Constructing an Adaptive Flexible Interphase on Halide Electrolytes by Lewis‐Acid‐Induced Polymerization for High‐Voltage All‐Solid‐State Lithium Batteries

All‐solid‐state lithium batteries (ASSLBs) combining halide solid electrolytes (HSEs) with high‐nickel layered oxide cathodes offer great promise for high energy density, yet they suffer from severe interfacial issues such as oxidative decomposition, mechanical degradation, and chemical incompatibility with sulfide electrolytes. Herein, we report a Lewis‐acid‐induced surface‐softening strategy that constructs a nanoscale adaptive flexible interphase (AFI) via in situ polymerization of acrylonitrile (AN) on a representative halide electrolyte, Li 3 InCl 6 (LIC). The resulting AFI layer concurrently provides ionic conductivity, excellent oxidation stability (>4.8 V), and favorable viscoelasticity, thereby synergistically enhancing the electrochemical, chemical, and mechanical stability of the interface without compromising bulk ion transport. When paired with a single‐crystal ultrahigh‑nickel cathode LiNi 0.94 Co 0.05 Mn 0.01 O 2 (Ni94), the surface‑softened LIC (S‑LIC) enables ASSLBs delivering specific capacities of 223.8 and 209.5 mAh g −1 at 2 C and 4 C, respectively, retaining over 85% of the initial capacity after 2000 cycles at 2 C, and operate stably for more than 9000 cycles at 10 C. Even under an ultrahigh cutoff voltage of 4.8 V, a capacity retention of 81% is achieved after 800 cycles at 2 C. This work provides a broadly applicable and efficient interfacial engineering route for high‑voltage halide‑based ASSLBs.