Investigation of LiMn1-xMxPO4 (M = Ni, Fe) as cathode materials for Li-ion batteries using density functional theory

Olivine LiMnPO4 (LMP) is attracted significant attention as cathode material for lithium ion batteries (LIBs), due to its high voltage and capacity. However, LMP suffers from low electronic and ionic conductivities, therefore in this current work we have focused to overcome these drawbacks by substitutions the Mn sites in LMP by different concentrations of Ni and Fe. That is, LiMn1-xMxPO4 (M = Ni, Fe; x = 0, 0.25, 0.5, 0.75, 1). The structural, electronic, electrochemical potential and magnetic properties of lithiated/delithiated (LiMn1-xMxPO4/Mn1-xMxPO4 (M = Ni, Fe; x = 0, 0.25, 0.5, 0.75, 1)) phases were investigated beside kinetic properties and capacity storage of Li-ion. The results reveal that Ni and Fe affected the structural, electronic, kinetic, electrochemical potential and magnetic properties. The unit cell volume of LMP were decreased with increasing Ni and Fe concentrations in Mn sites, this behavior could be explained by the difference in ionic radius of doping cations Mn2+, Ni2+ and Fe2+. The open circuit voltage of LiMn1-xNixPO4 was slightly increased from 4.39 V (for LMP) to 4.41 V (for LiMn0.25Ni0.75PO4) which indicate a good improvement of Li-intercalation voltages with increasing the concentration of Ni. Moreover, band gap (Eg) of LMP (3.62 eV) is reduced with substitutions of Mn sites by Ni and Fe especially for the compounds LiMn0.5Ni0.5PO4 (2.77 eV) and LiMn0.5Fe0.5PO4 (3.35 eV) and improve the electronic conductivity. Moreover, the diffusion energy barrier of Li-ion in Mn0.5Ni0.5PO4 (0.34 eV) and Mn0.5Fe0.5PO4 (0.39 eV) were lower than pristine MnPO4 (0.42 eV), indicates that 50 % of Fe and Ni are beneficial to improve the kinetic properties in LMP. This study suggest that LiMn0.5Ni0.5PO4 (LMNP) and LiMn0.5Fe0.5PO4 (LMFP) are great alternatives of LMP for cathode materials in LIBs due to their high performance compared to the pristine LMP.