First-Principle Studies Of Cathode Materials LiFePO₄ And Its Doped Systems For Lithium Ion Batteries

Lattice dynamical properties of LiFePO₄ are studied using first-principles density functional theory taking into account the on-site Coulomb interaction within the GGA+U scheme. Born effective charge tensors, phonon frequencies at the Brillouin zone center and phonon dispersion curves are calculated and analyzed. The Born effective charge tensors exhibit anisotropy, which gives an indirect evidence for the one-dimensional Li migration tunnel along the [010] direction in LiFePO₄, which has been proposed by other theoretical calculations and experimental observation. The calculated phonon frequencies at theГpoint of the Brillouin zone agree well with the available experimental results.Antisite defect, electronic conductivity and ionic dynamic properties of LiFePO₄ have been investigated using first-principles density functional theory taking into account the on-site Coulomb interaction within the GGA+U scheme. Results indicate the Li/Fe exchange defect is the most preferred to occur in LiFePO₄, which causes the Fe-O bond length to change in the direction favors the formation of Li⁺ diffusion channels, hence improving the ionic dynamic properties of the olivine LiFePO₄.Atomic and electronic structures of LiFePO₄ with Mg doping at Li and Fe sites have been investigated using the first-principles density functional theory with the on-site Coulomb interaction taken into account. It is demonstrated that Mg is preferentially doped at Fe site instead of Li site to form a new LiFe_1-yMg_yPO₄ solid solution, leading to a higher ionic conductivity. Besides, it is found that formation energies and the cell volumes decreased gradually with the increase of Mg concentration in LiFe_1-yMg_yPO₄, which agrees well with the available experimental data.