First-principles Studies Of Cathode Materials Of Lithium Ion Batteries And Ferroelectric Materials

The main content of this thesis includes: We present a first-principles investigation of the crystal and electronic structure as well as the average insertion voltage of LiCoPO₄ doped in Li-site (by Na and Cr) and metal-site (by isovalent Ni, Zn, Ca, Mg and Mn and aliovalent Cu, Al, In, Mo and Zr) ; We present a first-principles calculation for the structure, magnetic and electronic properties of LiMBO₃ (M=Mn, Fe, Co);the magnetic structure and resistance of Li₃V₂(PO₄)₃ is investigated by first-principles calculations; the crystal , elctronic characteristic and intercalated voltage is studied for a new novel cathode material: Li₉M₃(P₂O₇)₃(PO₄)₂ (M=Fe, Cr, Co, V). The stable chemical potential range and possible vacancies for ferroelectric compound BiAlO₃ are also investigated in this paper.The results show that, both the Li-site doping and metal-site in doping LiCoPO₄ may reduce the volume change of the material during lithium extraction/reinsertion process. The metal doped at Li-site will block the path of Li ion diffusion. The doping by aliovalent transition metals will introduce defect levels in the energy band. It could influence the conductivity insertion voltage.The Mn-doped LiCoPO₄ material is investigated by first-principles calculation in this work. Results indicate that the volume change of LiMn_xCo_1-xPO₄ to Mn_xCo_1-xPO₄ is smaller than that of pure LiCoPO₄ material, which is responsible for the excellent tolerance of LiCoPO₄ to the repeated cycling in lithium ion batteries. Combining first-principles calculation with basic thermodynamics, the average intercalation voltage of Mn-doped LiCoPO₄ is calculated. It shows that the redox couple Mn³⁺/Mn²⁺ is not observed when x<0.25, but with increasing Mn contents the redox couple Mn³⁺/Mn²⁺ is found. Therefore, the discharge/charge properties in LiMn_xCo_1-xPO₄ and LiFe_1-yMn_yPO₄ is similar, where Mn display some electrochemical active under the coexistence of Co and Mn .We present a first-principles calculation for the strcture, magnetic and electronic properties of LiMBO₃ (M=Mn, Fe, Co). For LiMBO3 structure, not only hexagonal lattice but also monoclinic lattice show the one dimensional antiferromagnet along the [001] derection, which is agreement with an oxygen-mediated superchange mechanism for metal-metal magnetic interaction. The calculated magnetic moment of 5?B is close to the experimental value of 5.4?B. The states of density show that these compound are the semicaonductor with the band gap of 0.4²eV, which all hold the low electronic conductivity. The corresponding calculated voltage 2⁴.8V is agreement with the experimental value.The first-principles calculation investigates the structure, magnetic and resistance of Li₃V₂(PO₄)₃. Results indicate that the volume change during extraction/insertion is small, which is responsible for the excellent tolerance to the repeated cycling in lithium ion batteries. The energy of spin structure is lower than that of the non-spin. The feimi level is located in the energy band, and then the conductivity of this material is good. It is agreement with the resistance that the resistance is 650?under room temperature and decrease with temperature and frequence increasing. When the temperature is above 300?, the resistance is almostly zero.A novel cathode material of lithium ion battery Li₉M₃(P₂O₇)₃(PO₄)₂ is studied by first-principles method. The calculated volumes and crystal parameters for different metal materials are agreement with the radus change of metal. The density of states indicate that structures for Co and V are stable, but structure for Fe and Cr is unstable due to fermi level located at the peak of energy band. By calculating the formation energy of Li vacancy, it is obtained that the formation energy of Li1vacancy is lowest and then Li1 is firstly extracted. Comparing the formation energy of Li3 with Li2 vacancy, that of Li3 vacancy is lower. Hence, the order of Li ion extraction is Li1, Li3 and Li2. Calculated voltage are agreement with the experimental value. Calculated voltages are agreement with experimental values.Through first-principles calculations and thermodynamic analysis, we investigate the stable chemical potential range for BiAlO₃ of R3c and Pm3m symmetry, and how the vacancy formation energies depend on the chemical environment in sample preparation. The calculated vacancies of BiAlO₃ are mostly at their charged state rather than the neutral state, including -3 and +2 charge states for the metal (Bi or Al) vacancies and O vacancy, respectively. The Bi-O divacancy is found to prefer -1 or -2 charge state, but it is hard to form due to its high formation energy. Furthermore, we find that all the studied vacancies are easier to form in the BiAlO₃ of R3C symmetry than the Pm3m structure at any given oxygen environment.