Beyond d-Band Catalysis: A Critical Review and Descriptor Framework for Rare-Earth Engineering in Lithium-Sulfur Batteries.

Rare earth elements (REEs) have emerged as a distinctive class of functional materials for lithium-sulfur (Li-S) batteries, offering catalytic behavior that extends beyond the conventional d-band paradigm of transition-metal systems. Their localized 4f orbitals, variable oxidation states, strong Lewis acidity, and defect chemistry enable multifunctional regulation of polysulfide adsorption and redox conversion. They also enhance interfacial stability. This review evaluates REE-based materials across cathodes, separators, electrolyte/additive systems, and integrated cell architectures, with emphasis on how REEs function as redox mediators, polar anchors, and electronic/ionic interface modulators. Some credible advances arise not from isolated material effects, but from conductive integration, balanced adsorption-conversion, and coordinated deployment across multiple cell components. By benchmarking reported systems against practical metrics, including high sulfur loading, areal capacity, rate capability, and pouch-cell relevance, we identify promising REE-enabled architectures while also highlighting persistent limitations in mechanistic validation and reporting consistency. We further argue that rational progress in this field requires a descriptor-guided framework tailored to 4f chemistry, in which crystal-field splitting, electronegativity, f-d hybridization, oxygen vacancy concentration, and Lewis acidity collectively govern catalytic behavior. Finally, we outline future directions centered on underexplored lanthanides, operando and multiscale characterization, hybrid REE/transition-metal designs, and circular-economy strategies for sustainable deployment.