Structural evolution of high energy density V3+/V4+ mixed valent Na3V2O2x(PO4)2F3−2x (x = 0.8) sodium vanadium fluorophosphate using in situ synchrotron X-ray powder diffraction

Abstract

Sodium-ion batteries have become good candidates for energy storage technology. For this purpose it is crucial to search for and optimize new electrode and electrolyte materials. Sodium vanadium fluorophosphates are considered promising cathodes but further studies are required to elucidate their electrochemical and structural behavior. Therefore, this work focuses on the time-resolved in situ synchrotron X-ray powder diffraction study of Na₃V₂O_2x(PO₄)₂F_3−2x (x = 0.8) while electrochemically cycling. Reaction mechanism evolution, lattice parameters and sodium evolution, and the maximum possible sodium extraction under the applied electrochemical constraints, are some of the features that have been determined for both a fresh and an offline pre-cycled cell. The reaction mechanism evolution undergoes a solid solution reaction with a two-phase region for the first lower-potential plateau while a predominantly solid solution behavior is observed for the second higher-potential plateau. Lattice and volume evolution is clearly dependent on the Na insertion/extraction mechanism, the sodium occupancy and distribution amongst the two crystallographic sites, and the electrochemical cycling history. The comparison between the fresh and the pre-cycled cell shows that there is a Na site preference depending on the cell and history and that Na swaps from one site to the other during cycling. This suggests sodium site occupancy and mobility in the tunnels is interchangeable and fluid, a favorable characteristic for a cathode in a sodium-ion battery.