Occurrence of anionic redox with absence of full oxidation to Ru5+ in high-energy P2-type layered oxide cathode

Abstract

The anionic redox has been widely studied in layered-oxide-cathodes in attempts to achieve high-energy-density for Na-ion batteries (NIBs). It is known that an oxidation state of Mn4+ or Ru5+ is essential for the anionic reaction of O2−/O− to occur during Na+ de/intercalation. However, here, we report that the anionic redox can occur in Ru-based layered-oxide-cathodes before full oxidation of Ru4+/Ru5+. Combining studies using first-principles calculation and experimental techniques reveals that further Na+ deintercalation from P2-Na0.33[Mg0.33Ru0.67]O2 is based on anionic oxidation after 0.33 mol Na+ deintercalation from P2-Na0.67[Mg0.33Ru0.67]O2 with cationic oxidation of Ru4+/Ru4.5+. Especially, it is revealed that the only oxygen neighboring 2Mg/1Ru can participate in the anionic redox during Na+ de/intercalation, which implies that the Na–O–Mg arrangement in the P2-Na0.33[Mg0.33Ru0.67]O2 structure can dramatically lower the thermodynamic stability of the anionic redox than that of cationic redox. Through the O anionic and Ru cationic reaction, P2-Na0.67[Mg0.33Ru0.67]O2 exhibits not only a large specific capacity of ∼172 mA h g−1 but also excellent power-capability via facile Na+ diffusion and reversible structural change during charge/discharge. These findings suggest a novel strategy that can increase the activity of anionic redox by modulating the local environment around oxygen to develop high-energy-density cathode materials for NIBs.