Effect of Composite Formation with Various Metals on Improving the Anode Performance of Hematite for Na-Ion Batteries

We prepared composite electrodes using hematite (α-Fe2O3) and various metals (Sn, Sb, Bi, In, Zn, and Al) and evaluated their charge–discharge performances as Na-ion battery anodes. When we used less Na-active metals (Al and Zn), steeper capacity decays were observed compared with an electrode of Fe2O3 alone. On the other hand, differences in performance were also observed when using various Na-active metals (Sn, Sb, Bi, and In). Only when we used Sb with a volumetric expansion ratio of 390% during its sodiation reaction, the cycle stability was significantly improved: a reversible capacity of 370 mA h g–1 was retained for 200 cycles. By using Bi with an expansion ratio of 300%, close to that of Sb, we confirmed that the capacity decline was improved to some extent. On the other hand, when we used Sn with a much larger expansion ratio of 520%, the cycle performance did not improve. From the above data, we have obtained the knowledge that the metal to be composited with Fe2O3 is preferably a material with (i) Na-active property, (ii) excellent electronic conductivity, and (iii) an appropriate ratio of volumetric expansion during the sodiation reaction.