An improved and scalable method for the preparation of Sn-doped hexagonal tungsten bronze-type iron fluoride materials as cathodes for sodium-ion batteries

Abstract

With their potentially high energy density and high safety, FeF₃ materials are viewed as very promising candidate cathode materials for the next generation of lightweight SIBs (sodium-ion batteries). However, so far, the fabrication procedures for FeF₃ materials have commonly entailed high-energy ball milling, solvothermal synthesis, utilization of costly ionic liquids, and even handling highly hazardous gases, rendering the preparation process intricate and impeding the successful incorporation of heteroatom doping into FeF₃ materials. Herein, an improved and scalable aqueous solution approach is developed and implemented to obtain FeF₃·0.33H₂O (hexagonal tungsten bronze-type) materials. Currently, in the field of improving the cycling performance of FeF₃ materials through metal doping, research focus has been primarily on transition metal doping, with limited investigation into non-transition metal doping. In non-transition metals, due to the similar electronegativity of Sn and Fe and the low cost of Sn, the electrochemical characteristics of Sn-doped FeF₃·0.33H₂O were investigated in this work. Meanwhile, the utilization of DFT reveals that Sn doping increases the atomic cloud density at neighboring Fe sites and expands the unit cell, consequently enhancing the conductivity and ion diffusion rate of the material. The synthesis method is simple and practical, providing a reliable experimental approach for promoting the commercialization of FeF₃·0.33H₂O.