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

Stacking 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-Na_0.75Li_0.2Mn_0.7Cu_0.1O₂ (P2-LMC) and P3-Na_0.75Li_0.2Mn_0.7Cu_0.1O₂ (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⁺, Mn^3.5+/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.