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

Abstract

The effect of the 4d Ru element in P2-Na-0.6[Mg0.2Ru0.2Mn0.6]O-2 is investigated. Ru-free Na-0.6[Mg0.2Mn0.8]O-2 is activated with Mn3+/Mn4+ redox, after which the charge is compensated by the sluggish oxidation of lattice oxygen (O2-) to O-2(n-) triggered by the evolution of the O-2 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 O-2 in the structure. The associated reaction progresses via the active Mn4+/Mn3+ and O2-/(O-2)(n-) reaction in addition to the Ru5+/Ru4+/Ru3+ redox pairs, enabling a capacity increase (similar to 210 mAh g(-1)), with similar to 72.1% retention for 300 cycles.