Understanding the Synergistic Effect of Mg/Co Ions Doping on the Layer-Tunnel Hybrid Structure of NaMnO Cathode Materials.

Sodium manganese oxide NaMnO has become a promising cathode material for sodium ion batteries due to its stable tunnel structure, but its low Na content and Mn 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 NaMnMgCoO cathode in sodium-ion batteries demonstrates a higher initial specific capacity (135.8 mAh g, 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.