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., LiMn0.5Ni0.25Fe0.25PO4/Mn0.5Ni0.25Fe0.25PO4), 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 (LMn0.5Fe0.5PO4/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.