Migration of Mg in Na–O–Mg Configuration for Oxygen Redox of Sodium Cathode

The effect of the 4d Ru element in P2-Na0.6[Mg0.2Ru0.2Mn0.6]O2 is investigated. Ru-free Na0.6[Mg0.2Mn0.8]O2 is activated with Mn3+/Mn4+ redox, after which the charge is compensated by the sluggish oxidation of lattice oxygen (O2–) to O2n– triggered by the evolution of the O2 from the oxide lattice. These effects are generally unfavorable and result in poor long-term cycle stability induced by the irreversible migration of Mg2+ from the transition metal (TM) to Na layers in the P2 structural framework. Benefiting from the covalent Ru bonded with O in the TM layers, the Mg migration reversibly progresses from the TM to sodium slabs without the evolution of the O2 in the structure. The associated reaction progresses via the active Mn4+/Mn3+ and O2–/(O2)n− reaction in addition to the Ru5+/Ru4+/Ru3+ redox pairs, enabling a capacity increase (∼210 mAh g–1), with ∼72.1% retention for 300 cycles.