Electrochemistry of hollandite α-MnO₂: Li-ion and Na-ion insertion and Li₂O incorporation

MnO₂ is attracting considerable interest in the context of rechargeable batteries, supercapacitors, and Li–O₂ battery applications. This work investigates the electrochemical properties of hollandite α-MnO₂ using density functional theory with Hubbard U corrections (DFT+U). The favorable insertion sites for Li-ion and Na-ion insertion are determined, and we find good agreement with measured experimental voltages. By explicit calculation of the phonons we suggest multiple insertion sites are accessible in the dilute limit. Significant structural changes in α-(Li,Na)_xMnO₂ during ion insertion are demonstrated by determining the low energy structures. The significant distortions to the unit cell and Mn coordination are likely to be active in causing the observed degradation of α-MnO₂ with cycling. The presence of Li₂O in the structure reduces these distortions significantly and is the probable cause for the good experimental cycling stability of α-[0.143Li₂O]-MnO₂. However, the presence of Na₂O is less effective in reducing the distortion of the Na-ion intercalated form. We also find a distinct change in the favored Li-ion insertion site, not identified in previous studies, for lithiation of α-Li_xMnO₂ at x > 0.5. The migration barriers for both Li-ions and Na-ions increase from <0.3 eV in the dilute limit to >0.48 eV for α-(Li,Na)_0.75MnO₂. Finally, the electronic density of states in α-MnO₂ with the incorporation of Li₂O has the character of a full metal, not a half metal as was suggested in previous work. This may be key to its good performance as a catalyst in Li–O₂ batteries.