NaF-FeF2 nanocomposite: New type of Na-ion battery cathode material

Na-ion batteries (NIBs) are considered one of the most attractive alternatives for Li-ion batteries (LIBs) because of the natural abundance of Na and the similarities between the NIB technology and the well-established LIB technology. However, the discovery of high-performance electrode materials remains a key factor in the success of NIBs. Herein, we propose a new type of cathode material for NIBs based on a nanocomposite of an alkali metal fluoride (NaF) and a transition metal fluoride (FeF2). Although neither of these components is electrochemically active with Na, the nanoscale mixture of the two can deliver a reversible capacity of similar to 125 mAh/g in the voltage range of 1.2-4.8 V vs. Na/Na+ via an Fe2+/Fe3+ redox couple. X-ray absorption spectroscopy reveals that the reversible Na storage is aided by the F-ions due to the decomposition of NaF, which are absorbed on the surface of FeF2, promoting the redox reaction of Fe and triggering the gradual transformation of the mother structure (FeF2) into a new (FeF3-like) host structure for the Na ions. This unique Na-ion storage phenomenon, which is reported for the first time, will introduce an avenue for designing novel cathode materials for NIBs.