Impact of Ti and Zn Dual-Substitution in P2 Type Na2/3Ni1/3Mn2/3O2 on Ni-Mn and Na-Vacancy Ordering and Electrochemical Properties

High-entropy layered oxide materials containing various metals that exhibit smooth voltage curves and excellent electrochemical performances have attracted attention in the development of positive electrode materials for sodium-ion batteries. However, a smooth voltage curve can be obtained by suppression of the Na+-vacancy ordering, and therefore, transition metal slabs do not need to be more multi-element than necessary. Here, the Na+-vacancy ordering is found to be disturbed by dual substitution of Ti-IV for Mn-IV and Zn-II for Ni-II in P2-Na-2/3[Ni1/3Mn2/3]O-2. Dual-substituted Na-2/3[Ni1/4Mn1/2Ti1/6Zn1/12]O-2 demonstrates almost non-step voltage curves with a reversible capacity of 114 mAh g(-1) and less structural changes with a high crystalline structure maintained during charging and discharging. Synchrotron X-ray, neutron, and electron diffraction measurements reveal that dual-substitution with Ti-IV and Zn-II uniquely promotes in-plane Ni-II-Mn-IV ordering, which is quite different from the disordered mixing in conventional multiple metal substitution.