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

摘要

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 Na0.67CoO_2 and Na0.6 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 Na0.6Co_(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 Na0.6Co_(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 4d Ru substitution and motivate the development of a new strategy for the design of high-capacity cathode materials for sodium-ion batteries.