Polymorphism and magnetic properties of Li2MSiO4 (M = Fe, Mn) cathode materials
Résumé
Transition metal-based lithium orthosilicates (Li2MSiO4, M=Fe, Ni, Co, Mn) are gaining a wide interest as cathode materials for lithium-ion batteries. These materials present a very complex polymorphism that could affect their physical properties. In this work, we synthesized the Li2FeSiO4 and Li2MnSiO4 compounds by a sol-gel method at different temperatures. The samples were investigated by XRPD, TEM, Li-7 MAS NMR, and magnetization measurements, in order to characterize the relationships between crystal structure and magnetic properties. High-quality Li-7 MAS NMR spectra were used to determine the silicate structure, which can otherwise be hard to study due to possible mixtures of different polymorphs. The magnetization study revealed that the Neel temperature does not depend on the polymorph structure for both iron and manganese lithium orthosilicates.