Crystal Structure and Electrochemical Properties of A₂MPO₄F Fluorophosphates (A = Na, Li; M = Fe, Mn, Co, Ni)

Abstract

We report new solid state and hydrothermal synthetic routes to (Li,Na)2FePO4F that incorporate carbon-containing additives and result in good electrochemical properties of this Li (or Na) ion electrode material. Single crystal X-ray diffraction analysis of Na2FePO4F prepared by flux growth confirms the unusual structural features of this compound that include pairs of face-sharing metal octahedra and [6 + 1] coordination of the sodium ions. Facile Na−Li ion-exchange occurs upon reflux with lithium salts, upon electrochemical cycling in a cell (vs. Li), and also in a cell simply equilibrated at OCV. The material does not exhibit typical two-phase behavior on electrochemical cycling. A combination of a redox process which occurs with little structural strain, and ion scrambling give rise to a solid solution-like sloping voltage profile on charge−discharge, although localization of the Fe2+/3+ in the mixed valence single phase intermediate, Na1.5FePO4F drives a very small structural distortion. Temperature-dependent Mössbauer spectroscopy measurements confirm this localization, at least on short time scales (10−8 s), which persists to 370 °C. Finally, polycrystalline powders of other members of this family of compounds (Na2CoPO4F, Na2NiPO4F) were synthesized for the first time. Na2CoPO4F has a Co2+/Co3+ potential near 4.8 V. Mixed-metal phosphates of the form Na2(Fe1−xMx)PO4F, where M = Co, Mg, were also synthesized and found to be promising positive electrode materials for Li-ion or Na-ion energy storage devices.