Theoretical understanding of oxygen stability in Mn–Fe binary layered oxides for sodium-ion batteries

Abstract

Exploiting oxygen redox reactions (ORRs) in sodium layered oxides is a breakthrough for overcoming the intrinsic low energy density of sodium-ion batteries (SIBs), where Li-excess transition metal (TM) layers are considered requisite for the ORRs during (de)sodiation. However, non-Li-excess Mn–Fe binary oxides have emerged as viable OR-based cathode materials, although stabilizing the reversible oxygen capacity to harness the full OR potential remains challenging. Considering the ORR mechanisms in NaFeO₂, those in Na_1−x[Mn_1/2Fe_1/2]O₂ were elucidated by using the “selective and successive ORRs” mechanism to unlock the origin of cycle retention degradation. The thermodynamic formation energies revealed that the oxygen stability in the Mn–Fe oxides with x = 0.75 and above varies with the coordination number of the TM neighboring the oxygen ions; that is, the oxygen stability dominantly declines at Fe-rich oxygen ions upon charging. The electronic structures of the Fe- and Mn-rich O(2p) ions reconfirmed the selective OR in Mn–Fe oxide with 0.5 ≤ x ≤ 0.75 and confirmed successive anion redox processes after the breakpoint (x = 0.75). The two-type OR mechanism mainly originates from the Fe-rich oxygen ions over the crystal framework. Analysis of the crystal orbital overlap populations showed that reorientation of the Fe³⁺(3d)–O(2p) bonds comprising Fe-rich oxygen ions was an intriguing trigger of the latter ORR upon deep desodiation. This unified concept of the Mn–Fe model over the full ORR reveals the origin of the oxygen (in)stability and consequent unstable cycle retention, and is expected to be universal for Mn-based binary oxide cathodes for advanced SIBs.