Impact of\nMg Substitution on the Structure, Stability,\nand Properties of the Na<sub>2</sub>Fe<sub>2</sub>F<sub>7</sub> Weberite\nCathode

Of the few weberite-type Na-ion cathodes explored to\ndate, Na<sub>2</sub>Fe<sub>2</sub>F<sub>7</sub> exhibits the best\nperformance,\nwith capacities up to 184 mAh/g and energy densities up to 550 Wh/kg\nreported for this material. However, the development of robust structure–property\nrelationships for this material is complicated by its tendency to\nform as a mixture of metastable polymorphs, and transform to a lower-energy\nNa<i><sub>y</sub></i>FeF<sub>3</sub> perovskite compound\nduring electrochemical cycling. Our first-principles-guided exploration\nof Fe-based weberite solid solutions with redox-inactive Mg<sup>2+</sup> and Al<sup>3+</sup> predicts an enhanced thermodynamic stability\nof Na<sub>2</sub>Mg<i><sub>x</sub></i>Fe<sub>2–<i>x</i></sub> F<sub>7</sub> as the Mg content is increased, and\nthe <i>x</i> = 0.125 composition is selected for further\nexploration. We demonstrate that the monoclinic polymorph (space group <i>C</i>2/c) of Na<sub>2</sub>Fe<sub>2</sub>F<sub>7</sub> (Mg0)\nand of a new Mg-substituted weberite composition, Na<sub>2</sub>Mg<sub>0.125</sub>Fe<sub>1.875</sub>F<sub>7</sub> (Mg0.125), can be isolated\nusing an optimized synthesis protocol. The impact of Mg substitution\non the stability of the weberite phase during electrochemical cycling,\nand on the extent and rate of Na (de)intercalation, is examined. Irrespective\nof the Mg content, we find that the weberite phase is retained when\ncycling over a narrow voltage window (2.8–4.0 V vs Na/Na<sup>+</sup>). Over a wider voltage range (1.9–4.0 V), Mg0 shows\nsteady capacity fade due to its transformation to the Na<i><sub>y</sub></i>FeF<sub>3</sub> perovskite phase, while Mg0.125 displays\nmore reversible cycling and a reduced phase transformation. Yet, Mg\nincorporation also leads to kinetically limited Na extraction and\na reduced overall capacity. These findings highlight the need for\nthe continued compositional optimization of weberite cathodes to improve\ntheir structural stability while maximizing their energy density.