| This dissertation focused on the fundamental problems, especially the relevant mechanisms at the atomic level, for the rational design of thermochromic material VO2 for energy-saving windows and cathode material LiFePO4 for Li-ion battery using density functional theory(DFT) calculations, concluded design criteria, and proposed possible experimental schemes. The dissertation included five chapters: The first chapter introduced the application background and the latest development of energy-saving windows based on thermochromic material VO2 and Li-ion battery using LiFePO4 as cathode material. The second chapter presented the basic methods which will be used in this study, i.e., the fundamentals of DFT calculations, as well as the software. The third chapter studied the influences of transition metal dopants on the metal-insulator transition(MIT) critical temperature and the phase stability of transition metal doped VO2. The fourth chapter investigated the surface structures and electric conductivities of the LiFePO4(010) surfaces coated with graphene and graphene-like B–C–N. The fifth chapter summarized the major achievements and conclusions and proposed outlooks of the future research of VO2 and Li FePO4. The main contents and results are summarized as follows:In order to optimize the thermochromic material VO2 being balanced among critical temperature, phase stability, large modulation ratio, and favorable color, a systematic investigation of the impacts of 29 transition metal dopants on MIT critical temperature and phase stability of VO2(M1) were carried out using DFT calculations. First, the transition metal doped VO2(M1) were studied using DFT calculations in terms of free energies U, formation enthalpies Hf, and Fermi energies Ef. And then, the cell volumes and bulk moduli were evaluated by fitting the calculated free energies to the Birch-Murnaghan equation of states. Third, the decomposition enthalpies and entropies of the transition metal doped VO2(M1), decomposing into pristine VO2(M1) and corresponding transition metal oxide, were evaluated by combination of DFT calculations and experimental formation enthalpies of transition metal oxides.The spin-polarized DFT calculations indicated:(1) The impacts of transition metal dopants on the MIT critical temperature could be evaluated in terms of the lattice distortion of VO2(M1). The cell volume expansion and the β-angle decreasing are associated with the MIT temperature decreasing.(2) The stabilities of transition metal doped VO2(M1) could be evaluated in terms of decomposition enthalpies and entropies of the decomposition products. It was concluded that VO2(M1) doped with high valence cations are more stable than those doped with low valence cations. These conclusions are consistent with experimental facts that W6+, Mo6+, and Re7+ doped VO2(M1) are stable and possess significantly lower MIT temperature and La3+, Hg2+, and Ag+ doped VO2(M1) undergoes phase separation.(3) Considering both the MIT critical temperature and the phase stability, Sc3+ was proposed as a potential dopant for the reduction of critical temperature, the blue-shift of absorption edge, and the enhancement of transmittance of visible light.In addition, the DFT calculations without spin-polarization were also carried out and the influences on calculation results from spin-polarization were evaluated. The energy differences between spin-polarization calculations and unspin-polarization calculations are relatively small and calculation errors for decomposition enthalpies cancel. Thus the influences on calculation results from spin-polarization to decomposition enthalpies can be neglected. However, the spin-polarization plays an important role for the prediction of lattice distortion, especially the β-angle. For W, Mo, and Re etc. transition metal dopants which effectively reduce the MIT temperature, the spin-polarization has little influences on the calculation results.Though the Li-ion battery cathode material LiFePO4 is low cost, environmental friendly, and chemical stable, the poor pristine electric conductivity impeded its practical application without the reduction of particular sizes or carbon coating. And now, the surface structure and conductivity of Li FePO4 is still an open issue attracting great research interests.This dissertation studied Li FePO4(010) surface using DFT calculations by comparison of the surface structures and band structures among neat surface, graphene coated and graphene-like B–C–N coated surfaces. The calculations demonstrated that the interactions between the coating layers and the P, Fe and O atoms on the LiFePO4(010) surface offset the contract of LiFePO4(010) surface without coating. Compared with neat LiFePO4(010) surface, the average bond length of Fe–O of LiFePO4(010) surface coated with graphene or graphene-like B–C–N is more close to that in the LiFePO4 bulk. The coating expands the Li+ transport channel, thus is advantageous for to the immigration and emigration of Li+.The density of states(DOS) of the three LiFePO4 surface structures demonstrated that the interactions between the coating layers and Li FePO4(010) surface improve the electric conductivity of Li FePO4(010) surface.(1) The band gap decreased from 3.3 eV(neat LiFePO4(010) surface) to 2.1 e V after coating with graphene.(2) When the LiFePO4(010) surface was coated with graphene-like B–C–N, the valence band maximum and the conduction band minimum are still dominated by Fe 3d orbits; however, two in-gap states appear with interval of 0.6 eV, attributing to the bonding interaction between the boron and nitrogen atoms of graphene-like B–C–N and the LiFePO4(010) surface. |
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