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

摘要

New metastable olivine phases of sodium metal phosphates, NaMn_(1-x)M_xPO4 (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 NH4Mn_(1-x)M_xPO4·H2O (M = Fe, Ca, Mg) to NaMn_(1-x)M_xPO4. Their crystal structures are characterized via XRD Rietveld refinement and electron diffraction. A full range of solid solution behavior was observed for olivine Na_(1-n)Mn_(0.5)Fe_(0.5)PO4, in contrast to that of LiMPO4 (M = Fe, Mn) olivine materials, and is ascribed to ion size effects. The solid solution behavior of NaMn_(0.5)Fe_(0.5)PO4 was confirmed by electrochemical characterization.