Solvothermal Synthesis of LiMn1–xFexPO4Cathode Materials: A Study of Reaction Mechanisms by Time-Resolved in Situ Synchrotron X-ray Diffraction

We applied time-resolved in situ synchrotron X-ray diffraction (XRD) to study the reaction processes and pathways during the solvothermal synthesis of the olivine-structured LiFePO4, LiMnPO4, and LiFe0.4Mn0.6PO4 with ethylene glycol C2H6O2 (EG) as the solvent by following the evolution of the crystal structures of the Fe/Mn-containing phases. We identified a stable intermediate phase in the synthetic reaction process of LiFePO4, viz., a ferrous oxalate EG complex (FeC2O4·C2H6O2), and resolved its structure; thus, we established a two-step reaction mechanism involving dissolution–precipitation followed by interface-coupled dissolution–reprecipitation for the synthesis of LiFePO4. The synthetic reactions in an LiFe0.4Mn0.6PO4 solid solution also followed a two-step process via the formation of a metastable intermediate phase bearing a structural similarity to FeC2O4·C2H6O2 that, however, has a slightly larger unit-cell, indicating that the substitution of Fe by Mn occurred at the intermediate stage. In contrast, the reaction in the synthesis of LiMnPO4 proceeds through a simple process of precipitation. Our findings provide important information for optimizing the synthesis of olivine cathode materials. The in situ XRD method we developed in this work offers a new way of exploring a wide range of solvothermal synthesis reactions, which is valuable for the rational design of new batteries.