Reversible Sodium and Lithium Insertion in Iron Fluoride Perovskites

Abstract

Li-ion batteries are omnipresent in consumer electronics and are seen as the most promising technology for electric vehicles. Na-ion batteries have emerged as viable and cheaper alternatives for stationary applications where Li-ion batteries are too expensive. However, the larger size of sodium ion compared to lithium makes traditional positive materials for Li-ion batteries not always suitable for the reversible insertion of sodium ions. Herein, a microwave-assisted solution synthesis of NaFeF3 perovskite nanoparticles from presynthesized rutile FeF2 colloidal particles, sodium ethoxide, and ammonium fluoride is presented. This NaFeF3 material shows a reversible electrochemical activity of 1Na or 1Li per iron with low polarization and excellent capacity retention after 100 cycles. The unexpected reversible insertion of both sodium and lithium ions, herein studied through ex situ and operando X-ray diffraction measurements, is attributed to a kinetic stabilization of corner-shared cubic AxFeF3 (A = Li, Na) frameworks along the cycles involving low volume change without high thermodynamic cost as supported by a polymorphism theoretical analysis.