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 (Na⁺), Na antisites (Na_Ti^3-), and Na vacancies (V_Na^-) are dominant defects depending on the synthesis conditions. Furthermore, our calculations reveal that O vacancies (V_O) and Ti antisites (Ti_Na) lower the electrode potential compared with the perfect system, whereas Ti vacancies (V_Ti) and Na_Ti 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.