Investigation And Structural Design Of Electrode Materials For Magnesium Ion Batteries

Magnesium-ion batteries?MIBs?are a front-runner among the alternative battery technologies suggested to sub-stitute the state-of the-art lithium-ion batteries?LIBs?due to high safety,high specific capacity and low price.Developing high energy density cathode materials with good electrochemical performance for MIBs is a hot topic in recent years.In this thesis,we used density functional theory?DFT?calculations to provide an atomistic understanding of the influencing factors of the electrochemical reaction during magnesium ion extraction and insertion process.We also reveal the microscopic mechanism of the effect of doping and surface factors on the magnesiation procedure of electrode materials and predict the possibility of several novel two-dimensional materials as as electrode materials for MIBs.1.Investigated the effect of Ni doping on the surface stability of spinel MgMn₂O₄ as cathode materials for MIBs.Results exhibite that the Jahn-Teller distortion of spinel MgMn₂O₄ can be effectively constrained by Ni with a doping level of 0.5 substituting Mn.The?001?surfaces of both the MgMn₂O₄ and MgNi_0.5Mn_1.5O₄ terminated with Mg-layer have the lowest energy values?0.08 J/m²?.For MgNi_0.5Mn_1.5O₄,most of surface energies are in the range of 0.10-0.13 J/m² which are very close to the lowest energies of?001?facet and surface energies show less dependence on the Ni-doping.It shows that Ni is less tendency for segregation at the surface of MgMn₂O₄ and spinel MgNi_0.5Mn_1.5O₄ can be used as the cathode material for MIBs.2.Studied Mg diffusion barriers in the Sn and Bi as anode materials for MIBs.Results show that the diffusion barriers for an isolated Mg ion in the Sn and Bi are 0.43and 0.67 eV,respectively.The diffusion barrier increases from 0.43 eV for an isolated Mg ion to 0.77 eV as Mg-Mg ions interaction appears in the Sn.However,there is no apparent change of the diffusion barrier in the Bi when there is an extra Mg ion appears near the diffusing Mg ion.Clearly,Mg ion diffusion is easier in the Bi than in the Sn.Mg₂Sn and Bi₂Mg₃ compounds were eventually formed after the fully magnesiation of the Sn and Bi,respectively.The Mg diffusion is easier in the Bi₂Mg₃ than that in the Mg₂Sn.The dynamic simulations show that Bi can be a better candidate than?-Sn as a promising anode material for the MIBs with fast charge/discharge rates.3.Studied surface magnesiation of tin and bismuth as anode materials for MIBs.Results show that the diffusion energy barriers are 0.31–0.55 eV and 0.37–0.54 eV for the diffusion of Mg in?-Sn and Bi,respectively,with a large barrier of 1.27 eV for Mg from the surface to the subsurface in Bi.The diffusion energy barriers are 1.05,1.58 and1.27 eV for Mg diffuses from the surface to subsurface by penetrating Sn?100?,Sn?110?and Sn?111?surfaces,respectively,which are larger than the value of Mg diffuses inside Sn.The large diffusion energy barrier induces a rate-limiting incorporation step.Surface modification is necessary to improve the electrochemical behaviour of Bi and?-Sn as the anodes for MIBs.4.Studied the adsorption and diffusion of Mg atoms on monolayer black P.Results show that because of the non-flat and puckered structure of the monolayer black P,Mg shows anisotropy diffusion behavior with diffusion barriers of 0.08 and 0.57 eV along the zigzag and armchair directions,respectively.Mg mobility along the zigzag direction is estimated to be much faster than that along the armchair direction.The monolayer of black P can keep the structural stability with Mg coverage to Mg_0.5P.The monolayer black P can be a promising two-dimensional anode material for the MIBs.5.Investigated the effect of asymmetrical functionalization with-F and-OH functional groups on the physical properties of monolayer M₂X?M=Sc,Ti,V,Mn,Nb,Mo,Hf;X=C,N?and the possibility of Janus MXenes as electrode materials for MIBs.Results show that?1?for pristine monolayer M2X?M=Sc,Ti,V,Mn,Nb,Hf;X=C,N?systems,T-phase is energetically stable than H-phase,whereas the stable structures of monolayers Mo₂C and Mo₂N are H-phase.?2?Asymmetrically functionalized MXenes are energy stable with type II configuration,except Janus Nb₂CFOH,Nb₂NFOH and Hf₂NFOH are with type III configurations.?3?All the asymmetrically functionalized monolayer M₂X except for Sc₂C are metallic materials regardless of whether the functional groups exist or not.?4?The mechanical stability of monolayer M₂X can be improved by Janus surface functionalization.?5?The adsorption of Mg on monolayer M₂X is stronger than that on the surface functionalized MXenes.?6?The diffusion barrier of Mg on Janus MXenes surface is lower and the diffusion rate of Mg on the surface with-F functional group is faster than the surface with-OH functional group.The results show that Janus MXenes can be a potential electrode material with a fast charge and discharge rate for MIBs.