Point defects and their impact on electrochemical performance in NaMnO for sodium-ion battery cathode application.

Sodium manganese oxide Na.MnO (NMO) in an open structure with large tunnels is of great interest for sodium-ion battery cathode materials due to its high electrode voltage and capacity. However, its practical application is limited by poor rate performance, which can be tuned and improved by controlling point defects. We herein present a comprehensive study of intrinsic point defects in NMO using density functional theory (DFT) calculations in combination with thermodynamics. Using the DFT+ approach, we determine the formation energies of elementary defects and defect complexes depending on the sets of atomic chemical potentials, corresponding to a certain thermodynamic condition for the synthesis of stable NMO. Sodium interstitials are found to have the lowest formation energies in the relevant ranges of temperature and pressure. Other intrinsic point defects such as oxygen vacancies, sodium vacancies and manganese antisites can also be formed with proper formation energies and have an impact on the cathode performance. Compared to the perfect system, oxygen vacancies lower the electrode voltage, whereas manganese vacancies and antisites increase the voltage. We find that most point defects and defect complexes improve the sodium ion diffusivity, highlighting a proper control of defect formation for enhancing the performance of sodium-ion batteries.