Revisiting the layered Na3Fe3(PO4)4 phosphate sodium insertion compound: Structure, magnetic and electrochemical study

Abstract

Layered sodium iron phosphate phase [Na3Fe3(PO4)4] was synthesized by solution combustion synthesis method, marking the first attempt of solvothermal synthesis of this phase. Its crystal structure was verified by synchrotron and neutron powder diffraction. Rietveld analyses proved the phase purity and formation of monoclinic framework with C2/c symmetry. It undergoes an antiferromagnetic ordering ∼27 K. This combustion prepared nanoscale Na3Fe3(PO4)4 compound was found to be electrochemically active with a stepwise voltage profile involving an Fe3+/Fe2+ redox activity centred at 2.43 V vs. Na/Na+. Despite various cathode optimization, only 1.8 Na+ per formula unit could be reversibly inserted into the Na3Fe3(PO4)4 framework leading to capacity close to 50 mAh g-1. This limited electrochemical activity can be rooted to (i) relatively large diffusion barrier (ca. 0.28 eV) as per Bond valence site energy (BVSE) calculations and (ii) possible structural instability during (de)sodiation reaction.