A vanadium oxy-phosphate Na4VO(PO4)2 as cathode material for Na ion batteries

The Na4VO(PO4)(2) phase has been synthesized using a solid state route in sealed tubes. This phase crystallizing in the orthorhombic system has a one-dimensional structure built up of corner shared VO6 and PO4 polyhedra. It is an ionic conductor with a conductivity of 10(-4)S/cm at 500 K and an activation energy of 0.63eV. The electrochemical properties of Na4VO(PO4)(2) as a positive electrode in a sodium ion battery have been studied. Results show the possibility of extracting almost one sodium through a two-phase process at 3.4 V leading to the phase Na3.2VO(PO4)(2) and to insert as well one sodium at low potential 1.5 V, to form through a solid solution process the reduced phase Na6VO(PO4)(2). This study is a proof of the great versatility of vanadium V3+/V4+/V5+ ions in Na-based batteries. (C) 2017 Elsevier Masson SAS. All rights reserved.