Impact of Mg Substitution on the Structure, Stability, and Properties of the Na2Fe2F7 Weberite Cathode.

Of the few weberite-type Na-ion cathodes explored to date, Na₂Fe₂F₇ exhibits the best performance, with capacities up to 184 mAh/g and energy densities up to 550 Wh/kg reported for this material. However, the development of robust structure-property relationships for this material is complicated by its tendency to form as a mixture of metastable polymorphs, and transform to a lower-energy Na _y FeF₃ perovskite compound during electrochemical cycling. Our first-principles-guided exploration of Fe-based weberite solid solutions with redox-inactive Mg²⁺ and Al³⁺ predicts an enhanced thermodynamic stability of Na₂Mg _x Fe_2-x F₇ as the Mg content is increased, and the x = 0.125 composition is selected for further exploration. We demonstrate that the monoclinic polymorph (space group C2/c) of Na₂Fe₂F₇ (Mg0) and of a new Mg-substituted weberite composition, Na₂Mg_0.125Fe_1.875F₇ (Mg0.125), can be isolated using an optimized synthesis protocol. The impact of Mg substitution on the stability of the weberite phase during electrochemical cycling, and on the extent and rate of Na (de)intercalation, is examined. Irrespective of the Mg content, we find that the weberite phase is retained when cycling over a narrow voltage window (2.8-4.0 V vs Na/Na⁺). Over a wider voltage range (1.9-4.0 V), Mg0 shows steady capacity fade due to its transformation to the Na _y FeF₃ perovskite phase, while Mg0.125 displays more reversible cycling and a reduced phase transformation. Yet, Mg incorporation also leads to kinetically limited Na extraction and a reduced overall capacity. These findings highlight the need for the continued compositional optimization of weberite cathodes to improve their structural stability while maximizing their energy density.