Na2FeF4 as a stable cathode material for Na-ion batteries

In the search of cathode materials for Na-ion batteries, iron-based compounds have attracted much attention due to the abundant resource, easy access, and environmental friendliness of iron. Herein, we report the synthesis, structure, and electrochemistry of a previously unknown compound in the Na–Fe–F system, formulated as Na2FeF4. It is prepared by an easy and mild hydrothermal reaction with oxalate as a Fe2+ protector. Based on the single crystal x-ray diffraction analysis, it crystallizes in space group Pmcb with a = 3.255 (3) Å, b = 5.591(7) Å, and c = 9.557(1) Å. The crystal structure features edge-sharing FeF6 octahedra to form [FeF4]∞ chains with Na+ ions located between chains. In electrochemical investigations, it is demonstrated that the material can deliver a reversible capacity of ∼90 mAh g−1 for 300 cycles in the window of 1.5–4.3 V with redox reactions at ∼3.0 V (vs Na+/Na). Such an activity originates from the Fe3+/Fe2+ redox couple, confirmed by x-ray absorption spectra and first-principles calculations.