Neutron diffraction studies of the Na-ion battery electrode materials NaCoCr2(PO4)(3), NaNiCr2(PO4)(3), and Na2Ni2Cr(PO4)(3)

The new compounds NaCoCr2(PO4)(3), NaNiCr2(PO4)(3), and Na2Ni2Cr(PO4)(3) were synthesized by sol-gel method and their crystal structures were determined by using neutron powder diffraction data. These compounds were characterized by galvanometric cycling and cyclic voltammetry. NaCoCr2(PO4)(3), NaNiCr2(PO4)(3), and Na2Ni2Cr(PO4)(3) crystallize with a stuffed alpha-CrPO4-type structure. The structure consists of a 3D-framework made of octahedra and tetrahedra that are sharing corners and/or edges generating channels along [100] and [010], in which the sodium atoms are located. Of significance, in the structures of NaNiCr2(PO4)(3), and Na2Ni2Cr(PO4)(3) a statistical disorder Ni2+/Cr3+ was observed on both the 8g and 4a atomic positions, whereas in NaCoCr2(PO4)(3) the statistical disorder Co2+/Cr3+ was only observed on the 8g atomic position. When tested as negative electrode materials, NaCoCr2(PO4)(3), NaNiCr2(PO4)(3), and Na2Ni2Cr(PO4)(3) delivered specific capacities of 352, 385, and 368 mA h g(-1), respectively, which attests to the electrochemical activity of sodium in these compounds. (C) 2016 Elsevier Inc. All rights reserved.The authors thank Qatar Foundation for the financial support allocated to the field of energy storage. They also thank the Center for Advanced Materials (CAM) in Qatar University for carrying out experiments