Phase formation and crystallization of LiMnxFe1−xPO4-C olivine material with different Mn2+ contents fabricated at lower calcination temperatures

Abstract

LiMnxFe1−xPO4-C cathode materials for lithium ion batteries were synthesized via solid-state method using Li2CO3, MnCO3, NH4H2PO4, FePO4 and sucrose as starting raw materials, followed by high-temperature reduction-annealing. A series of calcination experiments at different temperatures reveal that Mn2+-containing materials exhibit a lower temperature for olivine phase formation, for example LiMn0.5Fe0.5PO4 olivine phase forms at 275 °C, while manganese-free crystalline LiFePO4 generally forms at the required temperature of 350 °C. Increasing Mn2+ content is found to enhance crystallization degree of LiMnxFe1−xPO4 material prepared at lower calcination temperatures. X-ray photoelectron spectroscopy (XPS) results confirm that Mn valence state (+2) remains unchanged up to ~250 °C when calcined in ambient atmosphere. The above-mentioned beneficial effect of manganese on phase formation and crystallization of olivine can be well attributed to the stable nature of Mn2+ and its strong propensity to form olivine phases.