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.