Lattice dynamics, thermodynamics, and bonding strength of lithium-ion battery materials LiMPO ₄ (M = Mn, Fe, Co, and Ni): A comparative first-principles study

Gaps in our knowledge of phonon and thermodynamics still remain despite significant research efforts on cathode materials LiMPO ₄ (M = Mn, Fe, Co, and Ni) for rechargeable Li-ion batteries. Here, we employ a mixed-space approach of first-principles phonon calculations to probe the lattice dynamics including LO-TO splitting (longitudinal and transverse optical phonon splitting), quantitative bonding strength between atoms, and finite-temperature thermodynamic properties of LiMPO ₄. In order to take into account the strong on-site Coulomb interaction (U) presented in transition metals, the GGA + U calculations are used for LiMPO ₄. It is found that the oxygen-phosphorus (O-P) bond with the minimal bond length is extremely strong, which is roughly five times larger than the second strongest O-O bond. The atom P-containing bonds are apparently stronger than the corresponding atom O-containing bonds, indicating the stability of LiMPO ₄ is mainly due to atom P. It is observed that the equilibrium volume of LiMPO ₄ decreases from Mn, Fe, Co, to Ni, and the bulk modulus, zero-point vibrational energy, and Debye temperature increase. Phonon results indicate that the largest vibrational contribution to Gibbs energy is for LiMnPO ₄, followed by LiFePO ₄, LiCoPO ₄, and then LiNiPO ₄, due to the decreasing trend of phonon densities of state at the low frequency region of LiMPO ₄. Computed phonon and thermodynamic properties of LiMPO ₄ are in close accord with available experiments, and provide knowledge to be validated experimentally.