Crystal and Morphology Design of Dittmarite-Type Ammonium Iron–Manganese Phosphates, NH₄Mn_1–xFe_xPO₄·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 t...