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 (FeF3-1ike) 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.