Determination of the Lamb-Mössbauer factors of LiFePO4 and FePO4 for electrochemical in situ and operando measurements in Li-ion batteries

57Fe Mössbauer spectroscopy is a powerful tool to investigate redox reactions during in electrochemical lithium insertion/extraction processes. Electrochemical oxidation of LiFeIIPO4 (triphylite) in Li-ion batteries results in FeIIIPO4 (heterosite). LiFePO4 was synthesized by solid state reaction at 800 °C under Ar flow from Li2CO3, FeC2O4·2H2O and NH4H2PO4 precursors in stoichiometric composition. FePO4 was prepared from chemical oxidation of LiFePO4 using bromine as oxidative agent. For both materials a complete 57Fe Mössbauer study as a function of the temperature has been carried out. The Debye temperatures are found to be θM=336 K for LiFePO4 and θM=359 K for FePO4, leading to Lamb-Mössbauer factors f300 K=0.73 and 0.77, respectively. These data will be useful for a precise estimation of the relative amounts of each species in a mixture.