Brennhagen Anders, Skurtveit Amalie, Wragg David S, Cavallo Carmen, Sjåstad Anja O, Koposov Alexey Y, Fjellvåg Helmer
Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, Oslo 0315, Norway.
Department of Battery Technology, Institute for Energy Technology, Instituttveien 18, Kjeller 2007, Norway.
Chem Mater. 2024 Aug 2;36(15):7514-7524. doi: 10.1021/acs.chemmater.4c01503. eCollection 2024 Aug 13.
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 BiMoO 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 Mo 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 BiMoO, as flexibility of this matrix allows movement and coalescence of the Na Bi particles, which is detrimental to the electrochemical stability.
原位表征能够揭示电池材料的降解过程，对于电池化学的发展至关重要。本研究首次报道了对电池单元进行准同步原位对分布函数（PDF）和X射线吸收光谱（XAS）分析，从而对作为钠离子电池负极材料的BiMoO的循环机理有了详细的原子级理解。这种材料通过一种组合的转化-合金化反应进行循环，在首次 sodiation 过程中形成嵌入非晶态Na-Mo-O基体中的具有电化学活性的纳米晶Na Bi颗粒。原位PDF和XAS的结合表明，由于在Bi颗粒与Na-Mo-O基体的界面处形成了Bi-O键，Bi在脱 sodiation 结束时获得了正氧化态。此外，XAS证实Mo在整个（脱）sodiation 过程中的平均氧化态为 +6，因此对容量没有贡献。然而，Mo的局部环境从脱 sodiated 状态下的四面体配位变为 sodiated 状态下的扭曲八面体配位。这些结构变化与BiMoO较差的循环稳定性有关，因为这种基体的柔韧性允许Na Bi颗粒移动和聚结，但这对电化学稳定性不利。
