Brookhaven National Laboratory, Upton, New York 11973, USA.
J Am Chem Soc. 2011 Nov 23;133(46):18828-36. doi: 10.1021/ja206268a. Epub 2011 Sep 22.
Materials that undergo a conversion reaction with lithium (e.g., metal fluorides MF(2): M = Fe, Cu, ...) often accommodate more than one Li atom per transition-metal cation, and are promising candidates for high-capacity cathodes for lithium ion batteries. However, little is known about the mechanisms involved in the conversion process, the origins of the large polarization during electrochemical cycling, and why some materials are reversible (e.g., FeF(2)) while others are not (e.g., CuF(2)). In this study, we investigated the conversion reaction of binary metal fluorides, FeF(2) and CuF(2), using a series of local and bulk probes to better understand the mechanisms underlying their contrasting electrochemical behavior. X-ray pair-distribution-function and magnetization measurements were used to determine changes in short-range ordering, particle size and microstructure, while high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) were used to measure the atomic-level structure of individual particles and map the phase distribution in the initial and fully lithiated electrodes. Both FeF(2) and CuF(2) react with lithium via a direct conversion process with no intercalation step, but there are differences in the conversion process and final phase distribution. During the reaction of Li(+) with FeF(2), small metallic iron nanoparticles (<5 nm in diameter) nucleate in close proximity to the converted LiF phase, as a result of the low diffusivity of iron. The iron nanoparticles are interconnected and form a bicontinuous network, which provides a pathway for local electron transport through the insulating LiF phase. In addition, the massive interface formed between nanoscale solid phases provides a pathway for ionic transport during the conversion process. These results offer the first experimental evidence explaining the origins of the high lithium reversibility in FeF(2). In contrast to FeF(2), no continuous Cu network was observed in the lithiated CuF(2); rather, the converted Cu segregates to large particles (5-12 nm in diameter) during the first discharge, which may be partially responsible for the lack of reversibility in the CuF(2) electrode.
材料与锂发生转化反应（例如金属氟化物 MF(2)：M = Fe、Cu、...）通常每个过渡金属阳离子可容纳超过一个 Li 原子，是高容量锂离子电池阴极的有前途的候选材料。然而，对于转化过程中涉及的机制、电化学循环过程中产生的大极化的起源以及为什么有些材料是可逆的（例如 FeF(2)）而有些材料不是（例如 CuF(2)），人们知之甚少。在这项研究中，我们使用一系列局部和体相探针研究了二元金属氟化物 FeF(2)和 CuF(2)的转化反应，以更好地理解其电化学行为差异的原因。X 射线对分布函数和磁化率测量用于确定短程有序、颗粒大小和微结构的变化，而高分辨率透射电子显微镜 (TEM) 和电子能量损失光谱 (EELS) 用于测量单个颗粒的原子级结构并绘制初始和完全锂化电极中的相分布。FeF(2)和 CuF(2)均与锂发生直接转化反应，没有插层步骤，但转化过程和最终相分布存在差异。在 Li(+)与 FeF(2)的反应过程中，由于铁的扩散率较低，在转化的 LiF 相附近形核出直径小于 5nm 的小金属铁纳米颗粒。铁纳米颗粒相互连接并形成双连续网络，为通过绝缘 LiF 相的局部电子传输提供了途径。此外，纳米尺度固相之间形成的大界面为转化过程中的离子传输提供了途径。这些结果提供了第一个实验证据，解释了 FeF(2)中高锂可逆性的起源。与 FeF(2)相反，在锂化的 CuF(2)中没有观察到连续的 Cu 网络；相反，在第一次放电过程中，转化的 Cu 会分离成大颗粒（直径 5-12nm），这可能部分解释了 CuF(2)电极不可逆的原因。
