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, Li6+xGexSb1−xS5I, with different compositions is systematically synthesized and studied, among these compositions, an outstanding ionic conductivity of 1.6 mS cm−1 is achieved with Li6.6Ge0.6Sb0.4S5I. To improve the energy density and advance the interface compatibility, a metal sulfide FeS2 cathode with a high theoretical capacity (894 mAh g−1) and excellent compatibility with sulfide electrolytes is coupled with Li6.6Ge0.6Sb0.4S5I in ASLBs without additional interface engineering. The structural stabilities of Li6.6Ge0.6Sb0.4S5I and FeS2 during 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 Fe0 aggregation and polysulfides shuttle effects are well addressed. The ASLBs exhibit an outstanding initial capacity of 715 mAh g−1 at C/10 and are stable for 220 cycles with a high capacity retention of 84.5% at room temperature.

Conventional supported amine adsorbents to date are known to suffer from the trade-off between increasing amine content and decreasing access to amine sites. To address this challenge size selection of loaded amines may be a useful tool.

Achieving high energy density in all-solid-state lithium batteries will require the design of thick cathodes, and these will need to operate reversibly under normal use conditions. We use high-energy depth-profiling X-ray diffraction to measure the localized lithium content of Li1–x Ni1/3Mn1/3Co1/3O2 (NMC111) through the thickness of 110 μm thick composite cathodes. The composite cathodes consisted of NMC111 of varying mass loadings mixed with argyrodite solid electrolyte Li6PS5Cl (LPSC). During cycling at C/10, substantial lithiation gradients developed, and varying the NMC111 loading altered the nature of these gradients. Microstructural analysis and cathode modeling showed this was due to high tortuosities in the cathodes. This was particularly true in the solid electrolyte phase, which experienced a marked increase in tortuosity factor during the initial charge. Our results demonstrate that current distributions are observed in sulfide-based composites and that these will be an important consideration for practical design of all-solid-state batteries.