Operando 57Fe Mossbauer and XRD investigation of LixMnyFe1-yPO4/C composites (y=0; 0.25)

LiMn0.25Fe0.75PO4 and LiFePO4 powders have been synthesized by a ceramic route. A comparative investigation of their electrochemical behaviour using operando Fe-57 Mossbauer and X-ray diffraction is reported. The partial substitution of Fe by Mn atoms enhances the energy density of the olivine phase since the average potential of cycling is increased. The complementarity of the operando techniques used in this study allows the monitoring of changes in the local electronic environment and the lattice modifications that are directly linked to the redox reaction mechanisms. The lithium deintercalation/intercalation mechanism in LiMn0.25Fe0.75PO4 has been found to be completely different from LiFePO4. During the charge, the LiMn0.25Fe0.75PO4 phase has been found to undergo three well defined and reversible reactions; (i) a biphasic reaction at 3.46 V corresponding to a partial oxidation of Fe-II into Fe-III, (ii) the remaining Fe-II is oxidized in a monophasic reaction between 3.46 and 4.1 V. Finally, (iii) Mn-II is converted to Mn-III by mean of biphasic reaction at 4.2 V. Interestingly, the 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 in the quadrupole splitting. The non-optimized LiMn0.25Fe0.75PO4 electrode has been successfully cycled with constant capacity (120 mAh.g(-1)) for more than 40 cycles at room temperature.