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

ABSTRACT: 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 _x 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 _x Bi particles, which is detrimental to the electrochemical stability.

SUBMITTER: Brennhagen A 

PROVIDER: S-EPMC11325532 | biostudies-literature | 2024 Aug

REPOSITORIES: biostudies-literature