Enhancing the Electrochemical Performance of Olivine LiMnPO4 as Cathode Materials for Li-Ion Batteries by Ni-Fe Codoping

The cathode material is one of the components that play a key role in the safety, cost, and performance of Li-ion batteries. LiMnPO4 (LMP) has attracted significant attention as a potential cathode material for Li-ion rechargeable batteries due to its series of advantages. However, LMP suffers from low electronic and ionic conductivity. Therefore, this work aims to overcome these constraints of LMP by Ni-Fe codoping. In this regard, we used density functional theory simulations to investigate the effect of Ni-Fe codoping on the structural, electronic, magnetic, electrochemical potential, and kinetic properties of lithiated/ delithiated pristine phases (i.e., ), as well as on the thermodynamic stability, the theoretical capacity, the charge transfer, the average M-O bond lengths, and the electrical conductivity. We also evaluated the thermodynamic stability and charge transfer of Ni/Fe single doping in lithiated/delithiated (LiMnPO4/MnPO4) pristine phases, that is, LMNP/MNP (LiMn0.5Ni0.5PO4/Mn0.5Ni0.5PO4) and LMFP/MFP (LMn(0.5)Fe0.5PO4/Mn0.5Fe0.5PO4). We have found that Ni-Fe codoping affected the structural, electronic, kinetic properties, and electrical conductivity of pristine LMP. The volume of LMP decreased with Ni-Fe codoping. Moreover, a small change in unit cell volume between lithiated and delithiated phases was found for all structures, indicating good reversibility during Li insertion/ extraction. Ni-Fe codoping reduces the band gap of LMP from 3.62 to 1.55 eV, resulting in a good improvement in the electronic conductivity. The migration barrier energy was calculated to be 0.34 eV for Li-ions in MNFP, which is lower than that of MP (0.40 eV), indicating that Ni-Fe codoping is beneficial for enhancing the ionic conductivity of pristine LMP. This study may supply insights for the development of LMNFP cathode materials in lithium-ion rechargeable battery applications.