Local Construction of Mn-Based Layered Cathodes through Covalency Modulation for Sodium-Ion Batteries.

P2-type Mn-based layered oxides are among the most prevalent cathodes for sodium-ion batteries (SIBs) owing to their low cost, resource abundance, and high theoretical specific capacity. However, they usually suffer from Jahn-Teller (J-T) distortion from high-spin Mn and poor cycling stability, resulting in rapid degradation of their structural and electrochemical properties. Herein, a stable P2-type Mn-based layered oxide is realized through a local construction strategy by introducing high-valence Ru to overcome these issues. It has been revealed that the Ru substitution in the as-constructed NaMgMnRuO (NMMRO) renders the following favorable effects. First, the detrimental P2-OP4 phase transition is effectively inhibited owing to the robust Ru-O covalency bond. Second, the Mg/Mn ordering is disturbed and the out-of-plane displacement of Mg and in-plane migration of Mn are suppressed, leading to improved structural stability. Third, the redox ability of Mn is increased by weakening the covalence between Mn and O through the local Ru-O-Mn configurations, which contributes to the attenuated J-T distortion. Last, the strong Ru-O covalency bond also leads to enhanced electron delocalization between Ru and O, which decreases the oxidation of oxygen anion and thereby reduces the driving force of metal migration. Because of these advantages, the structural integrity and electrochemical properties of NMMRO are largely improved compared with the Ru-free counterpart. This work provides deeper insights into the effect of local modulation for cationic/anionic redox-active cathodes for high-performance SIBs.