Effect of Cu Substitution in P′2- and P2-Type Sodium Manganese-Based Oxides

Layered manganese oxides have been considerably studied toward sustainable sodium-ion battery materials. There has been a focus on the specific roles of Cu as a member of substituents in sodium manganese oxides. Herein, the effects of Cu substitution for Mn in P′2- and P2-type Na0.67[CuxMn1–x]O2 (0 ≤ x ≤ 0.33) are systematically investigated, focusing on the cooperative Jahn–Teller distortion of Mn3+ and Cu2+ ions. P2-type Na0.67[Cu0.2Mn0.8]O2 is selected to gain further insights on oxygen redox along with P2-type Na0.67[Cu0.1Mg0.1Mn0.8]O2 and P2-type Na0.67[Cu0.1Zn0.1Mn0.8]O2. Comparative studies among the three materials reveal that the type of substituent(s) for Mn has no impact on the activation of oxygen redox. In contrast, the nature and the concentration of substituted element(s) influence structural evolution during charge/discharge. P2-type Na0.67[Cu0.2Mn0.8O2] maintains a P2 phase without any secondary phase at the bulk and local scales at the end of charge to 4.5 V, leading to suppressed voltage hysteresis. At the end of discharge to 1.5 V, the presence of higher Cu2+ in P2-type Na0.67[Cu0.2Mn0.8]O2 results in a distorted P′2 phase with a distortion degree of 10.6(1)%. This can induce a large lattice stress related to the lattice mismatch between P2 and P′2, which jeopardizes cyclability.