Operando 57Fe Mössbauer and XRD investigation of LixMnyFe1-yPO4/C composites (y = 0.50; 0.75)

LiMn0.50Fe0.50PO4 and LiMn0.75Fe0.25PO4 powders have been synthesized by ceramic route. A comparative investigation of their electrochemical behaviour using operando Fe-57 Mossbauer and X-ray diffraction is reported. The complementarity of operando techniques used in this study allows the monitoring of changes of the local electronic environment and the lattice modifications that are directly linked to the redox reaction mechanisms. During the charge, LiMn0.50Fe0.50PO4 has been found to undergo three well defined and reversible reactions via an intermediate phase containing simultaneously Fe-II and Fe-III cations. LiMn0.75Fe0.25PO4 undergoes a two reaction mechanism. Fe-III Mossbauer signature has been found to be sensitive to the oxidation of Mn-II since this oxidation is accompanied with a significant increase of the iron quadrupole splitting. This study summarizes the different mechanisms observed for compositions with different manganese content.