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 ~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.