Relationship between Sodium Content and Copper Activity for High Voltage Stability in O3-Type Layered Oxide Cathodes

Layered oxides with an O3 framework have attracted attention as cathode materials for sodium-ion batteries due to their high discharge capacity. Yet they are hampered for commercialization by poor cyclability due to structural instability during the sodium (de)intercalation process. The introduction of Cu in small fractions to the transition metal layers has been empirically observed to improve reversible specific capacity and cycling stability. Understanding the redox activity of Cu in O3-type NaxTMO2 materials is crucial, as it could directly influence the charge compensation mechanism, voltage profile, and structural stability. However, the precise role of Cu in O3-type sodium cathodes under high-voltage cycling conditions (>4.1 V) remains insufficiently understood. To close this gap of knowledge, we studied the effect of Cu in two representative layered oxides with the same O3 stacking but different sodium stoichiometry, Na0.9Mn1/2Fe1/3Cu1/6O2 and NaMn1/2Fe1/3Cu1/6O2. X-ray spectroscopy reveals that in Na0.9Mn1/2Fe1/3Cu1/6O2, Cu exhibits dual redox activity, Cu+/Cu2+ in the pristine state and Cu2+/Cu3+ upon charging in the sodium-deficient material, whereas only the Cu+/Cu2+ redox couple is observed in the fully stoichiometric layered oxide. The results indicate that even a slight deficiency in sodium can significantly impact the electrochemical performance and material stability and alter the elemental redox activity of Cu.