Stacking Order Induced Anion Redox Regulation for Layer‐Structured Na0.75Li0.2Mn0.7Cu0.1O2 Cathode Materials

Abstract

AbstractStacking order plays a key role in defining the electrochemical behavior and structural stability of layer‐structured cathode materials. However, the detailed effects of stacking order on anionic redox in layer‐structured cathode materials have not been investigated specifically and are still unrevealed. Herein, two layered cathodes with the same chemical formula but different stacking orders: P2‐Na0.75Li0.2Mn0.7Cu0.1O2 (P2‐LMC) and P3‐Na0.75Li0.2Mn0.7Cu0.1O2 (P3‐LMC) are compared. It is found that P3 stacking order is beneficial to improve the oxygen redox reversibility compared with P2 stacking order. By using synchrotron hard and soft X‐ray absorption spectroscopies, three redox couples of Cu2+/Cu3+, Mn3.5+/Mn4+, and O2−/O− are revealed to contribute charge compensation in P3 structure simultaneously, and two redox couples of Cu2+/Cu3+ and O2−/O− are more reversible than those in P2‐LMC due to the higher electronic densities in Cu 3d and O 2p orbitals in P3‐LMC. In situ X‐ray diffraction reveals that P3‐LMC exhibits higher structural reversibility during charge and discharge than P2‐LMC, even at 5C rate. As a result, P3‐LMC delivers a high reversible capacity of 190.3 mAh g−1 and capacity retention of 125.7 mAh g−1 over 100 cycles. These findings provide new insight into oxygen‐redox‐involved layered cathode materials for SIBs.