Migration of Mg in Na–O–Mg Configuration for Oxygen Redox of Sodium Cathode
journal contribution
posted on 2023-12-13, 16:42 authored by 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 MyungThe
effect of the 4d Ru element in P2-Na<sub>0.6</sub>[Mg<sub>0.2</sub>Ru<sub>0.2</sub>Mn<sub>0.6</sub>]O<sub>2</sub> is investigated. Ru-free
Na<sub>0.6</sub>[Mg<sub>0.2</sub>Mn<sub>0.8</sub>]O<sub>2</sub> is
activated with Mn<sup>3+</sup>/Mn<sup>4+</sup> redox, after which
the charge is compensated by the sluggish oxidation of lattice oxygen
(O<sup>2–</sup>) to O<sub>2</sub><sup><i>n</i>–</sup> triggered by the evolution of the O<sub>2</sub> from the oxide
lattice. These effects are generally unfavorable and result in poor
long-term cycle stability induced by the irreversible migration of
Mg<sup>2+</sup> 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<sub>2</sub> in
the structure. The associated reaction progresses via the active Mn<sup>4+</sup>/Mn<sup>3+</sup> and O<sup>2–</sup>/(O<sub>2</sub>)<sup><i>n</i>−</sup> reaction in addition to the
Ru<sup>5+</sup>/Ru<sup>4+</sup>/Ru<sup>3+</sup> redox pairs, enabling
a capacity increase (∼210 mAh g<sup>–1</sup>), with
∼72.1% retention for 300 cycles.
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