Structural Evolution and Redox Processes Involved in the Electrochemical Cycling of P2–Na0.67Mn0.66Fe0.20Cu0.14O2

We report the synthesis and structural evolution of P2–Na0.67Mn0.66Fe0.20Cu0.14O2 during charge and discharge as a positive electrode for Na-ion batteries. Operando X-ray diffraction analysis revealed the existence of two phase transitions in the voltage window between 1.5–4.3 V. Ex situ pair distribution function analysis was used to characterize the local structure of the high-voltage phase which is disordered along the c-axis and is derived by the migration of a fraction of iron ions into the interlayer space. Operando X-ray absorption spectroscopy was employed to monitor the local structural evolution of the transition metals and shows the existence of Mn3+/4+ redox below ∼3.4 V, followed by the redox activity of Cu and Fe ions at higher voltages. However, no change was observed in the K-edge X-ray absorption near-edge spectrum of any of the transition metals at voltages above 4.1 V, where the growth of the high-voltage phase is initiated. The combination of these results implies the reversible contribution of oxide ion redox to the capacity, which is coincident with metal migration. The above processes result in voltage fading over cycling, similar to that exhibited by Li2MnO3-based positive electrode materials in Li-ion batteries.