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

Abstract The Na 4 VO(PO 4 ) 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 VO 6 and PO 4 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 Na 4 VO(PO 4 ) 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 Na 3.2 VO(PO 4 ) 2 and to insert as well one sodium at low potential 1.5 V, to form through a solid solution process the reduced phase Na 5 VO(PO 4 ) 2 . This study is a proof of the great versatility of vanadium V 3+ /V 4+ /V 5+ ions in Na-based batteries.