Transition Metal Fluorides as Advanced Cathodes for Lithium/Sodium-Ion Batteries: Rational Enhancement Strategies and Underlying Electrochemical Mechanisms

High-capacity and low-cost cathodes are urgently pursued to improve the energy density and realize the large-scale application of lithium/sodium ion batteries. Transition metal fluorides (MF_x), which involve multiple electron transfer based on conversion reactions, are widely regarded as promising candidates with the advantages of high capacity (≈500–800 mAh g⁻¹) and abundant resources. However, MF_x cathodes face significant challenges, including low conductivity, structural pulverization, sluggish kinetics, dissolution of MF_x in electrolyte, etc. Here, four rational strategies to overcome the shortcomings and improve the properties of MF_x are presented in detail. First, compositing MF_x with functional materials can enhance electronic conductivity, buffer volume changes, suppress agglomeration, and stabilize particle surface. Second, constructing hierarchical, porous, and hollow structures can strengthen structural stability, shorten ion transport distance, and expose rich reactive sites. Third, doping heterogeneous elements can improve electrical/ionic conductivity, promote reaction reversibility, and contribute additional capacity. Fourth, optimizing electrolytes and binders can form a stable electrode/electrolyte interface, widen voltage window, and inhibit dissolution of MF_x. Moreover, theoretical investigations are introduced to understand the underlying mechanisms for the reaction and enhancement. Finally, a perspective on the future prospects of MF_x is highlighted, to provide valuable inspiration and reference for the development of next-generation alkali-ion batteries.