Polaronic and ionic conduction in NaMnO$_2$: influence of native point defects

Layered NaMnO$_2$ has promising applications as a cathode material for sodium ion batteries. We will discuss strategies to improve the electrical performance of NaMnO$_2$, including how to optimize the conditions of synthesis and how impurity doping affects the performance. Using hybrid density functional theory, we explored the structural, electronic, and defect properties of bulk NaMnO$_2$. It is antiferromagnetic in the ground state with a band gap of 3.75 eV. Small hole and electron polarons can form in the bulk either through self-trapping or adjacent to point defects. We find that both Na and Mn vacancies are shallow acceptors with the induced holes trapped as small polarons, while O vacancies are deep defect centers. Cation antisites, especially ${\rm Mn_{Na}}$, are found to have low formation energies. As a result, we expect that ${\rm Mn_{Na}}$ exists in as-grown NaMnO$_2$ in moderate concentrations, rather than forming only at a later stage of the charging process, at which point it causes undesirable structural phase transitions. Both electronic conduction, via polaron hopping, and ionic conduction, through ${\rm V_{Na}}$ migration, are significantly affected by the presence of point defects. This work was supported by DOE.