Sulfur cathodes in Li–S batteries suffer significant volumetric expansion and lack of catalytic activity for polysulfide conversion. In this study, a confined self‐reduction synthetic route is developed for preparing nanocomposites using diverse metal ions (Mn2+, Co2+, Ni2+, and Zn2+)‐introduced Al‐MIL‐96 as precursors. The Ni2+‐introduced Al‐MIL‐96‐derived nanocomposite contains a "hardness unit", amorphous aluminum oxide framework, to restrain the volumetric expansion, and a "softness unit", Ni nanocrystals, to improve the catalytic activity. The oxygen‐potential diagram theoretically explains why Ni2+ is preferentially reduced. Postmortem microstructure characterization confirms the suppressive volume expansion. The in situ ultraviolet–visible measurements are performed to probe the catalytic activity of polysulfide conversion. This study provides a new perspective for designing nanocomposites with "hardness units" and "softness units" as sulfur or other catalyst hosts. [ABSTRACT FROM AUTHOR]