NaFeO ₂ : Possible Materials for Anode and Cathode of Na-Ion Batteries

The charge–discharge behaviors of α-NaFeO2 and β-NaFeO2 were evaluated for the first time as Na-ion battery anodes. We revealed that both phases showed reversible charge–discharge reactions based on the conversion reaction mechanism of Fe2O3 because of the phase change from NaFeO2 to Fe2O3 in the first sodiation. The mechanical milling treatment for NaFeO2 enhanced the discharge capacity, which is more significant for β-NaFeO2: it attained an excellent cycling performance with the discharge capacity of 590 mA g–1 at the 100th cycle. As the cathode material, no charge–discharge reaction of β-NaFeO2 was observed, whereas the α-NaFeO2 electrode exhibited a reversible charge–discharge reaction based on the insertion reaction mechanism. We assembled a full cell using of α-NaFeO2 anode and α-NaFeO2 cathode, and succeeded to operate reversible charge–discharge with the discharge voltage of 0.8–1.8 V and the discharge capacity of 90 mA h g–1. This is the first report that the full cell can operate using the same active material of NaFeO2 for the anode and cathode. Interestingly, the anode and the cathode can operate according to different charge–discharge mechanisms of the conversion reaction and the insertion reaction, respectively.