Mechanistic Insights into the Colloidal Synthesis of Monodisperse CoF₂ Nanorods

Transition metal fluorides (TMFs) are promising conversion-type cathode materials for high-energy lithium-metal batteries owing to their exceptionally high theoretical energy density, making them attractive for demanding applications such as electric aviation. However, their practical implementation is hindered by sluggish conversion kinetics, pronounced voltage hysteresis, and poor electrochemical reversibility. Understanding the structural and chemical evolution of TMFs and the electrode-electrolyte interphase formed during cycling is essential for improving their performance. Monodisperse nanocrystalline TMFs are ideal model systems for high-resolution transmission electron microscopy because their uniform size, morphology, and electron transparency enable representative nanoscale characterization. Here, we investigate a reproducible colloidal synthesis of monodisperse, single-crystalline CoF₂ nanorods and elucidate the mechanisms governing their formation to achieve precise control over particle size and aspect ratio. By independently varying reaction time, temperature, precursor composition, and ligand chemistry, we identify the key factors controlling precursor decomposition, burst nucleation, phase selection, and anisotropic crystal growth. These insights provide a transferable framework for the controlled synthesis of a broad range of transition metal fluoride nanocrystals.