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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.

Nanostructured materials can exhibit phase change behavior that deviates from the macroscopic phase behavior. This is exemplified by the ambiguity for the equilibrium phases driving the first open‐circuit voltage (OCV) plateau for the lithiation of Fe₃O₄nanocrystals. Adding complexity, the relaxed state for Li_xFe₃O₄is observed to be a function of electrochemical discharge rate. The phases occurring on the first OCV plateau for the lithiation of Fe₃O₄nanocrystals have been investigated with density functional theory (DFT) through the evaluation of stable, or hypothesized metastable, reaction pathways. Hypotheses are evaluated through the systematic combined refinement with X‐ray absorption spectroscopy (XAS), X‐ray diffraction (XRD) measurements, neutron‐diffraction measurements, and the measured OCV on samples lithiated tox= 2.0, 3.0, and 4.0 in Li_xFe₃O₄. In contrast to the Li–Fe–O bulk phase thermodynamic pathway, Fe⁰is not observed as a product on the first OCV plateau for 10–45 nm nanocrystals. The phase most consistent with the systematic refinement is LiFe₃O₄, showing Li+Fe cation disorder. The observed equilibrium concentration for conversion to Fe⁰occurs atx= 4.0. These definitive phase identifications rely heavily on the systematic combined refinement approach, which is broadly applicable to other nano‐ and mesoscaled systems that have suffered from difficult or crystallite‐size‐dependent phase identification.