Precursor-based methods for low-temperature synthesis of defectless NaMnPO4 with an olivine- and maricite-type structure

Abstract

We report precursor-based methods for low-temperature synthesis of two structure modifications of NaMnPO₄. The maricite phase is thermodynamically more stable, while the olivine phase is of great interest as a positive-electrode material for lithium and sodium ion batteries. The advantage of synthetic procedures is the formation of defectless NaMnPO₄ in the temperature range of 200–400 °C. The structure and morphology characterizations of two modifications are performed by powder XRD, SEM and TEM analyses. The oxidation state of the Mn ions in NaMnPO₄ is determined by X-ray photoelectron spectroscopy. The local environment of Na in both structure modifications is assessed by ²³Na MAS NMR spectroscopy. The synthesis methods are based on the formation of appropriate precursors that are easily transformed to target NaMnPO₄. Thermal decomposition of freeze-dried phosphate–formate precursor yields NaMnPO₄ with a maricite structure at 400 °C. KMnPO₄·H₂O with a dittmarite-type structure acts as a structure-template precursor for the preparation of NaMnPO₄ with an olivine structure by an ion exchange reaction. Both olivine and maricite NaMnPO₄ do not accommodate any anti-site mixing and Na,Mn deficiency. The morphology of NaMnPO₄ consists of nano-sized particles (less than 50 nm) that are closely bound together into aggregates, the shape of the aggregates being dependent on the synthesis procedure used.