayer transition metal chalcogenides MX₂?M=Ti,Zr,Hf,V,Nb,Ta,Mo,Cr,W;X=S,Se,Te?and their electrochemistry as electrode materials for lithium ion batteries were systematically studied by first-principles method.The results show that:The 2H structures of NbX₂ and TaX₂ are more stable;the 1T structures of TiX₂,VTe₂,VSe₂,CrX₂,ZrX₂ and HfX₂ are more stable;the 2H and 1T structures of VS₂ are all stable;the 2H and 1T structures of MoTe₂ and WTe₂ are all stable.The adsorption energy of Li on the stable structure ranges from-3.4 eV to-0.5eV,and the diffusion barrier values are mostly about 0.25 eV.On the other hand,the band gap is smaller with the better adsorption performance for these structures.These results indicate that single layer 1T-VSe₂ and CrX₂ are more suitable for anode materials of Li ion batteries.2.Based on density functional theory,the effects of defects?including single atom vacancies,polyatomic vacancies,inverse vacancies and grain boundaries?and doping on the adsorption and diffusion of lithium in single layer 2H-MoS₂ were studied.The results show that:?1?Defects can enhance the adsorption of lithium and increase the diffusion barrier.The diffusion barriers are still in the range of 0.26 eV⁰.42 eV.?2?The adsorption energy of Li near the doped atom decreases greatly,and the diffusion barrier does not increase in the doped structure where metal substitutes for Mo atom or non-metal substitutes for S atom.3.The effect of defects in graphene and black phosphene on the adsorption and diffusion properties of Na was studied using density functional theory.The intrinsic defects and grain boundaries of graphene and black phosphene enhance the adsorption properties of Na.On the other hand,the Na diffusion barriers on grain boundary graphene are in the range of 0.09 eV⁰.35 eV,which are smaller than those of Li diffusion barrier on grain boundary graphene 0.16 eV⁰.41 eV.Despite the existence of defects in monolayer black phosphene,the diffusion coefficients of SW1,SW2 and MV2 defects are still much smaller than those of original black phosphene.4.The mechanism of two-dimensional MoS₂ phase transition induced by lithium/sodium intercalation was studied by density functional theory.The results show that:After inserting additional electrons,the 2H-structure exhibits semiconductor properties and the 1T-structure exhibits metal properties.The injected electrons or ions from the interlayer occupy the 4dz² orbital of Mo,resulting in the loss of charge between Mo-S bonds,which makes the 2H structure unstable.On the contrary,the Mo-S bond of 1T'-MoS₂ does not lose charge.The insertion of electrons makes the crystal structure of 2H-MoS₂ unstable and transforms it into 1T'-MoS₂ structure.In addition,the critical concentration of phase transition from 2H-structure to 1T-structure decreases with the decrease of layers.This study shows that electron transfer is the origin of phase transition,which explains the micro-mechanism of phase transition between structures in charge and discharge process in theory,and provides scientific theoretical guidance for the development of more stable and safer anode materials for ion batteries.Electron doping is the microscopic mechanism that induces the phase transition of MoS₂ structure.5.The electrochemical characteristics of new two-dimensional materials for ion batteries were predicted.The Li/Na/Mg adsorptions are very strong and the diffusion barriers are small on SnO monolayer.The Li/Na/Mg diffusion barriers are 0.31 eV,0.24 eV and 0.21 eV,respectively;and the volume changes are small after Li/Na/Mg adsorption.On Cu₂Si monolayer,the Li/Na adsorption energy are about-2.5 eV with good charge-discharge voltage,and the diffusion energy of lithium/sodium are 0.29/0.23 eV,0.15/0.11 eV and 0.24/0.22 eV along three different diffusion paths,respectively.