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Abstract Sulfide solid‐state electrolytes have remarkable ionic conductivity and low mechanical stiffness but suffer from relatively narrow electrochemical and chemical stability with electrodes. Therefore, pairing sulfide electrolytes with the proper cathode is crucial in developing stable all‐solid‐state Li batteries (ASLBs). Herein, one type of thioantimonate ion conductor, Li_6+xGe_xSb_1−xS₅I, with different compositions is systematically synthesized and studied, among these compositions, an outstanding ionic conductivity of 1.6 mS cm⁻¹is achieved with Li_6.6Ge_0.6Sb_0.4S₅I. To improve the energy density and advance the interface compatibility, a metal sulfide FeS₂cathode with a high theoretical capacity (894 mAh g⁻¹) and excellent compatibility with sulfide electrolytes is coupled with Li_6.6Ge_0.6Sb_0.4S₅I in ASLBs without additional interface engineering. The structural stabilities of Li_6.6Ge_0.6Sb_0.4S₅I and FeS₂during cycling are characterized by operando energy dispersive X‐ray diffraction, which allows rapid collection of structural data without redesigning or disassembling the sealed cells and risking contamination by air. The electrochemical stability is assessed, and a safe operating voltage window ranging from 0.7≈2.4 V (vs. In–Li) is confirmed. Due to the solid confinement in the ASLBs, the Fe⁰aggregation and polysulfides shuttle effects are well addressed. The ASLBs exhibit an outstanding initial capacity of 715 mAh g⁻¹at C/10 and are stable for 220 cycles with a high capacity retention of 84.5% at room temperature.