Engineering Electronic Radial Effects for Fast Li⁺ Transport in Solid-State Electrolytes

Achieving high Li⁺ conductivity, near-unity transference numbers, and stable interfaces in solid-state electrolytes remains a major challenge for lithium-metal batteries. Here we introduce a radial-effect design principle: relativistic expansion and spin-orbit coupling of 5d orbitals enhance s-d/p-d hybridization, weaken Li-anion interactions, and lower migration barriers. An entropy-based descriptor, S_d, trained and validated with machine learning across >10,000 oxides, sulfides, and halides captures this effect. Machine-learning-guided high-throughput screening flags monoclinic HfO₂, whose 5d² radial expansion lowers migration barriers by ∼45% vs Sc₂O₃ or Y₂O₃. Guided by this insight, we employ millisecond flash-Joule heating to convert HfO₂ into nanosized single crystals, then embed them in a Li-conductive binder to create sc-HfO₂@LCB, whose radial coupling yields interconnected Li⁺ pathways (1.23 mS cm^-1, 30°C; t_{Li +} = 0.82, 25°C) and a 4.8 V electrochemical window. Operando Raman/XANES confirms faster Li⁺ transport. Consequently, 2 Ah LiNi_0.9Co_0.05Mn_0.05O₂‖Li pouch cells deliver ∼472 Wh kg^-1 (stack-level), maintain superior rate capability over hundreds of cycles, and survive 150°C hot-plate tests. These results establish radial-effect engineering as a sophisticated strategy for high-performance, thermally resilient solid-state batteries.