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.

One-dimensional ZnMn₂O₄ nanowires have been prepared and investigated as anode materials in Li rechargeable batteries. The highly crystalline ZnMn₂O₄ nanowires about 15 nm in width and 500 nm in length showed a high specific capacity of about 650 mAh·g^-1 at a current rate of 100 mA·g^-1 after 40 cycles. They also exhibited high power capability at elevated current rates, i.e., 450 and 350 mAh·g^-1 at current rates of 500 and 1000 mA·g^-1, respectively. Formation of Mn₃O₄ and ZnO phases was identified by ex situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies after the initial discharge-charge cycle, which indicates that the ZnMn₂O₄ phase was converted to a nanocomposite of Mn₃O₄ and ZnO phases immediately after the electrochemical conversion reaction.