Spinel-type MgxMn2-yFeyO4 as a new electrode for sodium ion batteries

Abstract

It is known that a certain amount (x) of lithium can be reversibly (de)inserted in the spinel-type LixMn2O4, while the structure is preserved. In contrast to lithium-containing spinel, it is believed that the sodium-containing spinel-type NaxMn2O4 is less stable compared to layered phases and this structure instability precludes from using as electrode for sodium batteries. The large size of sodium and the structure distortion which is induced by the Jahn-Teller effect of Mn(III) are the main reasons behind the instability of this spinel. The doping with other elements to improve the stability and electrochemistry of the sodium-spinels has been little explored. Replacing some manganese ions by non-Jahn-Teller elements can suppress the tetragonal distortion. Cations which bond to oxygen more strongly that sodium and that can be tetrahedrally coordinated may stabilize the framework, while sodium ions are reversibly (de)inserted. In addition, it is preferred employing abundant and nontoxic elements. Having all this in mind, we have evaluated nanostructured spinels MgxMn2-yFeyO4 (0 ≤ y ≤ 2) as new electrode active materials for sodium batteries, and the impact of several conditions on the electrochemistry are considered. Magnesium can stabilize the spinel framework and iron can decrease the decomposition of the electrolyte solution. The proton/metal exchange property has been also employed to change the spinel composition. On the other hand, theoretical calculations based on DFT are performed. The results open new possibilities for reversible intercalation of sodium into oxyspinels.