New Cathode Materials in the Fe‐PO4‐F Chemical Space for High‐Performance Sodium‐Ion Storage. Issue 22 (26th May 2022) - Liu, Xuelian
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‐PO4 ‐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 Na1.2 Fe1.2 PO4 F0.6 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‐PO4 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‐PO4 ‐F cathode materials and reveals the importance to explore the oft‐overlooked metastable or transient state materials for energy storage. Abstract : A series of new cathode materials are developed with combinations of not only the advantages in Na‐Fe‐PO4 ‐F building blocks but also compositional diversity and phase activity in this system to attain low‐cost, sustainable, and high‐performance sodium storage materials. The metastable Na1.2 Fe1.2 PO4 F0.6 material delivers an energy density of nearly 400 Wh kg −1 at the material level, holding promises for Na‐ion batteries.