Oxygen redox promoted by Na excess and covalency in hexagonal and monoclinic Na2−xRuO3 polymorphs

M. H.N. Assadi, Masashi Okubo, Atsuo Yamada, Yoshitaka Tateyama

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)

Abstract

Resorting to oxygen redox in addition to that of transition metal in oxide cathode materials can increase the capacity of rechargeable Na ions batteries. Through comprehensive density functional calculations, we demonstrate dominant oxygen participation in the redox reaction in cation disordered hexagonal and ordered monoclinic polymorphs of Na2xRuO3 (0 ≤ x ≤ 0.75). In both polymorphs, when O ions are coordinated by more than three Na ions, unhybridized orphaned O 2p states are lifted closer to the Fermi level and therefore become accessible for the redox reaction. Moreover, high Ru-O covalency promotes greater 2p-4d hybridization at the top of the valence band further increasing the density of states of the electrochemically labile O 2p orbitals near the Fermi level. Consequently, throughout cycling, the contribution of the O 2p states to the charge compensation mechanism becomes nearly as twice as that of Ru 4d states in both polymorphs. Due to broader dispersion of the O 2p states near the Fermi level, the cation disordered polymorph, nonetheless, has a higher voltage of 2.458 V compared to the voltage of the cation ordered polymorph of 2.243 V.

Original languageEnglish
Pages (from-to)A5343-A5348
JournalJournal of the Electrochemical Society
Volume166
Issue number3
DOIs
Publication statusPublished - 2019
Externally publishedYes

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Renewable Energy, Sustainability and the Environment
  • Surfaces, Coatings and Films
  • Electrochemistry
  • Materials Chemistry

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