Anionic redox is an effective way to increase the capacity of the cathode materials. Na₂Mn₃O₇ [Na_4/7[Mn_6/7□_1/7]O₂, □ for the transition metal (TM) vacancies] with native and ordered TM vacancies can conduct a reversible oxygen redox and be a promising high-energy cathode material for sodium-ion batteries (SIBs). However, its phase transition at low potentials (∼1.5 V vs Na⁺/Na) induces potential decays. Herein, magnesium (Mg) is doped on the TM vacancies to form a disordered Mn/Mg/□ arrangement in the TM layer. The Mg substitution suppresses the oxygen oxidation at ∼4.2 V by reducing the number of the Na–O–□ configurations. Meanwhile, this flexible disordering structure inhibits the generation of the dissolvable Mn²⁺ ions and mitigates the phase transition at ∼1.6 V. Therefore, the Mg doping improves the structural stability and its cycling performance in 1.5–4.5 V. The disordering arrangement endows Na_0.49Mn_0.86Mg_0.06□_0.08O₂ with a higher Na⁺ diffusivity and improved rate performance. Our study reveals that oxygen oxidation is highly dependent on the ordering/disordering arrangements in the cathode materials. This work provides insights into the balance of anionic and cationic redox for enhancing the structural stability and electrochemical performance in the SIBs.

中文翻译：

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层状氧化物正极材料中的 Mg 取代诱导 TM/空位无序化和增强的结构稳定性

阴离子氧化还原是提高正极材料容量的有效途径。Na ₂ Mn ₃ O ₇ [Na _4/7 [Mn _6/7 □ _1/7 ]O ₂ , □ for the transition metal (TM) vacancies] 具有天然和有序的TM空位可以进行可逆的氧氧化还原并且是有前途的用于钠离子电池 (SIB) 的高能正极材料。然而，它在低电位下的相变（~1.5 V vs Na ⁺/Na) 引起电位衰减。此处，镁(Mg)掺杂在TM空位上，在TM层中形成无序排列的Mn/Mg/□。Mg 取代通过减少 Na-O-□ 构型的数量来抑制 ~4.2 V 的氧氧化。^{同时，这种灵活的无序结构抑制了可溶性 Mn 2+}离子的产生并减缓了~1.6 V 的相变。因此，Mg 掺杂提高了结构稳定性及其在 1.5-4.5 V 的循环性能。无序排列赋予 Na _0.49 Mn _0.86 Mg _0.06 □ _0.08 O ₂具有较高的Na ⁺扩散率和改进的倍率性能。我们的研究表明，氧氧化高度依赖于阴极材料中的有序/无序排列。这项工作提供了对阴离子和阳离子氧化还原平衡的见解，以增强 SIB 的结构稳定性和电化学性能。