Ag3V2(PO4)2F3, a new compound obtained by Ag+/Na+ ion exchange into the Na3V2(PO4)2F3 framework

Abstract

Phosphate polyanionic compounds have been used for several technological applications and are especially widespread in battery research. Na₃V₂(PO₄)₂F₃ is a material that holds great promise as a positive electrode for Na-ion batteries. We study here the Ag⁺/Na⁺ ion exchange that is possible due to the high ionic mobility of Na⁺ in this material. A nearly complete ion exchange was obtained and we report the crystal structure of the new orthorhombic phase Ag₃V₂(PO₄)₂F₃, determined by synchrotron X-ray and neutron powder diffraction. Silver occupies the same crystallographic sites as sodium and, except for the differences caused by steric effects, Ag₃V₂(PO₄)₂F₃ preserves the symmetry constraints already present in the parent compound Na₃V₂(PO₄)₂F₃. We also followed the evolution of the crystal structure upon heating/cooling, to observe an order/disorder transition analogous to the one already reported for Na₃V₂(PO₄)₂F₃, but occurring at a significantly higher temperature.