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

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-Na_0.33[Mg_0.33Ru_0.67]O₂ is based on anionic oxidation after 0.33 mol Na⁺ deintercalation from P2-Na_0.67[Mg_0.33Ru_0.67]O₂ with cationic oxidation of Ru⁴⁺/Ru^4.5+. 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-Na_0.33[Mg_0.33Ru_0.67]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-Na_0.67[Mg_0.33Ru_0.67]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.