Synthesis, Crystal Structure and Properties of a New Phosphate, Na2Co2Cr(PO4)(3)

Abstract : In this work, a novel phosphate Na2Co2Cr(PO4)(3) was synthesized by sol-gel method, and its crystal structure was determined from powder X-ray diffraction data using Rietveld method. This new phosphate was further characterized by scanning electron microscopy, IR and Raman spectroscopy. This phosphate crystallizes in the well-known alluaudite-type structure with monoclinc symmetry (S.G. C 2/c). The structure of this phosphate consists of [CoO6] octahedra, [(Co,Cr)(2)O-10] dimer-octahedra and PO4 tetrahedra leading to an open three-dimensional framework containing two types of tunnels running along [001] direction, where Na+ cations are located. The magnetic measurements confirmed that the predominant interactions in this phosphate are antiferromagnetic with a Curie-Weiss constant of =-4.57K. The electrochemical properties of Na2Co2Cr(PO4)(3) as anode material for sodium-ion batteries were assessed using a galvanostatic mode and cyclic voltammetry in the potential window of 0.02-3V. The electrochemical behavior was investigated by ex situ X-ray diffraction. The results showed that this material undergoes a conversion-type reaction.