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 [Ni1/3 Mn2/3 ]O2 . Dual-substituted Na2/3 [Ni1/4 Mn1/2 Ti1/6 Zn1/12 ]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.
Keywords: cathode materials; intercalation; layered oxides; sodium batteries; structural analysis; substitution; superstructure.
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