Surface Air Stability of Layered Transition Metal Oxide NaMnO₂ with Different Crystalline Phases

As energy demand increases and environmental problems intensify, the demand for energy storage devices continues to rise. Sodium-ion batteries have become an alternative to lithium-ion batteries due to their low cost, abundant raw materials, and environmental protection. However, developing cathode materials for sodium-ion batteries still faces challenges. As a high-performance cathode material, NaMnO₂ has attracted much attention due to its high specific capacity, excellent cycle stability, and environmental friendliness characteristics, but its poor air stability (prone to adsorbing H₂O and CO₂) restricts its large-scale production and storage. This work takes NaMnO₂ as the research object, calculates the surface stability of different surfaces of various crystal phases based on density functional theory (DFT), and further investigates the charge density, density of states, and adsorption energies of H₂O and CO₂ on surface Na/Mn atoms of the most stable surfaces of different phases. The study indicates that variations in air stability among crystalline phases depend on surface structures, including exposed surfaces, electron distribution, and band centers. The study demonstrates that the monoclinic NaMnO₂ exhibits the best air stability. Therefore, the air stability of NaMnO₂ can be modulated by phase and surface engineering, providing new insights for designing sodium-ion battery cathode materials with superior stability.