How does intercalation reshape layered structures? A first-principles study of sodium insertion in layered potassium birnessite.

This work presents a first-principles study of the sodium intercalation process in a layered potassium birnessite (a layered manganese dioxide, δ-MnO2) using hybrid-level density functional theory. Understanding the intercalation processes of δ-MnO2 is a vital step in advancing its potential innovative applications. Using a formation energy formalism, we analyze the stability of the structure as sodium ions (Na+) are intercalated between layers. Simulated Raman spectra allow us to find relationships between the vibrational and structural properties of the material, i.e., we identify the most important vibrational modes and relate them to the structural/geometrical change. The diffusion of Na+ and K+ ions in birnessite is studied by transition state theory, determining the energy barriers to ion displacement in the interlayer. The symmetry and planar density of the system are characterized by simulated X-ray diffraction and geometrical analysis of the optimized structures. Through binding energy analysis, we also find that the Na+ ions are more loosely bound to the lattice as they reach the saturation limit. Finally, the electronic properties are studied via spin-polarized densities of states. As intercalants are added, the electronic properties are profoundly modified, resulting from modifications of Mn oxidation states, lattice distortions, and symmetry effects. Moreover, some of the intercalated structures behave as bipolar magnetic semiconductors with potential applications in spintronics devices. In other words, the bandgaps and magnetic behavior of the system can be controlled by intercalation. This work provides an overarching analysis of intercalated birnessite and describes the essential properties of potassium birnessite and co-intercalation with sodium as a next-generation energy, electronic, and spintronic material.