Investigation of the Inorganic Compounds NaMV(PO) (M = Fe, Co, Ni) as Anode Materials for Sodium-Ion Batteries.

The new compounds NaMV(PO) (M = Fe, Co, Ni) were synthesized via a sol-gel synthesis route, and their crystal structures were refined using the Rietveld method from X-ray powder diffraction data. NaCoV(PO) was also characterized by TGA, cyclic voltammetry, and galvanostatic cycling. The three phases crystallize with the orthorhombic symmetry and the space group . The structures are isotypic to the stuffed α-CrPO-type structure and contain two vacant sites in which two sodium atoms can be intercalated. When NaCoV(PO) is cycled at a 1C rate in the voltage ranges of 0.1-3 and 0.7-3 V vs Na/Na, it delivers specific capacities of 190 and 75 mA h/g, respectively, with an average operation potential of ∼1.4 V. This attests to the electrochemical activity of this compound and indicates that the α-CrPO-type compounds could be suitable for hosting other guest ions.