Unraveling the (De)sodiation Mechanisms of BiFeO₃ at a High Rate with Operando XRD

Development of new anode materials for Na-ion batteries strongly depends on a detailed understanding of their cycling mechanism. Due to instrumental limitations, the majority of mechanistic studies focus on operando materials' characterization at low cycling rates. In this work, we evaluate and compare the (de)sodiation mechanisms of BiFeO₃ in Na-ion batteries at different current densities using operando X-ray diffraction (XRD) and ex situ X-ray absorption spectroscopy (XAS). BiFeO₃ is a conversion-alloying anode material with a high initial sodiation capacity of ∼600 mAh g^-1, when cycled at 0.1 A g^-1. It does not change its performance or cycling mechanism, except for minor losses in capacity, when the current density is increased to 1 A g^-1. In addition, operando XRD characterization carried out over multiple cycles shows that the Bi ⇋ NaBi (de)alloying reaction and the oxidation of Bi at the interface with the Na-Fe-O matrix are detrimental for cycling stability. The isolated NaBi ⇋ Na₃Bi reaction is less damaging to the cycling stability of the material.