Dynamic phase transition behavior of a LiMn0.5Fe0.5PO4 olivine cathode material for lithium-ion batteries revealed through in-situ X-ray techniques

LiMn0.5Fe0.5PO4 has attracted great interest due to its good electrochemical performance and higher operating voltages. This has led to a greater than 30 percent higher energy density than for commercial LiFePO4 olivine cathodes. Understanding the phase transition behaviors and kinetics of this material will help researchers to design and develop next generation cathodes for Li-ion batteries. In this study, we investigated non-equilibrium phase transition behaviors in a LiMn0.5Fe0.5PO4 cathode material during charge–discharge processes by varying current rates (C-rates) using synchrotron in-situ X-ray techniques. These methods included wide angle X-ray scattering (in-situ WAXS) and X-ray absorption spectroscopy (in-situ XAS). The WAXS spectra indicate that the phase transition of LiMn0.5Fe0.5PO4 material at slow C-rates is induced by a two-phase reaction. In contrast, at a high C-rate (5C), the formation of an intermediate phase upon discharge is clearly observed. Concurrently, the oxidation numbers of the redox reactions of Fe2+/Fe3+ and Mn2+/Mn3+ were evaluated using in-situ XAS. This combination of synchrotron in-situ X-ray techniques gives clear insights into the non-equilibrium phase transition behavior of a LiMn0.5Fe0.5PO4 cathode material. This new understanding will be useful for further developments of this highly promising cathode material for practical commercialization.