Structure and Dynamics of Fluorophosphate Na-Ion Battery Cathodes

Fluorophosphate cathodes with the chemical formula NaxV2(PO4)2O2yF3–2y (0 ≤ x ≤ 4, 0 ≤ y ≤ 1) are some of the few known sodium-ion cathode materials with the potential to be competitive with conventional lithium-ion cathodes. However, the experimentally accessible performance of the fluorophosphates remains limited, primarily due to the fact that only half of the theoretical capacity has been reversibly cycled. In this article, we review the extensive body of literature on the fluorophosphate class of sodium-ion cathodes and, in combination with our own ab initio model of the material, investigate the mechanisms underlying the sodium-extraction limitations in the NaxV2(PO4)2F3 (y = 0) fluorophosphate. Specifically, we focus on the potential to reversibly extract sodium beyond the 1 ≤ x ≤ 3 range. We find that this limitation arises from a combination of the high voltage of the V4+/5+ oxidation reaction associated with sodium extraction in the 0 ≤ x ≤ 1 region and a precipitous drop in sodium diffusivity near the x = 1 composition due to the presence of a strong ordering, which prevents the formation of mobile defects in the structure. We conclude that the accessible capacity of NaxV2(PO4)2F3 can potentially be expanded to 0 ≤ x ≤ 3 by introducing defects into the material and reducing the voltage of the transition metal redox couple, both of which can likely be achieved via transition metal substitution and aliovalent anion doping.