Migration of Mg in Na–O–Mg Configuration for Oxygen Redox of Sodium Cathode
Jun Ho Yu, Natalia Voronina, Najma Yaqoob, Sungkyu Kim, Anil Kumar Paidi, Docheon Ahn, Kyuwook Ihm, Keun Hwa Chae, Min‐Gi Jeong, Hun‐Gi Jung, Olivier Guillon, Payam Kaghazchi, Seung‐Taek Myung
Abstract
The effect of the 4d Ru element in P2-Na 0.6 [Mg 0.2 Ru 0.2 Mn 0.6 ]O 2 is investigated. Ru-free Na 0.6 [Mg 0.2 Mn 0.8 ]O 2 is activated with Mn 3+ /Mn 4+ redox, after which the charge is compensated by the sluggish oxidation of lattice oxygen (O 2– ) 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 Mg 2+ 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 Mn 4+ /Mn 3+ and O 2– /(O 2 ) n − reaction in addition to the Ru 5+ /Ru 4+ /Ru 3+ redox pairs, enabling a capacity increase (∼210 mAh g –1 ), with ∼72.1% retention for 300 cycles.