Investigating the asymmetric electrochemical, structural and electronic properties of Mn-rich Li(Mn,Fe)PO4 electrode materials

Abstract

Olivine-type phosphates LiMn_xFe_1−xPO₄ are attracting increasing interest as positive electrode materials for lithium-ion batteries due to their low cost and good electrochemical performance. However, the effects of the mixed Mn/Fe composition on lithium intercalation and ion transport are not fully characterised, especially in Mn-rich compositions. In this study, the electrochemical, structural and ion transport properties of Mn-rich LiMn_xFe_1−xPO₄ (x = 0.6, 0.7, 0.8) (LMFP) are investigated using a combination of experimental and materials modelling techniques. Considerable asymmetry in charge/discharge profiles is found, which highlights the complexity of the mixed-metal system. An intricate lithium intercalation mechanism is observed, including both solid solution and two-phase regions. While the Fe/Mn cation disorder causes the oxidation to proceed mostly via a solid solution mechanism, the Mn plateau remains associated with a two-phase process. Ab initio simulations indicate that Li⁺ diffusion occurs along one-dimensional channels parallel to the crystallographic b-axis following a curved trajectory, and find favourable Li/Fe and Li/Mn anti-site defect formation. Analysing the band gaps of the lithiated and delithiated phases revealed that Mn substitution of Fe can improve the electronic conductivity, suggesting asymmetric electronic behaviour.