First Principles Studies Of Two-Dimensional Transition Metal Compounds In Advanced Secondary Battery Systems

Due to the continuous exploitation and utilization of fossil fuels,human beings are facing serious energy crisis and environmental pollution problems on the road of sustainable development.Clean and renewable energy conversion applications and storage technologies have become the key solution to such problems.Rechargeable batteries have been regarded as leading candidates for energy storage systems to satisfy soaring energy demands and ensure efficient energy use,and intensive efforts have thus been focused on enhancing the energy densities and power capabilities.First-principles calculations based on quantum mechanics have played an important role in obtaining a fundamental understanding of battery materials,thus providing insights for material design.By exploring and studying the new secondary battery systems and their storage mechanism,this thesis exploits the advantages of theoretical calculation to the full,and seeks the alternative solutions to the problems of limited lithium storage,expensive price and safety in lithium-ion batteries,for achieving higher energy density and power density of secondary battery system.Based on the first-principles calculation method of density functional theory,the crystal structure,physical and chemical intrinsic properties,ion storage mechanism and transmission mechanism,electrochemical properties of secondary battery electrode materials were studied.By studying the microcrystalline structure characteristics and macroscopic properties of electrode,the relationship between microstructures and electrochemical properties provides a reliable theoretical basis for exploring new electrode materials and energy storage mechanisms.Because of the open space for ion storage,abundant reactive sites,wide range of electronic structure types,specific electrocatalytic activity,weak interlayer interaction and good mechanical properties,two-dimensional materials have attracted an increasing attention in the application of secondary battery electrode materials.Moreover,two-dimensional materials are widely distributed from elementary to ternary compounds,including graphene and graphene derivatives（graphene oxide and reduced graphene oxide）,graphene-like elements（silene,germanium,phosphene,borene and stannum）,transition metal oxides,transition metal disulfides,transition metal carbides/nitrides and other two-dimensional structural materials.Therefore,this paper mainly uses two-dimensional materials as ion battery electrodes to predict their ion storage performance in lithium,sodium,potassium,magnesium,calcium,aluminum and other metal cation battery systems.In addition,in order to solve the problems of poor rate capacity,serious electrode volume change,pulverization of electrodes and fast capacity fading caused by electrode conversion reaction in anionic battery system,we first used two-dimensional materials based on adsorption/desorption mechanism as anionic battery electrodes to achieve stable cycling and rate performance.In lithium-sulfur batteries,the poor electrochemical performance is due to the low electronic conductivity of the sulfur cathode,shuttling effect of intermediate products and slow dynamic reaction.By introducing two-dimensional materials with good electronic conductivity,it successfully improved the conductivity of the electrode,but also realized the appropriate adsorption and potential electrochemical catalytic performance.The main research contents are as follows:（1）The potential of a Ti₂N monolayer and its Ti₂NT₂ derivatives（T=O,F,and OH）as anode materials for lithium-ion and beyond-lithium-ion batteries has been investigated by the first-principles calculations.The bare and terminated monolayers are metallic compounds with high electronic conductivity.The diffusion barriers on bare Ti₂N monolayer are predicted to be 21.5 meV for Li⁺,14.0 meV for Na⁺,7.0 meV for K⁺,75.9 meV for Mg²⁺,and 38.0 meV for Ca²⁺,which are the lowest values reported for state-of-the-art two-dimensional energy storage materials.The functional groups on Ti₂NT₂ increase the diffusion barriers by about 1 order of magnitude.The calculated capacities for the monovalent cations on Ti₂N and Ti₂NT₂ are close to that of the conventional graphite anode in lithium-ion batteries.In comparison,the capacities for Mg²⁺on Ti₂N and Ti₂NT₂ are more than 2000 mA·h·g^-1 due to the two-electron reaction and multilayer adsorption of Mg²⁺.Comparison of the electrochemical performances of Ti₂N and Ti₂C suggests that Ti₂N is a more promising anode material than Ti₂C due to its lower diffusion barriers for various cations（2）Based on adsorption/desorption storage mechanism of chloride ion on two-dimensional material,chloride ion storage on Ti₂C monolayers was theoretically investigated.The most favorable Cl^-adsorption configuration was identified using a partial particle swarm optimization algorithm and the results showed that Cl^- adsorption onto Ti₂C monolayers achieved a large theoretical capacity（331 mA·h·g^-1）,high working voltage（4.0-3.5 V）,and low diffusion barrier（0.22 eV）.This resulted in excellent rate capability and a large specific energy of 1269 W h kg-1 at the material level.The effects of terminal O,F,and OH groups on Cl^-adsorption onto Ti₂C monolayer were also studied,which showed that Cl^-could not be adsorbed onto O and F terminated Ti₂C monolayers.In comparison,Cl^-adsorption onto OH terminated Ti₂C was allowed but generated a smaller specific capacity (126 mA·h·g^-1)and lower working voltage（3.5-1.5 V）than a bare Ti₂C monolayer.（3）By first principles calculation,a general strategy was proposed for enhancing the electrochemical performance of Li-S batteries using surface-functionalized Ti₃C₂ MXenes.Functionalized Ti₃C₂T₂（T=N,O,F,S,and Cl）showed metallic conductivity as bare Ti₃C₂.In all Ti₃C₂T₂ investigated,Ti₃C₂S₂,Ti₃C₂O₂,and Ti₃C₂N₂ offered moderate adsorption strength,which effectively suppressed Li₂S_n dissolution and shuttling.This Ti₃C₂T₂ exhibited effective electrocatalytic ability for Li₂S decomposition.In particular,the Li₂S decomposition barrier was significantly decreased from 3.390 to⁰.4 eV using Ti₃C₂S₂ and Ti₃C₂O₂,with fast Li⁺diffusivity.On the basis of these results,O and S terminated Ti₃C₂ was suggested as promising host materials for S cathodes.Moreover,appropriate functional group vacancies could further promote anchoring and catalytic abilities of Ti₃C₂T₂ to boost the electrochemical performance of Li-S batteries.（4）First-principles calculations based on density functional theory were carried out to investigate the electrochemical performance of monolayer VS₂ for Li-,K-,Mg-and Al-ion batteries.A VS₂ monolayer shows differential storage ability for various cations,able to adsorb three layers of Li,two layers of Mg,one layer of K,and 1/9 layer of Al on both sides of the monolayer,producing theoretical capacities of 1397,1863,466,and 78 mA·h·g^-1 for Li,Mg,K,and Al,respectively.The average working voltages of VS₂ monolayers for Li⁺,K⁺and Mg²⁺are close to those of metallic Li,K,and Mg,suggesting that they can be used as anode materials in these rechargeable batteries.The adsorbed cations form a honeycombstacking lattice on VS₂ monolayers,similar to the plating process of Li,K,and Mg metal anodes.More interestingly,the honeycomb Li lattice is different from the body-centered cubic lattice of a Li metal anode,which provides very small diffusion barriers,resulting in the high rate capability of VS₂ monolayer in Li-ion batteries（5）The mechanisms for phase transitions,charge-transfer reactions,and ionic diffusion kinetics during Na⁺insertion were systematically investigated in this work via experimental testing and firstprinciples calculations using VS₂ nanosheets as an example material.The material showed a stable discharge capacity of 386 mA·h·g^-1 in the 0.3-3.0 V voltage window which then increased to 657 mA·h·g^-1 with further discharging to 0.01 V.It was discovered that Na⁺first intercalated into octahedral interstitial sites of Na_xVS₂,with 0<x≤1.0,accompanied by partial reduction of S anions.Afterwards,Na⁺intercalated into tetrahedral interstitial sites of Na_xVS₂, with 1.0<x≤2.0,causing partial reduction of both V cations and S anions.The electrode was finally converted into a V/Na₂S nanocomposite after insertion of 3.0 mol of Na⁺,giving rise to a large specific capacity.This work not only revealed the structural transformation and mixed anionic/cationic redox reactions of VS₂ during Na⁺intercalation,but also helped us to understand the electrochemical reaction mechanisms of transition metal disulfides in SIBs.（6）In this study,VO₂ and VS₂ nanosheets were systematically studied as host materials for Li-S batteries via experimental testing and theoretical calculations.The VS₂@S electrode exhibited superior electrochemical performance compared to VO₂@S,for its large capacity of 713 mA·h·g^-1 at 5C rate,and a low capacity fading rate of 0.13% per cycle in 200 cycles at 1C rate.First-principles calculations demonstrated that the superior electrochemical performance of VS₂@S benefited from the inherent semimetallic conductivity of VS₂,moderate adsorption strength for Li₂S_n,fast Li⁺ transport with a low diffusion barrier,and accelerated surface redox reactions with a low Li₂S decomposition barrier.In comparison,the low electronic conductivity and strong adsorption strength of VO₂ increased Li⁺diffusion as well as Li₂S decomposition barriers of the electrode,resulting in relatively poor rate capability and cycle stability.The constructed relationships between S cathode and host materials could guide the future design of high performance S cathodes for Li-S batteries.