Optimizing <i>f</i> ‐ <i>d</i> Hybridization Descriptor in Rare‐Earth Oxides for Efficient Sulfur Catalysis in all Solid‐State Lithium–Sulfur Batteries

All-solid-state lithium-sulfur batteries (ASSLSBs) offer high theoretical energy density and intrinsic safety, yet their development is hindered by sluggish sulfur redox kinetics at the solid-solid interfaces due to the weak orbital overlap and discontinuous electronic coupling at these regions. Here, we propose rare-earth oxide catalysts that accelerate the sulfur reduction reaction (SRR) in ASSLSBs by strengthening the f-d-p hybridization at catalyst-sulfur-electrolyte interfaces. We first define a hybridization-strength factor, I_f-d, derived from the electron densities of occupied 4f and unoccupied 5d states, which quantifies interfacial hybridization and directly correlates with the metal-sulfur coupling strength. Catalysts with higher I_f-d exhibit lower activation energy (E_a) and the overpotential (η) during SRR, validating its role as a structure-activity descriptor for catalyst screening and design. Guided by this descriptor, the screened Lu₂O₃ catalyst minimizes kinetic barriers (0.088 eV), and thus enabling stable cycling for over 20 000 cycles at 5 C, and an ultra-high areal capacity of 14.48 mAh cm^-2 at room temperature, among the highest performances reported for ASSLSBs. This work fills the mechanistic gap between interfacial orbital interactions and battery performance and paves the way toward catalyst design for high-energy, long-life ASSLSBs.