Crystal chemistry of Mg substitution in NaMnPO4 olivine: concentration limit and cation distribution

Abstract

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 NaMnPO₄. The preferential occupancy of the alkaline M1 position by Mg²⁺ ions has been found for the first time – a phenomenon which appears to be opposite to the case of Mg-substituted LiMnPO₄, where Mg²⁺ and Mn²⁺ reside in the M2 position. Mg solubility in NaMnPO₄ is limited in the range of 0.10 < Mg/(Mg + Mn) < 0.15 mole part. Mg-substituted NaMnPO₄ is prepared at 200 °C by ionic exchange reactions involving the participation of mixed dittmarite salts, KMn_1−xMg_xPO₄·H₂O. The structural aspects of Mg substitution in NaMnPO₄ 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 NaMnPO₄, we use solid solutions between LiMnPO₄ and LiMgPO₄ as references. The reference compositions of LiMn_1−xMg_xPO₄ are prepared using the same KMn_1−xMg_xPO₄·H₂O precursors as in the case of Mg-substituted NaMnPO₄.