Carbon-coated single crystal O3-NaFeO2 nanoflakes prepared via topochemical reaction for sodium-ion batteries

Layered O3-NaFeO2 with abundant raw material resources is a promising cathode material for sodium-ion batteries. However, the synthesis of O3-NaFeO2 without using Na2O2 as sodium source is greatly difficult with formation of the electrochemically inactive β-NaFeO2. In the present work, the single crystal O3-NaFeO2 nanoflakes have been synthesized via a facile solvothermal route without using Na2O2 as sodium source. Through the solvothermal treatment, the inactive α-Fe2O3 can be converted into active γ-Fe2O3 first and subsequently into the single crystal NaFeO2 nanoflakes via Na+/Fe3+ topochemical ion exchange reaction. A thin layer of carbon is further coated on NaFeO2 nanoflakes to enhance its electrode kinetics and structural stability. The carbon coated NaFeO2 cathode delivers a high reversible specific capacity of 89.6 mAh g−1 at 0.1C and exhibits 87.3% capacity retention after 100 cycles at 0.1C, maintaining the layered O3-structure. By using hard carbon as anode, a carbon coated NaFeO2//hard carbon full cell is successfully constructed, exhibiting good cyclability with 81.9% capacity retention after 100 cycles. The present work provides a novel synthesis strategy for developing O3-NaFeO2-based cathode for sustainable sodium-ion batteries.