Composition Variation Reveals the Non-Ideal Mixing of Components in (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O and the Benefits of this Variation in Sodium Ion-Battery

The reversible transformation between a multiphase and single-phase state has been used as a hallmark for entropy stabilization in the system (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O. it was shown that cations are fully random and homogeneous in this single-phase material, providing a way to estimate its configurational entropy. The deliberate variation of this entropy led to the conclusion that (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O is primarily stabilized by this configurational entropy. Here, we show that this phase transformation observed in (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O could originate from a Jahn-Teller distortion rather than entropy stabilization. We further demonstrate through composition variation that the consolute temperature depends on the nature of the components rather than the number of components. In addition, we show that there is significant cation ordering in these systems, further casting doubts on the role of configurational entropy as the main stabilizing term. Finally, we demonstrate that this compositional variation can be used to tune the voltage of these complex metal oxides to improve their reactivity in a sodium-ion cell.