Understanding the Synergistic Effect of Mg/Co Ions Doping on the Layer‐Tunnel Hybrid Structure of Na _0.44 MnO ₂ Cathode Materials

Sodium manganese oxide Na_0.44MnO₂ has become a promising cathode material for sodium ion batteries due to its stable tunnel structure, but its low Na⁺ content and Mn³⁺ induced Jahn Teller (JT) distortion pose challenges to its practical application. This study constructs a tunnel-layered hybrid material that combines the advantage of two structures through co-doping with Mg and Co ions. The Mg/Co co-substitution effect is crucial in regulating the hybrid crystal structure, as it compresses the TM-O layer (transition metal oxide plates) and expands the Na layer spacing, enhancing Na⁺ diffusion kinetics. This modification also mitigates lattice distortion from the JT effect, improving structural stability. Electrochemical studies reveal that the Na_0.44Mn_0.96Mg_0.02Co_0.02O₂ cathode in sodium-ion batteries demonstrates a higher initial specific capacity (135.8 mAh g^-1, 0.5C) and improved cycle stability. Density functional theory (DFT) calculations further confirm that Mg and Co ions reduce the band gap, enhance electronic conductivity, and improve rate performance. Additionally, the increased O 2p state density near the Fermi level favors oxygen redox reactions. This research provides a new method for the design of energy storage materials. Besides, the investigation into the mechanism of Mg and Co dual substitution offers a promising strategy for optimizing sodium-ion batteries.