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

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 Na1-x[Mn1/2Fe1/2]O-2 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.