The phase transition behaviors of Li_1-xMn_0.5Fe_0.5PO₄ during lithium extraction studied by in situ X-ray absorption and diffraction techniques

How the structural changes take place in LiMn_yFe_1-yPO₄-type cathode materials during lithium extraction/insertion is an important issue, especially on if they go through the single-phase reaction (i.e., solid solution reaction) or the two-phase reaction regions. Here we report the studies on the phase transition behaviors of a carbon coated Li_1-xMn_0.5Fe_0.5PO₄ (C{single bond}Li_1-xMn_0.5Fe0_.5PO₄, 0.0 ≤ x ≤ 1.0) sample during the first charge using in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques. The combination of in situ XAS and XRD results clearly identify two two-phase coexistence regions at two voltage plateaus of 3.6 (Fe²⁺/Fe³⁺) and 4.2 V (Mn²⁺/Mn³⁺) and a narrow intermediate region which proceeds via single-phase reaction in between two two-phase regions. In addition, simultaneous redox reactions of Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ in the narrow single-phase region are reported and discussed for the first time.