Crystal and Morphology Design of Dittmarite-Type Ammonium Iron–Manganese Phosphates, NH4Mn1–xFexPO4·H2O, as Precursors for Phospho-olivine Electrodes

This study provides the first data on the preparation of ammonium iron–manganese phosphates monohydrates, NH4Mn1–xFexPO4·H2O, with a dittmarite-type structure in the whole concentration range. The structure, Mn2+/Fe2+ distribution over 2a crystallographic site, and morphology of the mixed dittmarite salts are assessed by means of powder X-ray diffraction, infrared spectroscopy, electron paramagnetic resonance spectroscopy, and scanning electron microscopy. The mixed dittmarite salts participate in reactions of ion exchange with Li+ and Na+ ions, and as a result lithium and sodium phospho-olivines, LiMn1–xFexPO4 and NaMn1–xFexPO4, are formed at low temperature. The main advantage of this synthesis route is that the homogeneous Mn/Fe distribution in the metal-phosphate layers of the dittmarite structure is transmitted to the target olivine structure without any reorganization, thus providing phospho-olivines largely free of antisite defects. Despite the use of one and the same precursor, the morphology of the lithium and sodium phases differ remarkably. The electrochemical properties of LiMn0.8Fe0.2PO4 and NaMn0.8Fe0.2PO4 phospho-olivines are tested in model two electrode cells versus lithium anode and LiPF6-based electrolyte. It has been found that both phospho-olivines are able to intercalate alkali ions reversibly, which determines their potential for application as electrode materials for lithium and sodium ion batteries.