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 FeF₃ 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 FeF₂ and CuF₂. 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 FeF₃ forms FeF₂ on the particle’s surface, along with a cation-ordered and stacking-disordered phase, A-Li_xFe_yF₃, which is structurally related to α-/β-LiMn²⁺Fe³⁺F₆ and which topotactically transforms to B- and then C-Li_xFe_yF₃, before forming LiF and Fe. Lithiation of FeF₂ and CuF₂ results in a buffer phase between FeF₂/CuF₂ and LiF. The resulting principles will aid future developments of a wider range of isomorphic metal fluorides.

金属氟化物是一种很有前途的锂离子电池正极材料，由于广泛推测在锂化时发生的重建相变，已被归类为转换材料。_{我们通过使用 X 射线总散射和电子衍射技术研究 FeF 3}来挑战这一观点，这些技术在多个长度尺度上测量结构并结合密度泛函理论计算，并通过重新审视之前对 FeF ₂和 CuF ₂的实验研究。^{相反，金属氟化物锂化由扩散控制的位移机制主导，金属氟化物 F -}亚晶格与 LiF 亚晶格之间建立了清晰的拓扑关系。_{FeF 3}的初始锂化形成FeF₂在粒子的表面上，以及一个阳离子有序和堆积无序相，A-Li _x Fe _y F ₃，其在结构上与 α-/β-LiMn ²⁺ Fe ³⁺ F ₆相关，并且其拓扑转变为B- 然后是 C-Li _x Fe _y F ₃，然后形成 LiF 和 Fe。FeF ₂和CuF _{2的锂化导致FeF 2} /CuF ₂和LiF之间的缓冲相。由此产生的原理将有助于更广泛的同晶金属氟化物的未来发展。