First-principles study: Effect of lithium and sodium intercalation in transition metal phosphates, MPO4 (M: Mn, Fe, Co)

First-principles studies have been conducted to compare the effect of Li and Na atoms intercalation in transition metal phosphates for potential application as cathode materials in Li-ion and Na-ion batteries. We have employed the plane-wave pseudopotential method within the generalized gradient approximation functional with Hubbard (U) correction. The equilibrium lattice constants for MPO4, LiMPO4 and NaMPO4 systems are in good agreement with the available experimental data to within 5%. The equilibrium cell volumes for MPO4 expand to within 7% and 14% during Li and Na intercalation, respectively, suggesting better structural stability for LiMPO4 over NaMPO4. Li intercalation decreased the heats of formation values of MPO4 more than Na, suggesting thermodynamic stability on LiMPO4 over NaMPO4. This attribute encourages better cycling performance and safety on LiMPO4 systems. The predicted intercalation potentials for NaMnPO4 (3.70 V), LiFePO4 (3.43 V) and NaFePO4 (3.00 V) were found to fall within the minimum and maximum pulse limits, suggesting these systems may not undergo Li/Na deposition side reactions during battery operation. The electronic density of states predicted MnPO4, Fe-based and Co-based structure to be semiconductors, while LiMnPO4 and NaMnPO4 are insulators. Furthermore, all the independent elastic constants for MPO4, LiMPO4 and NaMPO4 structures satisfy the necessary mechanical stability conditions for orthorhombic lattices. Lastly, the phonon dispersion curves in the spin-polarized state suggest vibrational stability on MPO4, LiMnPO4, NaMnPO4, LiFePO4, NaFePO4, and LiCoPO4 systems, since no soft modes are observed. (c) 2019 Elsevier B.V. All rights reserved.