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

Abstract Sodium manganese oxide Na _0.44 MnO ₂ has become a promising cathode material for sodium ion batteries due to its stable tunnel structure, but its low Na ⁺ content and Mn ³⁺ 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 _0.44 Mn _0.96 Mg _0.02 Co _0.02 O ₂ 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.