Na_x(Cu–Fe–Mn)O₂ system as cathode materials for Na-ion batteries

Sodium-ion containing layered metal oxides have shown excellent performances as a high-energy cathode material for sodium-ion batteries. However, the role of 3d metal cations as well as their interactions in this materials system have not been well understood, which also limits their performance improvement. In this study, we systematically investigated the electrochemical behaviors and the roles of metal cations of layered Na_x(Cu–Fe–Mn)O₂ system with variable 3d transition metal elemental compositions. We observed three apparent phenomena. Firstly, a strong co-relationship between the pristine valence state of manganese (Mn) and the reversible capacity was observed, with the highest reversible capacity reaching 190 mA h g⁻¹ for the composition (i.e., Na_0.75(Cu_0.1Fe_0.1Mn_0.8)O₂) with the lowest pristine valence state of Mn. Secondly, it was observed that the average potential of the Na_x(Cu–Fe–Mn)O₂ system decreased with the increase of iron (Fe) composition. Thirdly, adding a proper amount of copper (Cu) was found to promote the rate performance and cyclability of the electrode material. To understand the underlying mechanism for such composition-dependent electrochemical behavior, we characterized the capacity of the electrode as a function of potential evolution and analyzed its relationship with the interactions between the metal cations. Interestingly, we found the variation in the Mn/Fe ratio could result in shifts in the redox potentials of Fe³⁺/Fe⁴⁺ and Mn³⁺/Mn⁴⁺ couples, which might be associated with superexchange interaction and Jahn-Teller effect. These findings shed light on design of new layered metal oxides for batteries with improved performances.