The first lithiation/delithiation mechanism of MFeOPO4 (M: Co, Ni) as revealed by 57Fe Mossbauer 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 (Fe-0, M-0, 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, Fe-57 Mossbauer spectroscopy study reported here in this paper demonstrated that Fe3+ is partially reduced to Fe2+ and Fe-0 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, Fe-57 Mossbauer spectroscopy technique surprisingly proved that iron is re-oxidized to + 2 and + 3. Therefore, the new data using Fe-57 Mossbauer spectroscopy has revealed insightful findings on the electronic changes of Fe upon the first lithiation/ delithiation. (c) 2022 Elsevier B.V. All rights reserved.