A layered sodium-ion host O3-NaFe0.5Ti0.5O2 enables Na-free cathodes through offering extractable Na-ions

Sodium-free cathodes and sodium-deficient cathodes are candidates for high performance sodium-ion batteries, for their high capacity when coupled with Na-metal anode that could provide extra Na+ ions. However, the growth of Na dendrites and safety concerns has long hindered their application. Herein, a new O3-type anode material NaFe0.5Ti0.5O2 was designed, which can provide extractable sodium-ions participating in electrochemical reactions mainly based on Fe2+/Fe3+ redox couple, that was realized for the first time in layered oxides. This anode possesses a reversible specific capacity of ∼90 mAh/g at 0.2 C rate, with excellent rate capability of 82.2 % at 10 C rate and a satisfactory capacity retention of 89.54 % after 650 cycles, showing a SEI-free feature. Coupled with the sodium-free cathode FeFe(CN)6, the full cell possesses a specific capacity of 135.9 mAh/g at 0.2 C rate with desired capacity retention of ∼90 % after 1000 cycles. Moreover, the O3-NaFe0.5Ti0.5O2 can also be applied as anode sodium compensation for the efficient utilization of the sodium-deficient cathode. This work provides a new approach for the development of sodium-ion full batteries.