Impact of Ti and Zn Dual-Substitution in P2 Type Na2/3Ni1/3Mn2/3O2 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 TiIV for MnIV and ZnII for NiII in P2-Na2/3[Ni1/3Mn2/3]O2. Dual-substituted Na2/3[Ni1/4Mn1/2Ti1/6Zn1/12]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 TiIV and ZnII uniquely promotes in-plane NiII–MnIV ordering, which is quite different from the disordered mixing in conventional multiple metal substitution.