Crystal and Morphology Design of Dittmarite-Type Ammonium Iron−Manganese Phosphates

The race to produce better battery systems for use in electric vehicles is at its peak and researchers around the globe have sized up to the matter. As a result, both Lithium and Sodium metal phosphates, known as phospho-olivines, appear to be most attractive electrodes to power electric vehicles. To design and utilize phospho-olivines as safe electrode materials the challenge is to develop suitable synthetic methods. Among the many quests undertaken, the dittmarite-type ammonium transition metal phosphate monohydrates(NH₄Mn_1−xFe_xPO₄·H₂O) -henceforth abbreviated DtAP- have shown effective ability to be structure-directed precursors for the synthesis of Lithium and Sodium phospho-olivine electrodes, Consequently, state-of-the-art research is focused on the exploitation of the structure and morphology of iron- and manganese-based dittmarites and their effects on the electrochemical performance of the corresponding phospho-olivines.

An in-depth assessment of the two structure modifications of the manganese dittmarite compound raises a critical question regarding its ability to form solid solutions between iron- and manganese-based dittmarites and if so, what would be the effect of structure and morphology of the mixed compounds during the preparation of the target phospho-olivines. As such, recent studies have been designed to examine the metal substituted phospho-olivines instead of the single representatives. Nonetheless, many publications have focused on lithium iron− manganese phospho-olivines, LiMn_1-xFe_xPO_4, but very few studies have reported on sodium analogues, NaMn_1-xFe_xPO₄.

In a recent publication, researchers from the Institute of General and Inorganic Chemistry at the Bulgarian Academy of Sciences: Professor Violeta  Koleva, Assistant Professor Tanya Boyadzhieva and Professor Radostina Stoyanova proposed to assess the structure peculiarities of the mixed salts in the series DtAP (0 ≤ x ≤ 1). Secondly, they focus on demonstrating the effectiveness of the mixed phosphate monohydrate salts for the preparation of lithium and sodium iron−manganese phospho-olivines with reliable electrochemical activity. Their work is currently published in the research journal, Crystal Growth & Design.

Technically, the structure, Mn²⁺/Fe²⁺ distribution over 2a crystallographic site, and morphology of the mixed dittmarite salts were assessed by means of powder X-ray diffraction, infrared spectroscopy, electron paramagnetic resonance spectroscopy, and scanning electron microscopy. Generally, the electrochemical properties of phospho-olivines were tested in model two electrode cells versus lithium anode and LiPF₆-based electrolyte.

The research team reported that the precipitation of Mn²⁺ and Fe²⁺ sulfate solution with (NH₄)₂HPO₄ yielded mixed salts DtAP with a dittmarite-type structure in the whole concentration range (i.e., 0 ≤ x ≤ 1). In fact, their synthesis technique allowed the Fe-to-Mn ratio in the dittmarite salts to be precisely controlled by a simple adjustment of the individual manganese and iron concentrations in the initial mixed sulfate solution. However, despite the use of one and the same precursor, the morphology of the lithium and sodium phases differed remarkably.

In summary, the study by Violeta Koleva and her colleagues provided the first data on the preparation of ammonium iron−manganese phosphates monohydrates, (DtAP), with a dittmarite type structure in the whole concentration range. Going by their results regarding optimization of the ratio between Fe and Mn, it would be possible to improve the electrochemical response of NaMn_1−xFe_xPO₄. Overall, the present data demonstrate the unique capability of the dittmarite precursors for the preparation of both lithium and sodium containing phospho-olivines with electrochemical activity.