The first lithiation/delithiation mechanism of MFeOPO4 (M: Co, Ni) as revealed by 57Fe Mössbauer spectroscopy

Iron (III) transition metal MFeOPO4@C (M: Co, Ni) oxyphosphates were synthesized using a solid-state reaction. The electrochemical properties of MFeOPO4@C were evaluated versus Li+/Li using two different binders (PVDF and CMC) in the voltage range 0.01–3.0 V. These phosphates show similar electrochemical profiles for both binders. They deliver a high discharge capacity during the first cycle around 755 mA h g−1 and 661 mA h g−1 for CoFeOPO4@C and 735 mA h g−1 and 789 mA h g−1 for NiFeOPO4@C when using PVDF and CMC binders, respectively. In our previous work, the electrochemical mechanism of MFeOPO4@C was investigated using in situ synchrotron XRD and X–ray absorption spectroscopy (XAS) revealing an irreversible amorphization of the crystal structure and the formation of new products at the end of discharge (Fe0, M0, Li2O, and Li3PO4). Moreover, XAS measurements showed that both transition metals M and Fe are active during the discharge/charge process. Our previous hypotheses suggested the oxidation of metallic iron to only divalent iron Fe2+ during charge. However, 57Fe Mössbauer spectroscopy study reported here in this paper demonstrated that Fe3+ is partially reduced to Fe2+ and Fe0 in the first stages of lithiation, and then totally reduced to metallic iron by the end of discharge. Furthermore, when charging MFeOPO4@C against Li+/Li, 57Fe Mössbauer spectroscopy technique surprisingly proved that iron is re-oxidized to + 2 and + 3. Therefore, the new data using 57Fe Mössbauer spectroscopy has revealed insightful findings on the electronic changes of Fe upon the first lithiation/ delithiation.