Copper Substitution Enhances Ni/Mn–O Hybridization for Improved Redox Kinetics and Stability in Sodium-Ion Cathodes

In sodium-ion battery layered oxide materials, research on copper doping mechanisms has primarily focused on regulating oxygen redox reactions (e.g., optimizing charge compensation, stabilizing TMO2 layers), enhancing material air stability, and inhibiting phase transitions. However, there has been limited exploration of the role of copper ions in modulating the local electronic structure of transition metals (such as Ni/Mn in the 2–4 V capacity region). In this study, we demonstrate that Cu2+ substitution restructures the Ni/Mn–O orbital hybridization network, synergistically optimizing the redox activity of Ni2+ and the structural stability of Mn3+. The Cu-substituted Na0.67(Ni0.2Mn0.7Mg0.1)0.9Cu0.1O2 (NMMC-0.10) significantly enhances the Ni2+/Ni3+ redox reaction in the 3–4 V region (with a marked increase of 29.7% in capacity contribution) and effectively suppresses the Jahn–Teller distortion of Mn3+ in the <3 V range. Density functional theory (DFT) calculations reveal that Cu2+'s 3d orbitals enhance Ni/Mn 3d–O 2p hybridization (orbital overlap integral 1.3 times that of the baseline), broaden the transition metal electronic state distribution (bandwidth increases by 15.6%), significantly strengthen the TM–O bond (12.5% increase), and form a robust Cu–O framework. Thanks to these synergistic effects, NMMC-0.10 exhibits outstanding electrochemical performance in the 2.0–4.5 V voltage window: 82.87% capacity retention after 200 cycles at 1 C (compared to 65.2% for the baseline) and an energy density of 412 Wh/kg, an 18.6% improvement over the baseline. This study provides theoretical insights into the design of layered oxide cathode materials by revealing the specific electronic structure modulation mechanism of Cu substitution.