Stacking Order Induced Anion Redox Regulation for Layer-Structured Na_0.75 Li_0.2 Mn_0.7 Cu_0.1 O₂ 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_0.75 Li_0.2 Mn_0.7 Cu_0.1 O₂ (P2-LMC) and P3-Na_0.75 Li_0.2 Mn_0.7 Cu_0.1 O₂ (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 O^2- /O^- are revealed to contribute charge compensation in P3 structure simultaneously, and two redox couples of Cu²⁺ /Cu³⁺ and O^2- /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.