The study demonstrates unusual capabilities of phospho-olivine NaMnPO4 to intercalate lithium and sodium reversibly, which makes it attractive electrode material instead of the conventional lithium analogue LiMnPO4.

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Metal ion substitution in phospho-olivines is an effective way to improve their performance as electrode materials in lithium ion and alternative sodium ion batteries. In this contribution, we examine in detail the crystal structure of Mg-substituted NaMnPO. The preferential occupancy of the alkaline M1 position by Mg ions has been found for the first time - a phenomenon which appears to be opposite to the case of Mg-substituted LiMnPO, where Mg and Mn reside in the M2 position. Mg solubility in NaMnPO is limited in the range of 0.10 < Mg/(Mg + Mn) < 0.15 mole part. Mg-substituted NaMnPO is prepared at 200 °C by ionic exchange reactions involving the participation of mixed dittmarite salts, KMnMgPO·HO. The structural aspects of Mg substitution in NaMnPO are studied by combination of powder X-ray diffraction using the Rietveld analysis with IR and electron paramagnetic resonance spectroscopy. The morphologies of precursors and target olivines are examined by means of SEM and EDS. In order to understand the crystal chemistry of Mg-substituted NaMnPO, we use solid solutions between LiMnPO and LiMgPO as references. The reference compositions of LiMnMgPO are prepared using the same KMnMgPO·HO precursors as in the case of Mg-substituted NaMnPO.