Fluoride-Driven Interfacial Engineering: Architecting Functional Layers for High-Performance Dendrite-Free Li Metal Batteries

High-capacity lithium metal anodes are essential for high-energy-density batteries. However, their practical deployment is hindered by notorious dendritic growth, severe volume variation and an unstable solid electrolyte interphase (SEI), giving rise to poor cyclic stability and serious safety hazards. Much effort has been devoted to exploring interfacial functional layers to enable dendrite-free and reversible plating/stripping. Metal fluorides (M_xFᵧ) are being actively exploited due to their high ionic conductivity, mechanical robustness and favorable reactivity with Li. Importantly, conversion and alloying reactions generate favorable interphases containing conductive LiF and alloying species, which effectively regulate Li⁺ flux and facilitate fast ion migration and uniform deposition, thereby enhancing both cycle life and energy density of Li metal batteries (LMBs). This review systematically summarizes the applications of M_xFᵧ in LMBs, including artificial interphase layers, lithiophilic 3D hosts, separator modification layers and electrolyte additives. Particular emphasis is placed on clarifying the role of M_xFᵧ for building a robust SEI and providing host framework that accelerates kinetics and guides homogeneous nucleation for dendrite-free deposition. Meanwhile, the performance of LMBs from a practical application viewpoint is discussed, especially in pouch cells. Finally, future directions and prospects for M_xFᵧ are presented to stimulate their commercialization and extension to other metal batteries.