Polymorphs of lithium transition-metal phosphates : synthesis and characterization

Lithium transition-metal phosphates, LiMPO₄ (M = Mn, Fe, Co, and Ni) have gained significant research interest over the past two decades as an important class of lithium-ion battery cathode materials. However, almost all of the investigations thus far have focused on the olivine polymorph which exists in orthorhombic Pnma space group. In this report, a distinct orthorhombic but non-olivine polymorph of LiMPO₄, described by a Cmcm space group symmetry, has been synthesized with M = Mn, Fe, Co, and Ni. Of these, LiMPO₄ in the Cmcm space group had never been reported before. A rapid microwave-assisted solvothermal (MW-ST) heating process with tetraethylene glycol (TEG) as the solvent and transition-metal oxalates as precursors facilitate the synthesis of these materials. The peak reaction temperatures and pressures, respectively, were below 300 °C and 30 bar, which is several orders of magnitude lower than the previously reported high pressure (GPa) method. The physiochemical and electrochemical properties of the synthesized materials are characterized with several techniques. X-ray diffraction (XRD) confirms the crystal structure with Cmcm space group and scanning electron micrographs (SEM) indicate a sub-micron thin platelet like morphology. The synthesis process conditions have been optimized to obtain impurity-free samples with correct stoichiometry, as characterized with XRD and inductively coupled plasma - optical emissions spectroscopy (ICP-OES). Upon heat treatment to higher temperatures, the transformation of the Cmcm polymorphs into olivine is observed with XRD and Fourier transform infrared spectroscopy (FTIR). Although the electrochemical activity of these polymorphs as lithium-ion cathodes turns out to be poor, the facile synthesis under mild conditions has enabled easy access to these materials, some of which were not even possible before.