Topochemical Synthesis of Sodium Metal Phosphate Olivines for Sodium-Ion Batteries

New metastable olivine phases of sodium metal phosphates, Na[Mn1–x M x ]PO4 (M = Fe, Ca, Mg), nanorods are synthesized by a simple solid-state reaction at low temperature (≤100 °C) by means of a topotactic molten salt reaction that converts NH4[Mn1–x M x ]PO4•H2O (M = Fe, Ca, Mg) to Na[Mn1–x M x ]PO4. Their crystal structures are characterized via XRD Rietveld refinement and electron diffraction. A full range of solid solution behavior was observed for olivine Na1–x Mn0.5Fe0.5PO4, in contrast to that of LiMPO4 (M = Fe, Mn) olivine materials, and is ascribed to ion size effects. The solid solution behavior of NaMn0.5Fe0.5PO4 was confirmed by electrochemical characterization.