Perfecting oxygen stoichiometry in cathode materials: A defect-control strategy for stable sodium-ion batteries

Abstract

The practical application of NaNi_1/3Mn_1/3Fe_1/3O₂ (NMF) is limited by irreversible phase transitions that lead to structural degradation, as well as pronounced air sensitivity. To address these challenges, we propose a protocol leveraging low-temperature annealing to engineer a defect-suppressed cathode architecture with tailored oxygen vacancies (OVs), as validated using neutron powder diffraction (NPD) and a series of characterizations. This approach remarkably improved lattice oxygen framework stability and promotes ionic charge redistribution within the crystal lattice. Additionally, the engineered material displays enhanced covalency of transition metal-oxygen bonds (TM-O) and optimized ionic transport pathways. As a result, modified material exhibits enhanced electrochemical stability, such as more stable cyclability, and smaller voltage hysteresis, etc. In situ and ex situ characterizations (e.g. XRD, XPS, SEM) reveal a highly ordered lattice, correlating with a robust and resilient crystal structure. This work establishes a defect-engineering paradigm to design high-stability layered oxide cathodes through modifying intrinsic OVs, addressing both bulk structural integrity and interfacial compatibility challenges in sodium-ion battery systems. Furthermore, the simple prolonged low-temperature annealing strategy not only enhances material performance in a cost-effective way but also promotes the commercial application of sodium-ion batteries.