(De)sodiation Mechanism of Bi₂MoO₆ in Na-Ion Batteries Probed by Quasi-Simultaneous Operando PDF and XAS

Operando characterization can reveal degradation processes in battery materials and are essential for the development of battery chemistries. This study reports the first use of quasi-simultaneous operando pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) of a battery cell, providing a detailed, atomic-level understanding of the cycling mechanism of Bi₂MoO₆ as an anode material for Na-ion batteries. This material cycles via a combined conversion-alloying reaction, where electrochemically active, nanocrystalline Na Bi particles embedded in an amorphous Na-Mo-O matrix are formed during the first sodiation. The combination of operando PDF and XAS revealed that Bi obtains a positive oxidation state at the end of desodiation, due to formation of Bi-O bonds at the interface between the Bi particles and the Na-Mo-O matrix. In addition, XAS confirmed that Mo has an average oxidation state of +6 throughout the (de)sodiation process and, thus, does not contribute to the capacity. However, the local environment of Mo⁶⁺ changes from tetrahedral coordination in the desodiated state to distorted octahedral in the sodiated state. These structural changes are linked to the poor cycling stability of Bi₂MoO₆, as flexibility of this matrix allows movement and coalescence of the Na Bi particles, which is detrimental to the electrochemical stability.