Effect of Mn Content on Li-Ion Diffusion and Interfacial Kinetics in Olivine LiMn x Fe 1−X PO 4 (x=0-0.6) Cathodes

Olivine lithium manganese iron phosphate (LMFP) cathodes offer a promising route toward higher energy density than lithium iron phosphate (LFP) by accessing the elevated Mn 2+ /Mn 3+ redox plateau at ~4.1 V vs. Li/Li + , while preserving the intrinsic safety and cost advantages of the olivine framework. However, increasing Mn content is accompanied by Jahn–Teller distortion, reduced electronic conductivity, and sluggish Li + transport. Although minor cation doping has been proposed to mitigate these kinetic limitations, the mechanistic basis of such improvements, particularly under real-time operating conditions, remains insufficiently understood. In this work, LiMn x Fe 1−x PO 4 (x = 0, 0.3, and 0.6) and 1% Mg-doped LiMn 0.6 Fe 0.39 Mg 0.01 PO 4 were synthesized via a hydrothermal route. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) revealed a clear compositional dependence of particle morphology, evolving from small particles at low Mn content to compact rod-like particles at higher Mn content, indicative of optimized crystal growth and improved interparticle connectivity. Carbon analysis showed 2.1–3.3% carbon in the final carbon-coated active materials. X-ray diffraction (XRD) confirmed the formation of a single-phase olivine structure for all compositions. Commercial LFP and LMFP cathodes were used as baseline references, reflecting industrially optimized carbon coating and particle engineering. The Li-ion transport kinetics of the cathodes were systematically investigated by the galvanostatic intermittent titration technique (GITT), cyclic voltammetry (CV), and time-resolved electrochemical impedance spectroscopy (TR-EIS) coupled with distribution of relaxation times (DRT) analysis. Apparent Li⁺ diffusion coefficients (D Li ) extracted from the GITT profiles exhibited two distinct plateaus associated with the Fe 2+ /Fe 3+ (~3.5 V) and Mn 2+ /Mn 3+ (~4.0 V) redox couples during lithiation. With increasing Mn content, LiMnPO 4 became the dominant phase in the solid solution, leading to reduced ionic conductivity and a decline in D Li , thereby confirming that an appropriate Mn level is critical for optimal electrochemical performance. In the phase-transition region, the Li⁺ diffusion coefficients derived from GITT were in the range of 10 ⁻20 to 10 ⁻19 cm 2 s -1 . The charge-transfer resistance (R ct ) exhibited a decreasing trend during charge and an increasing trend during discharge, reflecting dynamic variations in electronic and ionic transport throughout cycling. In addition, R ct increased consistently with Mn content, indicating that excessive Mn incorporation progressively deteriorates both electronic and ionic conductivity in LMFP, in agreement with the trends observed by GITT and CV. Overall, these results demonstrate that the combined TR-EIS/DRT/GITT approach provides a mechanistically informative framework for decoupling interfacial, charge-transfer, and transport limitations in LMFP cathodes and offers practical guidance for composition and electrode design optimization.