In situ techniques with high temporal, spatial and chemical resolution are key to understand ubiquitous solid-state phase transformations, which are crucial to many technological applications. Hard X-ray spectro-imaging can visualize electrochemically driven phase transformations but demands considerably large samples with strong absorption signal so far. Here we show a conceptually new data analysis method to enable operando visualization of mechanistically relevant weakly absorbing samples at the nanoscale and study electrochemical reaction dynamics of iron fluoride, a promising high-capacity conversion cathode material. In two specially designed samples with distinctive microstructure and porosity, we observe homogeneous phase transformations during both discharge and charge, faster and more complete Li-storage occurring in porous polycrystalline iron fluoride, and further, incomplete charge reaction following a pathway different from conventional belief. These mechanistic insights provide guidelines for designing better conversion cathode materials to realize the promise of high-capacity lithium-ion batteries.

中文翻译：

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使用操作性硬X射线光谱成像可视化电化学驱动的固态相变。

具有高时间，空间和化学分辨率的原位技术是理解普遍存在的固态相变的关键，这对于许多技术应用而言都是至关重要的。硬X射线光谱成像可以观察到电化学驱动的相变，但到目前为止需要具有强大吸收信号的相当大的样品。在这里，我们展示了一种概念上新颖的数据分析方法，可在纳米级对机械相关的弱吸收样品进行操作可视化，并研究氟化铁（一种有前途的高容量转化正极材料）的电化学反应动力学。在两个经过特殊设计的具有独特微结构和孔隙率的样品中，我们观察到放电和充电过程中均相变化，多孔多晶氟化铁中发生更快，更完全的锂存储，并且沿着不同于传统观念的途径进行不完全的电荷反应。这些机理见解为设计更好的转换阴极材料提供了指导，以实现大容量锂离子电池的前景。