In Situ Monitoring of Structural and Valence Evolution during Electrochemical Desodiation/Sodiation Process of Na2Fe0.5Mn0.5PO4F

Abstract

Na2Fe0.5Mn0.5PO4F as a cathode material for Na ion batteries is synthesized by a solid-state method. Synchrotron based in situ X-ray absorption near edge structure (XANES) and X-ray diffraction (XRD) techniques are used to study the electronic and crystal structure evolutions of Na2Fe0.5Mn0.5PO4F, complemented by electrochemical analyses during the electrochemical sodiation/desodiation. A reversible capacity of 107 mAh/g can be obtained at 12.4 mA/g with two well-defined voltage plateaus centered at 2.9 and 3.5 V vs. Na/Na+. Clear edge shifts in the Fe and Mn K-edge XANES are observed for Na2Fe0.5Mn0.5PO4F at low current density (i.e., 8 mA/g), relating to the two plateaus in the charging curve associated with the Fe2+/Fe3+ and Mn2+/Mn3+ redox reactions sequentially. Increasing the charge current density to 100 mA/g, similar spectroscopic behavior is observed for Fe K-edge XANES. While, the Mn K-edge shift is smaller compared with this observed at 8 mA/g, indicating the sluggish reaction kinetic of Mn3+/Mn2+ redox couple. Continuous diffraction peaks shifts in in-situ XRD patterns of Na2Fe0.5Mn0.5PO4F are observed throughout the charge-discharge process without emergence of new peaks, indicating of solid-solution processes. Na2Fe0.5Mn0.5PO4F shows good structural reversibility during the electrochemical sodiation/desodiation.