Enhanced Cycling Stability in the Anion Redox Material P3‐Type Zn‐Substituted Sodium Manganese Oxide

Abstract

Sodium layered oxides showing oxygen redox activity are promising positive electrodes for sodium-ion batteries (SIBs). However, structural degradation typically results in limited reversibility of the oxygen redox activity. Herein, the effect of Zn-doping on the electrochemical properties of P3-type sodium manganese oxide, synthesised under air and oxygen is investigated for the first time. Air-Na 0.67 Mn 0.9 Zn 0.1 O 2 and Oxy-Na 0.67 Mn 0.9 Zn 0.1 O 2 exhibit stable cycling performance between 1.8 and 3.8 V, each maintaining 96 % of their initial capacity after 30 cycles, where Mn 3 + /Mn 4 + redox dominates. Increasing the voltage range to 1.8-4.3 V activates oxygen redox. For the material synthesised under air, oxygen redox activity is based on Zn, with limited reversibility. The additional transition metal vacancies in the material synthesised under oxygen result in enhanced oxygen redox reversibility with small voltage hysteresis. These results may assist the development of high-capacity and structurally stable oxygen redox-based materials for SIBs.

Effect of Ti‐Substitution on the Properties of P3 Structure Na2/3Mn0.8Li0.2O2 Showing a Ribbon Superlattice

“…Changes in the integral of these peaks between 527 and 533 eV are given in Figure 8 (b), relative to the pristine material and relate to changes in the density of empty states just above the Fermi level which are influenced by the oxidation state of Mn and the covalency of the O 2 p band [43] . Figure 8 (b) reveals variations in the intensity which are consistent with changes observed in the Mn L‐edge spectra (Figure 7) and can be attributed to hybridization between the O 2 p states and TM 3 d states [22,27,44] . After charge to 4.1 V, Figure 8 (b) shows a decrease in intensity which is consistent with the appearance of Mn 3+ features (Figure 7 (a)).…”

Effect of Cu substitution on anion redox behaviour in P3-type sodium manganese oxides

“…Na + ion insertion), showing that this trend matches the level of sodiation (figure S2). These data are analogous to those observed for Zn-substituted P3-Na 0.67 Mn 0.9 Zn 0.1 O 2 [26].…”

“…Furthermore, figure 4(c) shows broader, asymmetric diffraction peaks suggestive of O-type stacking faults. Similar structural distortions (P3→P ′ 3) have been observed for P3-type Mg-and Zn-substituted counterparts after being charged to 4.3 V [26,39]. It should be noted that upon initial charge to 4.3 V, more Na + ions (0.43 Na) are extracted from Na 0.67 Mn 0.9 Cu 0.1 O 2 compared with Na 0.67 Mn 0.9 Zn 0.1 O 2 (0.35 Na) and Na 0.67 Mn 0.9 Mg 0.1 O 2 (0.39 Na) thereby resulting in greater Coulombic repulsion.…”

“…Whereas the Zn-substituted material, Na 0.67 Mn 0.9 Zn 0.1 O 2 shows a reduced initial discharge capacity of 185 mA h g −1 compared to the Cu-substituted material but reveals enhanced cycling performance, retaining 84% of its initial discharge capacity after 40 cycles. Comparatively, based on earlier structural studies of P3-Na 0.67 Mn 0.9 M 0.1 O 2 (M = Zn, Mg) [26,39], the substitution of Mn by Zn provides a more stable framework for Na + ion insertion/extraction than substitution by Mg or Cu. These data are consistent with O3-NaNi 0.45 M 0.5 Mn 0.4 Ti 0.1 O 2 (where M = Mg, Zn, Cu) which similarly showed that the Zn-substituted material displayed superior cycling stability [53].…”

“…To enhance the performance of oxygen anion redox, it is important to stabilise labile oxygen and suppress irreversible structural changes. Partial substitution of Mn by Ni 2+ , Zn 2+ , and Cu 2+ and/or the presence of transition metal vacancies promote reversible oxygen redox whilst maintaining structural stability and suppressing voltage hysteresis [17][18][19][20][21][22][23][24][25][26]. For instance, Kim et al reported a high initial discharge capacity of 204 mA h g −1 for P3-Na 0.67 Mn 0.8 Ni 0.2 O 2 , of which the Mn 3+ /Mn 4+ , Ni 2+ /Ni 3+ /Ni 4+ and O 2− /(O 2 ) n− redox couples contributed toward the charge mechanism, as evidenced by x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray spectroscopy (RIXS).…”