Solid Additive Engineering Breaks the Stability‐Conductivity Trade‐Off in Halide Electrolytes for Robust All‐Solid‐State Batteries

ABSTRACT Solid‐state electrolytes (SSEs) inherently face a stability–conductivity trade‐off, whereby strategies that enhance moisture tolerance and oxidative stability often compromise Li + transport. Here, inspired by additive engineering in liquid electrolytes, we demonstrate that a similar concept can be translated into solid electrolytes. A solid‐additive engineering strategy is developed by incorporating oxide nanoparticles into halide SSEs, enabling stability regulation without sacrificing fast ion transport. Combined experimental characterizations and atomistic simulations reveal that a fraction of the additives participates in local interfacial reconstruction that generates disordered Li‐ion coordination environments, whereas the majority remains as chemically stable nanodomains within the electrolyte matrix. This cooperative structural regulation simultaneously enhances ionic conductivity, electrochemical stability, and moisture tolerance. Moreover, ASSBs exhibit markedly improved cycling reversibility and high‐voltage stability. Full cells incorporating such solid additives sustain stable operation over 3300 cycles at 1 C and retain considerable reversibility even at 4.8 V. The solid additive effect is further demonstrated across multiple halide electrolytes. These findings establish solid additive engineering as a general and cost‐effective strategy for overcoming the intrinsic stability–conductivity trade‐off in SSEs, providing new opportunities for the design of robust solid‐state battery chemistries.