Atomic Imaging Reveals Low-Hysteresis Oxygen Redox in Transition Metal Vacancy-Engineered Sodium Ion Cathodes

Abstract

Voltage hysteresis has long been considered a bottleneck toward achieving fully reversible oxygen redox in layered oxide cathodes. Despite tremendous attempts, most reported strategies are based on elemental doping or compositional complexity to overcome this obstacle. Herein, we have surprisingly achieved a very small voltage hysteresis of ∼91 mV over repeated cycles in compositionally simple P2-type Na_3/4Mn_13/16□_3/16O₂. We have shown that introducing deliberate transition metal (TM) vacancies promotes the Mn⁴⁺ population that induces a ribbon-like superstructure, which improves the local structural framework and reduces lattice strain. Spectroscopic analysis in support of theoretical studies indicates that TM vacancy engineering promoted π-type Mn–O hybridization, which stabilized oxygen holes and inhibited O–O dimer formation. In situ X-ray diffraction in combination with HAADF-STEM reveals single-phase reversible lattice evolution without TM migration. These results indicate TM vacancy engineering is an effective parameter to tune key factors like the Mn⁴⁺ content and superstructure to achieve reversible oxygen redox by stabilizing the electronic structure and lattice framework, providing a promising strategy to develop sustainable high-capacity, low-hysteresis sodium ion cathodes.