In operando formation of new iron-oxyfluoride host structure for Na-ion storage from NaF–FeO nanocomposite

Abstract

Herein, we report a cubic FeOF structure as a new reversible Na ion host, which is formed during the electrochemical reaction from an electrode based on a nanosized mixture of NaF and FeO. This new FeOF host structure differs from the well-known rutile FeOF structure and displays distinct electrochemical properties as an electrode for Na-ion batteries. The X-ray absorption spectra measured at various cycles indicate that the cubic FeOF host structure gradually forms as cycling progresses. After formation of the cubic FeOF host structure, a discharge capacity of 165 mAh/g is delivered, corresponding to the intercalation of 0.7 Na+ ions per Fe with a volume change of 2.3% during charge and discharge, which is offered at a higher redox potential than that of the rutile FeOF electrode by approximately 0.2 V.