Synthesis, Crystal Structure, and Properties of the Alluaudite-Type Vanadates Ag2-xNaxMn2Fe(VO4)3

The new members of the Ag2-xNaxMn2Fe(VO4)3 (0 ≤ x ≤ 2) solid solution were synthesized by a solid-state reaction route, and their crystal structures were determined from single-crystal X-ray diffraction data. The physical properties were characterized by Mössbauer and electrochemical impedance spectroscopies, galvanostatic cycling, and cyclic voltammetry. These materials crystallize with a monoclinic symmetry (space group C2/c), and the structure is considered to be a new member of the AA'MM'2(XO4)3 alluaudite family. The A, A', M, and X sites are fully occupied by Ag(+)/Na(+), Ag(+)/Na(+), Mn(2+), and V(5+), respectively, whereas a Mn(2+)/Fe(3+) mixture is observed in the M' site. The Mössbauer spectra confirm that iron is trivalent. The impedance measurements indicate that the silver phase is a better conductor than the sodium phase. Furthermore, these phases exhibit ionic conductivities 2 orders of magnitude higher than those of the homologous phosphates. The electrochemical tests prove that Na2Mn2Fe(VO4)3 is active as positive and negative electrodes in sodium-ion batteries.