Mg Substitution Induced TM/Vacancy Disordering and Enhanced Structural Stability in Layered Oxide Cathode Materials

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.