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

Abstract

The cathode material is one of the components that play a key role in the safety, cost, and performance of Li-ion batteries. LiMnPO 4 (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., LiMn 0.5 Ni 0.25 Fe 0.25 PO 4 / Mn 0.5 Ni 0.25 Fe 0.25 PO 4 ), 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 (LiMnPO 4 /MnPO 4 ) pristine phases, that is, LMNP/MNP (LiMn 0.5 Ni 0.5 PO 4 /Mn 0.5 Ni 0.5 PO 4 ) and LMFP/MFP (LMn 0.5 Fe 0.5 PO 4 /Mn 0.5 Fe 0.5 PO 4 ). 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.

Comparative Analysis of LiMPO4 (M = Fe, Co, Cr, Mn, V) as Cathode Materials for Lithium-Ion Battery Applications—A First-Principle-Based Theoretical Approach

“…The doping in olivine LiMPO 4 structures has been identified as a highly efficient approach for engineering the material properties of different LiMPO 4 materials. In this context, the theoretical investigation suggests that the introduction of Ni and Fe co-doping presents a promising route to optimizing the electrochemical properties of LiMPO 4 (M = Mn) [19]. A similar exploration suggests that the Mn doping in LiMPO 4 (M = Co) can significantly influence the electrochemical activities of the material [20].…”

Tailoring the Electrochemical Performance of Olivine Phosphate Cathode Materials for Li-Ion Batteries by Strain Engineering: Computational Experiments

“…Since the energy differences between the anti-ferromagnetic and ferromagnetic states are small, the ferromagnetic order was selected. This approach has been used previously in various DFT works. ,,, The crystal structures of all of these systems have been visualized with the VESTA software …”

“…LiFePO 4 (LFP) is the most well-known cathode material of the olivine phosphate family and has been widely used commercially since its introduction. − However, LFP has a low energy density of 500 Wh/kg, compared with other olivine cathode materials like LiMnPO 4 (LMP), LiNiPO 4 (LNP), and LiCoPO 4 (LCP), which have a high energy density of 700 Wh/kg. − The LMP cathode material has an operating open-circuit voltage (OCV) of 4.1 V vs Li/Li + , which is compatible with the corresponding values for most conventional liquid electrolytes (below 4.5 V vs Li/Li + ). , Inversely, LCP and LNP exhibit high voltages of 4.8 and 5.1 V vs Li/Li + , respectively, which exceeds the electrochemical stability of commercial liquid electrolytes, resulting in a sharp drop in their efficiency. ,, In addition, LCP and LNP have some unresolved technical issues, such as poor electronic conductivity and sluggish Li + diffusion that require further work to be resolved. , Extensive research has been recently conducted to significantly influence the performance of these olivine cathode materials. These include, but are not limited to, surface functionalization, the substitution of other d-block transition metals, size dependence, and morphology control. − …”

Low-Cost Mn-Based Cathode Materials for Lithium-Ion Batteries

“…Li-site doping can reduce the charge transfer resistance and broaden the one-dimensional diffusion channels of Li + [134], but the transition metal in the Li layer will hinder Li + diffusion to a certain extent [135]. Thus, Mn-site doping has received a lot of attention, such as doping with Fe [136], V [137], Mg [138], Ni [139], Cu [140], Cr [141], and Zn [135]. Oukahou et al [136] synthesized LiMn 1-x M x PO 4 (M = Ni, Fe) with improved electronic conductivity and reduced Li + diffusion energy barrier, by doping Ni 2+ and Fe 2+ cations at Mn sites.…”