Lattice dynamics, thermodynamics, and bonding strength of lithium-ion battery materials LiMPO4(M = Mn, Fe, Co, and Ni): a comparative first-principles study

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Gaps in our knowledge of phonon and thermodynamics still remain despite significant research efforts on cathode materials LiMPO4 (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 LiMPO4. In order to take into account the strong on-site Coulomb interaction (U) presented in transition metals, the GGA + U calculations are used for LiMPO4. 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 LiMPO4 is mainly due to atom P. It is observed that the equilibrium volume of LiMPO4 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 LiMnPO4, followed by LiFePO4, LiCoPO4, and then LiNiPO4, due to the decreasing trend of phonon densities of state at the low frequency region of LiMPO4. Computed phonon and thermodynamic properties of LiMPO4 are in close accord with available experiments, and provide knowledge to be validated experimentally.