Impact of Ti and Zn Dual‐Substitution in P2 Type Na_2/3Ni_1/3Mn_2/3O₂ on Ni–Mn and Na‐Vacancy Ordering and Electrochemical Properties

Abstract 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[Ni_1/3Mn_2/3]O₂. Dual‐substituted Na_2/3[Ni_1/4Mn_1/2Ti_1/6Zn_1/12]O₂ 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.