The study of anionic redox in layered-oxide cathodes for Na-ion batteries (NIBs) has been extensive, aiming to attain high-energy density. The involvement of Mn 4+ or Ru 5+ oxidation states is known to be crucial for the anionic O 2 − /O − reaction during Na+ de/intercalation. Nevertheless, our findings indicate that anionic redox can activate in Ru-based layered-oxide cathodes prior to complete Ru 4+ /Ru 5+ oxidation. By employing first-principles calculation and experimental techniques, we disclose that additional Na + deintercalation from P2-Na 0.33 [Mg 0.33 Ru 0.67 ]O 2 relies on anionic oxidation after extracting 0.33 mol Na + from P2-Na 0.67 [Mg 0.33 Ru 0.67 ]O 2 , concomitant with cationic oxidation of Ru 4+ /Ru 4.5+ . Specifically, only the oxygen adjacent to 2Mg/1Ru is implicated in anionic redox during Na+ de/intercalation, indicating that the Na–O–Mg arrangement in the P2-Na 0.33 [Mg 0.33 Ru 0.67 ]O 2 structure significantly reduces the thermodynamic stability of anionic redox compared to cationic redox. Through O anionic and Ru cationic reactions, P2-Na 0.67 [Mg 0.33 Ru 0.67 ]O 2 demonstrates a substantial specific capacity of ∼172 mA h g − 1 and exceptional power capability, facilitated by facile Na + diffusion and reversible structural changes during charge/discharge. These findings propose an innovative strategy to enhance anionic redox activity by modifying the local oxygen environment, paving the way for high-energy-density cathode materials in NIBs.
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Ahn et al. (2024) studied this question.
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