Unveiling the Role of Ruthenium in Layered Sodium Cobaltite Toward High‐Performance Electrode Enabled by Anionic and Cationic Redox

Abstract The effect of Ru substitution on the structure and electrochemical properties of P2‐type Na 0.67 CoO 2 is investigated. The first‐discharge capacities of Na 0.67 CoO 2 and Na 0.6 [Co 0.78 Ru 0.22 ]O 2 materials are 128 and 163 mAh g −1 (23.5 mA g −1 ), respectively. Furthermore, the rate capability is improved due to the electro‐conducting nature of Ru doping. Operando X‐ray diffraction analysis reveals that the Na 0.67 CoO 2 does not undergo a phase transition; however, multiple Na + /vacancy ordered superstructures within the P2 phase appear during Na + extraction/insertion. In contrast, the Na 0.6 [Co 0.78 Ru 0.22 ]O 2 material undergoes a P2–OP4 phase transition during desodiation, with no formation of Na + /vacancy ordering within the P2 phase. The increased discharge capacity of Na 0.6 [Co 0.78 Ru 0.22 ]O 2 is most likely associated with additional cationic Ru 4+ /Ru 5+ redox and increased anionic O 2− /(O 2 n− ) redox participation. Combined experimental (galvanostatic cycling, X‐ray absorption spectroscopy, differential electrochemical mass spectrometry) and theoretical (density functional theory calculations) studies confirm that Ru substitution provokes the oxygen‐redox reaction and that partial O 2 release from the oxide lattice is the origin of the reaction. The findings provide new insight for improving the electrode performance of cathode materials via 4 d Ru substitution and motivate the development of a new strategy for the design of high‐capacity cathode materials for sodium‐ion batteries.