Hua Xiao, Eggeman Alexander S, Castillo-Martínez Elizabeth, Robert Rosa, Geddes Harry S, Lu Ziheng, Pickard Chris J, Meng Wei, Wiaderek Kamila M, Pereira Nathalie, Amatucci Glenn G, Midgley Paul A, Chapman Karena W, Steiner Ullrich, Goodwin Andrew L, Grey Clare P
Department of Chemistry, University of Cambridge, Cambridge, UK.
Adolphe Merkle Institute, Fribourg, Switzerland.
Nat Mater. 2021 Jun;20(6):841-850. doi: 10.1038/s41563-020-00893-1. Epub 2021 Jan 21.
Metal fluorides, promising lithium-ion battery cathode materials, have been classified as conversion materials due to the reconstructive phase transitions widely presumed to occur upon lithiation. We challenge this view by studying FeF using X-ray total scattering and electron diffraction techniques that measure structure over multiple length scales coupled with density functional theory calculations, and by revisiting prior experimental studies of FeF and CuF. Metal fluoride lithiation is instead dominated by diffusion-controlled displacement mechanisms, and a clear topological relationship between the metal fluoride F sublattices and that of LiF is established. Initial lithiation of FeF forms FeF on the particle's surface, along with a cation-ordered and stacking-disordered phase, A-LiFeF, which is structurally related to α-/β-LiMnFeF and which topotactically transforms to B- and then C-LiFeF, before forming LiF and Fe. Lithiation of FeF and CuF results in a buffer phase between FeF/CuF and LiF. The resulting principles will aid future developments of a wider range of isomorphic metal fluorides.
金属氟化物是很有前景的锂离子电池正极材料，由于普遍认为在锂化过程中会发生重构相变，它们被归类为转换材料。我们通过使用X射线全散射和电子衍射技术（这些技术可在多个长度尺度上测量结构）结合密度泛函理论计算来研究FeF，并重新审视之前对FeF和CuF的实验研究，从而对这一观点提出质疑。相反，金属氟化物的锂化过程由扩散控制的位移机制主导，并且在金属氟化物的F亚晶格与LiF的F亚晶格之间建立了明确的拓扑关系。FeF的初始锂化在颗粒表面形成FeF，同时形成一种阳离子有序且堆垛无序的相，即A-LiFeF，它在结构上与α-/β-LiMnFeF相关，并且在形成LiF和Fe之前，先拓扑转变为B-LiFeF，然后再转变为C-LiFeF。FeF和CuF的锂化会在FeF/CuF和LiF之间形成一个缓冲相。由此得出的原理将有助于更广泛的同构金属氟化物的未来发展。
