Defect formation and ambivalent effects on electrochemical performance in layered sodium titanate Na2Ti3O7

Point defects can be formed readily in layered transition metal oxides used as electrode materials for alkali-ion batteries but their influence on the electrode performance is yet obscure. In this work, we report a systematic first-principles study of intrinsic point defects and defect complexes in sodium titanate Na2Ti3O7, a low-voltage anode material for sodium-ion batteries. Within the density functional theory framework, we calculate the defect formation energies with a set of atomic chemical potentials, which define the synthesis conditions for the stable Na2Ti3O7 compound. Given the atomic chemical potential landscape and defect formation energies, we find that Na interstitials (Nai+), Na antisites (NaTi3−), and Na vacancies (VNa−) are dominant defects depending on the synthesis conditions. Furthermore, our calculations reveal that O vacancies (VO) and Ti antisites (TiNa) lower the electrode potential compared with the perfect system, whereas Ti vacancies (VTi) and NaTi increase the voltage. Finally, we evaluate the activation barriers for vacancy-mediated Na diffusion in the defective systems, finding that the intrinsic point defects improve the Na ion conduction. Our results provide a profound understanding of defect formation and influences on electrode performance, paving a way to designing high-performance anode materials.