Highly Stable CsPbX 3 /PbSO 4 Core/Shell Nanocrystals Synthesized by a Simple Post‐Treatment Strategy

All‐inorganic lead halide perovskite (CsPbX 3 , X = Cl, Br, I, or their mixture) nanocrystals (NCs) have recently emerged as a promising photoelectric material; however, developing highly stable CsPbX 3 NCs still remains a challenge due to their intrinsic ionic nature. This work presents a simple and facile post‐treatment strategy to encapsulate CsPbX 3 NC with PbSO 4 nanoshell at single‐particle level. Compared to pristine CsPbX 3 NCs, the as‐prepared CsPbX 3 /PbSO 4 NCs exhibit significant enhancement in the optical properties. Benefiting from the efficient protection of inert PbSO 4 nanoshell, the obtained CsPbX 3 /PbSO 4 core/shell NCs demonstrate great stability against polar solvents, including ethanol and acetone, and light irradiation. Furthermore, anion‐exchange reaction is strongly restricted upon the encapsulation of PbSO 4 shell. This work provides an alternative approach for the synthesis of highly stable CsPbX 3 NCs, and paves a new way for the preparation of core/shell heterostructures based on lead halide perovskite NCs.