Li Intercalation into a β-MnO2 Grain Boundary

MnO2 is well-known for its technological applications including Li ion, Li-air batteries, and electrochemical capacitors. Compared to the bulk material, nanostructuring of rutile (β-)MnO2 has been shown to vastly improve its electrochemical properties and performance. While the bulk material cannot readily intercalate Li, nanostructured mesoporous samples exhibit good Li intercalation. This observation is not yet fully understood. In this work, we use state-of-the-art theoretical techniques to investigate Li intercalation and migration at the β-MnO2 Σ 5(210)/[001] grain boundary (GB). We show how large tunnel structures in the GB can promote Li intercalation with voltages of up to 3.83 eV compared to the experimental value of 3.00 eV. Conversely, small tunnel structures resulting from overcoordination of ions at the GB can hinder Li intercalation with significantly reduced voltages. The size and shape of these tunnels also strongly influence the energetics of Li migration with energy barriers ranging from 0.15 to 0.89 eV, compared to a value for the bulk of 0.17 eV. Our results illustrate how GBs with large, open tunnel structures may promote electrochemical performance and could be a contributing factor to the excellent performance of nanostructured β-MnO2.