In Situ Building Halide‐Alloy Dual‐Phase Interfaces for Dendrite‐Free Sulfide Solid‐State Batteries

Argyrodite‑type sulfide solid electrolytes are promising candidates for all‑solid‑state lithium‑metal batteries due to their high Li‑ion conductivity and favorable mechanical compliance. Nevertheless, their inherent chemical reactivity toward Li metal induces continuous interfacial degradation and filamentary Li growth, undermining their practical viability. Herein, a surface-modified Li₆PS₅Cl electrolyte is engineered via co-treatment with GaCl₃ and InCl₃, enabling the in situ formation of a multifunctional interphase upon contact with lithium metal. These conversion reactions produce a LiCl-rich, electronically insulating matrix interlaced with lithiophilic Li-Ga and Li-In alloys. The LiCl matrix enhances interfacial energy to inhibit dendrite formation, while the alloy network promotes continuous and uniform lithium-ion transport. Concurrently, this interfacial layer scavenges irregular Li deposits formed during initial plating and acts as a conformal interlayer to maintain intimate contact. Consequently, Li||Li symmetric cells achieve exceptional cycling stability (>2600 h at 0.5 mA cm^-2). Paired with a Nickel-rich LiNi_0.9Mn_0.05Co_0.05O₂ cathode, full cells retain 80% capacity retention after 1000 cycles at 0.5 C with an ultra-thin 50 µm Li anode. This study highlights the effectiveness of engineered surface coating layers in stabilizing the anode interface with minimal disruption to the sulfide electrolyte framework.