Abstract
Sodium and iron make up the perfect combination for the growing demand for sustainable energy storage systems, given the natural abundance and sustainability of the two building block elements. However, most sodium-iron electrode chemistries are plagued by intrinsic low energy densities with continuous ongoing efforts to solve this. Herein, the chemical space of a series of (meta)stable, off-stoichiometric Fe-PO(4) -F phases is analyzed. Some are found to display markedly improved electrochemical activity for sodium storage, as compared to the amorphous or thermodynamically stable phases of equivalent composition. The metastable crystalline Na(1.2) Fe(1.2) PO(4) F(0.6) delivers a reversible capacity of more than 140 mAh g(-1) with an average discharge potential of 2.9 V (vs Na(+) /Na(0) ) resulting in a practical specific energy density of 400 Wh kg(-1) (estimated at the material level), outperforming many developed Fe-PO(4) analogs thus far, with further multiple possibilities to be explored toward improved energy storage metrics. Overall, this study unlocks the possibilities of off-stoichiometric Fe-PO(4) -F cathode materials and reveals the importance to explore the oft-overlooked metastable or transient state materials for energy storage.